Chapter 12. Gene expression: transcription and RNA modification

What is the organization of a protein-encoding gene?

A. regulatory regions (where and when a gene is expressed)

• Promoter: a DNA sequence that initiates transcription with RNA polym. and transcription factors (TATA box: core element within promoter that provides binding site for transcription factors)

• Enhancers and silencers: regulatory elements of a coding region that influence the rate of transcription through interactions with activators or repressors.

B. Transcription Unit (coding region transcribed into pre-mRNA)

• exons: coding sequences that will be spliced together to form final mRNA (contains actual instruction for protein synthesis)

• Introns (non-coding between exons, transcribed into pre-mRNA but spliced out before translation)

C. Termination: where transcription should stop

What is the organization of a mRNA transcript?

A. 5’ cap: modified guanine nucleotide added to 5’ end of pre-mRNA to protect from degradation and aids in ribosome binding during translation

B. Coding sequence (part that gets translated into a protein)

• start codon: AUG

• Stop codon: UAA, UAG, UGA

C. Poly-A Tail

• a sequence of adenine nucleotides is added to 3’ end of mRNA to stabilize and assist with export from nucleus to cytoplasm (Translation initiation)

Core promoter

• where RNA polym II binds to initiate transcription

• TATA box: 25-30 base pairs, strong regulated transcription

Regulatory elements

Enhancers: DNA sequences that bind activator proteins to increase transcription

• function at long distances being thousands of base pairs away

• DNA between enhancer and promoter brings the enhancer to proximity with transcriptional machinery

Silencers: bind repressor proteins to decrease/ prevent transcription

Transcription Factors

Regulate gene expression by binding to specific DNA sequences and interacting with RNA polymerase

• Basal: needed for assembly of machinery at core promoter and for recruitment of RNA polym II

• Activator proteins: bind to enhancers to stimulate transcription

• repressor proteins: bind to silencers or other regulatory elements to block transcription

what are the functions of RNA polymerase

Catalyzes the formation of RNA by adding ribonucleotides in 5-3 direction while using DNA template

• Initiation: binds to the promotor then unwinds DNA near start site and begins RNA synthesis

• Elongation: RNA polymer. moves along DNA reading 3-5 but synthesizes in 5-3. Phosphodiester bonds between ribonucleotides

• Termination: continues to elongate RNA chain until reaching termination signal, releases the newly synthesized RNA transcript

What are the different kinds of RNA synthesis?

• mRNA: encodes proteins and carries genetic information from DNA to ribosomes

• rRNA: ribosomes, involved in protein synthesis

• tRNA: transfers amino acids to ribosomes during translation

• snRNA: RNA splicing

What are the three key processing steps of eukaryotic mRNA’s

1. 5’ capping

5’ capping

• cap is added when mRNA is being synthesized, guanylyl transferase ads a nucleotide to 5’ linkage to provide stability and nuclear export

3’ Poly A tail

• poly A polymerase adds adenine nucleotides to the 3’ end to export mRNA from nucleus, protect from degredation, important for initiation of translation

Splicing

• process of removing introns from pre-mRNA and joining exons together to form a coding region

• carried out by spliceosome which consists of snRNAs to form snRNPS

• diversifies proteins from a single gene + protects and stabalizes