6.5.3 Transcription: Termination and RNA Protection

Question 1

terminator sequence

Answer 1

• a sequence of nucleotides on the DNA strand that signals the end of transcription.
  • After transcription, a series of enzymes in the nucleus modify the mRNA transcript before it undergoes translation in the cytoplasm. During mRNA processing, the ends of the transcript are altered, sections of the molecule are removed, and the remaining parts are sewn back together.

Question 2

note

Answer 2

• During transcription, a messenger RNA strand is
  constructed from a DNA template. Transcription factors are proteins that help RNA polymerase bind to a special site on the DNA (promoter) and initiate transcription. The assembly of transcription factors and RNA polymerase bound to the promoter is called the transcription initiation complex.
• Transcription ends when RNA polymerase reaches the
  termination sequence of the DNA strand. The sequence
  contains a high concentration of the base adenine. Uracil in RNA pairs with adenine in DNA, forming relatively weak hydrogen bonds between the DNA and mRNA strands. The weak bonding enables the RNA strand to break away from the template easily

Question 3

RNA processing

Answer 3

• In RNA processing, each end of the mRNA transcript is modified. The 5’ end is capped with a methylated guanine nucleotide. The 5’ capprotects the mRNA from degradation by hydrolytic enzymes as it travels through the cytoplasm prior to translation. The cap later serves as a signal for attachment of the ribosome.
• The 3’ end is also modified before the transcript leaves the nucleus. The poly(A) tail consists of 30–200 adenine nucleotides. The tail protects the mRNA from degradation and assists it in export from the nucleus.

Question 4

In eukaryotes, during transcription __________ occurs between the DNA template and the mRNA strand.

Answer 4

• hydrogen bonding

Question 5

The __________ is a sequence of DNA that signals the __________ of transcription.

Answer 5

• terminator sequence; end

Question 6

__________ protects the mRNA from degradation by hydrolytic enzymes and __________ assists mRNA in export from nucleus

Answer 6

• The 5′ cap; the poly (A) tail

Question 7

Which of the following is not a step in RNA synthesis?

Answer 7

• peptide bond formation

Question 8

RNA polymerase requires assistance from which molecules?

Answer 8

• transcription factors and promoter DNA sequences

Question 9

The mRNA transcript, 5′ AUCGUUA 3′, was just synthesized. Choose the DNA strand (including the correct orientation) that acted as the template.

Answer 9

• 5′ TAACGAT 3′

Question 10

What is the physical basis for the function of the terminator sequence?

Answer 10

• A sequence that minimizes the number of hydrogen bonds.