02 - January 10, 2013.docx

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Molecular Genetics and Microbiology
Johanna Rommens

January 10, 2013 Second lecture: Chromosome structure and function Protein coding genes -give ultra exquisite control over gene expression – at price of complex structure Gene density – gene desert – no protein coding genes there Orthologs can be in other species Gene like dystrophin – this large – takes 24 hours to transcribe at full rate – so long to make this gene – seems like a burden, - seems to not need this gene very much, true Largest protein 1/10 of size LISTEN TO RECORDING HERE Gene families – origins are? Relate to how they might behave, how present in genome Gene coding genes specifically Lots of time – generate non-functional gene copies – pseudogenes – specific function may be assigned to them for regulation – but rare Beta globulin – gene cluster – proximity to each other relates to how regulated through development Examples of clustered families Beta globin cluster – HLA family Coding genes in blue – pseudogenes in white has lost features to be expressed Also see fragments of genes – fragment order still maintained – Gene families continued Tandem – in contrast to them, widely dispersed – among different chromosomes Dispersed genes =/= tandem genes? Not sure – selection npressure to maintain reading frames if gene is absolutely essential but probably true – lots of dispersed gene family members will be across multiple species ___________________ Examples of interspersed gene families PAX genes – highly conserved through evolution – nine in humans – More examples of clustered multi-gene familes Globin clusters HOX genes – 38 in humans Gene families - genes that look like ach other – there are certainly genes that carry domains important for other genes – can distinguish four clusters by their structure – overall similarity but sub cluster Olfactory receptor genes – Non processed pseudogenes Many remanants in genome – two types: processed and non-processed May or may not be transcribed!!! Recognize by occurance of translation termination codons in middle of reading frame – can be at start, or middle or end If transcribed, due to remnant of how they cam about – Origins of nonprocessed psuedogenes Formation of pseudogenes P Processed psuedogenes Arose from complimentary DNA sequence generated rom RNA that was produced RNA to DNA back into genome Lots of pseudogenes might show remnant of repeated sequence Similar to virus processes - Probably where processed genes come from Expressed processed genes – hop into adjacent side in genome where there is local promoting sequences Can look tissue-specific and be misleading Have actual copy of gene and have processed psuedogene – be aware that the assay is looking at the authentic gene and not the pseudogene – need to distinguish For non-processed psuedogene – appropriate termination codon – Examples where genes enter genome as processed gene and are active – it might have been an advantage to carry two copies of this gene – one that looks processed and other non-processed Special groups of gene families Half of coding sequences Multi-families for tRNAs alone – difficult to distinguish gene and pseudogenes tRNA clustered on a number of chromosomes rRNAs – one major transcript that is chopped by cleavage acrocentric – tips of chromsomes with short arms – look like have no short arm – where ribosomal clusters are – many, many related pseudogenes – snRNA – very short RNAs, with own specific properties snRNAs – important in processing of mrNA – part of spliceosome – snoRNAs – in nucleolus
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