3.3 - Transcription: Mechanism of Transcription

Question 1

What is 5’-capping and polyA tails?

Answer 1

While the pre-mRNA is still being synthesized, a 7-methylguanosine cap is added to the 5′ end of the growing transcript by a 5′-to-5′ phosphate linkage. This moiety protects the nascent mRNA from degradation. In addition, initiation factors involved in protein synthesis recognize the cap to help initiate translation by ribosomes.

When a sequence called a polyadenylation signal shows up in an RNA molecule during transcription, an enzyme chops the RNA in two at that site. Another enzyme adds adenine (A) nucleotides to the cut end, forming a poly-A tail. The tail makes the transcript more stable and helps it get exported from the nucleus to the cytosol.

Source: Proudfoot NJ, Furger A, Dye MJ (February 2002). “Integrating mRNA processing with transcription”

Question 2

How RNA splicing can be used to produce different mature mRNA transcripts/proteins from the same pre-mRNA?

Answer 2

In alternative splicing, precursor messenger RNA is processed to produce many different messenger RNAs. The expression of these different RNAs from one gene makes possible the enormous protein diversity found in humans.

Question 3

A segment of one strand of a double-stranded DNA molecule has the sequence
5′-CATTCGACT-3′. What is the sequence of an RNA molecule synthesized from this DNA template?

Answer 3

The RNA transcript has the sequence

5’-AGUCGAAUG-3’ or 3’-GUAAGCUGA-5’

Question 4

The bases in the non‑template DNA strand of a bacteria gene includes 10% thymine. What percentage of the primary transcript synthesized from this gene is uracil?

A. 50%
B. 10%
C. 20%
D. 40%

Answer 4

B

(explanation:
10% thymine = 10% adenine
10% adenine = 10% uracil )

Question 5

What is the order of the steps for RNA Polymerase to synthesize RNA?
(ex. A,C,B)

A) transcription starts at the +1 site

B) transcription passes through the terminator and ends

C) RNA polymerase binds to a gene promoter

Answer 5

C,A,B

Question 6

What is the “-10 box and -35 box”?

A. -10 and -35 refer to the position upstream of the location of the +1 site

B. -10 is the location of the promoter, and -35 is the location of the terminator

C. none of the above

Answer 6

A

Question 7

MCQ fill in the blank:

-10 box –> prokaryotes
_____ –> eukaryotes

A. AUG
B. ATG
C. TATA box
D. + 1 site
E. -35 box

Answer 7

C

Question 8

Why isn’t the promotor region transcribed?

Answer 8

Since promotor is upstream of the +1 site where the transcription starts.