VI. Mutation Overview Changes in Ploidy Changes in ‘ Aneuploidy ’ (changes in chromosome number)

VI. MutationA.OverviewB.Changes in PloidyC.Changes in ‘Aneuploidy’ (changes in chromosome number)D. Change in Gene Number/Arrangement


1. Mechanism #1: Unequal Crossing-Over

a. process:

If homologs line up askew:

A

a b

B

A

a b

B



a. process:

If homologs line up askewAnd a cross-over occurs

A a b

B



a. process:

If homologs line up askewAnd a cross-over occurs Unequal pieces of DNA will be exchanged… the A locus has been duplicated on the lower chromosome and deleted from the upper chromosome



a. process: b. effects:

- can be bad:deletions are usually bad – reveal deleterious recessivesadditions can be bad – change protein concentration





- can be good:more of a single protein could be advantageous (r-RNA genes, melanin genes, etc.)





- can be good:more of a single protein could be advantageous (r-RNA genes, melanin genes, etc.)

source of evolutionary novelty (Ohno hypothesis - 1970)where do new genes (new genetic information) come from?

Gene A Duplicated A

Mutation – may even render the proteinnon-functional

But this organism is not selected against, relative to others in the population that lack the duplication, because it still has the original, functional, gene.

generations

Mutation – may even render the proteinnon-functional

Mutation – other mutations may render the protein functional in a new way

So, now we have a genome that can do all the ‘old stuff’ (with the original gene), but it can now do something NEW. Selection may favor these organisms.

Gene A Duplicated A

generations

If so, then we’d expect many different neighboring genes to have similar sequences. And non-functional pseudogenes (duplicates that had been turned off by mutation).These occur – Gene Families

And, if we can measure the rate of mutation in these genes, then we can determine how much time must have elapsed since the duplication event…

Gene family trees…


1.Mechanism #1: Unequal Crossing-Over2.Mechanism #2: Inversion (changes the order of genes on a chromosome)





Chromosomes are no longer homologous along entire length

B-C-D on topd-c-b on bottom



Chromosomes are no longer homologous along entire length

ONE “loops” to get genes across from each other…

And if a cross-over occurs….

The cross-over products are non-functional, with deletions AND duplications



The only functional gametes are those that DID NOT cross over – and preserve the parental combination of alleles



Net effect: stabilizes sets of genes. This allows selection to work on groups of alleles… those that work well TOGETHER are selected for and can be inherited as a ‘co-adapted gene complex’




1.Mechanism #1: Unequal Crossing-Over2.Mechanism #2: Inversion (changes the order of genes on a chromosome)3.Mechanism #3: Translocation (gene or genes move to another homologous set)

Translocation Downs.

Transfer of a 21 chromosome to a 14 chromosome

Can produce normal, carrier, and Down’s child.

VI. MutationA.OverviewB.Changes in PloidyC.Changes in ‘Aneuploidy’ (changes in chromosome number)D. Change in Gene Number/ArrangementE. Change in Gene Structure

1.Mechanism #1: Exon Shuffling

Crossing over WITHIN a gene, in introns, can recombine exons within a gene, producing new alleles.

EXON 1a EXON 2a EXON 3a Allele “a”

EXON 1A EXON 2A EXON 3A Allele “A”

VI. MutationA.OverviewB.Changes in PloidyC.Changes in ‘Aneuploidy’ (changes in chromosome number)D. Change in Gene Number/ArrangementE. Change in Gene Structure

1.Mechanism #1: Exon Shuffling

Crossing over WITHIN a gene, in introns, can recombine exons within a gene, producing new alleles.

EXON 1a EXON 2a EXON 3a Allele “a”

EXON 1A EXON 2A EXON 3A Allele “A”

EXON 2a EXON 3aEXON 1A

EXON 2A EXON 3AEXON 1a

Allele “α”

Allele “ά”

Throws off every 3-base codon from mutation point onward

VI. MutationA.OverviewB.Changes in PloidyC.Changes in ‘Aneuploidy’ (changes in chromosome number)D. Change in Gene Number/ArrangementE. Change in Gene Structure 1. Mechanism #1: Exon Shuffling 2. Mechanism #2: Point Mutations

a. addition/deletion: “frameshift” mutations

…C G T G T A C G T ….

Normal

…G C A C A U G C A …

ARG HIS ALA

Mutant: A inserted

…C G T A G T A C G T ….

…G C A U C A U G C A …

ARG SER CYS

DNA

m-RNA

VI. MutationA.OverviewB.Changes in PloidyC.Changes in ‘Aneuploidy’ (changes in chromosome number)D. Change in Gene Number/ArrangementE. Change in Gene Structure 1. Mechanism #1: Exon Shuffling 2. Mechanism #2: Point Mutations

a. addition/deletion: “frameshift” mutationsb. substitution

At most, only changes one AA (and may not change it…)

…C G T G T A C G T ….

Normal

…G C A C A U G C A …

DNA

m-RNA

ARG HIS ALA

Mutant: A for G

…C G T A T A C G T ….

…G C A U A U G C A …

ARG TYR ALA

VI. MutationA.OverviewB.Changes in PloidyC.Changes in ‘Aneuploidy’ (changes in chromosome number)D. Change in Gene Number/ArrangementE. Change in Gene StructureF. Summary

Sources of Variation Causes of Evolutionary Change

MUTATION: Natural Selection -New Genes: point mutation Mutation (polyploidy can make new exon shuffling species)

RECOMBINATION: - New Genes: crossing over -New Genotypes: crossing over independent assortment

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VI. Mutation Overview Changes in Ploidy Changes in ‘ Aneuploidy ’ (changes in chromosome number)