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Central Dogma
Chromosomes are composed of chromatin, a combination of DNA and protein.
Miescher and Griffith (early researchers who paved the way for understanding DNA as genetic material).
The monomers of DNA are nucleotides. These monomers have three parts:
A phosphate group
A deoxyribose sugar
A nitrogenous base
When they are linked together to build the polymers (called DNA strands), they form a sugar-phosphate backbone with nitrogenous bases off to the side. There are four types of nucleotides that make up a DNA strand: we call these nucleotides by the first initial of the base. The base pairing rules tell us that adenine (A) always pairs with thymine (T); and cytosine (C) always pairs with guanine (G). This explains Chargaff’s rule (which states that the amount of A always equals the amount of T; and the amount of C always equals G).
The two researchers who discovered the base pairing rules and who also described DNA as a double helix were James Watson and Francis Crick. They based some of their work on the X-ray pictures made by Rosalind Franklin.
When DNA is replicated, the double helix unwinds and this enzyme, DNA polymerase, builds a new strand of DNA alongside each of the old strands, using the base pairing rules. This is semi-conservative replication, since each new double helix consists of one old and one new strand of DNA.
According to Beadle and Tatum’s one gene-one enzyme hypothesis, DNA is the code for proteins.
In order to code for a protein, a gene must first be copied in the nucleus in the form of the other nucleic acid RNA. This copying process is called transcription. The difference in RNA and DNA is in the type of sugar in the molecule and in the base pairs: RNA does not have thymine (T); it has uracil (U) instead, so DNA A is copied as mRNA U. The enzyme that builds the new RNA strand is RNA polymerase.
mRNA leaves the nucleus and goes to the ribosome where the code is read in three-letter “words” called codons. Each codon codes for an amino acid, and the protein is built by hooking these together to form a polypeptide. This process (reading the code and building a protein) is translation.
There are three types of RNA:
mRNA (messenger RNA), which carries the genetic code from the nucleus to the ribosomes;
rRNA (ribosomal RNA), which is a part of the ribosome;
tRNA (transfer RNA), which brings the new amino acids into the ribosome when translation is happening.
Mutations
Any change in the genetic code is a mutation. These can occur when one base is put in the place of another, this is a base substitution. It can also occur when a base is left out of the DNA molecule, this is a base deletion, or when an extra one is put in, this is a base insertion.
Central Dogma Summary
The Central Dogma of Biology:
DNA → RNA → Protein
DNA Technology
Genetically Modified Organism (GMO): An organism whose genome has been altered in a way that does not occur naturally by mating or natural recombination.
Recombinant DNA: DNA that has been artificially manipulated to combine genes from different organisms.
Plasmid: A small, circular piece of DNA that is separate from the chromosomal DNA in bacteria and can be used to transfer genetic material.
Transgenic organism: An organism that contains genes from a different species.
Recombinant organisms (bacteria) are being used in the healthcare industry to produce:
Insulin
Growth hormones
Vaccines
Examples of transgenic organisms and why they were produced:
Glofish (for ornamental purposes and scientific research).
Pest-resistant crops (e.g., Bt corn to resist insect pests).
Herbicide-resistant crops (e.g., Roundup-ready soybeans).
Golden rice (produced to provide a source of vitamin A in regions with vitamin A deficiency).
Possible problems with GMOs:
Environmental impact (e.g., gene flow to wild relatives).
Health concerns (long-term effects unknown).
Loss of biodiversity.
Ethical issues (in some cases, altering an organism’s genome).
Stem Cells
Differentiated cells: Cells that have developed into a specific type and can no longer turn into other types of cells.
Pluripotent stem cells: Cells that can give rise to many, but not all, types of cells.
Unipotent stem cells: Cells that can only develop into one type of cell.
Undifferentiated cells: Cells that have not yet specialized.
Multipotent stem cells: Cells that can develop into several different types of cells.
Three general properties of stem cells:
Self-renewal: Stem cells can divide and produce copies of themselves.
Potency: Stem cells have the ability to differentiate into various types of cells.
Differentiation: Stem cells can specialize into different types of cells depending on the signals they receive.
Three types of stem cells:
Totipotent stem cells: Can form any cell in the body, including extra-embryonic tissues. (e.g., fertilized egg)
Pluripotent stem cells: Can form any type of cell except extra-embryonic tissues. (e.g., embryonic stem cells)
Multipotent stem cells: Can form a limited range of cell types. (e.g., adult stem cells)
The source of human embryonic stem cells is from the blastocyst stage of development.
CRISPR/Cas9
Gene editing is the process of altering the DNA of a living organism to achieve desired genetic outcomes.
CRISPR allows for the modification of an entire species by inserting the CRISPR/Cas9 complex into the genome.
This human condition is being treated with gene editing using CRISPR: Sickle cell anemia (and other genetic disorders).
The International Summit on Human Genome Editing recommended using gene editing only on somatic cells, which are not involved in reproduction.
PCR
PCR stands for Polymerase Chain Reaction.
What does PCR do? It amplifies (makes many copies of) a specific DNA segment.
Simple sequence repeats (SSRs) are short, repeating sequences of DNA that are used in genetic profiling and identifying individuals.
Three circumstances when PCR would be used:
DNA profiling (e.g., forensic analysis).
Cloning genes (to make more copies for research or therapeutic purposes).
Detecting genetic mutations (for medical diagnostics).
Gel electrophoresis is a technique used to separate bands of DNA based on their size.
Evolution
Evolution (definition) is:
The change in the genetic composition of a population over successive generations.
Five categories of evidence to support the theory of evolution:
Fossil record – Shows the progression of life forms and provides evidence of extinct species.
Comparative anatomy – Similarities and differences in the structures of different species (e.g., homologous and analogous structures).
Embryology – The study of the development of embryos; similar embryos in different species suggest common ancestry.
Molecular biology – DNA, RNA, and protein sequences show genetic similarities between species.
Biogeography – The distribution of species across different geographical areas and how it relates to their evolutionary history.
Natural Selection
Natural Selection (definition) is:
The process by which organisms better adapted to their environment tend to survive and produce more offspring, leading to the gradual change of species over time.
Artificial selection is different from natural selection because:
Artificial selection is driven by human intervention, where humans select individuals with desirable traits to reproduce, whereas natural selection is driven by environmental factors and survival pressures.
Polymorphism (definition) is:
The occurrence of two or more different forms (alleles) in a population, typically in reference to genetic variation.
Three ways natural selection acts on a polymorphic population:
Directional selection – Favors one extreme phenotype, causing the population's traits to shift in one direction (e.g., larger beaks in birds due to food source).
Stabilizing selection – Favors intermediate phenotypes, reducing variation in the population (e.g., human birth weight).
Disruptive selection – Favors both extreme phenotypes, potentially leading to the formation of two distinct groups (e.g., birds with either very large or very small beaks).
Evolutionary Study and Population Genetics
This branch of evolutionary study combines the work of Darwin (on populations) and Mendel (genes and alleles):
Population genetics.
All of the alleles for all of the genes in a population is that population’s:
Gene pool.
If a population is begun by a few individuals, we may see reduced variation in the future due to the:
Founder effect.
Any change in the allele frequency in a population is:
Evolution.
When individuals move into and out of populations, the change we see in allele frequency is due to:
Gene flow.
Random change in a population is:
Genetic drift.
If a population is reduced severely in size, we call that a:
Population bottleneck.
Sexual selection occurs when one gender selects mates based on some characteristic.
This can lead to a different appearance between males and females in the population (sexual dimorphism).
Examples of evolutionary processes:
Founder effect: A small group of individuals starts a new population with a limited gene pool.
Gene flow: Migration of individuals between populations results in the exchange of alleles.
Genetic drift: Random changes in allele frequencies, often in small populations.
Sexual selection: Traits that improve an individual's chances of attracting a mate are selected.
Sexual dimorphism: Males and females of the same species exhibit different physical characteristics.
Natural selection: The environment selects for traits that are beneficial for survival and reproduction.
Artificial selection: Humans select traits they find desirable in plants or animals.
Directional selection: Favors individuals at one end of the phenotypic spectrum.
Diversifying (disruptive) selection: Favors individuals at both extremes of the phenotypic spectrum.
Stabilizing selection: Favors the average or intermediate phenotypes, reducing variation.
Species and Speciation
We talked about 3 species concepts in class. The one that uses DNA similarities is the
Phylogenetic species concept.
The Morphological species concept is based on physical similarities.
The Biological species concept uses fertility and says that all members of a species have the potential to breed and produce fertile offspring.
A new species may be formed when a population is split for long periods of time by a geographic barrier (like a river or mountain range). This is:
Allopatric speciation.
Or a new species may be formed when a population splits without a geographic barrier, often due to polyploidy, sexual selection, or food preference. This is:
Sympatric speciation.
The study of evolution that involves studying how organisms are formed is:
Evo-devo (Evolutionary Developmental Biology).
This involves studying the genes that control development (these are homeotic genes) and often deals with the retention of juvenile characteristics into the adult stage of life (which is paedomorphosis).