RNA Expression and Processing

Transcription

RNA poly,erase uses a DNA template to guide the synthesis of a complementary RNA

Gene Expression

The process of synthesizing the product encoded by a gene begins with transcription

Amplifications of genetic information (caused by)

Having two genes can form hundreds of transcripts per cell.

Regulated by the cell to control gene expression

• Transcription

• mRNA stability (unstable and degraded within a few hours)

• Translation

RNA pol I (products)

rRNA

RNA pol II (Products)

mRNA (and some other RNAs)

RNA pol III (Products)

tRNA (and other small RNAs)

General Transcription Factors

Proteins that bind core promotes of all nuclear genes

• GTFs bind promoters in sequential order

• Each RNA polymerase has its own GTFs

• Nomenclature: TFIID is a GTF for RNA pol II

TFIID

Binds DNA sequences specifically found in promoters

TFIH

Last to arrive; has 2 jobs:

• Unwind DNA

• Phosphorylate RNA pol II

Phosphorylation is the ‘go’ signal for transcription initiation

Key Steps of Transcription

1) Initiation

2) Elongation

3) Termination

Transcription (elongation)

• Nucleosomes must be disassembled for RNA polymerase to access DNA

• Nucleosomes must be immediately reassembled after the enzyme passes

  • Requires chromatin remodelling proteins

• Elongation through the DMD gene takes approximately 16 hours.

Transcription elongation and termination

• RNA pol II termination is often coupled to polyadenylation

• Primary transcript cleaved approximately 10-35 nucleotides after polyadenylation signal

• The cleavage site is also the site for the addition of the poly(A) tail.

mRNA Splicing

Exons must be spliced together after introns are discarded

Exons

‘Expressed’ sequences

• Much smaller than introns

• Vary from gene to gene

• Some undergo alternative splicing (selectivyl included ro excluded)

Introns

The other ‘intragenic’ sequences that are removed.

• Much larger than exons

Alternative Splicing

• Introns allow alternative splicing of pre-mRNA

• Certain splice sites an be activated or skipped

• Allows one gene to encode multiple polypeptides

5’ Caps in mRNA 5’ end

• Modified guanosine with a ‘backward’ linkage to the mRNA

• Added soon after transcription is initiated

• Stabilizes mRNA and is important for translation

• Ribosomes assemble at 5’ caps and then scan mRNA until they find start codons.

C-terminal domain (CTD) of RNA during polymerase

• Recruits RNS processing proteins to the site of transcription

• Binds enzymes needed for capping and splicing, and cleavage/polyadenylation

RNA editing

Insertion, removal, or alteration of nucleotides in RNA. Alteration of nucleotides includes conversion to ‘modified’ bases.

• Frequent in tRNA and mRNA in the brain

rRNA types and named after

Types include:

• 25-28S

• 5.8S

• 5S

• 18S

Names for sedimentation rates during centrifugation (S — Svedberg Unit (correlates with size)