1. Genes, Chromosomes and DNA

2. Genes and alleles A gene is a section of DNA that carries the information for a particular trait (characteristic) eg. Eye colour, hair colour, production of a particular enzyme. Now a sequence of bases on a DNA molecule. Alleles are different forms of the same gene ie. The gene is eye colour, the alleles are blue, brown and green.

3. Chromosomes are long stretches of DNA, within which lie genes. A single chromosome may contain hundreds of genes. Below are the locations of some known genes on human chromosomes:

4. Chromosome structure Chromosomes are made up of chromatin – a complex of DNA and protein. The protein that DNA forms a complex with is mainly histone protein. Histone may regulate DNA functioning in some way. DNA is coiled very tightly so it can fit inside the nucleus.

5. Structure continued: Just before cell division, the chromosomes double but remain joined at a region called the centromere. The two arms of the chromosome are called chromatids.

6. Chromosome replicated and prepared for cell division. Chromosome in a non-dividing cell

7. Why are there two of each chromosome? When cells in an organism divide, the new cells contain exactly the same number of chromosomes as the original cell. In a human cell there are 46. These are arranged in pairs. Why? Because one chromosome in a pair comes from the female parent and one from the male parent.

8. Karyotypes A karyotype is a picture of the chromosomes in an organism arranged in homologous pairs according to shape and size.