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Lectures 9 and 10: Speciation and Genome Evolution (Gene duplication: Four…
Lectures 9 and 10: Speciation and Genome Evolution
Genome: Full set of genes plus noncoding regions of DNA
RNA polymerase makes RNA from DNA
Promoter sequences on DNA mark start of transcription
Total genome varies; genome size varies more than number of genes
In the human genome, 5% of sequence is constrained across mammals; 1.5% encodes proteins; 3.5% functional but non-coding
Evolutionary, biochemical, and genetic evidence is used to sequence functionality
Mechanisms to change gene number:
Transferred from other species
Duplicated within species
Gene transfer: Individual genes, organelles, or fragments of genomes move from one lineage to another
Virus -> host
Hybridization between species
DNA Fragments from the environment e.g. antibiotics
Gene duplication: Four fates for duplicated & original segment
Both copies retain original function but expression diverges
One copy becomes nonfunctional
Both copies retain original function
One copy accumulates substitutions that allow it to perform a new function
"Orphan genes" originate de novo from pre-existing sequences
Nonsynonymous substitution changes amino acid; silent or synonymous substitution doesn't change amino acid
If amino acid position is under purifying selection, rate of synonymous substitutions should be higher than nonsynonymous
If an amino acid replacement is neutral, the two rates are expected to be similar
If amino acid position is under positive selection for change, rate of nonsynonymous substitutions should be higher than synonymous
Large Genome can be an evolutionary constraint
Energetically costly
Target of mutation
Humans have an average of 6 billion bases