General Biology II: Molecular Biology, Evolution, and Population Genetics Study Notes

Chapter 10: Molecular Biology of the Gene

Chromosomes, DNA, and Proteins

Chromosomes are structures within cells that contain genetic material in the form of DNA (or RNA in some viruses) and proteins. The discovery of DNA as the genetic material was a pivotal moment in molecular biology.

• DNA as Genetic Material: The Hershey-Chase experiment used viruses with radioactively labeled sulfur or phosphorus to demonstrate that DNA, not protein, is the genetic material.

• Nucleotides: The building blocks of DNA, each composed of a phosphate group, a deoxyribose sugar, and a nitrogenous base.

• Base Pairing: DNA bases pair specifically: Adenine (A) with Thymine (T), and Cytosine (C) with Guanine (G).

• Hydrogen Bonds: These bonds hold the two DNA strands together between the bases.

Example: Using three components of a nucleotide, you can build a simple model of DNA and predict the complementary strand.

DNA Replication

DNA replication is the process by which DNA makes a copy of itself before cell division. It is semi-conservative, meaning each new DNA molecule consists of one old strand and one new strand.

• Semi-Conservative Model: Each daughter DNA molecule contains one parental strand and one newly synthesized strand.

• Experiment: The Meselson-Stahl experiment provided evidence for the semi-conservative model.

Central Dogma: DNA → RNA → Protein

The central dogma of molecular biology describes the flow of genetic information within a cell.

• Transcription: The process by which DNA is transcribed into RNA.

• Translation: The process by which RNA is translated into protein.

• Gene Expression: Different genes are expressed through transcription and translation, resulting in different proteins.

• Differences: There are structural differences between RNA and DNA.

• Function of mRNA: Messenger RNA (mRNA) carries genetic information from DNA to ribosomes for protein synthesis.

Example: The genetic code is read in triplets (codons), such as AUG for methionine (start codon).

Chapter 13: Evolution

Definition and Mechanisms of Evolution

Evolution is the change in inherited characteristics of organisms over generations. It can be measured by changes in allele frequencies within populations.

• Natural Selection: The process by which traits that enhance survival and reproduction become more common in a population.

• Genetic Drift: Random changes in allele frequencies, especially in small populations.

• Mutation: The source of new genetic variation.

• Non-Random Mating: Mating that is not random can affect allele frequencies.

Example: Homologous traits in different species indicate common ancestry (e.g., similar limb bones in mammals).

Requirements for Natural Selection

• Variation: There must be variation in traits.

• Heritability: Traits must be heritable.

• Differential Survival/Reproduction: Some traits must confer a survival or reproductive advantage.

Natural Selection is Not Random: It acts as a filter for genetic variation, favoring traits that increase fitness.

Example: The mantid that looks like a flower is an adaptation resulting from natural selection.

Evolution of Human Skin Color

Human skin color variation is an example of evolution influenced by natural selection. The roles of vitamin D production and folate protection are important in understanding the adaptive significance of skin color.

• Light Skin: Advantageous in low UV environments for vitamin D synthesis.

• Dark Skin: Protects against folate degradation in high UV environments.

Population Genetics

Hardy-Weinberg Equilibrium

The Hardy-Weinberg equilibrium describes a population that is not evolving. It provides a mathematical model for allele and genotype frequencies.

• Allele Frequency: The proportion of a specific allele among all alleles in the population.

• Genotype Frequency: The proportion of a specific genotype among all individuals.

Formulas:

• Allele frequency: $f(A) = \frac{A}{N}$ where N = population size

• Genotype frequency: $f(AA), f(Aa), f(aa)$

• Hardy-Weinberg equation: $p^2 + 2pq + q^2 = 1$

• Allele frequency: $p + q = 1$

Conditions for Hardy-Weinberg Equilibrium:

• No mutation

• No migration

• No selection

• Large population size

• Random mating

Example: If observed genotype frequencies match those predicted by Hardy-Weinberg, the population is not evolving.

Bottlenecks and Founder Effect

Bottlenecks and founder effects are examples of genetic drift that occur in small populations.

• Bottleneck Effect: A sharp reduction in population size due to environmental events, resulting in loss of genetic diversity.

• Founder Effect: When a small group establishes a new population, leading to random changes in allele frequencies.

Example: A population that survives a natural disaster may have different allele frequencies than the original population.

Negative Selection

Negative selection removes deleterious mutations from a population. It is detected by comparing the rates of non-synonymous (amino acid changing) and synonymous (silent) mutations.

• Non-synonymous Mutations: Change the amino acid sequence of proteins.

• Synonymous Mutations: Do not change the amino acid sequence.

Example: Genes under negative selection have fewer non-synonymous mutations than expected.

Evolution of Aging

Polygenic Traits and Environmental Effects

Longevity is affected by multiple genes and environmental factors. Selection acts differently on young and old individuals.

• Antagonistic Pleiotropy: Genes that are beneficial early in life may have harmful effects later.

• Selection Pressure: Stronger on the young than the old.

• Examples: Possums on islands live longer due to reduced predation.

Summary Table: Key Concepts in Population Genetics

Concept	Definition	Example
Hardy-Weinberg Equilibrium	Population allele and genotype frequencies remain constant in absence of evolution	Allele frequencies in a large, randomly mating population
Bottleneck Effect	Sharp reduction in population size leads to loss of genetic diversity	Population after a natural disaster
Founder Effect	Small group establishes new population with different allele frequencies	Island colonization by a few individuals
Negative Selection	Removal of deleterious mutations	Fewer non-synonymous mutations in essential genes
Genetic Drift	Random changes in allele frequencies	Allele frequency changes in small populations

Additional info:

• Sexual selection is mentioned as a possible topic to be added later.

• Some content inferred from context and standard biology curriculum.