Genes

Gene. A heritable factor — a section of DNA, usually coding for a polypeptide (some code for RNA only).

Locus. The fixed position of a gene on a chromosome.

Allele. An alternative form of a gene. Different alleles differ in their DNA sequence — usually by one or a few bases.

For example, the gene for ABO blood group is on chromosome 9 (locus). The alleles at that locus are I^A, I^B and i — different versions of the same gene.

Gene expression. From DNA to a functional protein:

1. Transcription — DNA gene → mRNA.
2. Translation — mRNA → polypeptide at ribosome.
3. Folding and post-translational modification → functional protein.

A protein's function (catalysis, structure, signalling, transport) carries out the trait associated with the gene.

Coding vs non-coding DNA. In eukaryotes, only ~1–2% of DNA codes for protein. The rest includes:

• Introns (within genes, spliced out).
• Regulatory sequences (promoters, enhancers).
• Repetitive sequences (telomeres, centromeres).
• Genes for RNA only (rRNA, tRNA, microRNA).

This non-coding DNA is far from "junk" — it regulates gene expression and shapes chromatin structure.

Point mutations — change a single base:

• Substitution — one base replaced by another.
• Insertion — extra base added.
• Deletion — base removed.

Effects on the protein:

Mutation type	What happens	Example
Silent	New codon codes for SAME amino acid	UCU → UCC (both Ser)
Missense	New codon codes for DIFFERENT amino acid	GAG → GUG (Glu → Val; causes sickle-cell anaemia)
Nonsense	New codon is a STOP codon	UAC → UAA → premature termination → truncated protein
Frameshift	Insertion/deletion (not multiple of 3) shifts reading frame	Whole sequence after the mutation altered

Sickle-cell anaemia. A single point mutation (A → T) in the β-globin gene changes amino acid 6 from glutamate (charged) to valine (non-polar). Mutant haemoglobin polymerises under low O₂ → distorts red blood cells into sickle shape. Heterozygotes have malaria resistance.

Chromosomal mutations alter large segments:

• Deletion — segment lost.
• Duplication — segment repeated.
• Inversion — segment reversed.
• Translocation — segment moves to a different chromosome.

Causes:

• Mistakes during replication.
• Mutagens: UV radiation, ionising radiation, chemicals (e.g. benzene), some viruses.

The genome. Total DNA of an organism. Human genome:

• ~3 × 10⁹ base pairs.
• ~20 000 protein-coding genes (~1% of DNA).
• Mostly diploid: each somatic cell has 2 copies (one from each parent).

The Human Genome Project (1990–2003) was an international effort that sequenced the human genome — enabling personalised medicine, ancestry tracking, and disease-gene identification.