Chickscope 1.5: Explore: Embryology: Day 7

Day 7: Jumpin' Genes!

Meiosis

The purpose of mitosis is to divide the cell into two new fully functional cells. Each new cell (in humans) contains 23 pairs of chromosomes (a total of 46 chromosomes). Each pair of chromosomes consists of a strand of DNA from your mother and a strand of DNA from your father. The chromosomes in a given pair code for the same genetic traits; consequently, the two chromosomes of a pair are called homologs. When a cell contains both sets of chromosomes, one from your mother and one from your father, it is considered diploid.

Egg and sperm cells also start out diploid. The purpose of meiosis is to produce cells by means of cell division with a single set chromosomes (rather than pairs of chromosomes), and with chromosomes consisting of a single molecule of DNA made up of a mixture of genes from your grandparents. The sperm cell and egg cell are haploid; each contains only one set of chromosomes. When an egg cell and a sperm cell fuse during fertilization, new cell is formed with two complete sets of homologous chromosomes - 23 from your father and 23 from your mother. The new cell is capable of becoming a new person -- you!

But how does meiosis happen? The germ cells, those cells in your mother or father which will one day become an egg or a sperm, are determined early in development, when the embryo is just a ball of cells. These specialized cells are the only cells in the body capable of undergoing meiosis.

DNA Replicates DNA Duplicating

In humans meiosis to produce the egg that would someday become you, actually began in your mother before she was born. The cells destined to become egg cells (or oocytes) in your mother start out diploid, containing a set of 23 chromosomes from your maternal grandmother and a set of 23 chromosomes from your maternal grandfather. A few months before your mother was born, the DNA of these cells replicated, as it does in mitosis. The cell, however, did not go on to divide within a few minutes as it would have in mitosis. Rather, it spent the next 12 or 13 years with duplicated DNA in its nucleus!

Homologous Chromosome Bind

But the egg cell was not idle. During this time, homologous chromosomes paired off and aligned themselves to each other trait by trait. Then the homologous chromosomes used a molecular zipper to temporarily bind.

Jumpin' Genes! According to this homologous pair of chromosomes, your mother carried both a gene for blue eyes from your grandfather and one for brown eyes from your grandmother. Since the gene for brown eyes is dominant over the gene for blue eyes, your mother phenotypically has brown eyes. Your mother also carried both a gene for blonde hair from your grandfather and one for brown hair from your grandmother. Since the gene for brown hair is dominant over the gene for blonde hair, your mother phenotypically has brown hair.

Cross-over Cross Over DNA

While joined, the chromosomes swapped genes which coded for homologous traits. This process of swapping genes is called cross-over. It occurs so that you would not just receive a copied molecule of DNA from your grandfather or one from you grandmother for each chromosome, but so that each chromosome you received from your mother would actually consist of a molecule of DNA mixed with traits from your maternal grandfather and your maternal grandmother. The set of 23 chromosomes you received from your father is also a mixture of genes from your paternal grandfather and grandmother.

Text and illustrations by Janet Sinn-Hanlon

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