lecture 4 - genome evolution - gene duplication Flashcards
How are new genes acquired by duplication events?
Gene duplication takes place
If there is selective pressure on both genes the genes stay similar
If there is selective pressure on just one copy of the genes one copy degrades and the other acquires a new function
What are homologous genes?
genes that share a common evolutionary ancestor
What are orthologous genes?
homologous genes located in the genomes of different organisms
What are paralogous genes?
two or more homologous genes located in the same genome
What is a pseudogene?
an inactivated & non-functional copy of a gene
How does gene duplication occur?
unequal crossing over between homologous chromosomes
unequal sister chromatid exchange
What happens after a gene has been duplicated?
Once there is more than one copy of a gene - sequence changes can occur in the extra copies
Each structural domain within a protein is an individual unit in a polypeptide chain & is coded for in continuous series of nucleotides
What is domain duplication?
Additional genes can be created by domain duplication
duplication of segments within a gene
What is domain shuffling?
New genes can be created by domain shuffling
Evidence: Domain shuffling in the tissue plasminogen activator (TPA)
TPA is required for the breakdown of blood clots in vertebrates and also used in clinical medicine to treat strokes
The most likely explanation is that domain shuffling has occurred in the tissue plasminogen activator (TPA) gene, but how this happened exactly is unclear
Domain shuffling has probably led to the evolution of a more sophisticated mechanism for the breakdown of blood clots in humans
This indicates that there may be an advantage to shuffling domains in this way
Further research is required to identify a mechanism
What are whole genome duplications?
Whole genome duplications generate extra copies of every gene (but no new genes)
This duplication does increase the potential for new genes
The sequence of each duplicated gene can begin to change without modifying the function of the original gene because there is an additional copy of the original gene
Is there any evidence for this? Yes, there is
Large, duplicated additional sets of genes in the same order have been identified in many organisms (recombination events may have moved some of the genes, or sets of genes, to new positions over time)
Where do the duplicated genes in S. cerevisiae come from?
Many sets of duplicated genes have been found in S. cerevisiae
~800 gene pairs
376 gene pairs occur in 55 duplicated sets
(set = at least 3 genes in the same order)
Where did these extra genes come from?
Kluyveromcyces lactis shares a common ancestor with S. cerevisiae that lived 100 million years ago (MYa)
K. lactis and S. cerevisiae do not share the same gene pairs
This is because 10% of S. cerevisiae genes are likely to be derived from a whole genome duplication event (about 100 MYa)
How did Saccharomyces evolve to become a good brewer?
In S. cerevisiae a whole genome duplication event occurred prior to the duplication of the ADH gene
The extra copies of the ADH gene allows new functions of the ADH enzyme to evolve
In S. cerevisiae ADH catalyses the conversion of acetaldehyde to ethanol
This enables S. cerevisiae to accumulate ethanol
This means S. cerevisiae can be used in fermentation and brewing
Conclusions
10% of S. cerevisiae genes are likely to be derived from a whole genome duplication event
Many sets of duplicated genes have been found in S. cerevisiae
The extra copy of every single gene has increased the potential for the sequence of the extra genes to change without affecting the function of the organism
This had lead to a significant change to sugar metabolism in S. cerevisiae which has been exploited by humans
