Genes and Chromosomes: How DNA Is Packaged

0:00The DNA in a typical cell of your body would stretch about 2 metres end to end. How does it all fit?

0:08Zoom in on the nucleus. It contains the cell's genetic material.

0:13Here, the DNA is packed into structures called chromosomes.

0:18Most human body cells have 46 chromosomes, in 23 pairs.

0:24In each pair, one chromosome came from your mother and the other from your father.

0:30Pull out one pair. Each chromosome is a single, very long molecule of DNA.

0:37Zoom in further: the DNA is wound around proteins, like thread on spools, then coiled up tightly.

0:46Unwound, the DNA is a double helix: two strands twisted around each other.

0:52A gene is a short section of DNA that codes for a protein: it carries the instructions for making that protein.

1:00Humans have about 20,000 genes that code for proteins, spread across the chromosomes.

1:07Its partner, from the other parent, carries the same gene in the same place.

1:13But the two copies can be different versions. Different versions of a gene are called alleles.

1:20A common mistake is to think that one gene controls each trait.

1:25In fact, many traits, like height, involve many genes.

1:31So, a chromosome is one long DNA molecule wound around proteins, and a gene is a section of that DNA.