Genes, Inheritance, and Selection (OCR)
Are reproductive cells, like sperm or egg cells, that carry half the genetic information (one set of chromosomes) an organism has.
When gametes combine during fertilization, they create offspring with a full set of chromosomes (one from each parent).
Formation
Meiosis - Gametes are formed through meiosis, which involves two rounds of cell division (meiosis I and meiosis II) resulting in four non-identical haploid cells from one diploid cell.
Meiosis I - Homologous chromosomes are separated, reducing the chromosome number by half.
Genetic diversity is created during prophase I through crossing over, a process in which homozygous chromosomes exchange genetic material to create new combinations of genes.
Meiosis II - Sister chromatids are separated, similar to mitosis.
Fusion
Fertilization - During fertilization, a sperm cell (male gamete) and an egg cell (female gamete) fuse to form a diploid zygote.
Zygote Development - The zygote undergoes multiple rounds of mitosis to develop into a multicellular organism.
Structure
DNA Packaging - Each chromosome consists of a single, long DNA molecule wrapped around histone proteins, forming a complex called chromatin.
Chromatin - Chromatin can be further compacted to form the visible structures known as chromosomes during cell division.
Types
Autosomes - Chromosomes that are not involved in determining sex. Humans have 22 pairs of autosomes.
Sex Chromosomes - Chromosomes that determine the sex of an individual. Humans have one pair of sex chromosomes (XX in females, XY in males).
Function
Protein Coding - Genes contain the instructions for synthesizing proteins, which perform a variety of structural, enzymatic, and regulatory functions in the body.
Non-coding RNA - Some genes code for RNA molecules that are not translated into proteins but have roles in gene regulation, splicing, and other cellular processes.
Example - tRNA, rRNA, and microRNAs are non-coding, but assemble amino acids into proteins, catalyze protein synthesis, and regulate gene expression, respectively.
Expression
Transcription - The process by which the information in a sequence of DNA nucleotides is copied to a newly synthesised messenger RNA (mRNA) strand. This involves the enzyme RNA polymerase binding to a noncoding DNA sequence called a promoter. This promoter sequence does not code for any amino acids and instead serves as a binding site for RNA Polymerase. Proteins called transcription factors assist in the binding of RNA Polymerase and the promoter.
The newly synthesized RNA strand must be antiparallel to the template DNA strand, so RNA Polymerase adds new nucleotides in the 5’ to 3’ direction and reads in the 3’ to 5’ direction.
Example - From a DNA sequence 3’ - ACG TAC GTA CGT - 5’, the RNA sequence will be 5’ - UTC AUG CAU GCA - 3’. This is because the mRNA sequence is not identical to the sequence of the DNA strand but instead is the complement of the DNA strand from which it was transcribed.
In Eukaryotic cells, three modifications must occur to the pre-RNA before it can leave the nucleus: the removal of introns and the joining of exons, the addition of guanosine triphosphate (GTP) gap to the 5’ end of the RNA, and the addition of a poly-adenine (poly-A) tail to the 3’ end of the RNA.
Translation - The mRNA is then translated into a protein at the ribosome, with the help of transfer RNA (tRNA) molecules with specific anticodons that link the appropriate amino acid to specific codons.
Translation begins when tRNA pairs with a specific start codon (AUG). After the first amino acid is placed in the ribosome by tRNA, the ribosome moves to the next codon, where a new tRNA with the appropriate anticodon and amino acid is paired. The ribosome then catalyzes a peptide bond between the amino acids brought to the ribosome.
Stop codons (also known as nonsense codons) do not code for amino acids. When ribosomes reach stop codons, proteins called release factors bind to the ribosome and cause it to disassemble and release the polypeptide chain.
Allele: One of two or more versions of a genetic sequence at a particular region on a chromosome.
Variant: Describes a subtype of a microorganism that is genetically distinct from a main strain, but not sufficiently different to be termed a distinct strain.
Dominance
Complete Dominance: The dominant allele's phenotype completely masks the recessive allele's effect in heterozygous individuals.
Incomplete Dominance: The phenotype of heterozygous individuals is a blend of both alleles.
Codominance
Example - In the case of AB blood type, both A and B alleles are expressed, resulting in the AB phenotype where both antigens are present on red blood cells.
An allele is dominant if it expresses its phenotype even when only one copy is present in the genotype.
A dominant allele will mask the effect of a recessive allele when both are present.
Example - Huntington's disease is caused by a mutation in the HTT gene. Individuals with even one copy of the mutated allele will develop the disease, typically in mid-adulthood.
An allele is recessive if it expresses its phenotype only when two copies of the allele are present in the genotype.
A recessive allele's effect is masked by the presence of a dominant allele.
Example - Cystic fibrosis is caused by mutations in the CFTR gene. Individuals must inherit two copies of the faulty allele (one from each parent) to exhibit the disease, which affects the respiratory and digestive systems.
An individual is homozygous for a trait if they have two identical alleles for that trait.
Example - A person with two identical alleles for a trait, such as bb for blue eyes, is homozygous recessive. Conversely, a person with two dominant alleles, such as BB for brown eyes, is homozygous dominant.
An individual is heterozygous for a trait if they have two different alleles for that trait.
Example - A person with one allele for brown eyes (B) and one allele for blue eyes (b) is heterozygous (Bb). The dominant brown allele will mask the expression of the recessive blue allele, resulting in brown eyes.
The genetic makeup of an organism.
Complex Traits
Polygenic Inheritance: Traits like height, skin color, and weight are influenced by multiple genes and their interactions. These traits often show continuous variation and are influenced by environmental factors.
Example - Height is influenced by several genes that affect bone growth, metabolism, and hormone levels, as well as nutritional and environmental factors during development.
An observable trait.
Phenotypic Plasticity: The ability of an organism to change its phenotype in response to environmental conditions, allowing for adaptation to different environments.
Example - The Himalayan rabbit has fur that changes color based on temperature. In colder temperatures, the fur on its ears, nose, feet, and tail turns black, while in warmer conditions, the fur remains white.
Importance - Phenotypic plasticity allows organisms to survive and reproduce in varying environments, contributing to their evolutionary success.
Genome: The complete set of DNA (deoxyribonucleic acid) in an organism, including all of its genes and non-coding sequences.
Components
Genes - Segments of DNA that code for proteins or functional RNA.
Non-coding DNA - Includes regulatory elements (promoters, enhancers, silencers), introns (non-coding sections within genes), and repetitive DNA sequences.
Human Genome
Size - Approximately 3 billion base pairs.
Gene Count - About 20,000-25,000 genes.
Chromosomes - Humans have 23 pairs of chromosomes (46 total), including 22 pairs of autosomes (non sex-related chromosomes) and 1 pair of sex chromosomes (XX or XY).
Importance
The genome contains all the instructions needed for the development, functioning, growth, and reproduction of an organism.
Variations in the genome are the basis for genetic diversity within a species.
Examples
Human Genome Project - An international research effort to sequence and map all the genes of Homo sapiens.
Comparative Genomics - Studies that compare the genomes of different species to understand evolutionary relationships and functional genomics.
Genotype: The genetic makeup of an organism.
Phenotype: The observable characteristics of an organism, influenced by the genotype and environmental factors.
The same genotype can produce different phenotypes under different environmental conditions.
Epigenetics: Study of changes in gene expression caused by mechanisms other than changes in the DNA sequence.
Environmental factors can cause epigenetic changes.
Discontinuous Variation: Traits with a limited number of distinct phenotypes.
Example:
Blood type (A, B, AB, O)
Eye color (blue, green, brown)
Continuous Variation: Traits that exhibit a range of phenotypes.
Example:
Height
Weight
Measurable values
Nutrition - Adequate nutrition is crucial for proper growth and development.
Example: Malnutrition during childhood can stunt growth, regardless of genetic potential.
Climate - Environmental conditions can influence phenotypic traits.
Example: Skin color adaptation to varying levels of UV radiation.
Lifestyle - Physical activity, stress, and exposure to toxins can all impact phenotype.
Example: Exercise can influence muscle mass and overall fitness.
Mutations: Changes in the DNA sequence that can occur spontaneously or due to environmental factors. Mutations cause genetic variation in populations, which can be acted upon by natural selection and can lead to the evolution of populations.
Types
Point Mutations: Single nucleotide changes.
Insertions/Deletions: Addition or loss of nucleotide segments.
Copy Number Variations: Duplications or deletions of large DNA segments.
Effects of Mutations
Neutral Mutations: Most mutations have no effect on the phenotype.
Example: Synonymous mutations that do not change the amino acid sequence of a protein.
Beneficial Mutations: Rare mutations that confer an advantage in the organism’s environment.
Example: CCR5-Δ32 mutation that provides resistance to HIV infection.
Harmful Mutations: Mutations that cause diseases or disorders.
Example: BRCA1 and BRCA2 mutations are associated with an increased risk of breast and ovarian cancer.
Phenotypic Influence
Silent Mutations: Do not affect protein function and thus have no impact on phenotype.
Missense Mutations: Change one amino acid in a protein, which can alter its function.
Example: Sickle cell anemia caused by a missense mutation in the hemoglobin gene.
Nonsense Mutations: Introduce a premature stop codon, leading to a truncated, usually nonfunctional protein.
Example: Duchenne muscular dystrophy caused by nonsense mutations in the dystrophin gene.
Mutations can be caused by environmental factors such as chemicals or radiation, or by random errors in DNA replication and repair mechanisms, or by mistakes in mitosis and meiosis. Some mutations do not change the amino acid sequence of a protein at all due to the redundancy of the genetic code, and the organism’s environment determines whether a mutation is beneficial, harmful, or has no effect.
Protein Structure and Function
Mutations in coding regions can change the amino acid sequence of a protein, affecting its structure and function, as well as the rate of expression of the gene.
Active Sites of Enzymes: Changes in amino acids at the active site can affect enzyme activity.
Example: PKU (phenylketonuria) caused by mutations in the PAH gene affecting the enzyme's ability to metabolize phenylalanine.
Promoters - Regions of DNA where RNA polymerase binds to initiate transcription. Mutations here can increase or decrease gene expression.
Example: Mutations in the promoter of the TERT gene, which affects telomerase activity and is linked to cancer.
Enhancers - DNA sequences that enhance the transcription of an associated gene. Mutations can disrupt normal gene regulation.
Example: Enhancer mutations affecting the SHH gene can lead to developmental disorders.
Silencers - DNA sequences that can repress transcription. Mutations can lead to inappropriate gene activation.
Example: Mutations in silencers can lead to overexpression of oncogenes in cancer.
Transcription Factors: Proteins that bind to specific DNA sequences to regulate transcription. Mutations can alter their binding affinity.
Example: Mutations in the p53 transcription factor, which plays a critical role in regulating the cell cycle and preventing cancer.
Epigenetic Changes: Modifications such as DNA methylation and histone acetylation can influence gene expression without altering the DNA sequence.
Example: DNA methylation patterns affecting the expression of tumor suppressor genes in cancer.
The production of offspring by a single organism without the fusion of gametes.
Advantages
Rapid Population Growth - Asexual reproduction allows organisms to reproduce quickly. For example, bacteria can divide every 20 minutes under ideal conditions.
No Mate Required - Organisms such as certain plants, bacteria, and fungi can reproduce without a mate, which is beneficial in isolated environments.
Energy Efficient - Since there is no need to find a mate, asexual reproduction saves the energy that would otherwise be spent on mating behaviors and structures.
Genetically Identical Offspring - This ensures the preservation of successful genetic traits. For example, a well-adapted plant can produce many identical copies of itself through cloning.
Disadvantages
Lack of Genetic Variation - In changing environments, this can be detrimental. For example, a disease that affects one individual could potentially wipe out the entire population due to genetic uniformity.
Higher Susceptibility to Diseases - A uniform genetic makeup means that a single disease can affect all individuals equally, such as the Irish potato famine caused by a lack of genetic diversity.
Overcrowding - Rapid reproduction can lead to competition for limited resources, leading to population crashes.
The production of new organisms by the combination of genetic information of two individuals of different sexes.
Advantages
Genetic Variation - Sexual reproduction generates genetic diversity, which enhances adaptability. For instance, human populations show a wide range of genetic diversity, increasing resilience to diseases.
Evolutionary Flexibility - Populations can better adapt to changing environments. This is evident in species that thrive in varied and changing habitats.
Disadvantages
Slower Reproduction Rate - The need for two individuals to mate and the longer gestation periods slow down population growth. For example, elephants have a long gestation period of about 22 months.
Need for a Mate - Finding a mate can be challenging, especially in sparse populations or those with skewed sex ratios.
Complexity - Processes like meiosis and fertilization are complex, and errors such as nondisjunction leads to conditions like Down syndrome.
Cells containing a single (n) set of chromosomes.
Examples:
Gametes (sperm and egg cells in animals), spores in fungi.
Human Context - Human gametes each have 23 chromosomes. Fertilization combines these to restore the diploid number.
Cells containing two (2n) sets of chromosomes, one from each parent.
Examples:
Somatic (body) cells in animals and plants.
Human Context - Human somatic cells have 46 chromosomes, organized into 23 pairs.
Process: Involves two rounds of division (Meiosis I and Meiosis II).
Meiosis I - Homologous chromosomes separate, resulting in two haploid cells.
Meiosis II - Sister chromatids separate, resulting in four haploid gametes.
Importance
Genetic Variation - Crossing over (exchange of genetic material between homologous chromosomes) and independent assortment (random distribution of homologous chromosomes) introduce genetic diversity.
Diploid to Haploid Transition - Reduces chromosome number by half, ensuring that fertilization restores the diploid state and offspring have the proper number of chromosomes.
Alleles: Different forms of a gene.
Example: The gene for flower color in peas has alleles for purple (P) and white (p).
Homozygous - Both alleles are the same.
Heterozygous - The alleles are different.
Dominant (P): Expresses the trait over the recessive allele.
Only one copy is needed to express the trait.
Recessive (p): Two copies are needed to express the trait.
Example: Pea Plants - Purple flower color (P) is dominant over white (p). A plant with Pp genotype will have purple flowers.
Punnett Squares
A tool used to predict the outcome of genetic crosses.
Monohybrid Cross: Involves one gene with two alleles.
Example Cross: Heterozygous parents (Pp x Pp).
Genotype Ratio - 1 PP : 2 Pp : 1 pp.
Phenotype Ratio - 3 purple : 1 white.
Example Problems:
Homozygous Dominant x Homozygous Recessive (PP x pp):
Offspring Genotype - 100% Pp (all heterozygous).
Offspring Phenotype - 100% dominant trait.
Heterozygous x Heterozygous (Pp x Pp):
Offspring Genotype Ratio - 1 PP : 2 Pp : 1 pp.
Offspring Phenotype Ratio - 3 dominant (purple) : 1 recessive (white).
Punnett Square Example:
Chromosomes Involved:
Females (XX) - Two X chromosomes.
Males (XY) - One X and one Y chromosome.
Punnett Square Example: Cross between a female (XX) and a male (XY):
Possible Combinations - XX (female), XY (male).
Probability - 50% XX (female), 50% XY (male).
Detailed Example
Mother's Gametes (X, X)
Father's Gametes (X, Y)
X-linked dominant and recessive genes N and n, respectively
Punnett Square:
The Y-Chromosome is typically much shorter than the X-Chromosome and carries fewer coding genes
Traits that are controlled by multiple genes, each contributing to the phenotype.
Examples:
Human Height - Controlled by many genes; shows a continuous range of variation.
Skin Color - Influenced by multiple genes, leading to a wide variety of skin tones.
Characteristics:
Continuous Variation: Traits do not fall into discrete categories but rather a spectrum.
Quantitative Traits: Often measured and influenced by environmental factors.
Experiments on Pea Plants
Studied seven traits.
Formulated the basic principles of inheritance.
Key Contributions:
Law of Segregation: Each individual has two alleles for each gene, which segregate during gamete formation, so each gamete receives one allele.
Law of Independent Assortment: Genes for different traits assort independently of one another during gamete formation.
Impact on Genetics:
Mendel's work, published in 1866, was largely unrecognized until the early 20th century.
Rediscovered by scientists such as Hugo de Vries, Carl Correns, and Erich von Tschermak.
Formed the foundation for modern genetics, including the understanding of genetic inheritance and the role of chromosomes.
Discovery of DNA - James Watson and Francis Crick's double-helix model in 1953 explained how genetic information is stored and replicated.
Human Genome Project - Completed in 2003, it mapped all human genes, enhancing our understanding of genetic diseases and variation.
CRISPR and Gene Editing - Modern techniques allow precise modifications to the genome, offering potential treatments for genetic disorders.
Refers to the differences in DNA sequences between individuals of the same species.
It's like a fingerprint that is unique to each organism but with some similarities within a population.
There is usually extensive genetic variation within a population of a species.
The process of grouping organisms based on shared characteristics has undergone a fascinating transformation alongside the understanding of biological advancement. The following are the impacts of developments on classification systems, considering both traditional and modern approaches.
Natural Classification Systems: Aims to group organisms based on their evolutionary relationships, reflecting their shared ancestry.
Early attempts relied on observable characteristics like morphology (physical form), anatomy (internal structure), and embryology (development).
Impact of Biology on Natural Systems:
Microscopy: The invention of powerful microscopes allowed scientists to observe and categorize organisms based on cellular structures, leading to the identification of microorganisms and a deeper understanding of cell types.
Paleontology: The study of fossils provided insights into extinct organisms and their relationships to living species, refining the evolutionary picture.
Limitations of Traditional Approaches:
Convergent Evolution: Organisms from different evolutionary lineages can develop similar physical features due to adaptation to similar environments. This convergence can be misleading for classification based solely on morphology.
Hidden Similarities: Microscopic features may not always reveal the true evolutionary relationships between organisms.
With the advent of DNA sequencing technology, a new era of classification dawned:
This approach analyzes the DNA or RNA sequences of organisms to construct evolutionary trees (phylogenetic trees).
Trees depict the relationships between different species based on their shared genetic heritage.
Advantages:
Objective and Universal: DNA sequences provide a more objective and universal measure of evolutionary relationships, overcoming limitations of morphology.
Identifying Deep Relationships: Genetic analysis based on DNA similarity can reveal relationships between organisms that may not be evident from physical appearance, especially for distantly related species.
Impact on Classification:
Reclassification of Species: DNA analysis has led to the reclassification of many species based on their true evolutionary relationships. This has reorganized the entire tree of life, placing organisms in more accurate positions.
Discovery of New Species: DNA sequencing has facilitated the discovery of new species with unique genetic signatures, even if their morphology is similar to known species.
Explains how life has changed and diversified over time.
A change in the inherited characteristics of a population over time, through a process of natural selection, may result in the formation of new species.
Is driven by the powerful force of natural selection, which favors variants (individuals with genetic differences) that are better suited to their environment.
Key Elements
Variation: Populations are not uniform.
Individuals within a population exhibit genetic variation due to mutations in their DNA and sexual reproduction (shuffling of genes).
These variations can affect traits, the physical or behavioral characteristics of an organism.
Heritability: Variations that are heritable, meaning they can be passed on from parents to offspring, play a key role in evolution.
Offspring that inherit beneficial traits from their parents are more likely to survive and reproduce.
Differential Reproduction: Not all individuals in a population reproduce equally.
Those with traits better suited to their environment have a higher chance of surviving and passing on their genes.
This differential reproduction leads to a gradual shift in the frequency of those advantageous traits over generations.
Action: Imagine a population of beetles living on a dark-colored tree trunk. Some beetles may have a naturally darker body color (melanism) due to a genetic variation, while others are lighter.
Environmental Pressure: If a predator bird starts hunting these beetles, the lighter-colored beetles become easier targets against the dark background.
Differential Survival: The darker beetles, due to their camouflage, are more likely to survive and reproduce.
Shift in Population: Over generations, the population will gradually have a higher proportion of dark-colored beetles as the genes for darker coloration become more prevalent.
Outcomes
Adaptation: Natural selection leads to adaptation, where populations become better suited to their environment. In the beetle example, dark coloration becomes an adaptation for camouflage.
Speciation: Over extended periods, significant genetic differences between populations can lead to speciation, the formation of new species.
Directional Selection: one extreme of the range of phenotypes is favored by natural selection, resulting in the frequency of that phenotype to increase over time.
Stabilizing Selection: The intermediate phenotype is favored and extreme phenotypes are selected against.
Disruptive Selection: Individuals on both extremes of the phenotypic range are more likely to survive and reproduce than individuals with an intermediate phenotype.
Important: Evolution occurs in populations, not individuals.
Fossil Record
Fossils are the preserved remains or traces of organisms from past eras as they offer a glimpse into the history of life and provide a timeline for how life has changed over time.
Transitional Fossils: Fossils that exhibit characteristics of both ancestral and descendant species.
They bridge the gap between different groups of organisms, demonstrating the gradual process of evolution. For example, fossils like Tiktaalik, with both fish-like and amphibian-like features, show the transition from water to land in vertebrates.
Fossil Distribution: The distribution of fossils across different geographic locations reflects the movement and diversification of life forms over time.
For instance, finding dinosaur fossils on continents that were once joined supports the theory of continental drift.
Antibiotic Resistance in Bacteria
Bacteria reproduce rapidly, and mutations in their DNA can occur during this process. Some mutations can provide bacteria with resistance to antibiotics, a phenomenon known as antibiotic resistance.
Antibiotic Use as a Selective Pressure: When antibiotics are used, they kill bacteria that are susceptible to them.
However, bacteria with resistance mutations survive and reproduce.
Increased Prevalence of Resistant Bacteria: Over time, the use of antibiotics selects resistant strains of bacteria, leading to an increase in their prevalence.
This rapid evolution within a few human generations demonstrates the power of natural selection.
Vestigial Structures
Some organisms contain anatomical features that no longer have a purpose in the modern organism but may have had a function in an ancestral organism.
Molecular Evidence
Comparing DNA sequences from different organisms can provide evidence of evolution. The more recently the organisms share a common ancestor, the more similar their DNA sequence will be.
For example, the GAPDH gene in humans and in chimpanzees is over 99% similar in sequence, but the similarity of the gene between humans and in dogs is only about 91% similar, indicating that humans and chimpanzees share a more recent common ancestor.
Voyage of the Beagle: Darwin's journey aboard the HMS Beagle exposed him to the diverse flora and fauna of South America and the Galapagos Islands. He observed distinct variations among species, particularly the finches on the Galapagos, which sparked his curiosity about the origin of these variations.
On the Origin of Species: In 1859, Darwin published his groundbreaking book, "On the Origin of Species," proposing the theory of evolution by natural selection. He argued that:
All living organisms share a common ancestor.
Species are not static but change over time through descent with modification.
The mechanism driving this change is natural selection, where individuals with traits best suited to their environment have a higher chance of survival and reproduction, passing on those advantageous traits to offspring.
Independent Discovery: While working as a naturalist in Southeast Asia, Wallace independently developed a similar theory of evolution by natural selection. He sent his ideas to Darwin, prompting Darwin to publish his own work earlier than originally planned.
Unifying Principle: Darwin and Wallace's theory provided a unifying principle for understanding the diversity of life on Earth. It explained how complex organisms could arise from simpler ones through a gradual process of change.
New Fields of Study: The theory stimulated the development of new fields of biology, such as comparative anatomy, paleontology (study of fossils), and evolutionary biology.
Understanding of Change: It changed the way we view the natural world, acknowledging the dynamic nature of life and the interconnectedness of all living things.
Crucial in conserving biodiversity by serving as a secure repository for the seeds of various plant species.
Function like a safety net, safeguarding the genetic diversity of our planet's flora in the face of potential threats.
Habitat Loss and Degradation: Human activities like deforestation, urbanization, and pollution contribute to habitat loss and degradation, pushing plant species towards extinction.
Seed banks provide a backup plan, preserving the genetic heritage of these threatened plants.
Climate Change: Disrupts ecosystems and alters growing conditions.
Seed banks can store seeds from populations adapted to different climates, ensuring the availability of genetic material for future restoration efforts.
Disease Outbreaks: Diseases can devastate entire plant populations.
Seed banks can house seeds from resistant varieties, ensuring the survival of the species and potentially providing sources for breeding disease-resistant crops.
Accidental Loss: Natural disasters or human errors can lead to the accidental loss of plant populations.
Seed banks offer a backup for such situations.
Are reproductive cells, like sperm or egg cells, that carry half the genetic information (one set of chromosomes) an organism has.
When gametes combine during fertilization, they create offspring with a full set of chromosomes (one from each parent).
Formation
Meiosis - Gametes are formed through meiosis, which involves two rounds of cell division (meiosis I and meiosis II) resulting in four non-identical haploid cells from one diploid cell.
Meiosis I - Homologous chromosomes are separated, reducing the chromosome number by half.
Genetic diversity is created during prophase I through crossing over, a process in which homozygous chromosomes exchange genetic material to create new combinations of genes.
Meiosis II - Sister chromatids are separated, similar to mitosis.
Fusion
Fertilization - During fertilization, a sperm cell (male gamete) and an egg cell (female gamete) fuse to form a diploid zygote.
Zygote Development - The zygote undergoes multiple rounds of mitosis to develop into a multicellular organism.
Structure
DNA Packaging - Each chromosome consists of a single, long DNA molecule wrapped around histone proteins, forming a complex called chromatin.
Chromatin - Chromatin can be further compacted to form the visible structures known as chromosomes during cell division.
Types
Autosomes - Chromosomes that are not involved in determining sex. Humans have 22 pairs of autosomes.
Sex Chromosomes - Chromosomes that determine the sex of an individual. Humans have one pair of sex chromosomes (XX in females, XY in males).
Function
Protein Coding - Genes contain the instructions for synthesizing proteins, which perform a variety of structural, enzymatic, and regulatory functions in the body.
Non-coding RNA - Some genes code for RNA molecules that are not translated into proteins but have roles in gene regulation, splicing, and other cellular processes.
Example - tRNA, rRNA, and microRNAs are non-coding, but assemble amino acids into proteins, catalyze protein synthesis, and regulate gene expression, respectively.
Expression
Transcription - The process by which the information in a sequence of DNA nucleotides is copied to a newly synthesised messenger RNA (mRNA) strand. This involves the enzyme RNA polymerase binding to a noncoding DNA sequence called a promoter. This promoter sequence does not code for any amino acids and instead serves as a binding site for RNA Polymerase. Proteins called transcription factors assist in the binding of RNA Polymerase and the promoter.
The newly synthesized RNA strand must be antiparallel to the template DNA strand, so RNA Polymerase adds new nucleotides in the 5’ to 3’ direction and reads in the 3’ to 5’ direction.
Example - From a DNA sequence 3’ - ACG TAC GTA CGT - 5’, the RNA sequence will be 5’ - UTC AUG CAU GCA - 3’. This is because the mRNA sequence is not identical to the sequence of the DNA strand but instead is the complement of the DNA strand from which it was transcribed.
In Eukaryotic cells, three modifications must occur to the pre-RNA before it can leave the nucleus: the removal of introns and the joining of exons, the addition of guanosine triphosphate (GTP) gap to the 5’ end of the RNA, and the addition of a poly-adenine (poly-A) tail to the 3’ end of the RNA.
Translation - The mRNA is then translated into a protein at the ribosome, with the help of transfer RNA (tRNA) molecules with specific anticodons that link the appropriate amino acid to specific codons.
Translation begins when tRNA pairs with a specific start codon (AUG). After the first amino acid is placed in the ribosome by tRNA, the ribosome moves to the next codon, where a new tRNA with the appropriate anticodon and amino acid is paired. The ribosome then catalyzes a peptide bond between the amino acids brought to the ribosome.
Stop codons (also known as nonsense codons) do not code for amino acids. When ribosomes reach stop codons, proteins called release factors bind to the ribosome and cause it to disassemble and release the polypeptide chain.
Allele: One of two or more versions of a genetic sequence at a particular region on a chromosome.
Variant: Describes a subtype of a microorganism that is genetically distinct from a main strain, but not sufficiently different to be termed a distinct strain.
Dominance
Complete Dominance: The dominant allele's phenotype completely masks the recessive allele's effect in heterozygous individuals.
Incomplete Dominance: The phenotype of heterozygous individuals is a blend of both alleles.
Codominance
Example - In the case of AB blood type, both A and B alleles are expressed, resulting in the AB phenotype where both antigens are present on red blood cells.
An allele is dominant if it expresses its phenotype even when only one copy is present in the genotype.
A dominant allele will mask the effect of a recessive allele when both are present.
Example - Huntington's disease is caused by a mutation in the HTT gene. Individuals with even one copy of the mutated allele will develop the disease, typically in mid-adulthood.
An allele is recessive if it expresses its phenotype only when two copies of the allele are present in the genotype.
A recessive allele's effect is masked by the presence of a dominant allele.
Example - Cystic fibrosis is caused by mutations in the CFTR gene. Individuals must inherit two copies of the faulty allele (one from each parent) to exhibit the disease, which affects the respiratory and digestive systems.
An individual is homozygous for a trait if they have two identical alleles for that trait.
Example - A person with two identical alleles for a trait, such as bb for blue eyes, is homozygous recessive. Conversely, a person with two dominant alleles, such as BB for brown eyes, is homozygous dominant.
An individual is heterozygous for a trait if they have two different alleles for that trait.
Example - A person with one allele for brown eyes (B) and one allele for blue eyes (b) is heterozygous (Bb). The dominant brown allele will mask the expression of the recessive blue allele, resulting in brown eyes.
The genetic makeup of an organism.
Complex Traits
Polygenic Inheritance: Traits like height, skin color, and weight are influenced by multiple genes and their interactions. These traits often show continuous variation and are influenced by environmental factors.
Example - Height is influenced by several genes that affect bone growth, metabolism, and hormone levels, as well as nutritional and environmental factors during development.
An observable trait.
Phenotypic Plasticity: The ability of an organism to change its phenotype in response to environmental conditions, allowing for adaptation to different environments.
Example - The Himalayan rabbit has fur that changes color based on temperature. In colder temperatures, the fur on its ears, nose, feet, and tail turns black, while in warmer conditions, the fur remains white.
Importance - Phenotypic plasticity allows organisms to survive and reproduce in varying environments, contributing to their evolutionary success.
Genome: The complete set of DNA (deoxyribonucleic acid) in an organism, including all of its genes and non-coding sequences.
Components
Genes - Segments of DNA that code for proteins or functional RNA.
Non-coding DNA - Includes regulatory elements (promoters, enhancers, silencers), introns (non-coding sections within genes), and repetitive DNA sequences.
Human Genome
Size - Approximately 3 billion base pairs.
Gene Count - About 20,000-25,000 genes.
Chromosomes - Humans have 23 pairs of chromosomes (46 total), including 22 pairs of autosomes (non sex-related chromosomes) and 1 pair of sex chromosomes (XX or XY).
Importance
The genome contains all the instructions needed for the development, functioning, growth, and reproduction of an organism.
Variations in the genome are the basis for genetic diversity within a species.
Examples
Human Genome Project - An international research effort to sequence and map all the genes of Homo sapiens.
Comparative Genomics - Studies that compare the genomes of different species to understand evolutionary relationships and functional genomics.
Genotype: The genetic makeup of an organism.
Phenotype: The observable characteristics of an organism, influenced by the genotype and environmental factors.
The same genotype can produce different phenotypes under different environmental conditions.
Epigenetics: Study of changes in gene expression caused by mechanisms other than changes in the DNA sequence.
Environmental factors can cause epigenetic changes.
Discontinuous Variation: Traits with a limited number of distinct phenotypes.
Example:
Blood type (A, B, AB, O)
Eye color (blue, green, brown)
Continuous Variation: Traits that exhibit a range of phenotypes.
Example:
Height
Weight
Measurable values
Nutrition - Adequate nutrition is crucial for proper growth and development.
Example: Malnutrition during childhood can stunt growth, regardless of genetic potential.
Climate - Environmental conditions can influence phenotypic traits.
Example: Skin color adaptation to varying levels of UV radiation.
Lifestyle - Physical activity, stress, and exposure to toxins can all impact phenotype.
Example: Exercise can influence muscle mass and overall fitness.
Mutations: Changes in the DNA sequence that can occur spontaneously or due to environmental factors. Mutations cause genetic variation in populations, which can be acted upon by natural selection and can lead to the evolution of populations.
Types
Point Mutations: Single nucleotide changes.
Insertions/Deletions: Addition or loss of nucleotide segments.
Copy Number Variations: Duplications or deletions of large DNA segments.
Effects of Mutations
Neutral Mutations: Most mutations have no effect on the phenotype.
Example: Synonymous mutations that do not change the amino acid sequence of a protein.
Beneficial Mutations: Rare mutations that confer an advantage in the organism’s environment.
Example: CCR5-Δ32 mutation that provides resistance to HIV infection.
Harmful Mutations: Mutations that cause diseases or disorders.
Example: BRCA1 and BRCA2 mutations are associated with an increased risk of breast and ovarian cancer.
Phenotypic Influence
Silent Mutations: Do not affect protein function and thus have no impact on phenotype.
Missense Mutations: Change one amino acid in a protein, which can alter its function.
Example: Sickle cell anemia caused by a missense mutation in the hemoglobin gene.
Nonsense Mutations: Introduce a premature stop codon, leading to a truncated, usually nonfunctional protein.
Example: Duchenne muscular dystrophy caused by nonsense mutations in the dystrophin gene.
Mutations can be caused by environmental factors such as chemicals or radiation, or by random errors in DNA replication and repair mechanisms, or by mistakes in mitosis and meiosis. Some mutations do not change the amino acid sequence of a protein at all due to the redundancy of the genetic code, and the organism’s environment determines whether a mutation is beneficial, harmful, or has no effect.
Protein Structure and Function
Mutations in coding regions can change the amino acid sequence of a protein, affecting its structure and function, as well as the rate of expression of the gene.
Active Sites of Enzymes: Changes in amino acids at the active site can affect enzyme activity.
Example: PKU (phenylketonuria) caused by mutations in the PAH gene affecting the enzyme's ability to metabolize phenylalanine.
Promoters - Regions of DNA where RNA polymerase binds to initiate transcription. Mutations here can increase or decrease gene expression.
Example: Mutations in the promoter of the TERT gene, which affects telomerase activity and is linked to cancer.
Enhancers - DNA sequences that enhance the transcription of an associated gene. Mutations can disrupt normal gene regulation.
Example: Enhancer mutations affecting the SHH gene can lead to developmental disorders.
Silencers - DNA sequences that can repress transcription. Mutations can lead to inappropriate gene activation.
Example: Mutations in silencers can lead to overexpression of oncogenes in cancer.
Transcription Factors: Proteins that bind to specific DNA sequences to regulate transcription. Mutations can alter their binding affinity.
Example: Mutations in the p53 transcription factor, which plays a critical role in regulating the cell cycle and preventing cancer.
Epigenetic Changes: Modifications such as DNA methylation and histone acetylation can influence gene expression without altering the DNA sequence.
Example: DNA methylation patterns affecting the expression of tumor suppressor genes in cancer.
The production of offspring by a single organism without the fusion of gametes.
Advantages
Rapid Population Growth - Asexual reproduction allows organisms to reproduce quickly. For example, bacteria can divide every 20 minutes under ideal conditions.
No Mate Required - Organisms such as certain plants, bacteria, and fungi can reproduce without a mate, which is beneficial in isolated environments.
Energy Efficient - Since there is no need to find a mate, asexual reproduction saves the energy that would otherwise be spent on mating behaviors and structures.
Genetically Identical Offspring - This ensures the preservation of successful genetic traits. For example, a well-adapted plant can produce many identical copies of itself through cloning.
Disadvantages
Lack of Genetic Variation - In changing environments, this can be detrimental. For example, a disease that affects one individual could potentially wipe out the entire population due to genetic uniformity.
Higher Susceptibility to Diseases - A uniform genetic makeup means that a single disease can affect all individuals equally, such as the Irish potato famine caused by a lack of genetic diversity.
Overcrowding - Rapid reproduction can lead to competition for limited resources, leading to population crashes.
The production of new organisms by the combination of genetic information of two individuals of different sexes.
Advantages
Genetic Variation - Sexual reproduction generates genetic diversity, which enhances adaptability. For instance, human populations show a wide range of genetic diversity, increasing resilience to diseases.
Evolutionary Flexibility - Populations can better adapt to changing environments. This is evident in species that thrive in varied and changing habitats.
Disadvantages
Slower Reproduction Rate - The need for two individuals to mate and the longer gestation periods slow down population growth. For example, elephants have a long gestation period of about 22 months.
Need for a Mate - Finding a mate can be challenging, especially in sparse populations or those with skewed sex ratios.
Complexity - Processes like meiosis and fertilization are complex, and errors such as nondisjunction leads to conditions like Down syndrome.
Cells containing a single (n) set of chromosomes.
Examples:
Gametes (sperm and egg cells in animals), spores in fungi.
Human Context - Human gametes each have 23 chromosomes. Fertilization combines these to restore the diploid number.
Cells containing two (2n) sets of chromosomes, one from each parent.
Examples:
Somatic (body) cells in animals and plants.
Human Context - Human somatic cells have 46 chromosomes, organized into 23 pairs.
Process: Involves two rounds of division (Meiosis I and Meiosis II).
Meiosis I - Homologous chromosomes separate, resulting in two haploid cells.
Meiosis II - Sister chromatids separate, resulting in four haploid gametes.
Importance
Genetic Variation - Crossing over (exchange of genetic material between homologous chromosomes) and independent assortment (random distribution of homologous chromosomes) introduce genetic diversity.
Diploid to Haploid Transition - Reduces chromosome number by half, ensuring that fertilization restores the diploid state and offspring have the proper number of chromosomes.
Alleles: Different forms of a gene.
Example: The gene for flower color in peas has alleles for purple (P) and white (p).
Homozygous - Both alleles are the same.
Heterozygous - The alleles are different.
Dominant (P): Expresses the trait over the recessive allele.
Only one copy is needed to express the trait.
Recessive (p): Two copies are needed to express the trait.
Example: Pea Plants - Purple flower color (P) is dominant over white (p). A plant with Pp genotype will have purple flowers.
Punnett Squares
A tool used to predict the outcome of genetic crosses.
Monohybrid Cross: Involves one gene with two alleles.
Example Cross: Heterozygous parents (Pp x Pp).
Genotype Ratio - 1 PP : 2 Pp : 1 pp.
Phenotype Ratio - 3 purple : 1 white.
Example Problems:
Homozygous Dominant x Homozygous Recessive (PP x pp):
Offspring Genotype - 100% Pp (all heterozygous).
Offspring Phenotype - 100% dominant trait.
Heterozygous x Heterozygous (Pp x Pp):
Offspring Genotype Ratio - 1 PP : 2 Pp : 1 pp.
Offspring Phenotype Ratio - 3 dominant (purple) : 1 recessive (white).
Punnett Square Example:
Chromosomes Involved:
Females (XX) - Two X chromosomes.
Males (XY) - One X and one Y chromosome.
Punnett Square Example: Cross between a female (XX) and a male (XY):
Possible Combinations - XX (female), XY (male).
Probability - 50% XX (female), 50% XY (male).
Detailed Example
Mother's Gametes (X, X)
Father's Gametes (X, Y)
X-linked dominant and recessive genes N and n, respectively
Punnett Square:
The Y-Chromosome is typically much shorter than the X-Chromosome and carries fewer coding genes
Traits that are controlled by multiple genes, each contributing to the phenotype.
Examples:
Human Height - Controlled by many genes; shows a continuous range of variation.
Skin Color - Influenced by multiple genes, leading to a wide variety of skin tones.
Characteristics:
Continuous Variation: Traits do not fall into discrete categories but rather a spectrum.
Quantitative Traits: Often measured and influenced by environmental factors.
Experiments on Pea Plants
Studied seven traits.
Formulated the basic principles of inheritance.
Key Contributions:
Law of Segregation: Each individual has two alleles for each gene, which segregate during gamete formation, so each gamete receives one allele.
Law of Independent Assortment: Genes for different traits assort independently of one another during gamete formation.
Impact on Genetics:
Mendel's work, published in 1866, was largely unrecognized until the early 20th century.
Rediscovered by scientists such as Hugo de Vries, Carl Correns, and Erich von Tschermak.
Formed the foundation for modern genetics, including the understanding of genetic inheritance and the role of chromosomes.
Discovery of DNA - James Watson and Francis Crick's double-helix model in 1953 explained how genetic information is stored and replicated.
Human Genome Project - Completed in 2003, it mapped all human genes, enhancing our understanding of genetic diseases and variation.
CRISPR and Gene Editing - Modern techniques allow precise modifications to the genome, offering potential treatments for genetic disorders.
Refers to the differences in DNA sequences between individuals of the same species.
It's like a fingerprint that is unique to each organism but with some similarities within a population.
There is usually extensive genetic variation within a population of a species.
The process of grouping organisms based on shared characteristics has undergone a fascinating transformation alongside the understanding of biological advancement. The following are the impacts of developments on classification systems, considering both traditional and modern approaches.
Natural Classification Systems: Aims to group organisms based on their evolutionary relationships, reflecting their shared ancestry.
Early attempts relied on observable characteristics like morphology (physical form), anatomy (internal structure), and embryology (development).
Impact of Biology on Natural Systems:
Microscopy: The invention of powerful microscopes allowed scientists to observe and categorize organisms based on cellular structures, leading to the identification of microorganisms and a deeper understanding of cell types.
Paleontology: The study of fossils provided insights into extinct organisms and their relationships to living species, refining the evolutionary picture.
Limitations of Traditional Approaches:
Convergent Evolution: Organisms from different evolutionary lineages can develop similar physical features due to adaptation to similar environments. This convergence can be misleading for classification based solely on morphology.
Hidden Similarities: Microscopic features may not always reveal the true evolutionary relationships between organisms.
With the advent of DNA sequencing technology, a new era of classification dawned:
This approach analyzes the DNA or RNA sequences of organisms to construct evolutionary trees (phylogenetic trees).
Trees depict the relationships between different species based on their shared genetic heritage.
Advantages:
Objective and Universal: DNA sequences provide a more objective and universal measure of evolutionary relationships, overcoming limitations of morphology.
Identifying Deep Relationships: Genetic analysis based on DNA similarity can reveal relationships between organisms that may not be evident from physical appearance, especially for distantly related species.
Impact on Classification:
Reclassification of Species: DNA analysis has led to the reclassification of many species based on their true evolutionary relationships. This has reorganized the entire tree of life, placing organisms in more accurate positions.
Discovery of New Species: DNA sequencing has facilitated the discovery of new species with unique genetic signatures, even if their morphology is similar to known species.
Explains how life has changed and diversified over time.
A change in the inherited characteristics of a population over time, through a process of natural selection, may result in the formation of new species.
Is driven by the powerful force of natural selection, which favors variants (individuals with genetic differences) that are better suited to their environment.
Key Elements
Variation: Populations are not uniform.
Individuals within a population exhibit genetic variation due to mutations in their DNA and sexual reproduction (shuffling of genes).
These variations can affect traits, the physical or behavioral characteristics of an organism.
Heritability: Variations that are heritable, meaning they can be passed on from parents to offspring, play a key role in evolution.
Offspring that inherit beneficial traits from their parents are more likely to survive and reproduce.
Differential Reproduction: Not all individuals in a population reproduce equally.
Those with traits better suited to their environment have a higher chance of surviving and passing on their genes.
This differential reproduction leads to a gradual shift in the frequency of those advantageous traits over generations.
Action: Imagine a population of beetles living on a dark-colored tree trunk. Some beetles may have a naturally darker body color (melanism) due to a genetic variation, while others are lighter.
Environmental Pressure: If a predator bird starts hunting these beetles, the lighter-colored beetles become easier targets against the dark background.
Differential Survival: The darker beetles, due to their camouflage, are more likely to survive and reproduce.
Shift in Population: Over generations, the population will gradually have a higher proportion of dark-colored beetles as the genes for darker coloration become more prevalent.
Outcomes
Adaptation: Natural selection leads to adaptation, where populations become better suited to their environment. In the beetle example, dark coloration becomes an adaptation for camouflage.
Speciation: Over extended periods, significant genetic differences between populations can lead to speciation, the formation of new species.
Directional Selection: one extreme of the range of phenotypes is favored by natural selection, resulting in the frequency of that phenotype to increase over time.
Stabilizing Selection: The intermediate phenotype is favored and extreme phenotypes are selected against.
Disruptive Selection: Individuals on both extremes of the phenotypic range are more likely to survive and reproduce than individuals with an intermediate phenotype.
Important: Evolution occurs in populations, not individuals.
Fossil Record
Fossils are the preserved remains or traces of organisms from past eras as they offer a glimpse into the history of life and provide a timeline for how life has changed over time.
Transitional Fossils: Fossils that exhibit characteristics of both ancestral and descendant species.
They bridge the gap between different groups of organisms, demonstrating the gradual process of evolution. For example, fossils like Tiktaalik, with both fish-like and amphibian-like features, show the transition from water to land in vertebrates.
Fossil Distribution: The distribution of fossils across different geographic locations reflects the movement and diversification of life forms over time.
For instance, finding dinosaur fossils on continents that were once joined supports the theory of continental drift.
Antibiotic Resistance in Bacteria
Bacteria reproduce rapidly, and mutations in their DNA can occur during this process. Some mutations can provide bacteria with resistance to antibiotics, a phenomenon known as antibiotic resistance.
Antibiotic Use as a Selective Pressure: When antibiotics are used, they kill bacteria that are susceptible to them.
However, bacteria with resistance mutations survive and reproduce.
Increased Prevalence of Resistant Bacteria: Over time, the use of antibiotics selects resistant strains of bacteria, leading to an increase in their prevalence.
This rapid evolution within a few human generations demonstrates the power of natural selection.
Vestigial Structures
Some organisms contain anatomical features that no longer have a purpose in the modern organism but may have had a function in an ancestral organism.
Molecular Evidence
Comparing DNA sequences from different organisms can provide evidence of evolution. The more recently the organisms share a common ancestor, the more similar their DNA sequence will be.
For example, the GAPDH gene in humans and in chimpanzees is over 99% similar in sequence, but the similarity of the gene between humans and in dogs is only about 91% similar, indicating that humans and chimpanzees share a more recent common ancestor.
Voyage of the Beagle: Darwin's journey aboard the HMS Beagle exposed him to the diverse flora and fauna of South America and the Galapagos Islands. He observed distinct variations among species, particularly the finches on the Galapagos, which sparked his curiosity about the origin of these variations.
On the Origin of Species: In 1859, Darwin published his groundbreaking book, "On the Origin of Species," proposing the theory of evolution by natural selection. He argued that:
All living organisms share a common ancestor.
Species are not static but change over time through descent with modification.
The mechanism driving this change is natural selection, where individuals with traits best suited to their environment have a higher chance of survival and reproduction, passing on those advantageous traits to offspring.
Independent Discovery: While working as a naturalist in Southeast Asia, Wallace independently developed a similar theory of evolution by natural selection. He sent his ideas to Darwin, prompting Darwin to publish his own work earlier than originally planned.
Unifying Principle: Darwin and Wallace's theory provided a unifying principle for understanding the diversity of life on Earth. It explained how complex organisms could arise from simpler ones through a gradual process of change.
New Fields of Study: The theory stimulated the development of new fields of biology, such as comparative anatomy, paleontology (study of fossils), and evolutionary biology.
Understanding of Change: It changed the way we view the natural world, acknowledging the dynamic nature of life and the interconnectedness of all living things.
Crucial in conserving biodiversity by serving as a secure repository for the seeds of various plant species.
Function like a safety net, safeguarding the genetic diversity of our planet's flora in the face of potential threats.
Habitat Loss and Degradation: Human activities like deforestation, urbanization, and pollution contribute to habitat loss and degradation, pushing plant species towards extinction.
Seed banks provide a backup plan, preserving the genetic heritage of these threatened plants.
Climate Change: Disrupts ecosystems and alters growing conditions.
Seed banks can store seeds from populations adapted to different climates, ensuring the availability of genetic material for future restoration efforts.
Disease Outbreaks: Diseases can devastate entire plant populations.
Seed banks can house seeds from resistant varieties, ensuring the survival of the species and potentially providing sources for breeding disease-resistant crops.
Accidental Loss: Natural disasters or human errors can lead to the accidental loss of plant populations.
Seed banks offer a backup for such situations.
