Gene structure (prokaryotic and eukaryotic)

human genome length

3200 Mbp

the genome size difference within and between prokaryotes and eukaryotes is due to

presence of more genes in eukaryotic genomes (in general, humans have ~25000 genes, E coli has ~4000)

significant differences in gene and, in eukaryotes, genome structure

gene density

much more dense in E coli, S cerevisaie than humans

prokaryotic genes

often arranged in operons

mRNAs can be polycistronic

mRNA is co-linear with DNA (no intervening sequences)

regulation of gene expression is at the level of transcription initiation and control, mediated by the binding of trans-acting factors and cis-acting sites

mRNAs are used directly for protein synthesis (no post-transcriptional modification)

transcription and translation are coupled

operon

a group of genes expressed in conjunction with one another, often encoding proteins involved in the same biochemical process or pathway

polycistronic

contain coding information for more than one protein

prokaryotic genes and transcription/translation

concurrent, RNA polymerases moves along active chromosome segment and at the same time ribosomes conduction protein synthesis in the direction towards the active chromosome

eukaryotic genes

extensive post-translational processing after synthesis of pre-RNA

regulatory elements positioned upstream and downstream of genes

as in prokaryotes, regulation of gene expression is at the level of transcription initial and control, trans-acting factors, cis-acting sites

transcripts are exported from nucleus before translation

post-translational processing in eukaryotes

addition of a 7-methylguanosine cap to the 5’ end (serves to position the mRNA on the ribosome)

removal of introns (splicing), needs to be done very precisely

poly A tail addition by poly A polymerase

alternative splicing

presence of multiple cleavage/poly A addition sites can give rise to different pre-mRNAs in different cell types

cleavage and polyadenylation of primary transcript gives rise to already different pre-mRNAs, then splicing to mature mRNA, then translation to immature protein, then more cleavage to form final protein

human dystrophin gene

encodes a protein involved in anchoring the internal cytoskeleton to the extracellular matrix

mutations in this gene leads to Duchenne muscular dystrophy

~2.4 Mb in size (~0.1% of the genome)

79 exons (99.4% of the gene is intronic)

~14 Kb mRNA

takes about 16 hours to be transcribed