Module 7 - Evolution of Genes and Genomes

included on exam 2

germ line

the cell lineage in a multicellular organism that contributes to the formation of the next generation by producing gametes (sperm or eggs)

(germ and gametes start with g, children (the result of sperm and eggs) are germs)

cells in this lineage are haploid (gametes)

gets aside early during development

(depicted in red in the picture)

somatic cell

body cell

any cell in a multicellular organism that is not a germ line cell

cells in this lineage are diploid

(depicted in blue in the picture)

if a mutation occurs in a germ line cell:

after the lineage is set aside, the mutation will not affect the individual but may be passed to the next generation; may result in a heritable trait

(mutation is indicated in green)

if a mutation occurs in a somatic cell:

the mutation may affect the individual in which the mutation occurred

(mutation is indicated in green)

genetic changes that contribute to gene alteration and genome evolution (4 of them)

mutation within a gene

mutation in a regulatory region

gene duplication

horizontal transfer

genetic innovation (result) of mutation within a gene

altered gene product

genetic innovation (result) of mutation in a regulatory region

altered gene expression

genetic innovation (result) of gene duplication

new gene family

genetic innovation (result) of horizontal transfer

gene acquisition

effect of regulatory regions on gene expression

differences in regulatory DNA may result in different developmental pathways, even in closely related species with the same sets of development-related genes

e.g. two species have (and express) gene 1 early on (gene 1 codes for a transcription regulator, which is active at a later embryonic stage)

in organism A, the regulator binds to regulatory DNA in front of gene 3

in organism B, the regulator binds to regulatory DNA in front of gene 2

as a result, different genes (gene 3 vs gene 2) are expressed despite the same genes, because of differences in the regulatory DNA

example: mutation in regulatory DNA in Drosophila

a mutation in regulatory DNA occurs

the antennapedia gene is ectopically expressed in the head

result: leg or leg-like structures form on the fly’s head

example: mutation in regulatory DNA of the lactase gene

a mutation occurs in the regulatory DNA of the lactase gene

mutation allows for lactase (an enzyme) to be made beyond infancy

result: the ability to digest lactose

the mutation was selected for in populations where humans first domesticated cattle (Northern Europe and Central Africa)

gene families

groups of genes that originate from a common ancestral gene

grouped by similarities in structure and/or function

originate by gene duplications

e.g. the globin gene family

the globin gene family

example of a gene family

genes code for subunits of hemoglobin

all the genes have the same structure: 3 exons and 2 introns

different versions are expressed during fetal, embryonic, and adult stages (embryonic and fetal globins have higher affinities for oxygen than adult globins)

some members of the family are pseudogenes

adult version contains 2 alpha and 2 beta subunits

the alpha-globin cluster (top in picture) is on chromosome 16

the beta-globin cluster (bottom in picture) is on chromosome 11

hemoglobin

the protein that transports O2 in the circulatory system

subunits are coded for by genes in the globin gene family

pseudogenes

non-functional genes

e.g. some members in the globin gene family

gene duplication by unequal crossing over

short repeated sequences within homologous chromosomes mauy cause misalignments and unequal crossing over

these interspersed repeats are non-coding, scattered throughout the genome

unequal crossing over → chromosomes with altered sizes and genetic contents

genetic rearrangement by mobile genetic elements

transposition of mobile genetic elements can mediate recombination of genes or regulatory sequences (in eukaryotes)

if two similar mobile genetic elements are inserted near each other in a chromosome, transposition may occur using the ends of the 2 different elements

result: movement of the entire segment of the chromosome between them

(if the ends of only 1 element were used, then just that element would be transposed, not the larger piece of chromosome)

transposition of regulatory sequences can alter the tissue-specific patterns of gene expression

exon transposition by mobile genetic elements

like normal genetic rearrangement by mobile genetic elements, but with a little extra

if the transposed segment includes an exon, then its insertion into a second gene alters and creates a new version of that second gene

exon shuffling

duplication and movement of DNA segments can result in the origin of new proteins

new proteins result by piecing together fragments of different genes

result: new proteins with multiple functional domains

genetic changes that contribute to gene alteration and genome evolution

vertical transfer

horizontal transfer

vertical transfer

genetic change from parent to progeny through gametes (normal genetic change)

contributes to gene alteration and genome evolution

horizontal gene transfer

genetic change from one organism to another (not progeny, and not through gametes)

contributes to gene alteration and genome evolution

e.g. viral infections, conjugation

conjugation

transfer of genetic information from one bacterium to another through a sex pilus

sex between bacteria, essentially

result: genetic recombination without reproduction

an example of horizontal gene transfer

human endogenous retroviruses (HERVs)

retroviruses that infect neurons and germ cells

linked to ALS and germ cell-derived tumors in testes and ovaries

ALS

amyotrophic lateral sclerosis, aka Lou Gehrig’s disease

a progressive neurodegenerative disease that affects neurons in the brain and spinal cord

believed to be caused by HERV-K

HERV-K: an ancient virus that was incorporated into human DNA 2-5 million years ago

in some examined patients of this disease, HERV-K was elevated, and the entire virus had come to life