Chromosomes
- In the nucleus of a cell, the DNA double helix supercoils to form structures called chromosomes
- They are only visible during cell division
- Ordinary human body cells contain 23 pairs of chromosomes
- This is a diploid number (often shown as 2n)
- One chromosome from a pair is inherited from each parent
- Each chromosome pair is called a homologous pair
- The 23rd pair of chromosomes are the sex chromosomes
- In females, the sex chromosomes are the same, a homozygous genotype (XX)
- In males, the sex chromosomes are different, a heterozygous genotype (XY)
- Gametes contain just 23 individual chromosomes
- This is half the full number of a body cell and is called the haploid number (or n)
- Genes are found in specific locations on the chromosomes, these locations were identified in the human genome project
Genes are short lengths of DNA that code for a protein. They are found on chromosomes.
Exam Tip
Remember that the number of chromosomes found in each species differs, for example, humans have 23 pairs whereas dogs have 39 pairs and rice has 12 pairs.
