Module 4: Section 2A

Question 1

Cap site - eukaryotic genes

Answer 1

• Found at beginning of mRNA
• Consists of a guanine nucleotide connected to the mRNA via a 5’-5’ triphosphate linkage
• Modification helps prevent mRNA degradation

Question 2

Poly (A) site - eukaryotic genes

Answer 2

• Found at 3’ end
• Stretch of RNA that only has adenine bases
• Modification is important for RNA stability and nuclear export

Question 3

Polycristronic mRNA

Answer 3

• Also called an operon
• Used in bacterial genes
• More than one gene is transcribed by a single promoter

Question 4

Monocistronic mRNA

Answer 4

• Used in eukaryotic genes
• A single gene codes for a single protein

Question 5

Role of sigma factors in bacterial transcription

Answer 5

• They bind to RNA polymerase and direct it to the promoter region of a gene
• Sigma first recognizes the -35 region, allowing polymerase to associate with promoter

Question 6

Transcription of bacterial genes - important components

Answer 6

• RNA polymerase core enzyme
• Sigma factor
• The target DNA

Question 7

Transcription of bacterial genes - Step 1

Answer 7

A functional core containing a sigma factor assembles and binds to DNA resulting in a closed complex

Question 8

Transcription of bacterial genes - Step 2

Answer 8

• The complex transitions to an open complex
• DNA is unwound near the transcriptional start site

Question 9

Transcription of bacterial genes - Step 3

Answer 9

• Nucleoside triphosphates are brought in
• Resulting in initiation of RNA synthesis

Question 10

Transcription of bacterial genes - Step 4

Answer 10

• Polymerase pauses and “scrunches” the DNA
• If it can’t continue, transcription aborts, releasing a short RNA fragment (abortive transcript)
• Pausing may help coordinate multiple RNA polymerases on the same DNA

Question 11

Transcription of bacterial genes - Step 5

Answer 11

Once polymerase synthesizes an RNA transcript of around 7-10 nucleotides in length, it progresses into the elongation phase and completes synthesis of mRNA

Question 12

When is the sigma factor released

Answer 12

Between the scrunched complex and elongation phases

Question 13

Transcription of bacterial genes - Step 6

Answer 13

Release of RNA polymerase occurs through 2 different processes:
1. Factor-independent release
2. Factor-dependent termination

Question 14

Factor-independent release

Answer 14

A stem-loop structure formed by mRNA causes the RNA polymerase to fall away

Question 15

Factor-dependent termination

Answer 15

• The factor (called Rho) interacts with RNA polymerase as it pauses at the end of the coding region
• Causes the polymerase to fall off the template and the transcript to be released