Molecular Evolution Study Notes

Module 03: Molecular Evolution

Learning Objectives

• Describe the organization of a typical eukaryotic genome in terms of coding and non-coding DNA.

• Define homologous, orthologous, and paralogous genes and explain which one is most useful for inferring evolutionary history.

• Define synonymous, non-synonymous, and non-coding DNA and explain how they can be used to infer evolutionary processes.

• Explain the rationale for the nearly-neutral theory and how it helps us detect selection for (or against) DNA variants.

• Compute Ka/Ks and interpret whether a region of DNA sequence is neutral, beneficial, or deleterious.

• Give examples of the three possible fates of duplicated genes and explain which is the most likely/common fate.

• Describe the contribution of transposable elements to genome variation.

• Describe the DNA "footprint" that each evolutionary force leaves on genomic variation.

What is Molecular Evolution?

• Definition: Molecular evolution refers to the evolution occurring at the level of nucleic acids and proteins.

• Key Processes: Evolution at the molecular level primarily occurs due to changes in the genome sequence, which subsequently influences protein structures and functions.

Understanding the Genome

• Definition of a Genome: The genome is the complete set of chromosomes (karyotype) found in an organism.

  • Human Genome Composition: Includes both autosomes (non-sex chromosomes) and sex chromosomes (X, Y).

Genome Structure Differences Among Domains

• Eukaryotes: Genome is linear.

• Bacteria and Archaea: Genome is circular.

• Viruses: Can have either circular or linear DNA or RNA structures.

Visualizing the Genome

• Zoom Levels:

  • Whole genome at ~290X zoom allows distinction of individual genes (exons) but not detailed visualization of DNA sequences.

  • At a further ~10,000X zoom, DNA sequences can be visualized in depth.

Eukaryotic Genome Organization

• Components of the Eukaryotic Genome:

  • Exons: Sequences that code for proteins.

  • Introns: Non-coding sequences between exons.

  • Intergenic Spaces: Non-coding sequences found between genes.

Contribution of Transposable Elements

• Transposable Elements Include:

  • LTR retrotransposons

  • DNA transposons

  • SINES (Short Interspersed Elements)

  • LINES (Long Interspersed Elements)

  • Miscellaneous unique sequences and heterochromatin.

• Contribution to Variation: These elements account for various percentages of genome structure and contribute to genetic diversity.

Gene Counts and Genome Sizes of Various Organisms

Evolutionary History in Genomes

• Gene Sequence Similarity: Related organisms share similar gene sequences, which can be utilized to determine evolutionary relationships.

• DNA Change Rates: The type of DNA influences its rate of change, affecting evolutionary interpretations:

  • Ribosomal RNA: changes slowly and is useful for detecting ancient relationships.

  • Mitochondrial DNA: evolves relatively quickly and is used for more recent evolutionary analyses.

Molecular Homology

• Process: DNA sequences are aligned to identify similar sequences between different species, where closely related species differ only slightly, while distant species may have significantly different sequences.

• Challenges: Insertions and deletions can complicate the sequence alignment of closely related species.

Gene Types

Homologous Genes

• Orthologous Genes: The same gene found in different species, arising from speciation events, providing insight into evolutionary history and facilitating phylogenetic tree construction.

• Paralogous Genes: Copies of the same gene within the same or different species, arising from duplication events.

• Two Main Types of Homologous Genes: Orthologous and paralogous genes must be correctly identified to avoid misinterpretations in molecular evolution studies.

Phylogenetic Trees

• Ortholog and Paralog Tree Construction: Illustrates the relationships between different species and their gene sequences based on evolutionary history.

Central Dogma of Molecular Biology

1. Process Overview: DNA is transcribed into RNA, which is then translated into protein.

2. Example:

   • Sequence: 5′ - A T G A C A C T - 3′ (Coding Strand)

   • Complement: 3′ - T A C T G T G A - 5′ (Template Strand)

Alterations of Chromosome Structure

• Types of Changes:

  • Deletion: Removal of a chromosomal fragment.

  • Duplication: Repetition of a chromosomal segment.

  • Inversion: Reversal of segment orientation within a chromosome.

  • Translocation: Movement of a segment from one chromosome to another.

Mutations

• Definition: Mutations are modifications in the genetic information of a cell, which can occur at various scales.

• Point Mutations: Changes in a single nucleotide pair, which may lead to abnormal protein production or genetic disorders.

Insertions and Deletions (Indels)

• Effects on Protein: Indels can have drastic effects on resultant proteins, often more than substitutions, and may lead to frameshift mutations.

Alleles and Their Variation

• Heterozygous vs. Homozygous: Discuss how alleles at a specific locus vary between organisms (heterozygous) or are identical (homozygous).

Point Mutation Types

• Silent Mutation: No change to amino acid; often synonymous due to redundancy in genetic code.

• Missense Mutation: Leads to one amino acid change; may be harmful or occasionally beneficial.

• Nonsense Mutation: Results in a stop codon and a non-functional protein.

Likelihood of Silent Mutation

• Changes to the third base pair in a codon are less likely to result in a non-silent mutation.

• Changes to start codons are never silent.

Nucleotide Substitutions and Typology

• Synonymous Substitutions: Do not alter amino acid sequence and are considered neutral.

• Non-synonymous Substitutions: Change the amino acid; may be adaptive, deleterious, or neutral.

• Non-coding DNA Effects: Substitutions can affect gene expression levels and potential phenotypic outcomes.

Causes of Genomic Diversity Among Species

• Genetic Variation: Mutations change base pair sequences; lethal mutations are typically removed by natural selection.

• Hypotheses of Variation:

  • Selectionist Hypothesis: Genetic variations are primarily due to natural selection.

  • Neutral Hypothesis: Genetic variations occur due to random chance.

  • Nearly Neutral Hypothesis: A combination of natural selection and random chance causes genetic variation.

Selectionist vs. Nearly Neutral vs. Neutral Theories

• Selectionist Theory:

  • All mutations significantly impact fitness through natural selection.

• Neutral Theory:

  • Most mutations are neutral, driven by random genetic drift.

  • Implications are significant for understanding evolutionary processes across various populations, with population size influencing the balance between selection and drift.