B2- Genes

Genetics: DNA is found in the nucleus of all cells and is an organism’s genetic information. A gene is a section of DNA that contains the information for single characteristics e.g. eye colour. Genes are found on structures called chromosomes. A gene is a section of DNA that determines a particular protein and a particular characteristic. Chromosomes are found in pairs- one chromosome from each pair is inherited from the mother and the other from the father. Humans have 23 pairs of chromosomes, so have 46 chromosomes.

Mitosis: Body cells divide by mitosis. Mitosis takes place wherever cells are dividing such as the shoots and roots of plants, cancer cells and hair follicles. It occurs during growth or to produce replacement cells, for example after you have a cut. It is also used during asexual reproduction.

1) Before mitosis, the DNA replicates so there are two copies of each chromosome (this is when the DNA becomes two strands).

2) During mitosis, the copies of the chromosome separate and the cells divide

3) The result is two ‘daughter cells’ which are genetically identical to each other and the parent cell- they are clones

The DNA replicates so that the ‘daughter’ cells can have one copy of each chromosome and be genetically identical to the parent cells and have a full set of chromosomes- called diploid. If the DNA didn’t replicate then each cell would only have half the amount of chromosomes needed.

Meiosis: A gamete (sex cell) contains half the number of chromosomes that a normal body cell contains- they only have one set of chromosomes as there are no pairs- this is called haploid. They only have half the number of chromosomes so that when two gametes fuse during fertilisation, they will produce a zygote (fertilised egg) with a complete set of chromosomes. Meiosis is a type of cell division that results in four daughter cells each with half the number of chromosomes of the part cell, as in the production of gametes. Cells in the testes and ovaries and divide in this way.

1) The DNA in a diploid cell from a reproductive organ replicates forming identical copies of the chromosomes.

2) The first division happens, forming 2 cells with different chromosomes.

3) The cell divides again and the copies of the chromosomes split apart.

4) This gives four sex cells, each with a single set of chromosomes (half the number of the original). They are all genetically different and this is what causes variation.

When gametes join at fertilisation, a single body cell with new pairs of chromosomes is formed. A new individual then develops by this cell repeatedly dividing by mitosis. Most types of animal cells differentiate at an early stage whereas many plant cells retain the ability to differentiate throughout life. In mature animals, cell division is mainly restricted to repair and replacement.

Genetic variation: Genetic variation is a result of meiosis and sexual reproduction. These processes allow an individual with a completely unique combination of…