Transcription

Question 1

What is trascription?

Answer 1

ribonucleic acid synthesis from DNA template

Question 2

What is the primary and secondary structure of RNA?

Answer 2

primary: existing in single strands

secondary: folds back upon itself in regions where complimentary base pairing is possible

Question 3

What are the three types of RNA and how much they make up the RNA in a cell’s cytosol?

Answer 3

mRNA: 1-2% cytosol RNA
tRNA: 15% cytosol
rRNA: 85% cytosol RNA

Question 4

What does the stucture of RNA affect and why?

Answer 4

affects longevity as mRNA are only in primary structure which degrades easily and tRNAs/rRNAs are in secondary structure which prevent ribonuclease attack

Question 5

What are the features in a bacterial gene for transcription?

Answer 5

1. Promoter: -35 (recognized by sigma factors) and -10 boxes (pribnow box)
2. Leader sequence: transcribed into mRNA but not translated
3. Coding region: transcribed and translated into polypeptide
4. Trailer: preps mRNA to release template strand; transcribed into mRNA but not translated
5. Terminator: RNA polymerase releases the template strand

Question 6

What is RNA polymerase’s structure and function?

Answer 6

Structure: Large multimeric enzyme with 2 alpha (a) subunits, beta subunit (B), beta’ subunit (B’), and omega (w)

Function: responsible for RNA synthesis

Question 7

What is the sigma factor?

Answer 7

Controls binding of RNA polymerase to promoter and detaches once first few RNA nucleotides have been joined together; different sigma factors initiate binding of RNA polymerases to different promoters.

Question 8

What are the steps of transcription initiation?

Answer 8

1. sigma factor recognizes -35 box on promoter and binds first, then binds to -10 box forming a closed complex
2. Complex opens with the melting of the -10 region by RNA polymerase and 16-20bp are unwound
3. Sigma factor dissociates and RNA polymerase core enzyme responsible for elongation of RNA transcript

Question 9

How does transcription elongation work?

Answer 9

1. Core enzyme reads strand in 3’-5’ direction and adds rNTPs (ribonucleoside triphosphates)
2. As nucleotides are added, a temporary double stranded RNA/DNA hybrid is formed, but the new RNA transcript separate from template
3. Single stranded DNA rewinds to reform double stranded DNA helical structure

Question 10

How does transcription termination work?

Answer 10

Inverted repeats at the end of gene sequence in the coding strand allow for hairpin/stemloop formation at the end of the RNA

Question 11

What is the purpose of the hairpin structure at the end of an RNA transcript?

Answer 11

Secondary structure presents physical obstruction to cause RNA polymerase to stall and dissociate via two mechanisms of rho dependent termination

Question 12

How does the poly-U mechanism of termination work?

Answer 12

A rich region present after inverted repeats in DNA allow for poly-U in RNA that causes destabilization of RNA/DNA hybrid in the open complex and releases RNA polymerase

Question 13

How does rut site mechanism of termination work?

Answer 13

1. rut site is activated when transcribed into RNA and will be bound by multi-rho hexameric protein that assembles around the RNA transcript.
2. Multi-Rho protein uses ATP hydrolysis to act as helicase, knocking off RNA polymerase as it is stalled by the hairpin structure.