Genetic Organization in Eukaryotes
Chromosome structure in eukaryotes
Multiple chromosomes
Larger than prokaryotic chromosomes; more genes
Prokaryote - 2,000-3,000 genes
Humans - 50,000-100,000 genes
Multiple origins of replication (ori)
Linear chromosomes: Middle = centromere; ends = telomeres
Telomeres
DNA replication leaves gap at 5' end following removal of the RNA primer
Telomeres have 6 bp repeats repeated approx. 2000 times in humans
Telomerase - adds back a few 6 bp repeats after every round of replication
Recognizes telomeres because of complementary RNA complexed with the protein (similar to antisense RNA)
Repetitive sequences
Moderately repetitive - rRNA and tRNA genes and "junk"
Highly repetitive - "junk"
e.g. Alu element - 300 bp repeats; 300,000-500,000 copies in humans
Pseudogenes
Defective copy of a genuine gene; isn't expressed
Exons (coding region) and introns (intervening sequences)
Introns may make up to 90% of a gene
Eukaryotic transcription
Comparison of prokaryotic and eukaryotic transcription/translation
RNA pol I - transcribes 18S, 5.8S and 28S rRNA genes
RNA pol II - transcribes protein-coding genes
RNA pol III - transcribes tRNAs and 5S rRNA genes
Regulation of RNA polymerases
Pol I and III don't need much regulation because transcribe housekeeping genes that are constitutively expressed
Pol II regulation
Promoters, enhancers, and transcription factors
3 regions:
1) Initiator box - start site of transcription
2) TATA box - at -25 position; AT rich
3) Upstream elements - around 10 bp long; may be more than one (more complex)
May be located upstream or downstream of gene
Transcription factors
4 domains:
1) DNA binding
2) RNAP II binding
3) entry into nucleus
4) stimulus response
RNA processing
Primary transcript > (capping, tailing, splicing) > mature mRNA
Cap
Guanosine monophosphate added to 5' end, then G residue is methylated
Tail
Poly A site is AAUAAA; located at 3' end
Splicing
1) Self-splicing - in genes of some protozoan nuclei, fungal mitochondria and plant chloroplasts
Reactive adenosine residue interacts with the internal guide sequence to form a lariat structure that is then removed from the RNA (This is found in p. 404 of your text)
Spliceosome made up of snRNPs (small nuclear ribonucleoproteins) (This is found in Chapter 11 of your text)
Regulation by alternative splicing
Insert or delete single U residues; occurs in trypanosomes
RNA export
Regulated by proteins around the nuclear pores; only spliced RNAs may exit
Overview of genetic regulation in eukaryotes
Translation in eukaryotes
40S + 60S = 80S
Ribosome uses the 5'-most (first) AUG codon as the start codon
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Last updated: 9-April-98 / laa