Chromosomes, Chromatids, Bivalents, Tetrads, Mitosis, Meiosis I, and Meiosis II.
ID: 58091 • Letter: C
Question
Chromosomes, Chromatids, Bivalents, Tetrads, Mitosis, Meiosis I, and Meiosis II. Can you please break down using an example of a diploid organism of lets say 2n = 30, and explain how many chromosomes, chromatids, bivalents, and tetrads would be present in each of the following stages of cellular division? MORE IMPORTANTLY, SPECIFY THE BREAKDOWN OF MITOSIS, MEIOSIS I, AND MEIOSIS II, AND HOW MANY OF THE PREVIOUSLY MENTIONED CHROMASOMES, CHROMATIDS, BIVALENTS, AND TETRADS THERE WOULD BE IN FOR EXAMPLE PROPHASE OF MITOSIS, METAPHASE OF MITOSIS ETC FOR EACH 3 DIFFERENT PROCESSES OF CELLULAR DIVISION.
Explanation / Answer
If a cell has 15 pairs of chromosomes (n = 15), it has 30 chromosomes (2n = 30). At the end of mitosis, the two daughter cells will be exact copies of the original cell. Each daughter cell will have 30 chromosomes. At the end of meiosis II, each cell (i.e., gamete) would have half the original number of chromosomes, that is, 15 chromosomes.
Explanation for meiosis what happens exactly in each stage...
What is meiosis I?
prophase I
DNA replication precedes the start of meiosis I. During prophase I, homologous chromosomes pair and form synapses, a step unique to meiosis. The paired chromosomes are called bivalents, and the formation of chiasmata caused by genetic recombination becomes apparent. Chromosomal condensation allows these to be viewed in the microscope. Note that the bivalent has two chromosomes and four chromatids, with one chromosome coming from each parent.
prometaphase I
The nuclear membrane disappears. One kinetochore forms per chromosome rather than one per chromatid, and the chromosomes attached to spindle fibers begin to move.
Metaphase I
Bivalents, each composed of two chromosomes (four chromatids) align at the metaphase plate. The orientation is random, with either parental homologue on a side. This means that there is a 50-50 chance for the daughter cells to get either the mother's or father's homologue for each chromosome.
If 2n=30,15 Bivalents,60 chromatids align at the metaphase plate.(Synapsis (joining) of homologous chromosomes produces tetrads (also called bivalents)
Anaphase I
Chiasmata separate. Chromosomes, each with two chromatids, move to separate poles. Each of the daughter cells is now haploid (15 chromosomes), but each chromosome has two chromatids.
Telophase I
Nuclear envelopes may reform, or the cell may quickly start meiosis II.
Meiosis II is similar to mitosis. However, there is no "S" phase. The chromatids of each chromosome are no longer identical because of recombination. Meiosis II separates the chromatids producing two daughter cells each with 15 chromosomes (haploid), and each chromosome has only one chromatid.
In meiosis I, chromosomes in a diploid cell resegregate, producing four haploid daughter cells. It is this step in meiosis that generates genetic diversity.Related Questions
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