1.13. Expression of genes - translation

Question 1

materials needed for translation

Answer 1

• mRNA
• tRNA
• free a-a inside cytoplasm
• ribosomes -> made out of rRNA + proteins (not organelles, no membrane)

Question 2

ribosomes

Answer 2

• composed of two subunits which are usually separated but join together during translation
• mRNA binding site on the small subunit
• tRNA 3 binding sites on the big subunit (E-exit, P-peptidyl, A-acceptor)

Question 3

tRNA

Answer 3

• clover-shaped molecule, has 4 loops
• complementary base pairing (result of attraction forces) when not in loops
• transport amino acids towards the ribosomes during translation
• specific a-a joins to the 3’ of the tRNA (a-a binding site) - determined by anticodon
• attaches to the binding sites on the big ribosome subunit
• simplified representation in letter T
• there are various tRNAs each for a specific a-a

Question 4

anticodon

Answer 4

triplet of N-bases on the tRNA complementary to mRNA codon (defines which a-a binds to the tRNA)

Question 5

3 stages of translation

Answer 5

1|initiation
2|elongation
3|termination

Question 6

1|initiation

Answer 6

• mRNA binds to small subunit
• initiator tRNA binds to the start codon on mRNA (always carrying a-a Met)
• big subunit joins the complex (initiator tRNA fits into the P-site)
• next tRNA fits into A-site - peptide bond formed
• 1st a-a detaches from its tRNA - dipeptide carried on the 2nd tRNA

Question 7

2|elongation

Answer 7

• ribosome moves for one codon (towards 3’ of mRNA)
• tRNA at the P-site moves to E-site and detaches off the ribosome
• next tRNA attaches to A-site - polypeptide chain grows (on P-site)

Question 8

3|termination

Answer 8

• A-site above stop codon - no tRNA or amino acid for it so release factor (protein) binds instead
• release factor releases the polypeptide chain (everything is disassembled - subunits detach)

Question 9

mRNA is connected to the mRNA binding site and anti-codons on tRNA are connected to codons on mRNA by…

Answer 9

H-bonds

Question 10

draw the mRNA-tRNA-ribosome complex (in translation)

Answer 10

…

Question 11

replication and transcription might…

Answer 11

occur simultaneously

Question 12

repair mechanisms

Answer 12

since during transcription, the sense strand is more exposed and subjected to the impact of mutagens (inducing mutations), the repair mechanisms exist to minimize these mutations and maintain the stability of the DNA templates
- however, since they deteriorate as we age, mutations on the strand accumulate and this leads to diseases

Question 13

mRNA vs gene length in pro- and in eukaryotes

Answer 13

P: the same length, no introns
E: (pre-) mRNA much longer than genes - introns get spliced out in the post-transcriptional modification

Question 14

The anti-sense strand has the same base sequence as __, and the sense strand has the same base sequence as ___.

Answer 14

tRNA, (pre) mRNA

Question 15

When asked how would a certain DNA base sequence be transcribed:

Answer 15

the chain showing complementary RNA bases to that DNA strand