4.2 DNA and Protein Synthesis

Question 1

Define ‘genome’ and ‘proteome’

Answer 1

Genome = the complete set of genes in a cell (including those in mitochondria and / or chloroplasts)
Proteome = the full range of proteins that a cell can produce (coded for by the cell’s DNA / genome)

Question 2

Describe the two stages of protein synthesis

Answer 2

Transcription = production of messenger RNA (mRNA) from DNA, in the nucleus
Translation = production of polypeptides from the sequence of codons carried by mRNA, at ribosomes

Question 3

Compare and contrast the structure of tRNA and mRNA

Answer 3

Similarities:
- both single polynucleotide strand

Question 4

Compare and contrast the structure of tRNA and mRNA

Answer 4

Differences:
- tRNA is folded into a ‘clover leaf shape’, whereas mRNA is linear / straight
- tRNA has hydrogen bonds between paired bases, mRNA doesn’t
- tRNA is a shorter, fixed length, whereas mRNA is a longer, variable length (more nucleotides)
- tRNA has an anticodon, mRNA has codons
- tRNA has an amino acid binding site, mRNA doesn’t

Question 5

Describe how mRNA is formed by transcription in eukaryotic cells

Answer 5

1. Hydrogen bonds between bases break
2. Only one DNA strand acts as a template
3. Free DNA nucleotides align next to their complementary bases on the template strand
   - in RNA, uracil is used in place of thymine (pairing with adenine in DNA)
4. RNA polymerase joins adjacent nucleotides
5. This forms phosphodiester bonds via condensation reactions
6. Pre-mRNA is formed and this is spliced to remove introns, forming (mature) mRNA

Question 6

Describe how the production of mRNA in a eukaryotic cell is different from the production of mRNA im a prokaryotic cell

Answer 6

• Pre-mRNA procured in eukaryotic cells whereas mRNA is produced directly in prokaryotic cells
• because genes in prokaryotic cells don’t contain introns so no splicing in prokaryotic cells

Question 7

Describe how translation leads to the production of a polypeptide

Answer 7

1. mRNA attaches to a ribosome and the ribosome moves to a start codon
2. tRNA brings a specific amino acid
3. tRNA anticodon binds to complementary mRNA codon
4. Ribosome moves along to next codon and another tRNA binds so 2 amino acids ids can be joined by a condensation reaction forming a peptide bonds
   - using energy from hydrolysis of ATP
5. tRNA released after amino acid joined polypeptide
6. Ribosome moves along mRNA to form the polypeptide, until a stop codon is reached

Question 8

Describe the role of ATP, tRNA and ribosomes in translation

Answer 8

ATP:
- hydrolysis of ATP to ADP + Pi releases energy
- so amino acids join to tRNAs and peptide bonds form between amino acids
tRNA:
- attaches to / transports a specific amino acid, in relation to its anticodon
- tRNA anticodon complementary base pairs to mRNA codon, forming hydrogen bonds
- 2 tRNAs bring amino acids together so peptide bonds can form
Ribosomes:
- mRNA binds to ribosome, with space for 2 codons
- allows tRNA with anticodons to bind
- catalyses formation of peptide bond between amino acids (held by tRNA molecules)
- moves along (mRNA to the next codon) / translocation