Structure and role of DNA
DNA (deoxyribonucleic acid) is a very long molecule found mainly in the nucleus. It stores the information that tells a cell which proteins to make, and so how an organism looks and works. A gene is a short piece of DNA that holds the instruction for one trait or protein.
DNA is made of small units called nucleotides. Each has a sugar, a phosphate and one of four bases: A (adenine), T (thymine), G (guanine) and C (cytosine). The order of these four letters is the code, just as the order of letters makes words.
Role: it passes information from parents to children, and it directs the working of the cell.
The double helix and DNA replication
DNA has two strands twisted together like a spiral staircase. This is the double helix. The strands are held together by pairs of bases, the steps of the staircase: A pairs only with T, and G pairs only with C (base pairing rule).
Because of this rule each strand tells what the other strand must be. This makes copying easy. In replication the helix unzips. Each old strand is used as a pattern to build a new matching strand. The result is two DNA molecules, each with one old and one new strand (that is why it is called semi-conservative). Replication happens before a cell divides, so both new cells get a full set of DNA. Rare copying mistakes are called mutations.
Chromosomes: chromatids and centromere
The DNA is far too long to float loose in a nucleus, so it is wound around proteins and coiled tightly into chromosomes. Chromosomes are easiest to see when a cell is about to divide.
- Before replication a chromosome is a single thread.
- After replication it is made of two identical chromatids (sister chromatids), each one a full DNA molecule.
- The chromatids are joined at a narrow point called the centromere. This is the part that spindle fibres pull on when the cell divides.
When the cell divides, the chromatids separate and each goes to a different new cell.
Human chromosomes: autosomes and sex chromosomes
A normal human body cell has 46 chromosomes, arranged as 23 pairs. In each pair one chromosome came from the mother and one from the father. Pairs 1 to 22 are autosomes, which carry genes for most body traits. Pair 23 is the sex chromosomes.
- A girl has two X chromosomes: XX.
- A boy has one X and one small Y: XY.
The mother can pass only an X to her child. The father passes either an X (child is a girl) or a Y (child is a boy). Other species have other numbers: a fruit fly has 8, a dog 78.
Haploid and diploid cells; why mitosis and meiosis matter
A cell with two sets of chromosomes (pairs) is diploid, written 2n (human: 46). A cell with one set is haploid, written n (human: 23). Body cells are diploid. Egg and sperm are haploid.
- Mitosis makes two identical diploid cells (46 to 46). It gives growth, repair of wounds and replacement of old cells, and in some organisms it makes new individuals.
- Meiosis makes four haploid sex cells (46 to 23), each a little different. It is needed so that when egg and sperm join at fertilisation, the child gets 23 + 23 = 46 and not 92. It also mixes genes, so children differ from their parents and from each other.
Key formulas and definitions
- Base pairing: A with T, G with C
- Human body cell: 2n = 46 (23 pairs = 22 autosome pairs + 1 sex pair)
- Human egg or sperm: n = 23
- Mitosis: 2n to 2n. Meiosis: 2n to n
- Fertilisation: n + n = 2n (23 + 23 = 46)
- Female XX, male XY
Worked examples
1. One strand of DNA reads A-T-G-C-C-A. What is the matching strand?
Use A-T and G-C: T-A-C-G-G-T.
2. A cell has 46 chromosomes before replication. How many chromatids does it have after replication, and how many chromosomes?
Still 46 chromosomes, but each now has 2 chromatids, so 92 chromatids.
3. How many autosomes does a human body cell have?
46 chromosomes minus 2 sex chromosomes = 44 autosomes (22 pairs).
4. A sperm cell has 23 chromosomes. How many does the fertilised egg (zygote) have?
The egg has 23 and the sperm 23, so the zygote has 23 + 23 = 46.
5. A father has XY and a mother XX. Which sex chromosome decides the sex of the child, and who passes it?
The mother always gives X. The father gives X or Y, so the father's sperm decides: X gives a girl, Y gives a boy.
6. A fruit fly body cell has 8 chromosomes. How many does its sperm have?
Sperm are haploid (n), so half of 8 = 4 chromosomes.
Common mistakes
- Using DNA, gene and chromosome as the same word. DNA is the molecule, a gene is a short piece of it, a chromosome is a package of DNA.
- Saying A pairs with G. The rule is A with T and G with C.
- Saying meiosis makes identical cells. Mitosis makes identical cells. Meiosis makes cells with half the chromosomes, each different.
- Thinking a chromatid is a whole chromosome pair. Two chromatids of one chromosome are copies of each other; the chromosome pairs come from two parents.