GENES AND CHROMOSOMES

Important points
• In their body cells, humans have 46 chromosomes, made up of 23 pairs. There are 44 chromosomes numbered 1-22 (called
autosomes) according to size from the smallest to the largest and two sex chromosomes: X and Y
• Women’s chromosomes are described as 46,XX; men’s as 46,XY
• A mother passes 23 chromosomes to her child through her egg and a father passes 23 chromosomes through his sperm
• The chromosomes consist of two very long thin strands of DNA chains twisted into the shape of a double helix and are
located in the nucleus (the ‘control centre’) of our body cells
• The chromosomes are long strands of genes
• Since the chromosomes come in pairs, the genes also come in pairs
• Genes are also located in very small compartments called mitochondria that are randomly scattered in the cytoplasm of the cell
outside the nucleus
• In each of the approximate 20,000 genes there is a piece of genetic information which guides our growth, development and
health and is in the form of a chemical code, called the genetic code
• The genetic code in the DNA, is virtually identical across all living organisms and is like a recipe book for the body to make
proteins
• The DNA code is made up of very long chains of four chemical ‘letters’: Adenine (A), Guanine (G), Thymine (T) and
Cytosine (C)
– In the DNA information, each ‘word’ is a combination of three of these four chemical ‘letters’ A, G, C and T
– Each three-letter word (triplet) tells the cell to produce a particular amino acid that form proteins
– The sequence of three-letter words in the gene enables the cells to assemble the amino acids in the correct order to make
up a protein
• Different genes are active in different cell types, tissues and organs, producing the necessary specific proteins; some genes are
‘switched off’ and others are ‘switched on’
• Changes to the genetic code can mean that a particular protein is not produced properly, produced in the wrong amounts or
not produced at all
• In some cases, the change in the genetic code can result in a genetic condition, affecting our growth, development and how
our bodies work