DNA, Chromosomes and Cell Division

In short: DNA is a double helix of nucleotides in which the base A pairs with T and C pairs with G. Before a cell divides, DNA copies itself, then mitosis makes two identical cells for growth and repair, and meiosis makes four different gametes with half the chromosomes.

Starting point: what you already know

• DNA carries genetic information and is found in the nucleus.
• Genes are sections of DNA. Humans have 46 chromosomes in body cells (Year 10 science).
• New cells come from existing cells (cell theory).

The structure of DNA

DNA (deoxyribonucleic acid) is a double helix: two strands twisted like a spiral ladder. Each strand is a chain of nucleotides. Each nucleotide has three parts:

1. a phosphate group,
2. a sugar called deoxyribose, and
3. a nitrogen base: adenine (A), thymine (T), cytosine (C) or guanine (G).

The sugars and phosphates make the sides of the ladder. Pairs of bases make the rungs, joined by weak hydrogen bonds.

The two strands run in opposite directions (antiparallel). James Watson and Francis Crick built the model in 1953, using X-ray images made by Rosalind Franklin and Maurice Wilkins.

From DNA to chromosomes

In eukaryotes, DNA wraps around proteins called histones, then coils and folds again and again into a chromosome. Before a cell divides, each chromosome is copied, so it looks like an X: two identical sister chromatids joined at a centromere.

Human body cells have 46 chromosomes in 23 pairs. One of each pair comes from each parent. Pairs that carry the same genes are homologous chromosomes. One pair are the sex chromosomes: XX in females, XY in males.

• Diploid cells have two sets of chromosomes, written $2n$. Body cells are diploid. In humans $2n=46$.
• Haploid cells have one set, written $n$. Gametes are haploid. In humans $n=23$.

In prokaryotes, the DNA is a single circular chromosome, not wrapped in histones, plus small loops called plasmids.

DNA replication

Before a cell divides, its DNA must be copied. Replication is semi-conservative: each new DNA molecule keeps one old strand and gets one new strand.

1. Helicase unzips the double helix by breaking the hydrogen bonds between bases.
2. Free nucleotides pair with the exposed bases (A with T, C with G).
3. DNA polymerase joins the new nucleotides into a strand and checks for mistakes.
4. Two identical DNA molecules result.

Mitosis and meiosis

Mitosis stages: prophase (chromosomes condense), metaphase (they line up in the middle), anaphase (sister chromatids pulled apart), telophase (two nuclei form). Then the cytoplasm divides.

Quick summary

• DNA is a double helix of nucleotides (phosphate, deoxyribose, base). A pairs with T, C with G.
• DNA wraps around histones to form chromosomes. Humans have 46 (diploid). Gametes have 23 (haploid).
• Replication is semi-conservative: helicase unzips, DNA polymerase builds the new strands.
• Mitosis makes 2 identical diploid cells for growth and repair.
• Meiosis makes 4 different haploid gametes. Crossing over and independent assortment create variation.

Quiz