Protein Synthesis I

Question 1

1) What is the difference between the DNA template strand and the coding strand?

Answer 1

• Template (sense) strand is used for RNA production and is involved in complementary base pairing
• Coding (antisense) strand is complementary to the template strand, so is similar to the RNA strand produced though T/U bases are different

Question 2

2) Which enzyme is involved in the production of RNA and what does it do?

Answer 2

• RNA polymerase
• Addition of nucleotides in a 5’ -> 3’ direction (the template strand is read from 3’ -> as its antiparallel
• the RNA nucleotides are joined by covalent phosphodiester bonds

Question 3

3) What are the three stages of RNA synthesis and where do they take place?

Answer 3

• Initiation : RNA polymerase binding to template strand and beginning transcription
• Elongation : reading of DNA sequence on template and synthesis of mRNA
• Termination : reaching transcriptional termination site, mRNA synthesis finishes
• In the nucleus

Question 4

4) How do the RNA nucleotides reach the DNA template strand for complementary base pairing?

Answer 4

• Via a ribonucleoside triphosphate (RNA nucleotide) uptake channel on the RNA polymerase

Question 5

5) Describe how RNA polymerase finds the transcription initiation site in prokaryotic cells (e.g. E.Coli)

Answer 5

• During initiation, RNA polymerase is directed to the start site of transcription on the double stranded DNA. - The enzyme is able to find the transcriptional start site, after the promoter region and UTRs (untranslated regions, which are shorter in prokaryotes 3-10 nucleotides long) on the template strand

Question 6

6) What is the purpose of the 5’ UTR and 3’ UTR (on either end) of the template strand?

Answer 6

• 5’ signals for initiation of transcription

- 3’ signals transcriptional termination

Question 7

7) Which two areas on the template strand are important for RNA polymerase to locate the transcription initiation site?

Answer 7

• The -35 and -10 groups/boxes

- [-10 aka Pribnow box]

Question 8

8) What does the RNA polymerase bind to, on the template strand, before transcription occurs?

Answer 8

• Sigma factor

- This dissociates shortly after transcription starts

Question 9

9) Describe the sequence of events after transcription of the mRNA strand has completed

Answer 9

• mRNA strand grows in length as the RNA polymerase moves along the DNA strand
• RNA polymerase reaches the 3’ UTR which signals the termination of transcription
• mRNA dettachment from the DNA template is facilitated by the ‘hairpin’ shape (with a G-C rich stem loop) at the end of mRNA strand, which has weak A-U base pairing to DNA

Question 10

10) Describe the structures and functions of mRNA, tRNA and rRNA

Answer 10

mRNA: messenger RNA, transport RNA to a ribosome in cytosol/on RER for translation
tRNA: transfer RNA, to bring correct amino acids to ribosome, which are coded for by the mRNA strand
rRNA: ribosomal RNA, associates with proteins to form ribosomes

Question 11

11) What are the differences between eukaryotic and prokaryotic transcription?

Answer 11

• Eukaryotic is more complex with more proteins involved (polymerases, initiation and elongation factors, other helper proteins)
• In eukaryotes, the main enzyme involved is RNA Polymerase II
• Eukaryotic has additional mRNA processing

Question 12

12) What are the two regions eukaryotic genes are split into?

Answer 12

• Introns (non-coding regions), these are spliced out so aren’t present in mature mRNA
• Exons (coding regions) found in mature mRNA

Question 13

13) Define the term hnRNA

Answer 13

heteronuclear RNA
-heterogeneous RNA which is the primary transcript from DNA, these are unprocessed mRNA molecules in the nucleus that haven’t undergone any post-transcriptional changes yet

Question 14

14) Name 3 post-transcriptional modifications that occur in eukaryotic cells during conversion of the hnRNA to mRNA.

Answer 14

• Capping : formation of 7-methylguanosine ‘cap’ at 5’ end (Guanosine triphosphate -GTP- reacts with 5’ end of mRNA so its added to a 5’ phosphate group -> 5’ -5’ bond). Guanosine is methylated at 7’ position
• Splicing : removal of introns from pre-mRNA/ hnRNA
• Polyadenylation : addition of polyadenyl tails (long stretcch of adenosines to stabilise mRNA) to the 3’ end of RNA [adds 50-250 adenosine residues]

Question 15

15) State one way in which hnRNA can be spliced differently, from simply intron removal, and how this is beneficial

Answer 15

• Some exons may be removed as well, to produce different protein products from the same gene