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Genetic Variation within Species Evolution Biol 4974/5974 D F Tomback Biology 4974/5974 Evolution Genetic Variation and Gene Regulation Figs. From Hall and Hallgrimsson 2008, 2014, unless otherwise credited Google images Learning goals Review and understand: • The structure of genes and chromosomes • Polygenic inheritance: epistasis and pleiotropy • The major kinds of genetic variation and potential evolutionary significance: – – – – Recombination and independent assortment Polyploidy, aneuploidy (changes in number) Point mutations Changes within and between chromosomes and evolutionary significance • Mechanisms of gene regulation, including pre and post transcriptional regulation and both genetic and epigenetic regulation. Genes and chromosomes The genetic material is nucleic acid: Usually DNA; in retroviruses RNA. In all prokaryotes, forms a circular chromosome. In eukaryotes, within the cell nucleus organized into linear chromosomes and associated with histones. Eukaryotes also have genetic material in cytoplasm: mitochondria, chloroplasts, plasmids (circular DNA). 1 Genetic Variation within Species Evolution Biol 4974/5974 D F Tomback Gene structure Each chromosome Contains a series of genes, e.g., lac operon. Genes have regulation sites—signal areas for beginning transcription, stopping. Genes include introns and exons Exons (expressed sequences) are coding regions for transcription of m-RNA and translation into proteins Introns are non-coding regions, often called “nonsense” DNA, but may be involved in gene regulation, speciation, and evolution. Genetic variation and meiosis Meiosis is a major source of genetic variation in eukaryotes Recombination: In prophase of meiosis, crossing over may occur between homologous chromosomes, leading to new gene associations Independent assortment: When chromosomes line up on the spindle during meiosis, they are mixed up in terms of parental identity, forming new chromosomal associations Expression of genotype Polygenic inheritance: Almost every character is formed by the interaction of several to many genes. Epistatic interactions (epistasis): Each trait is the result of epistatic interactions among non-allelic genes— “Interactions between genes in different allelic systems”— Sometimes referred to as modifier genes. Pleiotropy: Effects of a single gene on more than one trait. 2 Genetic Variation within Species Evolution Biol 4974/5974 D F Tomback Whole genome changes Euploid variations: changes in numbers of entire sets of chromosomes. New species Autopolyploidy: extra sets of Allopolyploidy chromosomes within a species. Allopolyploidy: extra sets of chromosomes from hybridization between species. Hybrid then undergoes polyploidy. Autopolyploids Fig. 16.2 Aneuploid variations When more or fewer than the normal two copies of a chromosome occurs: May produce lethality, sterility, or abnormality, depending on the chromosome E.g., sex chromosome aneuploidies: XO Turner’s & XXY Klinefelter’s syndrome E.g., 3 copies of chromosome #21: Down’s syndrome Changes within and between chromosomes Changes within chromosomes Fig. 16.3 Deletion: potentially harmful. Duplication: evolutionary significance, e.g., isozyme evolution, such as hemoglobin, or α and β subunits of a protein. Also, neofunctionalization.P. 311-12. Chromosomal rearrangements Inversions: suppress cross-over in heterokaryotes; results in 1/2 sterile gametes. Reciprocal translocation: results in 2/3 sterile gametes. (The sterility problems disappear when the chromosomal change spreads throughout the population) Both have implications for speciation. Fig. 16.4 3 Genetic Variation within Species Evolution Biol 4974/5974 D F Tomback Point mutations Point mutations arise spontaneously during DNA replication or from mutagens. Some point mutations in DNA are of evolutionary significance. Types Substitutions: one nucleotide base for another Transitions: between purines (A and G) or pyrimidines (T and C) Transversions: exchange of a purine for a pyrimidine or vice versa Also: deletions, duplications, and insertions Consequences of point mutations Synonomous mutation: no amino acid change. Frameshift mutation (e.g., insertion/deletion): change. reading sequence for every amino acid after mutation. Missense mutation: new amino acid (evolutionary potential). Nonsense mutation: stop codon results in middle of a gene. Chromosomal evolution Inversions and translocations and chromosome fusions rearrange the karyotypes of species. Important implications for speciation: Speciation may be associated with chromosomal changes. E.g., Speciation in muntjac deer (p. 308). Chinese muntjac have 23 chromosome pairs (left) and Indian muntjac have 3 pairs (and males have an additional Y chromosome) (right). Fig. 16.5 4 Genetic Variation within Species Evolution Biol 4974/5974 D F Tomback Chromosome rearrangements among humans and our closest relatives Fig. 16.7 Gene regulation and evolution Changes in gene regulation are considered important for evolution: Logical pathway to change within species Gene sequences and products are conserved Fewer genes in metazoans than previously thought; most genes are involved in pleiotropic and epistatic interactions Similarities in metabolic pathways among organisms: change caused by differences in regulation pathways Noncoding DNA may function in gene regulation Mechanisms of gene regulation Regulation of transcription: Operons in prokaryotes cis-and trans-regulation in eukaryotes --cis when regulatory sequences adjacent to gene promoter --trans when regulatory sequences distant from promoter Small interference RNAs (siRNAs or RNAi) Post-transcription regulation: microRNA (miRNAs) RNA editing Rates of mRNA degradation Piwi-interacting RNAs (piRNAs) regulate insertion of transposons into genomes 5 Genetic Variation within Species Evolution Biol 4974/5974 D F Tomback Study questions • What is the genetic material and how is it organized in a. Prokaryotes b. Eukaryotes? • How do recombination and independent assortment result in genetic variation? • What is the evolutionary significance of gene duplication? Of inversion and translocation among chromosomes? • List the different consequences of these point mutations: substitution, insertion, deletion, duplication. • Are any point mutations more likely to be of evolutionary significance than others? Why? • What is the likely role of gene regulation in evolution? • What are examples of pre and post transcriptional regulatory processes? 6