Download Evolution notes lecture Genetic Variation and Gene Regulation Fall

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:
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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).
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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.
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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
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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
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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
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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?
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