Unit IV: Cell Division, Genetics, and Biotechnology

Principles of Reproduction

• Definition of Reproduction: Reproduction is the biological process by which new individuals are created from existing ones. It is considered one of the defining characteristics of life.
• Asexual Reproduction:
  • Definition: The production of offspring from a single parent.
  • Genetic Fidelity: Offspring are genetically identical to their parent.
  • Mechanism: Occurs through the process of mitosis.
  • Advantage: Organisms do not need to find a mate of the same species to produce offspring.
  • Biological Examples: Includes organisms such as sea stars, sea anemones, and sponges.
  • Disadvantages: Because all offspring are genetically identical, the entire population is vulnerable to sudden environmental changes. A population lacks the genetic diversity required to adapt to a changing environment.
• Sexual Reproduction:
  • Definition: The production of offspring from two parents.
  • Mechanism: Occurs through the fusion of gametes (reproductive cells).
  • Genetic Variation: Offspring are not genetically identical to either parent. This leads to genetic variation within a species, which is essential for adaptability to changing environmental conditions.
  • Prevalence: The majority of organisms reproduce sexually.

Cell Division and Chromosomal Foundations

• Mitosis Overview:
  • This type of cell division occur when a cell makes an exact duplicate of itself.
  • Main Functions: Used for growth (increasing the number of cells) or the repair and maintenance of the body (replacing damaged, infected, or worn-out cells).
• Meiosis Overview:
  • This cell division occurs specifically for the production of gametes.
  • Function: In humans, it is performed to create egg and sperm cells.
• Key Terminology:
  • Mother Cell: The name given to the original cell that undergoes division.
  • Daughter Cells: The cells produced at the end of cell division.
• Chromosome Numbers:
  • Diploid Number ($2n$): The number of chromosomes found in somatic (body) cells. In humans, the normal diploid number is $46$ chromosomes.
  • Species Variation: The diploid number varies between different species.
  • Haploid Number ($n$): This number is exactly half the diploid number.
  • Human Haploid Number: For humans, the haploid number is $23$.
  • Location of Haploid Cells: Haploid numbers are only found in gametes ($\text{egg and sperm}$).

The Phases and Mechanics of Mitosis

• Interphase: This is where a cell spends the majority of its existence, performing tasks necessary to support the body. During interphase, DNA replication occurs where a copy of each chromosome is made.
  • Sister Chromatids: These are the two identical copies of a single replicated chromosome that are connected by a centromere.
• Prophase:
  • The chromatin condenses into visible chromosomes.
  • The nuclear envelope begins to break down.
  • Spindle fibers begin to form.
• Metaphase:
  • Chromosomes line up along the center (equator) of the cell.
  • Spindle fibers attach to the centromere of each chromosome.
• Anaphase:
  • Sister chromatids are pulled apart and move toward opposite poles of the cell.
• Telophase:
  • Chromosomes arrive at opposite poles and begin to decondense back into chromatin.
  • The nuclear envelope reforms around each set of chromosomes.
• Cytokinesis:
  • The cytoplasm of the mother cell divides, resulting in two separate and identical daughter cells.

The Mechanics of Meiosis and Sexual Reproduction

• Fertilization: The union of two unique reproductive cells (gametes) to form a new individual.
  • The Process: Two haploid ($n$) cells unite to produce one diploid ($2n$) cell ($23 + 23 = 46$ chromosomes).
• Meiotic Purpose: To produce gametes while reducing the chromosome number by half ($46 \rightarrow 23$). It occurs in specialized diploid cells located in the gonads (ovaries in females and testes in males).
• Homologous Pairs: Humans have $23$ pairs of chromosomes. Each pair consists of one paternal copy and one maternal copy. These are known as homologous pairs.
• Process Overview: Like mitosis, DNA replication occurs during interphase. However, the cell must undergo two rounds of division to complete the process, resulting in four haploid daughter cells.
• Meiosis I:
  • Prophase I: Homologous chromosomes pair up. A critical process called Crossing Over occurs, where homologous chromosomes exchange genetic material. This results in different combinations of alleles, increasing genetic variation/diversity within a species.
  • Metaphase I: Homologous pairs line up.
  • Anaphase I: Homologous chromosomes (not sister chromatids) are separated.
  • Telophase I and Cytokinesis: Two daughter cells form.
• Meiosis II:
  • Prophase II, Metaphase II, Anaphase II, Telophase II: These steps mirror mitosis, but the result is the separation of sister chromatids within the two cells formed in Meiosis I, leading to four unique haploid cells.

Cell Division Errors and Chromosomal Screening

• Nondisjunction: This occurs when homologous chromosomes fail to separate correctly during Meiosis I or when sister chromatids fail to separate during Meiosis II.
  • Impact on Fertilization: If a gamete affected by nondisjunction participates in fertilization, the resulting child will have an abnormal number of chromosomes.
  • Trisomy: Having three copies of a chromosome (e.g., Down syndrome).
  • Monosomy: Having only one copy of a chromosome (e.g., Turner syndrome).
  • Lethality: Missing a somatic chromosome or having an extra somatic chromosome is usually fatal, with the exception of sex chromosomes and specific trisomies like Down syndrome.
• Specific Conditions:
  • Down Syndrome (Trisomy $21$): Results in specific facial characteristics and shorter stature. Incidence increases with maternal age, particularly for women over $40$.
  • Turner Syndrome: Characterized by a single X chromosome ($XO$).
  • Klinefelter Syndrome: Characterized by an $XXY$ genotype.
  • XYY Males: An extra Y chromosome.
  • XXX Females: An extra X chromosome.
• Karyotypes: An analytical tool used for assessing chromosomal abnormalities. It is a photographic representation of the chromosomes arranged in order from largest to smallest.
• Medical Procedures for Fetal Cells:
  • Maternal Blood Tests: The least invasive method; searches for markers in maternal blood indicating the likelihood of chromosomal abnormalities.
  • Amniocentesis: Uses a large needle to draw amniotic fluid surrounding the fetus for analysis.
  • Chorionic Villi Sampling (CVS): Involves obtaining and analyzing cells from the placenta.

Mendelian Genetics

• Foundational Principles: Derived from Gregor Mendel's work with pea plants.
  • Heredity: The passing of characteristics from parents to offspring via genes.
  • Allele: A copy or variation of a gene passed from a parent.
  • Inheritance Rule: For any given trait, an individual receives exactly one copy of a gene from each parent, resulting in two copies in diploid cells.
• Dominance and Recessivity:
  • Dominant Traits: Mask or cover up other traits; represented by capital letters (e.g., $D$).
  • Recessive Traits: Are masked by dominant traits; represented by lowercase letters (e.g., $d$).
• Genotype and Phenotype:
  • Genotype: The specific alleles present in an individual.
  • Homozygous Dominant: Inheriting two dominant alleles (e.g., $DD$).
  • Homozygous Recessive: Inheriting two recessive alleles (e.g., $dd$).
  • Heterozygous: Inheriting two different alleles (e.g., $Dd$).
  • Phenotype: The physical appearance or outward expression of the individual (what they look like).
• Punnett Squares: A device used to determine the probability of genotypes and phenotypes in offspring. They provide theoretical ratios for large numbers of offspring.
  • Phenotypic Ratios: Comparison of the number of offspring with the dominant phenotype vs. the recessive phenotype.
  • Genotypic Ratios: Comparison of homozygous dominant vs. heterozygous vs. homozygous recessive.
• Genetic Disorders:
  • Recessive Trait Disorders: Requires two recessive alleles to manifest. Examples: Tay-Sachs disease, Cystic fibrosis, and Phenylketonuria (PKU).
  • Carriers: Individuals with one dominant and one recessive allele ($heterozygous$) who do not display the disease but can pass the allele to offspring.
  • Dominant Trait Disorders: Requires only one dominant allele. Examples: Marfan syndrome, Hypercholesterolemia, Huntington’s disease, and Achondroplasia.
• Pedigrees: Tools for determining inheritance patterns. Females are circles; males are squares.

Non-Mendelian Inheritance Patterns

• Incomplete Dominance: Neither allele is completely dominant. Heterozygotes show an intermediate phenotype (e.g., Redsnapdragon $RR$ + White snapdragon $WW$ = Pink snapdragon $RW$).
• Codominance: Both alleles are fully expressed in the heterozygote.
  • Example: Blood type $AB$ (both $A$ and $B$ antigens are present).
  • Example: Sickle cell disease vs. sickle cell trait.
• Multiple Allelism: When a single gene has more than two alleles in the gene pool.
  • ABO Blood Group: Includes alleles $I^A$, $I^B$, and $i$.
  • Type A: Genotypes $I^AI^A$ or $I^Ai$; Antigens: A.
  • Type B: Genotypes $I^BI^B$ or $I^Bi$; Antigens: B.
  • Type AB: Genotype $I^AI^B$; Antigens: A and B.
  • Type O: Genotype $ii$; Antigens: None.
  • Rh Factor: Determines positive or negative blood status.
  • Human Leukocyte Antigen (HLA): Multitude of alleles coding for cell membrane proteins that help the immune system recognize ‘self’ vs. ‘foreign.’ Essential for organ donor matching.
• Polygenic Traits: Phenotypes influenced by multiple genes. These typically display a normal distribution (bell curve). Examples: skin color, height, body weight, eye color.
• Environmental Effects: Phenotypes are shaped by genes interacting with the environment. Example: Hydrangea flowers are blue in acidic soil and pink in alkaline soil.
• Sex-Influenced Traits: Traits not on sex chromosomes but influenced by hormones like estrogen or testosterone (e.g., male pattern baldness, fraternal twins).
• Sex-Linked Traits: Coded for on sex chromosomes (usually the X).
  • Female Genotype: $XX$.
  • Male Genotype: $XY$.
  • Males: Have only one allele for X-linked traits; cannot be carriers; cannot pass X-linked traits from father to son. Males are more likely to exhibit sex-linked recessive traits.
  • Examples: Red-green color blindness, hemophilia.

Tracing Human Ancestry

• Y Chromosome Analysis: The Y chromosome is passed directly from father to son without genetic exchange. It contains mostly male-related traits (e.g., hairy pinna) and changes very little over generations, making it ideal for tracing paternal lines.
• Mitochondrial DNA (mtDNA) Analysis:
  • Extranuclear DNA: Found in mitochondria outside the nucleus.
  • Maternal Inheritance: mtDNA is inherited solely from the mother via the egg.
  • Tracing: Used to determine maternal relationships and trace human populations back to a common origin approximately $200,000 \text{years ago}$.

DNA Structure and the Genetic Code

• DNA (Deoxyribonucleic Acid): Encoded set of instructions for growth and development. It is composed of building blocks called Nucleotides.
• Nucleotide Structure: Consists of a deoxyribose sugar, a phosphate group, and one of four nitrogenous bases.
• Nitrogenous Bases: Adenine ($A$), Thymine ($T$), Guanine ($G$), and Cytosine ($C$).
• Base Pairing Rules: $A$ pairs with $T$; $C$ pairs with $G$.
• The Gene: A specific section of DNA ranging from a few to several thousand base pairs. The sequence of nucleotides codes for instructions to build proteins.
• Codons: A group of three nucleotides that code for one specific amino acid. Proteins are polymers of these amino acids, and the sequence determines the protein’s structure and function.
• RNA (Ribonucleic Acid): Single-stranded; uses Uracil ($U$) instead of Thymine ($T$).
• Protein Synthesis Steps:
  1. Transcription: Copying a gene’s DNA sequence into mRNA (messenger RNA). This occurs in the nucleus.
  2. Translation: Converting mRNA into a sequence of amino acids to form a protein. This occurs in the cytoplasm at the ribosome.
• Mutation Types:
  • Point Mutation: Change in a single nucleotide.
  • Insertion: Adding nucleotides.
  • Deletion: Removing nucleotides.
  • Effects: Mutations can be beneficial, harmful, or neutral, and serve as the source of genetic variation and evolution.

DNA Fingerprinting and Profiles

• Noncoding DNA: DNA not used in protein production, also called introns or (formerly) ‘junk DNA.’
• Short Tandem Repeats (STRs): Repetitive sequences in non-coding DNA. The length of these repeats at specific loci varies person to person.
• DNA Fingerprint (Profile): A unique pattern based on multiple STR loci. Because STRs are inherited (one from each parent), they can identify individuals or determine paternity.
• Gel Electrophoresis: Process used to separate DNA fragments by size. Fragments move through a gel at different rates to form distinct bands.
• Applications: Forensics, paternity testing, and identifying deceased individuals in disasters or wars.

Genetic Testing and Counseling

• Types of Genetic Tests:
  • Diagnostic: Determines if someone has a specific condition (e.g., sickle cell anemia).
  • Predictive/Presymptomatic: Predicts likelihood of future disease (e.g., Alzheimer’s).
  • Carrier: Determines if an individual can pass on a condition (e.g., Tay-Sachs).
  • Prenatal: Screens unborn babies (e.g., Down syndrome).
  • Newborn Screening: Early identification for treatment (e.g., PKU).
  • Pharmacogenomic: Analyzes response to drugs based on genes.
  • Identification: For paternity or mass casualty victims.
• Genetic Counseling: A healthcare process that helps individuals understand genetic risks, complex medical information, and management of conditions. Counselors assess risk, provide education, offer emotional support, and facilitate decision-making.
• Home Testing Kits: Services like AncestryDNA, MyHeritage, and 23andMe use saliva or cheek swabs. 23andMe specifically provides health risk information for $10$ conditions.

Transgenes and Recombinant DNA Technology

• Transgene: Genetic material transferred from one organism to another. Because the genetic code is universal, organisms like bacteria can read human genes to produce human proteins.
• Applications:
  • Gene Replacement Therapy: Replacing a mutated gene with a functional one using a virus as a vector. The virus integrates the functional gene into the host DNA.
  • Genetically Modified Organisms (GMOs): Crops/animals produced via recombinant DNA. Example: Bt corn produces a bacterial toxin targeting insects.
  • Bioremediation: Using engineered organisms (like bacteria) to clean environmental pollutants (e.g., oil spills).
  • Medicine: Production of Human Growth Hormone (HGH), insulin, and erythropoietin.
• Genome Editing (CRISPR-Cas9): A fast, efficient, and cheap technology that allows specific genetic material to be added, removed, or altered.
• Ethical Considerations:
  • Genetic Privacy: Potential misuse of data by employers or insurance companies.
  • GMO Safety: Impact on human health and natural ecosystems.
  • Gene Editing: Ethical concerns regarding editing human embryos and long-term societal effects.

Questions & Discussion

• Collaborative Catch-up: Meiosis: What would happen if mitosis produced gametes? Answer: If gametes were produced by mitosis, they would be diploid ($2n$). During fertilization, the chromosome number would double every generation ($46 + 46 = 92$), resulting in non-viable offspring.
• Collaborative Catch-up: Blood Types: If a mother is Type A and a baby is Type B, Suspected Father #1 (Type O) is excluded because he cannot provide a $B$ allele ($genotype ii$). Suspected Father #2 (Type B) is the likely father because he can provide the $I^B$ allele. Incompatibilities in transfusions lead to agglutination (clumping of red blood cells), which can be fatal.
• Collaborative Catch-up: Genetic Testing: The primary purpose of genetic counseling is to provide information and support to people at risk of genetic disorders to help them make informed medical decisions.
• Extra Credit Mutation Logic: A deletion (like removing a base in triplet $7$) causes a "frameshift," meaning every subsequent triplet is altered. A base substitution only affects a single triplet. Therefore, deletions generally cause much greater alterations in the protein sequence than substitutions.