MITOSIS
Mary was a very attractive young woman who liked to play ice hockey. Her high school gym teacher took an interest in her ability and gave her extra coaching. She hoped that one day Mary would play on an Olympic team. But something was wrong. Mary was sixteen and still not menstruating. Her parents decided to have her undergo a series of medical tests. Much to the surprise of everyone, Mary had an X and Y chromosome in the nucleus of her cells. She was a chromosomal male.
The doctor explained to Mary and her parents that Mary had testicular feminization syndrome. She has internal testes that produce testosterone but her cells won’t respond to it. Her genitals are like those of a female and she has well-developed breasts. However, she will never be able to have children. Mary will be able to go on and play hockey in the Olympics but she has to always carry a letter explaining her condition. Otherwise she will be disqualified because of her sex chromosomes.
This is an example of various syndromes that occur when people have chromosomal abnormalities.
Chromosomal Inheritance
In the nucleus of a cell there are chromosomes, arranged in pairs which are joined in the center. Each of these pairs are called a called a karyotype ( type of chromosome pair). Humans have 23 pairs of chromosomes. When they are taken out of a cell for genetic testing, scientists arrange the pairs in order of their assigned name (which is a number from 1-23). This procedure is called karyotyping.
Males have one pair of chromosomes that are not the same length. The larger chromosome in this pair is the X chromosome and the smaller is the Y chromosome. Females have two X chromosomes. This pair is called the sex chromosomes because they contain the genes that determine the sex. Therefore, males are called XY and females are called XX. The male is the parent that determines the sex of the child.
GENES are made of DNA and control the characteristics of the individual.
Genes are like small dots on each chromosome. Each dot contains the blueprint for some aspect of the body. One gene is for eye color, one for liver function, one for right or left handedness, etc. Every aspect of the body corresponds to a gene on one of the chromosomes.
Human Cell Division
All cells in our body divide by a process called mitosis, which ensures that each and every cell has a complete number of chromosomes. Mitosis produces two daughter cells with the same number and kind of chromosomes as the parent cell.
Here’s a TQ hint: If a parent cell has 7 chromosomes prior to mitosis, how many chromosomes will the daughter cells have? Answer = 7.
In addition to mitosis, the sex cells (and only the sex cells) undergo a second process call meiosis, in which each daughter cell has only half of the chromosomes. In males, it produces the cells that become sperm; in females, it produces the cells that become eggs. The sperm and the egg are the sex cells, or gametes. GAMETES contain half the number of chromosomes compared to the rest of the body cells.
Therefore, meiosis requires two nuclear divisions, resulting in four daughter cells, each with half of the parent chromosomal number
When a sperm cell fertilizes an egg, the resulting zygote has the all of the chromosomes from each of the sex cells, for a total of 46 (or 23 pairs). Thereafter, when the cells of the zygote/embryo/fetus/infant/child/adult divide by mitosis, each cell has 23 pairs of chromosomes.
MITOSIS
Mitosis is cell division that produces two daughter cells, each with the same number and kinds of chromosomes as the parent cell (the cell that divides). Therefore, following mitotic cell division, the parent cell and the daughter cells are genetically identical.
Stages of Mitosis
The process is divided into five phases: interphase, prophase, metaphase, anaphase, and telophase.
You need to know the order of these phases (you can skip interphase for this test).
Interphase
The chromosomes make an identical clone of themselves within the nucleus. For a human with 46 chromosomes, they now have 92 chromosomes (just for a few minutes!). The clone is joined to the original at the middle so they look like an “X”.
Prophase
The nuclear envelope dissolves away, and the chromosomes are released into the cytoplasm of the cell. The chromosome pairs start to shorten and thicken as they get ready to separate.
Metaphase
During metaphase, the chromosomes line up at the center of the cell.
Anaphase
At the start of anaphase, the sister chromosomes separate and move toward opposite sides of the cell. There will now be 23 chromosomes on each side of the cell.
Telopbase
Telophase begins when the chromosomes arrive at the poles. A nuclear envelop now forms around each set of chromosomes, so at this phase, the parent cell has two nuclei, each with a complete set of chromosomes. Telophase is characterized by the formation of two daughter nuclei. At the very end of telophase, the cell membrane pinches in two so that there are two new cells.
MEIOSIS
Meiosis only occurs in the testes and ovaries when they are ready to make an egg cell or a sperm cell. First, mitosis occurs as normal. But right after that, the two daughter cells divide again, but this time there is no prophase, so the chromosomes have not made a clone of themselves.
When the second cell division is at the metaphase stage, the chromosomes touch each other and exchange a few genes. This is called crossing over. That is what allows for genetic variation.
Meiosis results in four daughter cells, each having half the number of chromosomes as the parent cell. The daughter cells are not genetically identical, and neither is identical to the parent cell.
The daughter cells that are formed are sperm cells in males and egg cells in females (these are gametes because each one has half the number of chromosomes). When a sperm and egg combine and contribute their chromosomes, the fertilized egg (called a zygote) will now have 23 pairs of chromosomes.
Nondisjunction
This is when chromosomes can become abnormal if the sister chromosomes do not separate properly during meiosis in the formation of a sex cell. This is what causes a sex cell to wind up with XXY or other chromosome abnormalities.
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