March 2007

Tags: chromosomes, basics, inheritance

How do we inherit chromosomes from our parents?
We inherit one copy of each chromosome from mom, and one copy of each from dad. The picture below illustrates that typically the egg contains 23 chromosomes and the sperm contains 23 chromosomes. When they join at conception, the baby ends up with a full set of 46 chromosomes.

Meiosis: making egg and sperm cells
You’re probably wondering how the egg and sperm cells end up with only 23 chromosomes when the rest of our body’s cells have 46.

A process called ‘meiosis’ creates new reproductive cells with half as many chromosomes as the original cell. Without meiosis, joining of the egg and sperm at fertilization would produce offspring with twice the original number of chromosomes as its parents. Meiosis starts with one reproductive cell (containing 46 chromosomes) that replicates its DNA only once but divides twice to produce four new cells, each having only 23 chromosomes.

The diagram on the left shows one chromosome pair going through normal meiosis. The pink chromosome came from mom, whereas the blue chromosome came from dad. The pink and blue chromosomes are homologues, meaning they are matching but have slightly different information within them. Remember, all 23 pairs go through this process but for simplicity, we’re just showing one chromosome pair.

Importantly, the 4 new cells produced are not genetically identical to each other or to the original cell. The exchange of DNA material (recombination) between the chromosomes in meiosis ensures the newly produced egg and sperm cells are not identical. This mixing up of genes between generations helps preserve human diversity.

Meiosis happens in three stages:

1. First, the chromosomes replicate – but do not separate – so the chromosomes will now have the X shape. The X shaped chromosome consists of two identical ‘sister chromatids’
2. The homologous chromosomes exchange DNA material (genetic recombination) along their length. Then the first round of division happens (meiosis I) and the two homologues split.
3. Next, the sister chromatids separate during meiosis II. The end result of meiosis is 4 cells with half as much material as the starting point.

Meiosis differs slightly in sperm and egg cells.

• In males, one reproductive cell (the primary spermatocyte) undergoes meiosis to produce 4 equal sperm cells, each with 23 chromosomes.
• In females one reproductive cell (the primary oocyte) undergoes meiosis to form 4 products, each with 23 chromosomes. But, when meiosis 1 occurs in females, two egg cells are produced, with an unequal division of the cytoplasm and organelles. After meiosis is completed, three of the egg cells disintegrate. The remaining large egg cell has most of the cytoplasm and is ready to be fertilized.