Chapter 10
Meiosis
Meiosis occurs in the sex organs of organisms. Meiosis
is a type of cell division that reduces the number of chromosomes in a cell to
one-half the original number. It is a process
that organisms capable of sexual reproduction use to produce reproductive cells
called gametes. A typical cell has chromosomes that occur in
pairs. (Ex: If a fish has 30 chromosomes, then it will
have 15 pairs.) Each pair is considered
to be homologous. If the cell has homologous pairs of
chromosomes then it is considered to be diploid. If it is a gamete, it has undergone a special
form of cell division that has reduced its number to one-half. A gamete has one from each homologous pair
and is therefore a haploid
cell. **Every cell has two pairs of chromosomes, one set from its mother and
one set from its father.
The cell division that produces haploid gametes is called meioses. Meiosis is how sperm and egg are produced. When a haploid sperm fertilizes a haploid egg, a diploid zygote is formed. This is called sexual fertilization. Meiosis occurs in two phases: Meiosis I and Meiosis II.
In Meiosis I all of the steps of the cell cycle occur:
Interphase
G1, S, and G2
Mitosis
Prophase
Metaphase
Anaphase
Telophase
Cytokinesis
In late Prophase I of Meiosis I, the chromosomes that had duplicated in S phase of Interphase begin to lay close together to form tetrads.
During this time the chromatids lay across each other and exchange genetic information. This is called crossing over.
In Anaphase I of Meiosis I, the centromeres holding the sister chromatids of each chromosome together DO NOT separate. The chromosomes are pulled to opposite sides of the cell, each consisting of two chromatids held together by a centromere.
At the end of Cytokinesis of Meiosis I, there are two cells. Each of these cells has chromosomes that all consist of sister chromatids held together by a centromere.
There will be NO INTERPHASE before Meiosis II begins
and the two cells each move directly to Prophase II of Meiosis II.
Everything proceeds as normal and during Anaphase II of Meiosis II, the centromeres holding the sister chromatids together are separated by the spindles and are moved to opposite sides of each of the two cells and Telophase II of Meiosis II begins.
Following Telophase II of Meiosis II, Cytokinesis produces 4 new haploid cells containing one chromosome from each of the homologous pairs in the original cell.
Genetic Variation:
Crossing over in Prophase I provides for genetic variation. Genes can be exchanged between chromosomes before gametes are formed. This is very important for variety in the gene pools of organisms. Without variety in the gene pool, evolutionary adaptations would be limited.
The segregation of chromosomes at random during anaphase I explains Mendel’s observation that factors, or genes, for different traits are inherited independently of each other.
Mistakes in Meiosis:
If the chromatids do no separate properly during meiosis, then there is an odd number of chromosomes (too many or too few). This result is called nondisjunction. Some types of nondisjunction can lead to trisomy (three of each chromosome), monosomy (only one from each pair) and triploidy or tetraploidy (three or four SETS of chromosomes). Organisms with an unusual number of chromosome SETS are called polyploids. Polyploidy is fatal in animals, but can be beneficial in plants.
Examples of nondisjunction:
Turner’s Syndrome only one X sex chromosome abnormal female
Klinefelter’s Syndrome two X sex chromosomes and one y abnormal male
Down’s Syndrome three number 21 chromosomes abnormal child (sex chromosomes are normal)
**Within the human genome there are 22 pairs of autosomes and 1 set of sex chromosomes. (46 total)