The cell cycle, mitosis and meiosis — University of Leicester
Mitosis produces two daughter cells that are genetically identical to each other, and from the parent cell at the start? why does the cartoon illustration of the . Chromosomes, chromatids, what is the difference and how many. Asexual reproduction produces offspring genetically identical to the one The parent cell simply divides to form two daughter cells that are. Mitotic divisions create two new daughter cells with the same chromosome results in daughter cells with half the number of chromosomes as the parent cell. Molecular Cell Biology 2, 72–75 () doi/ (link to article).
NOVA has a good interactive side-by-side comparison of mitosis and meiosis on this page: How cells divide Meiosis sets the stage for Mendelian genetics. Consider the X and Y chromosomes. They pair in prophase I, and then separate in the first division. Each cell now has only one sex chromosome, like a haploid cell.
One way of thinking about ploidy is the number of possible alleles for each gene a cell can have.
The cell cycle, mitosis and meiosis
Right after meiosis I, the homologous chromosomes have separated into different cells. Each homolog carries one copy of the gene, and each gene could be a different allele, but these two homologs are now in two different cells. 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.
In prophase I, homologous chromosomes pair and separate in the first division Meiosis I. In Meiosis II, sister chromatids separate. 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: Meiotic Segregation tutorial Chromosomes, chromatids, what is the difference and how many chromosomes are there at different times of the cell cycle and after mitosis and meiosis?
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: 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 phasebut the chromosome number stays the same.
A chromatid, then, is a single chromosomal DNA molecule. The number of chromatids changes from 2X in G1 to 4X in G2 and back to 2X, but the number of chromosomes stays the same.
The chromosome number is reduced from 2N to 1N in the first meiotic division, and stays at 1N in the second meiotic division. Because homologous chromosomes separate in the first division, the daughter cells no longer have copies of each chromosome from both parents, so they have haploid genetic information, and a 1N chromosome number.
Cell division: mitosis and meiosis
The second meiotic division, where sister chromatids separate, is like mitosis. Chromosome number stays the same when sister chromatids separate. Using the information above, compare these two simplified diagrams of mitosis and meiosis to visualize why cells are haploid after meiosis I. Specifically, compare the chromosomes in cells at the end of mitosis vs the end of meiosis I, recognizing that the diagram of mitosis tracks just a single pair of homologous chromosomes, whereas the diagram of meiosis tracks two pairs of homologous chromosomes one long chromosome and short chromosome: Meiosis Overview from Wikipedia by Rdbickel The video below is geared toward a high school audience, but it does present a helpful way for recognizing how many chromosomes are present in a cell and thus the ploidy level of that cell.
While watching, see if you can recognize why the products of meiosis 1 are haploid cells: Answers to questions about the mitosis figure: The two daughter cells are the same as each other, and same as the parental cell Each rod represents a chromatid, and DNA replication results in two sister chromatids joined at their centromeres. Meiosis Some simple organisms - such as bacteria - can reproduce by simply dividing into two new individuals.
Other organisms, including human beings, reproduce through sexual reproduction. New individuals are formed by the joining together of two special cells: The cells in our bodies contain 23 pairs of chromosomes - giving us 46 chromosomes in total. Sperm cells and egg cells contain 23 single chromosomes, half the normal number, and are made by a special form of cell division called meiosis.
Cell division: mitosis and meiosis | Biology Biological Principles
Meiosis separates the pairs of matching or 'homologous' chromosomes, so that sperm cells and egg cells have only one copy of each. That way, when an egg cell fuses with a sperm cell, the fertilised egg has a full set: Meiosis involves two cell divisions: Meiosis I and Meiosis II. Meiosis I separates the matching - or 'homologous' - pairs of chromosomes. Meiosis II divides each chromosome into two copies much like mitosis.
In Meiosis I, each daughter cell receives a mix of chromosomes from the two sets in the parent cell. In addition, the chromosomes in each matching pair swap some genetic material before they are parted in a process called crossing over. These processes produce new combinations of genes in the sperm cells and egg cells. A similar process in females produces egg cells rather than sperm cells.
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