3.4.2 DNA and protein synthesis

Describe how the base sequence of nucleic acids can be related to the amino acid sequence of polypeptides when given data. (4 marks)

- The genetic code shows which triplets or codons specify each amino acid.

- tRNA anticodons are complementary to mRNA codons.

- mRNA codons are complementary to DNA triplets on the template strand.

- In RNA, uracil replaces thymine.

Define 'genome' and 'proteome'. (2 marks)

- Genome is the complete set of genes in a cell, including those in mitochondria and/or chloroplasts.

- Proteome is the full range of proteins a cell can produce, coded for by its DNA.

Describe the two stages of protein synthesis. (2 marks)

- Transcription is the process of producing messenger RNA (mRNA) from DNA in the nucleus.

- Translation is the process of producing polypeptides from the codon sequence on mRNA at the ribosome.

State a similarity and five differences between tRNA and mRNA. (6 marks)

- Both are single polynucleotide strands.

- tRNA is folded into a clover-leaf shape, whereas mRNA is linear.

- tRNA contains hydrogen bonds between complementary bases; mRNA does not.

- tRNA has a fixed, shorter length; mRNA is longer and varies in length.

- tRNA contains an anticodon; mRNA contains codons.

- tRNA has an amino acid binding site; mRNA does not.

Describe how mRNA is formed by transcription in eukaryotic cells. (6 marks)

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, with uracil replacing thymine.

4. RNA polymerase joins the RNA nucleotides.

5. Phosphodiester bonds form by condensation reactions.

6. Pre-mRNA is produced and then spliced to remove introns, forming mature mRNA.

Explain how mRNA production in eukaryotic cells differs from prokaryotic cells. (2 marks)

- In eukaryotic cells, transcription produces pre-mRNA, which is spliced to remove introns before becoming mRNA.

- In prokaryotic cells, mRNA is produced directly because their genes do not contain introns.

Describe how translation leads to the production of a polypeptide. (6 marks)

1. mRNA attaches to a ribosome, which moves to the start codon.

2. A tRNA molecule carrying a specific amino acid binds to the mRNA through complementary base pairing between its anticodon and the mRNA codon.

3. The ribosome moves along to the next codon, allowing another tRNA to bind so the amino acids can join by a condensation reaction, forming a peptide bond.

4. The peptide bond formation uses energy from ATP hydrolysis.

5. The tRNA is released after its amino acid is added to the growing polypeptide.

6. This process continues until a stop codon is reached.

Describe the role of ATP in translation. (2 marks)

- Hydrolysis of ATP to ADP and Pi releases energy.

- This energy allows amino acids to bind to tRNA and peptide bonds to form between amino acids.

Describe the role of tRNA in translation. (3 marks)

- Attaches to and transports a specific amino acid according to its anticodon.

- Anticodon forms complementary base pairs with mRNA codon by hydrogen bonding.

- Two tRNAs bring amino acids together for peptide bond formation.

Describe the role of ribosomes in translation. (4 marks)

- mRNA binds to the ribosome, which has space for two codons.

- Allows tRNA with anticodons to bind.

- Catalyses peptide bond formation between amino acids carried by tRNA.

- Moves along mRNA to the next codon (translocation).