Transcription (DNA to RNA)

Types of RNA (5)

• mRNA - template for protein synthesis

• rRNA - major component of ribosomes

• tRNA - carries amino acids in activated form to the ribosome

• snRNA - participated in RNA splicing

• miRNA - bind to mRNA, promoting degredation or inhibiting their translation

Transcription initiation

• RNA polymerase

• Does not require a primer

• 5’ to 3’

• 17 base pairs in the DNA duplex are unwound

• RNA polymerase is a holoenzyme made up of a core enzyme and an initiation factor which directs the enzymes to the correct initiation sites

• the start of the transcriptional units are marked by special sequences called promoters

Difference between prokaryotes and eukaryotes?

• in eukaryotes there are 3 different RNA polymerases: 1,2 and 3

• In eukaryotes RNA synthesis (in nucleus) is separated from translation (in cytoplasm)

• Eukaryotic pre-mRNA is processed: addition of a polyA tail to the 3’ end of the RNA trascript (increases stability), addition of a cap to the 5’ end

Termination

• One is a palindromic GC rich region followed by an AT rich region

• the RNA transcript forms a hairpin structure with stem and loop

Promoter regions

• a conserved AT rich region at around -10 (Pribnow box)

• A consensus sequence at -35

Splicosomes

• allows synthesis of several protiens from one gene

• may be cell type specific: finronectin is a component of the extra cellular matrix - fibroblasts produce fibronectin mRNA that includes exons EIIIA and EIIIB - encode amino acids that bind tightly to proteins in fibroblast plasma membrane

Splicing mutations

Have been estimated to cause 15% of all genetic diseases

Control of transcription

Can be enhanced by activators or blocked by repressors

• repressor e.g. Lac operon (Lac Z, Lac y, Lac A, involved in metabolism of lactose)

• Allolactose (side product of beta-galactosidase reaction) is an inducer of this operon, it binds to repressor and prevents it from binding to operator

Control of transcription in eukaryotes

• basal transcription factors bind to promotors, common in most genes

• Regulatory transcription factors bind to promotor proximal elements and enhancers (specific for particular genes)

miRNA

• small non-coding RNA - 21 to 25 nucleotides

• Synthesised as a precursor from nuclear DNA and then processed into miRNA

• Function by base-pairning complementary sequences of mRNA, silencing of specific mRNA’s and repression of protein translation