The nucleus of a cell contains chromosomes, which are long threads of the molecule DNA. DNA carries the coded instructions that control how an organism develops and functions.
From gene to protein
A gene is a length of DNA that codes for one protein. The sequence of bases in a gene determines the sequence of amino acids in a protein. Because proteins include enzymes, hormones and structural molecules, genes ultimately control an organism's characteristics.
Key idea
A gene is a length of DNA that codes for a protein. The order of bases codes for the order of amino acids that are assembled to make a specific protein.
Chromosomes and cells
Human body cells contain 46 chromosomes arranged in 23 pairs — this is the diploid number. Gametes contain 23 single chromosomes — the haploid number. Every body cell of an individual carries the same genes because they all come from the same original zygote by mitosis.
Worked example
Only some genes are switched on in each cell type. A muscle cell and a nerve cell contain the same genes, but different genes are active, so different proteins are made and the cells do different jobs. This explains cell specialisation.
Why proteins matter
Enzymes control the rate of chemical reactions; structural proteins build tissues. A mistake in the base sequence (a mutation) can change the protein produced, which may alter a characteristic.
Remember
- Chromosome → made of DNA → contains many genes.
- Gene = length of DNA coding for one protein.
- Base sequence → amino acid sequence → protein.