Chromosomes & Genes (AQA GCSE Combined Science: Synergy: Life & Environmental Sciences): Revision Note

Sexual Reproduction

• Sexual reproduction is a process involving the fusion of the nuclei of two gametes (sex cells) to form a zygote (fertilised egg cell) and the production of offspring that are genetically different from each other

  • The gametes of animals are the sperm cells and egg cells

  • The gametes of flowering plants are the pollen cells and egg cells

• Fertilisation is defined as the fusion of gamete nuclei, and as each gamete comes from a different parent, there is variation in the offspring

• The formation of gametes involves meiosis

The Structure of DNA

• The genetic material in the nucleus of a cell is composed of a chemical called DNA

• DNA, or deoxyribonucleic acid, is the molecule that contains the instructions for growth and development of all organisms

• DNA is a polymer made up of two strands forming a double helix

• DNA is contained in structures called chromosomes

  • Chromosomes are located in the nucleus of cells

  • Human body cells contain 23 pairs of chromosomes

Genes are short lengths of DNA that code for a protein. They are found on chromosomes

Genes & the Genome

Genes

• A gene is a short length of DNA found on a chromosome

• Each gene codes for a particular sequence of amino acids

• These sequences of amino acids form different types of proteins

• There are many different types of proteins but some example of these could be:

  • structural proteins such as collagen found in skin cells

  • enzymes

  • hormones

• Genes control our characteristics as they code for proteins that play important roles in what our cells do

The genome

• The entire set of the genetic material of an organism is known as its genome

• Biologists now know the entire human genome (they have worked out all the genes that are found in humans)