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

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 4

Describe how mRNA is formed by transcription in eukaryotic cells

Answer 4

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 5

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

Answer 5

● 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 6

Describe how translation leads to the production of a polypeptide

Answer 6

1. mRNA attaches to a ribosome and the ribosome
   moves to a start codon (AUG)
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 can be joined by a
   condensation reaction forming a peptide bond
   ● 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 7

Describe the role of ATP, tRNA and ribosomes in translation

Answer 7

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 8

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

Answer 8

● 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