Chapter 10: Molecular Biology of the Gene – Mutations

Definition and Types of Mutations

Mutations are changes in the nucleotide sequence of DNA. They can affect the structure and function of proteins, leading to various effects on an organism's phenotype.

• Substitution: One nucleotide is replaced by another. This may result in a different amino acid in the protein sequence.

• Insertion: One or more nucleotides are added to the DNA sequence, potentially altering the reading frame.

• Deletion: One or more nucleotides are removed from the DNA sequence, which can also shift the reading frame.

Types of Point Mutations

• Silent Mutation: Alters a nucleotide but does not change the amino acid due to the redundancy of the genetic code.

• Missense Mutation: Changes a codon so that a different amino acid is inserted into the protein.

• Nonsense Mutation: Converts a codon into a stop codon, leading to premature termination of translation.

• Frameshift Mutation: Caused by insertions or deletions that are not multiples of three nucleotides, shifting the reading frame and altering downstream amino acids.

Predicting Effects of Mutations

• Given a DNA sequence, a mutation can be analyzed to predict changes in the resulting protein's amino acid sequence and function.

• For example, a substitution in the third position of a codon may be silent, while an insertion may cause a frameshift, drastically altering the protein.

Impact of Mutations on Phenotype

• Negative Impact: May cause diseases or loss of function (e.g., sickle cell anemia from a missense mutation).

• Positive Impact: Can confer advantages (e.g., antibiotic resistance in bacteria).

• Neutral Impact: No observable effect on phenotype (e.g., silent mutations).

Chapter 13: How Populations Evolve – Evolution and Natural Selection

Natural Selection as a Mechanism of Evolution

Natural selection is the process by which individuals with advantageous traits survive and reproduce more successfully, leading to the accumulation of those traits in a population over generations.

• It is the primary mechanism driving evolutionary change.

Darwin’s Argument for Natural Selection

• Observations:

  • Members of a population vary in their traits.

  • Traits are inherited from parents to offspring.

  • All species are capable of producing more offspring than the environment can support.

  • Many offspring do not survive due to limited resources.

• Inferences:

  • Individuals with traits that give them a higher probability of surviving and reproducing will leave more offspring.

  • This will lead to the accumulation of favorable traits in the population over generations.

Examples of Natural Selection

• Antibiotic resistance in bacteria.

• Beak size variation in Galápagos finches in response to food availability.

Evidence for Evolution

• Fossil Record and Radiometric Dating: Fossils show changes in organisms over time; radiometric dating provides absolute ages of rocks and fossils.

• Comparative Anatomy and Embryology: Homologous structures indicate common ancestry; similar embryonic development patterns suggest evolutionary relationships.

• Molecular Biology: Similarities in DNA and protein sequences among species reflect shared ancestry.

Sources of Genetic Variation

• Mutations (random changes in DNA)

• Sexual reproduction (recombination of alleles)

• Gene flow (movement of alleles between populations)

Population, Gene Pool, and Microevolution

• Population: A group of individuals of the same species living in the same area.

• Gene Pool: The total collection of genes and their alleles in a population.

• Microevolution: Small-scale changes in allele frequencies within a population over time.

Mechanisms of Microevolution

• Natural Selection: Differential survival and reproduction based on inherited traits.

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

  • Bottleneck Effect: A sudden reduction in population size due to a disaster, reducing genetic diversity.

  • Founder Effect: A few individuals colonize a new area, leading to a gene pool that differs from the original population.

• Gene Flow: Movement of alleles between populations through migration.

• Non-random Mating: Mating that is not random can change genotype frequencies (e.g., inbreeding).

• Mutation: Introduces new genetic variation into a population.

Patterns of Natural Selection

Natural selection can alter the phenotypic variation in a population in three main ways:

Additional info: Microevolution is often measured by changes in allele frequencies using the Hardy-Weinberg equation: , where p and q are the frequencies of two alleles in a population.