CSB349H1 Lecture Notes - Lecture 2: Horizontal Gene Transfer, Open Reading Frame, African Clawed Frog
14 May 2018
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Lecture 2(c): Genes & Genome Structure
Recap:
• Humans have ‘only’ 20,000-25,000 genes – they come from gene duplication & divergence:
• Gene duplication and divergence occurs within the same mechanism that generate copy
number variations in DNA repeats:
a) Errors in recombination; unequal crossing over
b) Errors in single strand break repair
c) Retrotransposition (rare)
ð Another major source of new genes is whole genome duplication – extremely rare
o Once it occurs there is a lot of new genes; example is barely (cereal crop) their genome
is 5GB which is larger than the human genome
§ Has underwent rounds of whole genome duplication
Whole Genome Duplication:
• Occurs when there is an error in meiosis; instead of
generating haploid gametes, diploid gametes are generated
o If diploid gametes fuse it will give rise to a
tetraploid organism – i.e. Xenopus Laevis
ð When whole genome duplication occurs; over
evolutionary history the vast majority of the gene
duplicates are lost
o Some genes may remain intact perhaps due to
a benefit
Horizontal Gene Transfer:
• Small number of genes come from symbionts, parasites or prey
o Bacteria transfer genes with one another via horizontal gene transfer
o Controversial when looking at human genome
Examples of Organisms w/ Horizontal Gene Transfer:
a) Sea Slug:
o Adult form of the sea slug is capable of undergoing photosynthesis
§ Does not require the organism to consume nutrients
§ Sea slug obtains chloroplast in its larval stage when it
physically ‘sucks’ out the chloroplast from algae
ð It is known that plants require the expression of certain genes in order to
maintain the functionality of the chloroplast
o Sea Slugs do not have those genes but it has been hypothesized that they were able
to acquire psbO gene from the algae which helps maintain the chloroplast
ð Cannot be said with confidence that there is horizontal gene transfer within the Sea Slug
b) Aphids:
o Lateral transfer of genes from fungi; some of the aphids are red; and the pigments that
make it red is obtained from fungi – there are similarities between fungi and aphids
Document Summary
Has underwent rounds of whole genome duplication. Whole genome duplication: occurs when there is an error in meiosis; instead of generating haploid gametes, diploid gametes are generated, if diploid gametes fuse it will give rise to a tetraploid organism i. e. xenopus laevis. When whole genome duplication occurs; over evolutionary history the vast majority of the gene duplicates are lost: some genes may remain intact perhaps due to a benefit. Horizontal gene transfer: small number of genes come from symbionts, parasites or prey, bacteria transfer genes with one another via horizontal gene transfer, controversial when looking at human genome. Examples of organisms w/ horizontal gene transfer: sea slug, adult form of the sea slug is capable of undergoing photosynthesis. Does not require the organism to consume nutrients. Sea slug obtains chloroplast in its larval stage when it physically sucks" out the chloroplast from algae.
