Meiosis
The process by which the chromosome number is halved during gamete formation is called meiosis. In meiosis in the honey bee, a cell containing the diploid number of chromosomes (32) is converted into four cells, each having the haploid number of chromosomes (16). As a result of what happens in meiosis each of these cells is genetically distinct i.e. each honey bee, whether it be queen, worker, or drone, is genetically unique. Only one of these haploid cells appears in the egg the other three are reabsorbed.
Two major phases of meiosis occur: meiosis I and meiosis II. During meiosis I, a single cell divides into two. During meiosis II, those two cells each divide again.
Visual and narrated animation of Meiosis
Each phase of meiosis which can be summarized as follows:
before meiosis begins in the preceding
interphase, DNA replication takes place.
Meiosis I:
a honey bee cell contains 32 chromosomes, or 64 chromatids. Meiosis I proceeds through the following phases:
Prophase I: The chromatids shorten and thicken and become visible under a microscope.
During prophase I, the two homologous chromosomes associate. Because each homologous chromosome consists of two chromatids, there are four chromatids aligned next to one another. This combination of four chromatids forms a tetrad; the coming together is synapsis.
After synapsis has taken place, the process of crossing over occurs. In this process, segments of DNA from one chromatid in the tetrad pass to another chromatid in the tetrad. They result in a genetically new chromatid. After crossing over has taken place, it is important to realize that the four chromatids of the tetrad are genetically different from the original four chromatids.
Metaphase I: the tetrads align randomly on the equatorial plane. The centromeres attach to spindle fibers, which extend from the poles of the cell. (There are two possibilities; only one is shown on the diagram).
Anaphase I: the homologous chromosomes separate. One homologous chromosome (consisting of two chromatids) moves to one pole, while the other homologous chromosome (consisting of two chromatids) moves to the other pole. The result is that 16 chromosomes (each consisting of two chromatids) move to one pole, and 16 chromosomes (each consisting of two chromatids) move to the other pole. Essentially, the chromosome number of the cell is halved.
Telophase I: the nucleus reorganizes, the chromosomes become chromatin, and a cytoplasmic division into two cells takes place. Each daughter cell (with 16 chromosomes each consisting of two chromatids) then enters interphase, during which there is no duplication of the DNA.
Meiosis II
In meiosis II, the two cells, formed in Meiosis I, containing 32 chromatids undergo division into four cells, each with 16 chromosomes. Meiosis II proceeds through the following phases:
Prophase II: The chromatin material condenses, and each chromosome contains two chromatids attached by the centromere. The 16 chromatid pairs, a total of 32 chromatids, then move to the equatorial plane.
Metaphase II: the 16 chromatid pairs gather at the center of the cell prior to separation.
Anaphase II: the centromeres divide, and the 32 chromatids become 32 chromosomes. Then the 32 chromosomes separate from one another. Spindle fibers move one chromosome from each pair to one pole of the cell and the other member of the pair to the other pole. In all, 16 chromosomes move to each pole.
Telophase II: the chromosomes gather at the poles of the cells and become indistinct. Again, they form a mass of chromatin. The nuclear envelope develops, the nucleoli reappear, and the cells undergo
cytokinesis (cell division). As a result 4 cells are formed each containing just one set of chromosomes i.e. they are haploid.
Only one of these haploid cells then is found in the egg; the other three are reabsorbed by the system.