L15: Genome Evolution & Multigene Families Flashcards
What are syntenic genes?
Genes that are grouped in the same way on the chromosomes of two (or more) species
What are syntenic blocks?
Regions containing the homologous genes
How are chromosomal rearrangements defined?
By looking for blocks of synteny between chromosomes of different organsims
What is an example of chromosome rearrangement?
Translocation
What disease is an example of rearrangement of chromosomes?
Acute Myelogenous Leukaemia
What are the rearrangements that take place in Acute Myelogenous Leukaemia?
Translocations and inversions
What is centric fusion and Robertsonian translocation of chromosomes?
2 telocentric chromosomes fuse to generate 1 new chromosome, short arms lost so individual only has 45 chromosomes
What are the consequences of centric fusion and Robertsonian translocation?
- If no essential material in short arms no phenotype is produced
- After meiosis may generate trisomy or monosomy of long arms
- eg down syndrome & patau syndrome
Why do the chromosomes have bands on them?
The stain is used to clearly discriminate different chromosomes/regions of them
How can inversion of chromosomes take place?
As they may be paracentric in one or both arms
How do changes to genomes occur?
Mainly through accidents or mistakes
In what processes do accidents/mistakes occur?
DNA replication, recombination & repair leading to simple/complex changes (point mutation, translocations, deletions, duplications & inversions)
How is the genome made more complex?
-Small scale gene duplications
-Large scale gene duplications followed by loss of superfluous genes and divergence of the retained homologues
What is sister chromatid exchange?
-2 pairs of sister chromatids line up during meiosis. A repetitive region of one chromatid (the third one) does not line up exactly with corresponding region in others
- Strand breaks on non-sister chromatids result in unequal crossover, and different number of repeat units
- Strand breaks on sister chromatids also produce different repeat numbers
What is an example of an organism that has gone through genome duplication?
S. cerevisae
What are the benefits of genome duplication?
The extra set of genes do not have the same restriction on mutation rate as a single copy
What are multigene families?
Arise after duplication events and are followed by mutation to generate alteration in expression/function
What are homeotic (hox) genes?
The proteins encoded by hox genes are transcription factors that influence expression patterns of many genes during development
What are examples of hox genes?
bithorax, Antennapedia and Ultrabithorax in Drosophila
How does the position of the gene influence the expression?
The gene can be located in a specific segment which when expressed are located in that area (eg genes for wings are located in a specific segment)
How does mutation of a hox gene lead to changes in segmental identity?
Altered expression of downstream targets i.e. genes for which the hox protein is a transcription factor
Which chromosome are the 6 beta globin genes location on?
Chromosome 11
Where is the expression of beta globins?
Epsilon - During embryogenesis
Gamma - during fetal development
Beta - switch from gamma at around time of birth
Delta - low level expression in children & adults
Where is the expression of alpha globins?
Xi - fist few weeks of embryogenesis the alpha
What is Thalassemia?
Unbalanced production of globins which results in abnormal hemoglobin
How does Thalassemia take place and what measures the severity?
Result from mutations in either alpha or beta and severity depends on how many genes are affected and how severe the mutations are
What is Beta Thalassemia?
Impacts 2 beta genes if both mutated - major (death by ~20 years), if 1 mutated - asymptomatic
Excess alpha chains do not tetramerise so they bind to RBC membrane and can aggregate
What are other examples of hox mutations?
Exon shuffling - exons corresponding to discrete domains
De novo generation of genes - when a gene is neutral or advantages the gene is retained as a new gene
