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Meiosis

Sex cells, also referred to as gametes (i.e. sperm and egg) are produced in the sex organs (gonad) - testes in males, ovaries in females, through a process called meiosis. The adjective for meiosis is meiotic. In summary, meiosis involves the replication of genetic material followed by two distinct divisions to arrive at half the chromosome number of the other cells in the body (somatic cells). Haploid is the term applied to a cell with half the number of chromosomes. An explanation of meiosis is useful in understanding how genes segregate. It is also useful in understanding how various genetic events (such as, crossover, translocation or inversion) occur during the production of gametes.

Each phase of meiosis is discussed below. Following each explanation is a simple sketch representative of the phase. The sketches only shows six chromosomes for simplicity, a cockatiel has many chromosomes, the exact number is unknown. I also labeled two genes ("A" and "a") that occupy the same loci or chromosomal locations in order to show how genes segregate during the meiotic process. This labeling starts at late prophase I.

1. Early Prophase I
The six chromosomes (three homologous pairs) within the cell become visible. They do not appear double stranded although replication of genetic material has already occurred.


2. Middle Prophase I
Homologous chromosomes synapse (pair together) and become shorter and thicker. Each pair of synapsed chromosomes is called a bivalent (two chromosomes).

3. Late Prophase I
Chromosomes become clearly double stranded. Now that all the chromatids are visible, I have labeled the hypothetical genes A and a.

4. Metaphase I
Each synaptic pair of chromosomes moves to the center of the cell.

5. Anaphase I
Centromeres on the chromosomes do not divide. Double stranded chromosomes move apart to opposite poles.

6. Telophase I
Two haploid cells (cells with half the chromosome number) start to form. Chromosomes are half the number, 3 rather than 6, but still double stranded.

7. Interkinesis
Chromosomes have faded from view. There is no replication of genetic material during this stage.

8. Prophase II
Double stranded chromosomes condense and become visible. Note that genes "A" and "a" have segregated into different cells.

9. Metaphase II
Double stranded chromosomes in each haploid cell move to the center of the cell.

10. Anaphase II
The centromeres divide and the single stranded chromosomes move away from each other toward opposite poles.

11. Telophase II
Haploid cells with single stranded chromosomes begin to develop.

12. Interphase
Chromosomes fade from view. A total of four haploid cells are produced.

The end result of a single meiosis is four gametes each containing half the genetic complement of the parent. The next phase would be fertilization, where the male gamete combines with the female gamete to restore the normal doubled somatic cell chromosome number (i.e. diploid number), which then develops into an offspring with the genetic contributions from both parents.



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