state that a gene mutation is a change in the sequence of base pairs in a DNA molecule that may result in an altered polypeptide

Protein Synthesis

Objective

State that a gene mutation is a change in the sequence of base pairs in a DNA molecule that may result in an altered polypeptide.

Overview

Protein synthesis is the process by which cells build proteins according to the genetic instructions stored in DNA. It occurs in two main stages: transcription in the nucleus and translation in the cytoplasm (ribosomes).

Transcription

1. Initiation – RNA polymerase binds to the promoter region of a gene.
2. Elongation – The enzyme unwinds the DNA helix and synthesises a complementary messenger RNA (mRNA) strand using base‑pairing rules (A↔U, C↔G).
3. Termination – A termination signal causes RNA polymerase to release the newly formed mRNA.

Translation

1. mRNA leaves the nucleus and attaches to a ribosome.
2. Transfer RNA (tRNA) molecules bring specific amino acids to the ribosome. Each tRNA carries an anticodon that pairs with a codon on the mRNA.
3. The ribosome moves along the mRNA, linking amino acids together to form a polypeptide chain.
4. When a stop codon (UAA, UAG, or UGA) is reached, the ribosome releases the completed polypeptide.

Key Terms

• Gene: A segment of DNA that encodes a functional product, usually a protein.
• Codon: A sequence of three nucleotides on mRNA that specifies a particular amino acid.
• Anticodon: A three‑nucleotide sequence on tRNA that is complementary to a codon.
• Polypeptide: A chain of amino acids linked by peptide bonds; folds to become a functional protein.

DNA–RNA–Protein Coding Table

Gene Mutations

A gene mutation is a change in the sequence of base pairs in a DNA molecule. Mutations can be classified as:

• Point mutations – substitution of a single nucleotide (e.g., missense, nonsense, silent).
• Insertions or deletions – addition or loss of one or more nucleotides, often causing a frameshift.
• Chromosomal mutations – larger scale changes such as duplications, inversions, or translocations.

Potential Effects on the Polypeptide

1. Missense mutation: A single base change results in a different amino‑acid being incorporated, possibly altering protein function.
2. Nonsense mutation: A codon is changed to a stop codon, leading to premature termination and a truncated, usually non‑functional protein.
3. Silent mutation: The altered codon still codes for the same amino acid; the polypeptide remains unchanged.
4. Frameshift mutation: Insertion or deletion shifts the reading frame, changing every downstream codon and typically producing a non‑functional protein.

Illustrative Diagram