Topic 4 .2 DNA and protein synthesis

Question 1

Define ‘genome’ and ‘proteome’

Answer 1

Genome : The complete set of genes in a cell

Proteome : The full range of proteins that a cell can produce

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

Comparison (similarities):

● Both single polynucleotide strand

Contrast (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 3

Describe how mRNA is formed by transcription in eukaryotic cells

Answer 3

1. Hydrogen bonds between DNA bases break
2. Only one DNA strand acts as a template
3. Free RNA 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 RNA 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 4

Describe how production of messenger RNA (mRNA) in a eukaryotic cell is
different from the production of mRNA in a prokaryotic cell

Answer 4

● Pre-mRNA produced 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 5

Describe how translation leads to the production of a polypeptide

Answer 5

1. mRNA binds to ribosome at start codon
2. tRNA brings amino acids, binding anticodon to mRNA
3. Ribosome moves, adding amino acids and forming peptide bonds (ATP required)
4. tRNA released, process continues until stop codon

Question 6

Describe the role of ATP, tRNA and ribosomes in translation

Answer 6

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 bond 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

Question 7

Describe how the base sequence of nucleic acids can be related to the
amino acid sequence of polypeptides when provided with suitable data

Answer 7

● You may be provided with a genetic code to identify which
triplets / codons produce which amino acids (example shown)
● tRNA anticodons are complementary to mRNA codons
○ Eg. mRNA codon = ACG → tRNA anticodon = UGC
● Sequence of codons on mRNA are complementary to sequence
of triplets on DNA template strand
○ Eg. mRNA base sequence = ACG UAG AAC
→ DNA base sequence = TGC ATC TTG
● In RNA, uracil replaces thymine