17.1 - DNA & Chromosomes

DNA, chromosomes and genes

• Genetic information that an organism inherits and passes on is stored in a molecule called DNA.
• A DNA molecule is extremely long - far too long to sit loosely in a cell - so it is coiled up very tightly and packaged into structures called chromosomes. Each chromosome is a single molecule of DNA wound up compactly.
• Chromosomes are held in the nucleus of the cell.
• In body cells, chromosomes are normally found in pairs. One chromosome of each pair came from each parent, so the two carry information for the same characteristics in the same positions.

Genes and the proteins they code for

• A gene is a section of DNA on a chromosome.
• Each gene carries the code for making one particular protein - it sets the order in which amino acids are joined, and that sequence forms a specific protein.
• The proteins a cell makes determine how the cell behaves and what type of cell it is, so an organism's genes control its inherited characteristics.
• An organism has thousands of different genes, each a separate section of DNA coding for its own protein.

Proteins do many different jobs, including:

• Enzymes - Speed up the cell's chemical reactions
• Carrier proteins - Sit in the cell membrane and move substances across it
• Receptors - Let the cell pick up chemical signals from neurotransmitters

Diploid and haploid nuclei

• Diploid - A nucleus containing two sets of chromosomes, with one pair of each type. A human body cell is diploid, with 23 pairs of chromosomes - 46 in total.
• Haploid - A nucleus containing only a single set of chromosomes, with one of each type rather than a pair.