Protein synthesis and mutations — Grade 12 Life Sciences

DNA and RNA compared

	DNA	RNA
Strands	Double	Single
Sugar	Deoxyribose	Ribose
Bases	A, T, C, G	A, U, C, G (uracil replaces thymine)
Where found	Nucleus (and mitochondria)	Nucleus, cytoplasm and ribosomes

Transcription

Transcription happens in the nucleus:

The DNA double helix unwinds and the hydrogen bonds between bases break.
One strand acts as the template.
Free RNA nucleotides pair with the template bases (A–U, T–A, C–G, G–C) to form a strand of mRNA.
The mRNA leaves the nucleus through a nuclear pore and moves to a ribosome.

Translation

Translation happens at a ribosome in the cytoplasm:

The mRNA attaches to the ribosome. Every three bases on the mRNA form a codon.
Each tRNA molecule carries a specific amino acid and has an anticodon that is complementary to a codon.
tRNA anticodons pair with the mRNA codons in order, bringing amino acids in the correct sequence.
Peptide bonds form between adjacent amino acids, building a polypeptide chain until a stop codon is reached.

mRNA codon	Amino acid	mRNA codon	Amino acid
AUG	Methionine (start)	GCU	Alanine
UUU	Phenylalanine	GAC	Aspartic acid
GGC	Glycine	AAA	Lysine
CCU	Proline	UAA	Stop

✏️ Worked example (use the codon table above)

1DNA template strand: TAC AAA CCG CGA ATT

2mRNA (complementary, with U for T): AUG UUU GGC GCU UAA

3tRNA anticodons: UAC AAA CCG CGA (none for the stop codon)

4Amino acids: methionine – phenylalanine – glycine – alanine, then stop.

Mutations

A mutation is a change in the sequence of bases in DNA.

Substitution — one base is replaced by another. It may change one amino acid, or none if the new codon codes for the same amino acid.
Insertion or deletion — a base is added or removed. This shifts the reading frame (a frameshift), changing every codon after it, so the protein is usually non-functional.
Chromosome mutations — changes in chromosome structure or number, such as trisomy from non-disjunction.

✏️ Effect of a substitution

1In the example above, the third DNA triplet CCG changes to CTG.

2The mRNA codon changes from GGC to GAC, so glycine is replaced by aspartic acid.

3A different amino acid can change the shape of the protein and therefore its function.

Mutations can be harmful (for example haemophilia), harmless (no change in the protein), or useful (a new variation that increases survival, which natural selection can act on).

Answer in your exercise book, then open each memo to mark your work.

Question 1 · (3 marks)

State THREE differences between DNA and RNA.

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Any THREE: DNA double-stranded, RNA single-stranded ✓; DNA has deoxyribose, RNA has ribose ✓; DNA has thymine, RNA has uracil ✓; DNA mainly in the nucleus, RNA in nucleus and cytoplasm.

Question 2 · (5 marks)

A DNA template strand reads TAC CCA AAA GGA. GGU codes for glycine, CCU for proline and UUU for phenylalanine, and AUG is the start codon (methionine). (a) Write the mRNA. (b) Write the tRNA anticodons. (c) Name the amino acids.

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(a) mRNA: AUG GGU UUU CCU ✓✓
(b) tRNA anticodons: UAC CCA AAA GGA ✓
(c) Methionine – glycine – phenylalanine – proline ✓✓

Question 3 · (4 marks)

Describe the process of translation.

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mRNA attaches to a ribosome ✓. tRNA molecules carry specific amino acids; their anticodons pair with complementary mRNA codons ✓. Amino acids are brought in the order coded by the mRNA ✓ and joined by peptide bonds to form a polypeptide ✓.

Question 4 · (3 marks)

Explain why a deletion of one base usually has a greater effect on a protein than a substitution of one base.

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A deletion shifts the reading frame (frameshift) ✓, so every codon after the deletion changes ✓ and many amino acids change, while a substitution changes at most one codon and one amino acid ✓.