Termination of Transcription

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Questions and Answers

What causes RNA polymerase to stop transcribing and dissociate from the DNA template?

A promotor

A primase

A helicase

A terminator (correct)

Which type of terminator requires the Rho protein to function?

Rho-independent terminator

Rho-suppressive terminator

Rho-dependent terminator (correct)

Rho-enhanced terminator

What is the primary activity of the Rho protein during termination?

Polymerase activity

Ligase activity

Helicase activity (correct)

Synthase activity

Which characteristic is typical of a Rho-independent terminator?

G-C rich stem as part of a hairpin Signup and view all the answers

Which stage of RNA synthesis immediately precedes termination?

Elongation Signup and view all the answers

How does the Hot Pursuit model describe the function of Rho during termination?

Rho binds to the RNA tail and moves along the transcript until it catches the polymerase. Signup and view all the answers

Which subunit of the E. coli RNA polymerase is responsible for the initiation of transcription?

σ Signup and view all the answers

What is the function of the β' subunit in E. coli RNA polymerase?

DNA template binding region Signup and view all the answers

Which of the following sequences is known as the Pribnow box in E. coli promoters?

-10 sequence Signup and view all the answers

Which of the following steps occurs first during the initiation of transcription in E. coli?

Localized unwinding of DNA strands Signup and view all the answers

What happens after the σ factor is released during the transcription process in E. coli?

Nus A protein binds Signup and view all the answers

How is the transcription initiation site designated in a transcription unit?

+1 Signup and view all the answers

What is the main function of RNA polymerase in transcription?

Copying a DNA sequence into an RNA molecule Signup and view all the answers

Which structure is responsible for carrying out translation?

Ribosome Signup and view all the answers

In what direction does RNA polymerase synthesize the RNA molecule?

5' to 3' Signup and view all the answers

What unwinds the DNA during transcription?

RNA polymerase Signup and view all the answers

During which stage of transcription does RNA polymerase begin synthesizing RNA?

Elongation Signup and view all the answers

Which steps are involved in the stages of transcription?

Initiation, elongation, termination Signup and view all the answers

What is a key difference between transcription and DNA replication?

Transcription is carried out by RNA polymerase Signup and view all the answers

Which of the following is a similarity between transcription and DNA replication?

Both processes require the unwinding of the DNA double helix Signup and view all the answers

Study Notes

Termination of Transcription

RNA polymerase transcribes until it meets a terminator, then transcription stops and the RNA product dissociates from the DNA template.
Many terminators are hairpin forming sequences.
Termination signals in E. coli:
- Rho-dependent terminators: require a protein factor (Rho)
- Rho-independent terminators: do not require Rho

Rho-dependent Termination

Rho Factor:
- 46-kD protein, active as a hexamer
- Binds to RNA
- Has helicase activity, causing RNA-DNA to separate
Hot pursuit model: Rho binds to RNA tail and moves along transcript until it catches the polymerase

Rho-independent Termination

Rho-independent terminator:
- G-C rich stem
- U-DNA pairing is very weak, allowing dissociation
- U-run: 7-9 bases after loop

Central Dogma

Describes the flow of genetic information from DNA to RNA to protein
Transcription, translation, and degradation of mRNA molecules often occur simultaneously in prokaryotes

Transcription

Process by which a DNA sequence is copied into an RNA molecule
RNA polymerase synthesizes the RNA molecule in the 5' to 3' direction using one of the DNA strands as a template

Stages of Transcription

Initiation: RNA polymerase binds to the promoter region and begins to unzip the DNA strands
Elongation: RNA polymerase synthesizes the RNA molecule in the 5' to 3' direction using one of the DNA strands as a template
Termination: RNA polymerase reaches the terminator sequence and stops transcribing the DNA

Differences Between Transcription and DNA Replication

Transcription produces an RNA molecule, while DNA replication produces a DNA molecule
Transcription is carried out by RNA polymerase, while DNA replication is carried out by DNA polymerase
Transcription only requires one strand of DNA as a template, while DNA replication requires both strands of DNA as a template

E.Coli RNA Polymerase

Core enzyme: α₂ β β' ω
Holoenzyme: α₂ β β' ωσ
Functions of the subunits:
- α: assembly of the tetrameric core
- β: ribonucleoside triphosphate binding site
- β': DNA template binding region
- σ: initiation of transcription

Numbering of a Transcription Unit

Transcript initiation site is +1
Bases preceding the initiation site are given minus (-) prefixes and are referred to as upstream sequences
Bases following the initiation site are given plus (+) prefixes and are referred to as downstream sequences

Promoters

In E. coli, must be >12 bp to avoid occurrence by chance
Promoters have only small conservation in sequence
Four main regions:
- Startpoint
- -10 sequence (Pribnow box)
- -35 sequence
- 17 nucleotide spacer sequence between -10 and -35 sequences

Starting Transcription

Binding of RNA polymerase holoenzyme to a promoter region in DNA
Localized unwinding of both DNA strands by RNA polymerase to provide a single-stranded template
Formation of phosphodiester bonds between the first few ribonucleotides in the nascent RNA chain
Conformational change in enzyme, promoter is cleared
Sigma Factor is released
Nus A protein binds instead, ready for elongation "antitermination complex"

Elongation

RNA polymerase is bound to DNA and is covalently extending the RNA chain
The polymerase moves downstream from its position, processively extending the nascent RNA chain