Explain how a gene is expressed through transcription and translation to produce a polypeptide

What this dot point is asking

SCSA wants you to trace the flow of information from DNA to protein, name the molecules involved at each step, and explain how the genetic code links codons to amino acids. A strong answer keeps transcription and translation clearly separate and explains where each happens.

The central dogma

The flow of genetic information runs DNA to RNA to protein. DNA stays in the nucleus as the master copy; a working copy (mRNA) is made and sent to the ribosome, where the protein is built. This two-step design protects the original DNA and allows many copies of a protein to be made from one gene.

Transcription: making mRNA

Transcription occurs in the nucleus.

1. RNA polymerase binds to the start of the gene and unwinds a section of the double helix.
2. Using one strand (the template strand) as a pattern, it builds a complementary single strand of messenger RNA, adding RNA nucleotides in the 5 prime to 3 prime direction.
3. RNA uses uracil (U) instead of thymine, so the template base A is matched with U.
4. The completed mRNA detaches and leaves the nucleus through a nuclear pore into the cytoplasm.

The genetic code

The mRNA sequence is read in groups of three bases called codons. Each codon specifies one amino acid, or a start or stop signal. With four bases in triplets there are 64 codons for 20 amino acids, so the code is:

• degenerate: most amino acids have more than one codon;
• non-overlapping: each base belongs to one codon only;
• near universal: the same codons mean the same amino acids in almost all organisms, which is what makes biotechnology across species possible.

Translation: building the polypeptide

Translation occurs at a ribosome in the cytoplasm.

1. The ribosome attaches to the mRNA at the start codon.
2. Transfer RNA (tRNA) molecules each carry a specific amino acid and have a three-base anticodon. A tRNA whose anticodon is complementary to the next mRNA codon binds in place.
3. The ribosome joins the amino acid carried by each tRNA to the growing chain with a peptide bond, then moves one codon along.
4. When a stop codon is reached, the finished polypeptide is released.

From polypeptide to functional protein

The chain of amino acids folds into a precise three-dimensional shape, determined by the order of amino acids. This shape gives the protein its function, whether it is an enzyme, a hormone such as insulin, or a structural protein such as collagen. A change in the base sequence of the gene can change an amino acid, alter the folding, and so change or destroy the protein function, which links expression to mutation.

Why this matters for continuity

Because the genetic code is nearly universal, the sequence of a gene reliably produces the same protein in every cell that expresses it and in every generation that inherits it. This dependable link between gene and protein is what allows traits to be passed on faithfully, while controlled differences in which genes are expressed allow one genome to build many different cell types.