What is mitosis? What are kinetochores made of? What are centromeres composed of? See all questions in Mitosis. Impact of this question views around the world. Though it looks like there are two of each chromosome in each cell, these are duplicated chromosomes; ie, it is one chromosome which has been copied, so there is only one possible allele in the cell just two copies of it. The second meiotic division is where sister duplicated chromatids separate. It resembles mitosis of a haploid cell.
At the start of the second division, each cell contains 1N chromosomes, each consisting of a pair of sister chromatids joined at the centromere. Here is a simplified diagram illustrating the overall process and products of meiosis:. Meiosis Overview from Wikipedia by Rdbickel. And here is a video that walks through the steps of meiosis: It is very important that you recognize how and why cells become haploid after meiosis I.
To confirm for yourself that you understand meiosis, work through one or more of these interactive tutorials:. Chromosomes by definition contain the DNA that makes up the fundamental genome of the cell. In a prokaryote, the genome is usually packaged into one circular chromosome consisting of a circular DNA molecule of a few million base pairs Mbp.
In eukaryotes, the genome is packaged into multiple linear chromosomes, each consisting of a linear DNA molecule of tens or hundreds of Mbp. Chromosomes exist at all different phases of the cell cycle. The chromosome number, N, in eukaryotes, refers to the number of chromosomes in a haploid cell, or gamete sperm or egg cell. Diploid cells all the cells in our body except our gametes have 2N chromosomes, because a diploid organism is created by union of 2 gametes each containing 1N chromosomes.
A pair of sister chromatids is one chromosome because it has genetic information alleles inherited from only one parent.
A pair of homologous chromosomes, each consisting of a single chromatid in a daughter cell at the end of mitosis, has alleles from the father and from the mother, and counts as 2 chromosomes. This chromosome number stays the same after chromosome replication during S phase: each chromosome entering cell division now consists of a pair of sister chromatids joined together at the centromere.
Then in mitosis, the sister chromatids of each chromosome separate, so each daughter cell receives one chromatid from each chromosome.
The result of mitosis is two identical daughter cells, genetically identical to the original cell, all having 2N chromosomes. So during a mitotic cell cycle, the DNA content per chromosome doubles during S phase each chromosome starts as one chromatid, then becomes a pair of identical sister chromatids during S phase , but the chromosome number stays the same.
When mitosis is not regulated correctly, health problems such as cancer can result. The other type of cell division, meiosis, ensures that humans have the same number of chromosomes in each generation. It is a two-step process that reduces the chromosome number by half—from 46 to 23—to form sperm and egg cells. When the sperm and egg cells unite at conception, each contributes 23 chromosomes so the resulting embryo will have the usual Meiosis also allows genetic variation through a process of gene shuffling while the cells are dividing.
Other chapters in Help Me Understand Genetics. The "2" signifies the fact that each pair of chromosomes is represented by two chromosomes. The "c" is the total number of pairs of chromosomes in the cell. When this cell replicates its DNA in preparation for division it makes copies of every chromosome. Each of the replicated chromosomes consists of two chomatids that are connected at the centromere and, as above, each of these chromatids represents a complete chromosome.
Thus, we have now doubled the number of chromosomes in the cell and ploidy in terms of chromosome number is 4c. The "4" signifies the fact that each pair of chromosomes is now represented by 4 chromosomes i. If we now apply these facts to the process of meiosis, we can see how ploidy in terms of genetic content and chromosome number differs as meiosis occurs. Initially before replication of the DNA of the chromosomes , the primary gametocyte is 2n diploid in terms of genetic content and 2c diploid in terms of the number of chromosomes , as described for a diploid cell above.
The primary gametocyte then replicates its chromosomes its DNA. After this replication is complete, the cell is still 2n no new gene alleles have been added , but since the total number of chromosomes has doubled as described above it is now 4c a tetraploid number of chromosomes. Next the first meiotic division takes place.
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