Birth And Death Of Genomes

Front 1

How many human genes have a directly equvalent orthologue in mice?

Back 1

- approximately 80%

Front 2

What causes the biggest difference between gene content of different vertebrate genomes?

Back 2

- changes in the number of members of genes within gene families

e.g. olfactory receptors - 396 in human, 1035 in mouse

Front 3

How do new genes arise?

Back 3

- reverse transcr. mRNA (generate retrogenes)

- exon shuffling - exons from 1 gene become inserted into or fused with another gene

- de novo - mutation of non-coding DNA

- Gene duplication

Front 4

What is Gene duplication

Back 4

- most new genes and new members of gene family arise through gene duplication

- every gene is believed to undergo a duplication event once every 100 mln years

Front 5

How does gene duplication occur?

Back 5

- Replication slippage

- unequal crossing over between repetitive sequences

- unequal crossing over between

duplicated genes

- whole genome duplications

Front 6

What is replication slippage?

Back 6

Front 7

What is unequal crossing over between repetitive sequences?

Back 7

- May occur where gene is flanked by two copies of a repetitive sequence (e.g. Alu),

Front 8

What is unequal crossing over between duplicated genes?

Back 8

- may occur where there is a cluster of two or more related genes (during meiosis)

Front 9

Give an example of a whole genome duplication:

Back 9

- Xenopus tropicalis - diploid genome

- Xenopus laevis - tetraploid genome

- may have undergone one or two rounds of gene duplication about 500 mln years ago

Front 10

What is the fate of duplicated genes?

Back 10

- nonfunctionalization - 1 copy becomes inactivated through mutation (pseudogene)

- neofunctionalization - one copy acquires new function

- subfunctionalization - one copy acquires a subdivision of function

Front 11

Give an example of neufunctionalization:

Back 11

- antifreeze glycoprotein gene in Antarctic fish encodes a glycoprotein with 41 copies of the motif Thr-Ala-Ala, evolved from a duplicate of the trypsinogen gene

Front 12

Neofunctionalization - scheme

Back 12

Front 13

Give an example of subfinctionalisation

Back 13

- if gene is expressed in several tissues, each duplicate may become expressed in a subset of the tissue that the parent gene was expressed in

- most vertebrates: single aromatase gene expressed in brain and ovary

- fish - duplicate aromatase genes, one expressed in brain one expressed in ovary

Front 14

How did the hypothetical scheme for evolution of duplicated aromatase genes in fish?

Back 14

- The original gene had one sequence controlling expression in brain and another controlling expression in brain

- in fish the ovary mutated one sequence and the brain - the other

Front 15

What is an example of gene family generated by gene duplication?

Back 15

- Haemoglobin - tetramer

- different isoforms are used in different stages of development

Front 16

Evolution in the globin family:

Back 16

Front 17

When do pseudogenes originate?

Back 17

- inactivating mutation occur in a newly duplicated genes

- inactivating mutations occurn in an existing gene that has become surplus to requirement

Front 18

How many pseudogenes are there in humans?

Back 18

- 3230 known; 425 are from olfactory receptors

Front 19

Where do processed pseudogenes and retrogenes arise from?

Back 19

- reverse transcription of mRNA molecules

- either become inactive processed pseudogene

- or potentially active retrogene

Front 20

What are the differences between processed pseudogenes and pseudogenes>

Back 20

- processed are derived from mRNA -no introns, no promoter, polyA tail;

- processed are distant from active related genes, normal genes may be in clusters with active genes

- processed are flanked by direct repeats