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Day 7: Jumpin' Genes!

Meiosis Continued

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A single human trait is represented in every cell of your body by two genes - one on a chromosome you received from your father and one on the homologous chromosome you received from your mother. For germ cells to be capable of fusing and producing a new individual with two genes for each trait, the germ cell must only possess one gene for each trait. A germ cell in the stage of meiosis during which cross-over occurs actually has four genes for each trait. The germ cell must divide twice without replicating its DNA to reduce the number of genes representing a single trait to one. The first division separates homologous chromosomes to produce two haploid cells and is called the first maturation. Each resulting cell contains a set of 23 chromosomes, but the DNA of each chromosome has been replicated for cross-over; so there are still two genes for every trait. The second division, called the second maturation, separates the remaining two chromatids.

First Maturation

Homologous Chromosomes


Second Maturation

Second Maturation


Jumpin' Genes
As you can see from the oocytes above, the single set of chromosomes that your mother passed on to you were made up of a mixture of genes from your maternal grandfather and your maternal grandmother. Let's say that you ended up with the oocyte containing a chromosome with genes for blue eyes and brown hair. That combination of traits could not have been passed on to you if the genes for those traits had not been swapped during cross-over. In other words, if cross-over had not occurred, your mother would have passed on to you either an exact copy of your grandfather's genes (blonde hair, blue eyes) or an exact copy of your grandmother's genes (brown hair, brown eyes). The combination of blonde hair, brown eyes or brown hair, blue eyes could not have been passed from your mother. Because of cross-over, you may have your maternal grand mother's hair color, your paternal grandmother's smile, your maternal grandfather's walk and your paternal grandfather's ears.

Meiosis in the Oocyte (Egg)

Just before an oocyte is released from the human ovary, its nucleus divides. This results in two haploid nuclei. One nucleus remains with the oocyte, while the other, referred to as the first polar body, fades away. This event is referred to as the first maturation of the oocyte. The oocyte now contains a single set of chromosomes, but the DNA of each chromosome is still doubled. Shortly after the egg is released from the ovary, the remaining nucleus divides again without duplicating its DNA. Once again, one nucleus stays with the oocyte, while the other - the second polar body - dies off. This event, called the second maturation, leaves the oocyte with a nucleus containing a haploid set of chromosomes and each chromosome consists of a single molecule of DNA. The egg is now ready to be fertilized.

Meiosis in the Sperm Cell

Sperm also undergo meiosis, but it does not begin until puberty. DNA of the immature diploid sperm cell duplicates. For several weeks, cross-over occurs between the four molecules of DNA. When the sperm cell undergoes its first maturation, it divides the cell completely, resulting in two haploid sperm cells whose chromosomes each consist of two molecules of DNA. Both cells divide again (second maturation) without duplicating their DNA to produce four viable haploid sperm cells whose chromosomes are each a single molecule of DNA. The sperm are now ready to fertilize an egg. Each chromosome is made up of a different combination of genes from your paternal grandfather and grandmother.

Text and illustrations by Janet Sinn-Hanlon

- Page 9 of 10 -

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