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Biology · Practice book · Senior 5 · Unit 5 of 12

Protein synthesis

Senior 5 Biology · 12 questions with answers. Try each question on paper, then tap it to see the answer.

Unit 5 Protein synthesis

Key unit competence: Justify the relationship between a gene and the sequence of nucleotides in DNA, and the process of protein synthesis in eukaryotes.

Key ideas

  • Proteins are needed as enzymes, structural proteins (collagen, keratin), transport proteins (haemoglobin, carriers), hormones (insulin), antibodies and contractile proteins. Protein synthesis needs DNA (the gene), mRNA, tRNA, ribosomes (rRNA + protein), amino acids, enzymes (RNA polymerase, aminoacyl-tRNA synthetases) and ATP.
  • The central dogma: DNA → (transcription) → mRNA → (translation) → polypeptide. The sequence of bases in a gene determines the sequence of amino acids in the polypeptide (primary structure), which determines its folding and function.
  • Transcription (in the nucleus): RNA polymerase binds to the promoter of a gene; the DNA unwinds and hydrogen bonds break; only the template (antisense) strand is read 3′→5′; free RNA nucleotides pair with it (A–U, T–A, G–C, C–G) and are joined 5′→3′; transcription stops at a terminator; the pre-mRNA has the same sequence as the coding (sense) strand, with U instead of T.
  • Post-transcriptional modification in eukaryotes: splicing removes non-coding introns and joins exons (by spliceosomes); a 5′ cap (modified guanine) and a 3′ poly-A tail are added for stability and export through nuclear pores. Alternative splicing lets one gene produce several different proteins.
  • Translation (on ribosomes in the cytoplasm/RER): (1) Initiation: the small subunit binds mRNA; the start codon AUG pairs with a tRNA anticodon UAC carrying methionine; the large subunit joins. (2) Elongation: the next tRNA enters the A site; a peptide bond forms between amino acids (catalysed by rRNA, peptidyl transferase); the ribosome moves one codon along (translocation); the empty tRNA leaves from the E site. (3) Termination: a stop codon (UAA, UAG, UGA) is reached; release factors free the polypeptide. Several ribosomes may translate one mRNA (polysome).
  • Activation of amino acids: each amino acid is attached to its specific tRNA by an aminoacyl-tRNA synthetase using ATP. tRNA has a cloverleaf shape with an anticodon loop and a CCA end where the amino acid binds.
  • The genetic code (on mRNA codons) is triplet, universal, degenerate and non-overlapping, with start and stop codons. After translation, polypeptides fold (secondary, tertiary structure) and may be modified in the Golgi apparatus (e.g. glycosylation) before secretion.
  • Gene mutations change the base sequence: substitution (one base replaced) may be silent (same amino acid, because of degeneracy), missense (different amino acid, e.g. sickle-cell: GAG→GTG, glutamic acid → valine in β-globin) or nonsense (creates a stop codon, short protein). Insertions or deletions of bases (not multiples of three) cause a frameshift, changing all following codons and usually producing a non-functional protein.
  • DNA stores the information and stays in the nucleus (protected); RNA copies and carries it (mRNA), brings amino acids (tRNA) and forms the catalytic machinery (rRNA), so many proteins can be made from one gene quickly.

Common misconceptions

  • "Both DNA strands are transcribed to make mRNA." No: only the template strand of a gene is transcribed; mRNA matches the coding strand (with U for T).
  • "Translation takes place in the nucleus." No: translation occurs on ribosomes in the cytoplasm or on the rough ER.
  • "Every base change alters the protein." No: because the code is degenerate, some substitutions are silent.
  • "Introns are translated into protein." No: introns are removed by splicing before mRNA leaves the nucleus.

Worked examples

Example 1. The template strand of DNA reads 3′-TAC GGA CTT ATT-5′. Write the mRNA and the amino acid sequence (AUG = Met, CCU = Pro, GAA = Glu, UAA = stop).

Solution

mRNA (complementary to template, 5′→3′): AUG CCU GAA UAA. Polypeptide: Met–Pro–Glu, then translation stops at UAA.

Example 2. Explain why a deletion of one base usually has a bigger effect than a substitution.

Solution

A deletion causes a frameshift: every codon after the deletion is read in a new frame, so many amino acids change and a premature stop codon may appear; the protein is usually non-functional. A substitution changes only one codon, so at most one amino acid changes, and it may be silent.

Practice questions

  1. State the requirements for protein synthesis.

    Show answer

    DNA (gene), RNA polymerase, free RNA nucleotides, mRNA, tRNA, ribosomes, amino acids, aminoacyl-tRNA synthetases, ATP/GTP.

    Common mistake: Leaving out ATP.

  2. Describe transcription.

    Show answer

    RNA polymerase binds the promoter; DNA unwinds; the template strand is read; free RNA nucleotides pair by complementary base pairing (U with A) and are joined; transcription ends at the terminator; pre-mRNA is released.

    Common mistake: Saying both strands are transcribed.

  3. Describe the post-transcriptional modification of mRNA in eukaryotes.

    Show answer

    Introns removed and exons spliced together; a 5′ cap added; a 3′ poly-A tail added; mature mRNA exits through nuclear pores; alternative splicing may produce different mRNAs.

    Common mistake: Saying exons are removed.

  4. Describe the three stages of translation.

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    Initiation: small subunit binds mRNA; tRNA-Met pairs with AUG; large subunit joins. Elongation: tRNAs bring amino acids to the A site; peptide bonds form; ribosome moves one codon. Termination: stop codon; release factors free the polypeptide.

    Common mistake: Saying tRNA carries codons.

  5. Give the mRNA and amino acid sequence for the template strand 3′-TAC AAA CCG ACT-5′ (AUG Met, UUU Phe, GGC Gly, UGA stop).

    Show answer

    mRNA 5′-AUG UUU GGC UGA-3′; polypeptide Met–Phe–Gly, then stop.

    Common mistake: Writing T instead of U.

  6. Distinguish between silent, missense and nonsense mutations.

    Show answer

    Silent: base change gives the same amino acid. Missense: a different amino acid (e.g. sickle cell). Nonsense: creates a stop codon, shortened protein.

    Common mistake: Saying all mutations are harmful.

  7. Explain how sickle-cell anaemia arises from a mutation.

    Show answer

    In the β-globin gene, a substitution changes the codon GAG to GUG (DNA CTC to CAC), replacing glutamic acid with valine; haemoglobin S polymerises at low oxygen, making red cells sickle-shaped, block capillaries and break down (anaemia).

    Common mistake: Saying it is caused by a frameshift.

  8. Explain why the genetic code is described as degenerate and why this is useful.

    Show answer

    Most amino acids are coded by more than one codon (64 codons for 20 amino acids); many substitutions, especially in the third base, are silent, protecting proteins from some mutations.

    Common mistake: Saying one codon codes for several amino acids.

Matching and drag-and-drop

  1. Matching

    Match each item in column A with the correct answer in column B. Write the letter next to each number.

    Column AColumn B
    1. Proteins for export are modified and packaged in the:a. uracil
    2. Amino acids are joined during translation by:b. template (antisense) strand
    3. The DNA strand read during transcription is the:c. Golgi apparatus
    4. In transcription, adenine on DNA pairs with:d. peptide bonds
    Show answer

    1 → c (Golgi apparatus) · 2 → d (peptide bonds) · 3 → b (template (antisense) strand) · 4 → a (uracil)

    • Then secreted by exocytosis.
    • Formed by condensation.
    • mRNA is complementary to it.
    • RNA has uracil.

  2. Drag and drop

    Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

    900  ·  299  ·  exons  ·  nonsense mutation
    1. An mRNA has 900 bases including the stop codon. The maximum number of amino acids in the polypeptide is .
    2. The coding parts of a eukaryotic gene are .
    3. A substitution that creates a stop codon is a .
    Show answer

    1: 299 · 2: exons · 3: nonsense mutation

    • 900 ÷ 3 = 300 codons, minus the stop codon.
    • They are expressed.
    • The protein is shortened.

  3. Matching

    Match each item in column A with the correct answer in column B. Write the letter next to each number.

    Column AColumn B
    1. Making mRNA from DNA is called:a. anticodon
    2. The start codon AUG codes for:b. ribosomes
    3. The tRNA region that pairs with an mRNA codon is the:c. transcription
    4. Translation takes place on:d. methionine
    Show answer

    1 → c (transcription) · 2 → d (methionine) · 3 → a (anticodon) · 4 → b (ribosomes)

    • It happens in the nucleus.
    • It starts every polypeptide.
    • It ensures the correct amino acid.
    • In the cytoplasm or on RER.

  4. Drag and drop

    Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

    base substitution in the β-globin gene  ·  frameshift mutation in the insulin gene  ·  polysome  ·  5′-AUG-3′
    1. Several ribosomes translating one mRNA together form a .
    2. Sickle-cell anaemia is caused by a .
    3. The template DNA triplet 3′-TAC-5′ gives an mRNA codon of .
    Show answer

    1: polysome · 2: base substitution in the β-globin gene · 3: 5′-AUG-3′

    • Many copies of the protein are made quickly.
    • Glutamic acid is replaced by valine.
    • Complementary bases with U for T.

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