Chromosomes, Genes and Proteins

DNA.

• Long molecule, double helix.
• Made of nucleotides (sugar + phosphate + nitrogenous base).
• Bases: adenine (A), thymine (T), guanine (G), cytosine (C).
• A pairs with T; G pairs with C — the COMPLEMENTARY pairs.

Chromosome.

• DNA wrapped around proteins (histones) → coiled tightly.
• Visible under microscope when cell is dividing.
• Humans: 23 PAIRS = 46 total in body cells.
• Sex chromosomes: XX (female) or XY (male).

Gene.

• A SECTION of DNA on a chromosome.
• Codes for ONE specific protein (or part of one).
• Different genes code for different proteins.
• Humans have ~20,000-25,000 genes.

Allele.

• A particular version of a gene.
• Each gene typically has 2 alleles in a diploid cell (one from each parent).
• Examples: blood type gene has alleles A, B, O.

Protein.

• The PRODUCT of a gene.
• Made of amino acids in a specific sequence.
• Determines a TRAIT (e.g. enzyme catalysing a colour pigment → eye colour).

Worked qualitative. Why do CHROMOSOMES form pairs in body cells but not in gametes?

• Body cells diploid (2n) — have one chromosome from each parent → forms pairs.
• Gametes haploid (n) — one of each chromosome → no pairs.
• This way, fertilisation (sperm + egg) restores the pairs.

Cambridge tip. State 'GENE = section of DNA coding for a protein'. Cambridge marks this directly.

Stage 1 — TRANSCRIPTION (in the NUCLEUS).

1. The relevant gene 'unzips' (DNA opens).
2. An RNA copy of the gene is made — mRNA (messenger RNA).
3. mRNA detaches from DNA.
4. DNA closes back up (intact).

Stage 2 — TRANSLATION (at a RIBOSOME, in cytoplasm).

1. mRNA leaves the nucleus through nuclear pores.
2. mRNA attaches to a ribosome.
3. Ribosome reads mRNA in groups of THREE bases (called codons).
4. Each codon specifies one AMINO ACID.
5. Amino acids brought in by tRNA (transfer RNA), joined together in order.
6. Chain of amino acids = the PROTEIN.

Why mRNA?

• DNA stays safe in the nucleus.
• mRNA = a temporary copy that can be sent out to where work is done.
• Multiple ribosomes can read the same mRNA → many proteins from one gene.

Worked qualitative. Why does a mutation in DNA usually change the protein?

• Mutation = change in DNA sequence.
• Changed DNA → changed mRNA → changed codons → different amino acid → different protein.
• A different protein may not work properly → genetic disease.
• Example: sickle-cell anaemia is caused by a single base change in the haemoglobin gene.

Cambridge tip. Cambridge IGCSE wants the OUTLINE — DNA → mRNA → ribosome → protein. Don't go too deep into the molecular detail.

In humans:

Cell	Chromosomes	Notes
Body cell (diploid)	46	23 PAIRS
Sperm or egg (gamete, haploid)	23	One of each pair
Zygote	46	23 from each parent — diploid restored

Sex chromosomes (the 23rd pair).

• XX = female (two X chromosomes).
• XY = male (one X, one Y).
• Eggs always carry an X.
• Sperm carry either X or Y → determines baby's sex.
• 50:50 chance.

Worked qualitative. Why is the SEX of a baby determined by the FATHER, not the mother?

• Mother provides only X (in eggs).
• Father provides X OR Y (50:50 in sperm).
• If father's sperm has X → baby = XX = girl.
• If father's sperm has Y → baby = XY = boy.

Cambridge tip. Memorise 46 (body) / 23 (gamete). Cambridge often gives a chromosome count and asks 'is this body cell or gamete?'.