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Mutation and genetic variation. Mutations are raw material of evolution. No variation means no evolution and mutations are the ultimate source of variation. Where do new alleles come from?.
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Mutation and genetic variation • Mutations are raw material of evolution. • No variation means no evolution and mutations are the ultimate source of variation.
Where do new alleles come from? • DNA made up of sequence of nucleotides. Each nucleotide includes a sugar, phosphate and one of four possible nitrogenous bases (adenine and guanine [both purines], and thymine and cytosine [both pyrimidines]).
4. 4.1b + 4.1d
Where do new alleles come from? • The opposite strands of the DNA molecule are complementary because the strands are held together by bonds between the opposing bases and adenine bonds only with thymine and cytosine only with guanine. • Thus, knowing the sequence on one strand enables one to construct the sequence on the other strand.
Where do new alleles come from? • Sequence of bases in DNA codes for protein structure as each three base sequence codes for one amino acid in the protein chain. • [To refresh yourself on basic DNA structure and protein synthesis see any Introductory Biology textbook]
Where do new alleles come from? • When DNA is synthesized an enzyme called DNA polymerase reads one strand of DNA molecule and constructs a complementary strand. • If DNA polymerase makes a mistake and it is not repaired, a mutation has occurred.
Types of mutations • A mistake that changes one base on a DNA molecule is called a pointmutation. • Two forms: • Transition: one pyrimidine (T or C) substituted for the other pyrimidine or one purine substituted for the other purine (A or G). • Transversion: purine substituted for pyrimidine or vice versa
Types of mutations • Transitions more common than transversions. Perhaps because transitions cause less disruption to the DNA molecule and so are less likely to be noticed by DNA repair molecules.
Types of mutations • Not all mutations cause a change in amino acid coded for. These are called silent mutations. • Mutations that do cause a change in amino acid are called replacementmutations.
Types of mutations • Another type of mutation occurs when bases are inserted or deleted from the DNA molecule. • This causes a change in how the whole DNA strand is read (a frame shift mutation) and produces a non-functional protein.
Mutation rates • Most data on mutations comes from analysis of loss-of-function mutations. • Loss-of-function mutations cause gene to produce a non-working protein. • Examples of loss-of-function mutations include: insertions and deletions, mutation to a stop codon and insertion of jumping genes.
Mutation rates • Some mutations cause readily identified phenotypic changes. • E.g. Achrondoplastic dwarfism is a dominant disorder. An Achrondoplastic individual’s condition must be the result of a mutation, if his parents do not have the condition.
Mutation rates • Human estimate is 1.6 mutations/genome/generation. • In Drosophila rate is only 0.14 m/g/g, but when corrected for number of cell divisions needed to produce sperm (400 in humans 25 in Drosophila) mutation rates per cell division are very similar.
Mutation rates • These rates are underestimates as they are based on loss-of-function mutations. • Direct estimate of number of mutations of all kinds made for roundworm Caenorhabditiselegans by sequencing mitochondrial DNA.
Mutation rates • Roundworms can self-fertilize so researchers tracked 74 family lines derived from one female and followed each for 214 generations. • At end sequenced 771,672 base pairs of mitochondrial DNA. Found 26 mutations giving rate of 1.6X10-7 mutations per site per generation. Ten mutations were insertion/deletions and 16 substitutions.
Mutation rates • Applying mutation rates to entire genome gives estimate of approximately 15 mutations/individual/generation.
Where do new genes come from? • Mutation can produce new alleles, but new genes are also produced and gene duplication appears to be most important source of new genes.
Gene duplication • Duplication results from unequal crossing over when chromosomes align incorrectly during meiosis. • Result is a chromosome with an extra section of DNA that contains duplicated genes
Gene duplication • Extra sections of DNA are duplicates and can accumulate mutations without being selected against because the other copies of the gene produce normal proteins. • Gene may completely change over time so gene duplication creates new possibilities for gene function.
Globin genes • Human globin genes are examples of products of gene duplication. • Globin gene family contains two major gene clusters (alpha and beta) that code for the protein subunits of hemoglobin.
Globin genes • Hemoglobin (the oxygen-carrying molecule in red corpuscles) consists of an iron-binding heme group and four surrounding protein chains (two coded for by genes in the Alpha cluster and two in the Beta cluster).
Globin genes • Ancestral globin gene duplicated and diverged into alpha and beta ancestral genes about 450-500 mya. • Later transposed to different chromosomes and followed by further subsequent duplications and mutations.
Globin genes • Lengths and positions of exons and introns in the globin genes are very similar. Very unlikely such similarities could be due to chance.
Exons (blue), introns (white), number in box is number of nucleotides. 4.9
Globin genes • Different genes in alpha and beta families are expressed at different times in development. • For example, in very young human fetus, zeta (from alpha cluster) and epsilon (from beta cluster) chains are present initially then replaced. Similarly G-gamma and A-gamma chains present in older fetuses are replaced by beta chains after birth.
4.8 Gestation (weeks) Post-birth(weeks) Fetal hemoglobin has a higher affinity for oxygen than adult hemoglobin. Enhances oxygen transfer from mother to offspring.
Chromosomal alterations • Two major forms important in evolution: inversions and polyploidy.
Inversions • A chromosome inversion occurs when a section of chromosome is broken at both ends, detaches, and flips. • Inversion alters the ordering of genes along the chromosome.
Inversions • Inversion affects linkage (linkage is the likelihood that genes on a chromosome are inherited together i.e., not split up during meiosis). • Inverted sections cannot align properly with another chromosome during meiosis and crossing-over within inversion produces non-functional gametes. • Genes contained within inversion are inherited as a set of genes also called a “supergene”
Inversions • Inversions are common in Drosophila (fruit flies) • Frequency of inversions shows clinal pattern and increases with latitude. • Inversions are believed to contain combinations of genes that work well in particular climatic conditions.
Inversions • Drosophila introduced to Washington and Chile from Old World. Spread along both coasts and within a few years had developed similar clines in number of inversions to those found in native range.
Polyploidy • Polyploidy is the duplication of entire sets of chromosomes. • A polyploid organism has more than two sets of chromosomes. • E.g. A diploid (2n chromosomes) organism can become tetraploid (4n), [where n refers to one set of chromosomes].
Polyploidy • Polyploidy is common in plants, rare in animals. • Half of all angiosperms (flowering plants) and almost all ferns are polyploid.
Polyploidy • Polyploid animal taxa include earthworms and flatworms, which can self-fertilize, and some other groups including insects that can reproduce asexually (parthenogenesis).
Polyploidy • Polyploidy can occur if an individual produces diploid gametes and self-fertilizes generating tetraploid offspring. • If an offspring later self fertilizes or crosses with its parent, a population of tetraploids may develop.
Polyploidy • If a sterile plant undergoes polyploidy and self-fertilization a new species can develop essentially immediately.
Polyploidy • Cross-fertilization of different species, followed by polyploidy, was responsible for the development of many crop plants e.g. wheat. • Initial cross-fertilization produces sterile offspring, because chromosomes cannot pair up during meiosis.
