4. Nucleic Acids and Gene Expression

Question 1

Describe the structure of mRNA.

Answer 1

Single-stranded sequence of ribonucleotides synthesised from DNA template strand during transcription

Question 2

Describe the role of mRNA.

Answer 2

• carrier molecule that conveys DNA sequence specifying amino acid sequence of a polypetide in the nucleus to ribosomes in cytoplasm
• each codon codes for an amino acid

Question 3

Describe the structure of tRNA.

Answer 3

• folds back upon itself to form distinct cloverleaf structure; held in shape by H bonds between complementary base pairs in certain regions
• 3’ CCA stem where specific amino acid binds
• anticodon complementary to mRNA codon

Question 4

Describe the role of tRNA.

Answer 4

• carries amino acid on 3’ CCA stem, from cytosol to ribosome before translation
• amino acid carried dependent on anticodon and corresponding complementary mRNA codon

Question 5

Describe the structure of rRNA.

Answer 5

Single-stranded sequence of ribonucleotides that folds back upon itself to form structure with many H bonds and loops

Question 6

Describe the role of rRNA.

Answer 6

• rRNA associated with ribosomal proteins to form both small and large ribosomal subunits, which then combine to form functional ribosomes
• catalytic function by peptidyl transferase in large ribosomal subunit; catalyses peptide bond formation between amino acids

Question 7

Stages of translation

Answer 7

1) Amino acid activation
2) Binding of mRNA to ribosome and initiation
3) Elongation
- codon recognition
- peptide bond formation
- translocation
4) Termination

Question 8

Translation - Amino acid activation

Answer 8

• amino acids activated by being matched with specific anticodon on tRNA, catalysed by aminoacyl-tRNA synthetase
• amino acid attached to 3’ CCA stem of tRNA through covalent ester bond; coupled with hydrolysis of ATP

Question 9

Translation - Binding of mRNA to ribosome and initiation

Answer 9

• small ribosomal subunit binds to 5’ end of mRNA at recognition sequence upstream of start codon, AUG
• initiator aminoacyl-tRNA with complementary anticodon UAC, carrying methionine, base pairs with AUG codon
• large ribosomal unit binds, forming functional ribosome and transcription-initiation complex, mediated by transcription initiator factors and coupled with hydrolysis of GTP
• initiator aminoacyl-tRNA positioned at P site of large ribosomal subunit, vacnat A site for tRNA carrying next amino acid

Question 10

Translation - Elongation

Answer 10

i) - anticodon of second aminoacyl-tRNA enters A site and base pairs with complementary second mRNA codon by hydrogen bonds
- facilitated by elongation factors, coupled with hydrolysis of GTP
ii) - peptidyl transferase catalyses peptide bond formation between carboxyl end of methionine and amino group of second amino acid; breaks ester bond between methionine and 3’ CCA stem of initiator tRNA
iii) - ribosome moves to next codon in 5’→3’ direction, coupled with hydrolysis of GTP
- second aminocacyl-tRNA translocated from A to P site while first tRNA occupies E site and is released into cytosol where it is recycled
- third aminoacyl-tRNA now occupies vacant A site

Question 11

Translation - Termination

Answer 11

• when stop codon is reached, release factor enters A site and adds H2O, hydrolysing ester bond between polypeptide and tRNA in P site, thereby freeing carboxyl end
• polypeptide released; ribosome dissociates into subunits and releases mRNA