NCEA Level 1 Science/Evolution

A section of DNA; the sequence of the plate-like units (nucleotides) in the center carries information.

Introduction

Genetics is the study of how living organisms inherit features from their ancestors – for example, children often look like their parents. Genetics tries to identify which features are inherited, and work out the details of how these features are passed from generation to generation.

Roles and relationships

Chemical structure of DNA

Replication and reproduction

There are two ways a cell can divide:

• mitosis
• meiosis

Mitosis

Mitosis occurs when genetically identical cells are created , for either growth or repair. In mitosis, two genetically identical 'daughter' cells are produced from a single parent cell.

Stages of Mitosis

1. Chromosomes in a cell become smaller and thicker, allowing it to be seen under a microscope.
2. The cell nucleus disappears
3. Chromosomes replicate
4. The chromosome in each pair move to opposite ends of the cell.
5. There are now two identical groups of chromosomes on either side of the cell. Nuclear membranes form around each one of these groups.
6. The cell starts to split into two.
7. The outermost layer of the cell grows in between the splitting daughter cells. In animal cells the outermost layer is the cell membrane, whereas in plants, it is the cell wall.
8. If daughter cells are not genetically identical then a mutation has occurred.

Meiosis

Meiosis occurs in the sex organs and produces sex cells, which have half the number of chromosomes than that of a regular body cell. During Meiosis 4 daughter cells are produced from a single parent. During cell division, genetic information mixes, creating daughter cells which are all nonidentical to each other. The original number of cells is restored during fertilisation.

Importance of variation

Terminology

• gamete: Cell involved with reproduction
• zygote: Fertilised female reproductive cell
• dominant: A gene causing the parental characteristics it controls to occur in offspring
• recessive: A gene that produces an effect in an organisms only when its matching allele is identical
• homozygous: Having two identical genes
• heterozygous: Having two or more different alleles of one of its genes
• pure breeding: Homozygous for either the dominant or recessive allele
• Punnett square: Diagram showing possible combinations of alleles during fertilisation
• pedigree chart: Chart showing all the know visible characteristics for an organism and its ancestors