gene expression in eukaryotes

2 reasons why eukaryotic genomes are large

number of genes (complex organisms) & amount of non-coding dna (around 97%)

promoter

part of gene that controls transcription

2 types of repetitive sequences

interspersed repetitive dna & tandemly repetitve satellite dna

interspersed repetitive dna

repeated units scattered throughout genome, single unit - 100-10,000 bp, makes up 25-40% of mammalian genomes

tandemly repetitive dna

classified according to length of repetitive region, a lot is located at telomeres and centromeres, cause of some genetic diseases

regular satellite dna length

100,000 - 10 million bp per site

minisatellite dna length

100 - 100,000 bp per site

microsatellite dna length

10 -100 bp per site

centromeres are important for

chromosome organisation

telomeres are involved in

protecting ends of chromosomes

chromatin structure

intricate form of packaging for dna - 10,000 fold compaction

heterochromatin

highly condensed during interphase, not actively transcribed, occurs during interphase in some regions of chromosome

euchromatin

less condensed during interphase, able to be transcribed - more of the chromosome is in this form during interphase

nucleosome

basic unit of chromatin

histones

proteins with positively charged amino acids that bind to the negatively charged dna, plays key role in chromatin structure

2 ways of chemical modification of chromatin

dna methylation for gene silencing, histone acetylation for gene activation

dna methylation

attachment of CH3 groups to bases, triggers formation of compact chromatin structure, associated with inactive dna

histone acetylation

attachment of COCH3 to histones, acetylated histones grip dna less tightly, involved in switching genes on and off

closed chromatin

DNA methylated, histones not acetylated

open chromatin

dna unmethylated, histones acetylated

rna pol 1

ribosomal rna

rna pol 2

mRNA

rna pol 3

small rna eg. tRNA

promoter determines ..

where transcription starts, rate of transcription

TATA box

part of promoter, provides site of initial binding of the transcription initiation machinery

preinitiation complex

forms before transcription - TF2D binds to TATA box to form initial committed complex, addition of other TFs, binding of TF2F and RNA pol2, then TF2E and TF2H

how do TFs affect rate of transcription

TFs bind to proximal/distal control elements - dna folding brings distal sequences into proximity with the promoter so they can work

post transcriptional regulation

capping of 5' end, polyadenylation of 3' end, splicing to remove introns