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RNA and protein synthesis

Reproduction and inheritance · DNA, genes and protein synthesis · note 3 of 3

Spec 3.17B, 3.18B
Biology only (what this means)Biology only: only in International GCSE Biology. Double Award students can skip it. What the labels mean
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RNA and protein synthesisSpec 3.17B, 3.18B

DNA stays in the nucleus, but proteins are made at ribosomes in the cytoplasm. A different molecule, RNA, carries the instructions from the gene to the ribosome.

DNA and RNA
DNARNA
Number of strandsTwo (double helix)One (single stranded)
Base that pairs with adenineThymine (T)Uracil (U)
Bases presentA, T, C, GA, U, C, G

A codon is a sequence of three bases on mRNA. Each codon codes for one amino acid. An anticodon is a sequence of three bases on a tRNA molecule that is complementary to a codon.

Stage 1: transcription (in the nucleus)

  1. The two DNA strands separate along the gene.
  2. A single strand of mRNA (messenger RNA) is made as a copy of the gene, by pairing complementary bases. A pairs with U, T pairs with A, C pairs with G and G pairs with C.
  3. The mRNA leaves the nucleus through a pore and goes to a ribosome in the cytoplasm.

Stage 2: translation (at a ribosome)

  1. The mRNA attaches to a ribosome.
  2. tRNA (transfer RNA) molecules bring amino acids to the ribosome. Each tRNA carries one specific amino acid.
  3. The anticodon of each tRNA pairs with the matching codon on the mRNA.
  4. The amino acids are joined together in the order set by the codons, making a chain called a polypeptide. The polypeptide folds up to form a protein.

Transcription and translation of a short gene

The DNA template strand reads T A C G G T. Give the mRNA codons and the tRNA anticodons.

  1. Transcription: pair each DNA base with its RNA partner. T gives A, A gives U, C gives G, and so on. mRNA = A U G C C A.
  2. Codons: AUG and CCA.
  3. Translation: each anticodon is complementary to its codon. AUG pairs with UAC and CCA pairs with GGU.

Answer: mRNA codons AUG CCA; tRNA anticodons UAC and GGU.

Two panels. Transcription in the nucleus: the DNA strands separate along a gene and mRNA is made by complementary base pairing (template TAC GGT gives mRNA AUG CCA), then the mRNA leaves through a pore. Translation at a ribosome in the cytoplasm: tRNA molecules with anticodons CUG and AAU pair with the mRNA codons GAC and UUA, each carrying an amino acid that joins the growing polypeptide. (opens full size in a new tab)
Transcription makes mRNA from a gene in the nucleus; translation at a ribosome joins amino acids in the order set by the codons.
Common mistake:

Do not mix up the two stages. Transcription makes mRNA from DNA in the nucleus. Translation makes a polypeptide from mRNA at a ribosome.

Exam tip:

Codons are on mRNA. Anticodons are on tRNA. Name the molecule when you use either word.

Quick check

  1. What is a gene?

    Show answer

    A section of a molecule of DNA that codes for a specific protein.

  2. What is the genome?

    Show answer

    The entire DNA of an organism.

  3. Biology only Which base pairs with adenine in DNA?

    Show answer

    Thymine.

  4. Biology only Which base is in RNA instead of thymine?

    Show answer

    Uracil (U).

  5. Biology only Where are amino acids joined together to make a polypeptide?

    Show answer

    At a ribosome (translation).

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