The karyotype image above shows the homologous pairs for all the autosomes. Homologous chromosomes are not identical to each other, unlike sister chromatids. They frequently have different variants of the same hereditary information — such as blue eye color vs brown eye color, or blood type A versus blood type B.
Mitosis Mitosis produces two daughter cells that are genetically identical to each other, and to the parental cell. Each chromosome now consists of a joined pair of identical sister chromatids. During mitosis the sister chromatids separate and go to opposite ends of the dividing cell. All eukaryotic cells replicate via mitosis, except germline cells that undergo meiosis see below to produce gametes eggs and sperm. This animation below shows the packaging of DNA and condensation of chromosomes as a cell undergoes mitosis.
This is a special sequence of 2 cell divisions that produces haploid gametes from diploid germline cells. 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.
Students need to know that most of the genetics action occurs in the first meiotic division:. The last point appears to be the most difficult for students to grasp. 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. 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. However, these chromosomes are not arranged in the same way as they were during mitosis.
Rather than each chromosome lining up individually across the center of the cell, homologous pairs of chromosomes line up together forming tetrads , also known as bivalents :. Here, the homologous chromosome pairs have been color coded:. When anaphase I begins, you may expect the chromosome number to change, but it does not. Remember — it is only after the sister chromatids separate that the chromosome number changes. Since anaphase I only separates the homologous chromosomes, neither the chromosome number nor the chromatid number changes during anaphase.
Visualized below:. As you can see, the separation of homologous chromosomes does not change the chromosome number or the chromatid number. There are still 8 chromosomes and 16 chromatids. In fact, until the completion of meiosis I, the chromosome and chromatid numbers remain the same through all stages. Similarly in a human, we do not see a change in chromosome or chromatid number until the end of meiosis I when division of the cell in two results in half the chromosome and chromatid count.
Below is a table summarizing the chromosome and chromatid number during meiosis I in humans:. The second division of meiosis meiosis II appears similar to mitosis, with the only difference being that there are now half as many chromosomes as before. During metaphase II, the chromosomes are lined up individually across the center of the cell. Division is not complete until the cell components have been apportioned and completely separated into the two daughter cells. Although the stages of mitosis are similar for most eukaryotes, the process of cytokinesis is quite different for eukaryotes that have cell walls, such as plant cells.
In cells such as animal cells that lack cell walls, cytokinesis follows the onset of anaphase. A contractile ring composed of actin filaments forms just inside the plasma membrane at the former metaphase plate. The actin filaments pull the equator of the cell inward, forming a fissure. The furrow deepens as the actin ring contracts, and eventually the membrane is cleaved to separate the two new cells.
In plant cells, a new cell wall must form between the daughter cells. During interphase, the Golgi apparatus accumulates enzymes, structural proteins, and glucose molecules prior to breaking into vesicles and dispersing throughout the dividing cell. During telophase, these Golgi vesicles are transported on microtubules to form a phragmoplast a vesicular structure at the metaphase plate.
There, the vesicles fuse and coalesce from the center toward the cell walls; this structure is called a cell plate. As more vesicles fuse, the cell plate enlarges until it merges with the cell walls at the periphery of the cell. Enzymes use the glucose that has accumulated between the membrane layers to build a new cell wall. The Golgi membranes become parts of the plasma membrane on either side of the new cell wall. Consider the events of interphase and mitosis.
What cell cycle events will be affected in a cell that produces mutated non-functional condensin proteins? If condensin is not functional, chromosomes are not packaged after DNA replication in the S phase of interphase. It is likely that the proteins of the centromeric region, such as the kinetochore, would not form.
Even if the mitotic spindle fibers could attach to the chromatids without packing, the chromosomes would not be sorted or separated during mitosis. John W. This content is distributed under a Creative Commons Attribution 3. After DNA replication copies the chromosomes during S phase, sister chromatids remain held together by cohesin proteins.
These sisters are still "counted" as one chromosome, until they separate during anaphase of mitosis, which ensures that each of the two new cells inherits one copy sister chromatid of each chromosome. If a cell has 10 chromosomes total , how many chromosomes will be present during metaphase of mitosis? During anaphase? In the two daughter cells? After telophase and cytokinesis, the new daughter cells will each have 10 chromosomes, which is identical to the parental cell.
How many chromosomes and sister chromatids are present at each of the following stages: G 1 , G 2 , anaphase of mitosis, and after cytokinesis? Human cells have 23 pairs of chromosomes. How many chromosomes are present during metaphase? In the cells that have completed mitosis? Learning Objectives List and describe the key events that occur during the cell cycle and mitosis. Describe the movement of chromosomes and chromatids during mitosis and the role of microtubules.
Determine the chromosome content of cells during various stages given information about the n or 2n number of the cell. The chromosome was treated to remove its histones. Because the centrosomes are located outside the nucleus in animal cells, the microtubules of the developing spindle do not have access to the chromosomes until the nuclear membrane breaks apart.
Prometaphase is an extremely dynamic part of the cell cycle. Microtubules rapidly assemble and disassemble as they grow out of the centrosomes, seeking out attachment sites at chromosome kinetochores, which are complex platelike structures that assemble during prometaphase on one face of each sister chromatid at its centromere.
As prometaphase ensues, chromosomes are pulled and tugged in opposite directions by microtubules growing out from both poles of the spindle, until the pole-directed forces are finally balanced. Sister chromatids do not break apart during this tug-of-war because they are firmly attached to each other by the cohesin remaining at their centromeres.
At the end of prometaphase, chromosomes have a bi-orientation, meaning that the kinetochores on sister chromatids are connected by microtubules to opposite poles of the spindle. Next, chromosomes assume their most compacted state during metaphase, when the centromeres of all the cell's chromosomes line up at the equator of the spindle.
Metaphase is particularly useful in cytogenetics , because chromosomes can be most easily visualized at this stage. Furthermore, cells can be experimentally arrested at metaphase with mitotic poisons such as colchicine. Video microscopy shows that chromosomes temporarily stop moving during metaphase. A complex checkpoint mechanism determines whether the spindle is properly assembled, and for the most part, only cells with correctly assembled spindles enter anaphase.
Figure 10 Figure Detail. Figure 9. The progression of cells from metaphase into anaphase is marked by the abrupt separation of sister chromatids. A major reason for chromatid separation is the precipitous degradation of the cohesin molecules joining the sister chromatids by the protease separase Figure Two separate classes of movements occur during anaphase.
During the first part of anaphase, the kinetochore microtubules shorten, and the chromosomes move toward the spindle poles. During the second part of anaphase, the spindle poles separate as the non-kinetochore microtubules move past each other.
These latter movements are currently thought to be catalyzed by motor proteins that connect microtubules with opposite polarity and then "walk" toward the end of the microtubules. Mitosis ends with telophase, or the stage at which the chromosomes reach the poles. The nuclear membrane then reforms, and the chromosomes begin to decondense into their interphase conformations. Telophase is followed by cytokinesis, or the division of the cytoplasm into two daughter cells.
The daughter cells that result from this process have identical genetic compositions. Cheeseman, I. Molecular architecture of the kinetochore-microtubule interface. Nature Reviews Molecular Cell Biology 9 , 33—46 doi Cremer, T. Chromosome territories, nuclear architecture and gene regulation in mammalian cells. Nature Reviews Genetics 2 , — doi Hagstrom, K. Condensin and cohesin: More than chromosome compactor and glue. Nature Reviews Genetics 4 , — doi Hirano, T. At the heart of the chromosome: SMC proteins in action.
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Theodor and Marcella Boveri: Chromosomes and cytoplasm in heredity and development.
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