3.3 - DNA, Chromosomes & the Genome

DNA, chromosomes and genes

• The genetic information 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 inside a cell.
• To fit, it's 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.

Chromosomes come in pairs

• In body cells, chromosomes are normally found in pairs.
• One chromosome of each pair came originally from each parent.
• The two chromosomes in a pair carry information for the same characteristics in the same positions (apart from the X and Y chromosomes of a male).

Genes code for proteins

• 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 together, 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 the genes an organism carries control its inherited characteristics.
• An organism has thousands of different genes, each a separate section of DNA coding for its own protein.

The genome

• The genome of an organism is its entire genetic material - all of the DNA an organism has, taken together.
• In humans, that's the complete set of DNA across all 46 chromosomes. This includes:
  • Every gene the organism has
  • The stretches of DNA between and around the genes
• Together this makes up the complete set of genetic instructions for building and running that organism.
• The genome is the same in nearly every cell, because each cell carries a full copy of the DNA.