What are chromosomes?

Chromosomes are packages of DNA held within the nucleus of most of the trillions of cells which make up our bodies. Humans have a total of 46 chromosomes made up of 22 matching pairs and two additional chromosomes which determine gender. These sex determining chromosomes can be either two X chromosomes (female) or an X and a Y chromosome (male).

A cell showing a magnified chromosome and its component DNA

The DNA held within the chromosomes hold the "blueprints" for all the 20,000 or so proteins that our cells can produce. When a protein is produced, the cell machinery transcribes DNA to produce RNA which is then translated into the amino acids needed to assemble the protein.

The count of 46 chromosomes is made up of 22 pairs of homologous chromosomes, one of each pair having been contributed by each parent. In addition each parent contributed one sex determining chromosome (an X from the mother and an X or Y from the father), giving 46 chromosomes in total. When cells need to multiply in order to build or heal our bodies, the cells go through a process called mitosis. This process divides the original cell into two and ensures that each daughter cell ends up with the same number of chromosomes as the original cell.

The process to create gametes (egg or sperm) is slightly different because each parent only contributes half of their genes to the next generation, this means that the number of chromosomes must be halved during gamete production. To achieve this, the cells go through a process called meiosis. During this process, each of the chromosomes find their matching partner and swap sections of the DNA in a process called recombination which randomly mixes the genetic material between the two chromosomes to provide variety (Ref 1).

The process of meiosis
  1. A pair of homologous (matching) chromosomes in a cell, one chromosome from each parent
  2. Each chromosome duplicates itself, but the duplicates (called chromatids) remains attached
  3. The chromosomes line up with their partner
  4. And swap random regions of genetic material
  5. The cell divides into two cells, separating the two chromosomes
  6. The cell divides again separating the two chromatids
  7. Resulting in four gametes, each of which contains one chromosome

This process means that eggs and sperm end up with half the number of chromosomes each i.e. 23 rather than 46 chromosomes. When the egg is fertilised the DNA from both the egg and sperm are added together, bringing the number of chromosomes back up to 46 again.

It is vital that little or no genetic material is missing when the egg is fertilised and almost as important that no material is duplicated. Either of these circumstances would usually lead to either a non-viable fetus or one with severe health problems.

From the above description of meiosis it would appear to be impossible for the chromosome number to change. Each chromosome matches up with its partner when the gametes are formed, and no significant amount of genetic material can be added or lost without causing serious problems. So how is it possible for the chromosome number to change?

The rest of this web site explains how it can happen.

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  • Top image from the National Human Genome Research Institute
  • Ref 1 : Hartl and Jones, Essential Genetics, P78-88