Meiosis The process of creating sex cells
What is Meiosis? Meiosis involves two successive nuclear divisions that produce four haploid cells. The first division (meiosis I) is the reduction division; the second division (meiosis II) separates the chromatids.
Advantages of Meiosis Mitotic cell division produces new cells genetically identical to the parent cell. Meiosis increases genetic variation in the population.
Interphase Interphase is an important stage preceding meiosis. Without this stage meiosis would not occur. During this stage, each individual chromatid replicates, similar to mitosis.. At this stage, the chromosomes are long and stringy and are not visible. **Remember: All somatic cells are diploid in number (2n), therefore for each chromatid there also exists its homolog, which also replicates during interphase.
The graphical representation here shows that each chromotid pair is connected by a centromere (the gold structure). Each pair of the chromosome are referred to as a sister chromatidscentromere
Prophase I Prophase I is one of the most important stages of meiosis. The chromotid threads begin to twist and condense, creating chromosomal structures which are visible to the microscope. Each chromosome then actively seeks out its homologous chromosome, as shown in the graphical representation. After the homologous chromosomes pair, the structure is referred to as a tetrad (four chromatids). The point at which two non-sister chromatids intertwine is known as a chiasma. Sometimes a process known as crossing over occurs at this point. This is where two non-sister chromatids exchange genetic material. This exchange does not become evident, however, until the two homologous pairs separate.
Prophase I Cell Photographs of Prophase IProphase I
Metaphase I At metaphase, each chromosome has reached its maximum density. The homologous pairs and their sister chromatids also prepare for separation. They interact with spindle fibers which form from either side of the nuclear envelope of the cell. The spindle fibers are produced from a structure called a centriole. There is a centriole at opposite ends of the cell, which is referred to as poles. During metaphase, the chromosomes are lined by the spindle fibers at what is known as the metaphase plate. As in the cell picture, it looks like a line through the center of the cell.
Metaphase I
Anaphase I Anaphase I pulls apart the tetrad, separating each homologous chromosome. In the graphical representation in the next slide, the affects of crossing over are evident. It is by random chance that a certain chromosome is pulled to a certain pole.
Anaphase I
Telophase I Telophase I varies from species to species. Sometimes Telophase I is skipped and meiosis starts its second division immediately. In general, however, two nuclear envelopes begin to surround the separate chromosomes and cytokinesis (splitting of the cytoplasm into two separate entities) will sometimes occur. Then a phase called interkinesis will follow, which essentially is a resting period from Telophase I to Prophase II. This differs from mitosis because DNA replication does not occur.
Telophase I
Prophase II During Prophase II, each dyad (1/2 a tetrad) is composed of a pair of sister chromatids and they are connected by a centromere. The centrioles (replicated during Telophase I) which produce the spindle fibers also start to move toward the poles of the cell.
Prophase II
Metaphase II Metaphase II is similar to Metaphase I in that the dyads are lined up at a metaphase plate by the spindle fibers.
Anaphase II Anaphase II separates the dyads into individual chromatids. Each sister chromatid ends up on one side of the cell.
Telophase II At the end of Telophase II, the nuclear envelopes forms around each set of DNA and the cytoplasm divides once again. As a result, four haploid cells have formed from one diploid cell. The chromosomal content of a haploid cell in one-half the chromosomal content of a diploid cell ( n as opposed to 2n ) one-half the chromosomal content
Cell Photograph of Telophase II