DNA, genes and chromosomes
DNA (deoxyribose nucleic acid) molecules are large and complex. They carry the genetic code that determines the characteristics of a living thing.
Except for identical twins, each person’s DNA is unique. This is why people can be identified using DNA fingerprinting. DNA can be cut up and separated, forming a sort of 'bar code' that is different from one person to the next.
A gene is a short section of DNA. Each gene codes for a specific protein by specifying the order in which amino acids must be joined together.
The cell’s nucleus contains chromosomes made from long DNA molecules.
Mitosis is the type of cell division that leads to growth or repair. When a cell divides by mitosis:
- two new cells form
- each cell is identical to the other one, and the cell they were formed from
1, parent cell 2, chromosomes make identical copies of themselves 3, they line up along the centre 4, they move apart 5, two daughter cells form with identical chromosomes to the parent cell
Gametes and fertilisation
Human body cells have 23 pairs of chromosomes in the nucleus. One of these pairs controls the inheritance of gender - whether offspring are male or female:
- In males, the two sex chromosomes are different. They are XY.
- In females, the two sex chromosomes are the same. They are XX
Gametes are sex cells. The male gametes are the sperm, and the female gametes are the eggs.
Gametes contain one set of genetic information, while body cells contain two sets of genetic information.
Fertilisation is the joining or fusion of a male gamete and a female gamete. The new cell that is formed divides over and over again by mitosis. This creates the many cells that eventually form a new individual
Gametes are formed by meiosis have half as many chromosomes as the cell that formed them. Human body cells contain 23 pairs of chromosomes, while human gametes contain 23 single chromosomes.
The main features of meiosis are:
- the chromosomes are copied
- the cell divides twice, forming four gametes
1, parent cell 2, chromosomes make identical copies of themselves 3, similar chromosomes pair up 4, sections of DNA get swapped 5, pairs of chromosomes divide 6,chromosomes divide 7, 4 unidentical cells are formed