The mitotic spindle plays a critical role during the later phases of mitosis as it orchestrates the movement of sister chromatids to opposite poles of the cell Figure 2. After prophase is complete, the cell enters prometaphase. During prometaphase, the nuclear membrane disintegrates and the mitotic spindle gains access to the chromosomes. During this phase, a protein structure called the kinetochore is associated with the centromere on each sister chromatid.
Stringlike structures called microtubules grow out from the spindle and connect to the sister chromatids at their kinetochores; one microtubule from one side of the spindle attaches to one sister chromatid in each chromosome, and one microtubule from the other side of the spindle attaches to the other sister chromatid Figure 3a.
Figure 3: a Metaphase and b Anaphase. In metaphase a , the microtubules of the spindle white have attached and the chromosomes have lined up on the metaphase plate. During anaphase b , the sister chromatids are pulled apart and move toward opposite poles of the cell.
Figure Detail. After metaphase is complete, the cell enters anaphase. During anaphase, the microtubules attached to the kinetochores contract, which pulls the sister chromatids apart and toward opposite poles of the cell Figure 3c. At this point, each chromatid is considered a separate chromosome. Figure 4: During telophase, two nuclear membranes form around the chromosomes, and the cytoplasm divides. Finally, once anaphase is complete, the cell enters the last stage of the division process — telophase.
During telophase, the newly separated chromosomes reach the mitotic spindle and a nuclear membrane forms around each set of chromosomes, thus creating two separate nuclei inside the same cell. As Figure 4 illustrates, the cytoplasm then divides to produce two identical cells. Why is mitosis important? As previously mentioned, most eukaryotic cells that are not involved in the production of gametes undergo mitosis.
These cells, known as somatic cells , are important to the survival of eukaryotic organisms, and it is essential that somatic parent and daughter cells do not vary from one another. With few exceptions, the mitotic process ensures that this is the case. Therefore, mitosis ensures that each successive cellular generation has the same genetic composition as the previous generation, as well as an identical chromosome set. Watch this historic video from to see mitosis in action.
Key Questions How do centromeres work? Key Concepts chromosomes replication meiosis. Topic rooms within Genetics Close. No topic rooms are there. Browse Visually. Consequently, females are born with a finite number of oocytes arrested in the first meiotic prophase.
Within the ovary, these oocytes grow within follicle structures containing large numbers of support cells. The oocytes will reenter meiosis only when they are ovulated in response to hormones. Human females, for example, are born with hundreds of thousands of oocytes that remain arrested in the first meiotic prophase for decades. Over time, the quality of the oocytes may deteriorate; indeed, researchers know that many oocytes die and disappear from ovaries in a process known as atresia.
Two divisions, meiosis I and meiosis II , are required to produce gametes Figure 3. Meiosis I is a unique cell division that occurs only in germ cells; meiosis II is similar to a mitotic division.
Before germ cells enter meiosis, they are generally diploid , meaning that they have two homologous copies of each chromosome. Then, just before a germ cell enters meiosis, it duplicates its DNA so that the cell contains four DNA copies distributed between two pairs of homologous chromosomes. Compared to mitosis, which can take place in a matter of minutes, meiosis is a slow process, largely because of the time that the cell spends in prophase I.
During prophase I, the pairs of homologous chromosomes come together to form a tetrad or bivalent , which contains four chromatids. Recombination can occur between any two chromatids within this tetrad structure. The recombination process is discussed in greater detail later in this article. Crossovers between homologous chromatids can be visualized in structures known as chiasmata, which appear late in prophase I Figure 4. Chiasmata are essential for accurate meioses.
At the end of prometaphase I, meiotic cells enter metaphase I. Here, in sharp contrast to mitosis, pairs of homologous chromosomes line up opposite each other on the metaphase plate , with the kinetochores on sister chromatids facing the same pole.
Pairs of sex chromosomes also align on the metaphase plate. In human males, the Y chromosome pairs and crosses over with the X chromosome. These crossovers are possible because the X and Y chromosomes have small regions of similarity near their tips. Crossover between these homologous regions ensures that the sex chromosomes will segregate properly when the cell divides. Next, during anaphase I , the pairs of homologous chromosomes separate to different daughter cells.
Before the pairs can separate, however, the crossovers between chromosomes must be resolved and meiosis-specific cohesins must be released from the arms of the sister chromatids. Failure to separate the pairs of chromosomes to different daughter cells is referred to as nondisjunction , and it is a major source of aneuploidy.
Overall, aneuploidy appears to be a relatively frequent event in humans. Meiosis II resembles a mitotic division, except that the chromosome number has been reduced by half. Thus, the products of meiosis II are four haploid cells that contain a single copy of each chromosome. In mammals, the number of viable gametes obtained from meiosis differs between males and females. In males, four haploid spermatids of similar size are produced from each spermatogonium.
In females, however, the cytoplasmic divisions that occur during meiosis are very asymmetric. Fully grown oocytes within the ovary are already much larger than sperm, and the future egg retains most of this volume as it passes through meiosis. As a consequence, only one functional oocyte is obtained from each female meiosis Figure 2. The other three haploid cells are pinched off from the oocyte as polar bodies that contain very little cytoplasm.
Prophase I is the longest and arguably most important segment of meiosis, because recombination occurs during this interval. For many years, cytologists have divided prophase I into multiple segments, based upon the appearance of the meiotic chromosomes. Thus, these scientists have described a leptotene from the Greek for "thin threads" phase, which is followed sequentially by the zygotene from the Greek for "paired threads" , pachytene from the Greek for "thick threads" , and diplotene from the Greek for "two threads" phases.
In recent years, cytology and genetics have come together so that researchers now understand some of the molecular events responsible for the stunning rearrangements of chromatin observed during these phases. Recall that prophase I begins with the alignment of homologous chromosome pairs. Historically, alignment has been a difficult problem to approach experimentally, but new techniques for visualizing individual chromosomes with fluorescent probes are providing insights into the process.
Recent experiments suggest that chromosomes from some species have specific sequences that act as pairing centers for alignment. In some cases, alignment appears to begin as early as interphase, when homologous chromosomes occupy the same territory within the interphase nucleus Figure 5. The formation of DSBs is catalyzed by highly conserved proteins with topoisomerase activity that resemble the Spo11 protein from yeast.
Genetic studies have shown that Spo11 activity is essential for meiosis in yeast, because spo11 mutants fail to sporulate. As the invading strand is extended, a remarkable structure called synaptonemal complex SC develops around the paired homologues and holds them in close register, or synapsis.
The stability of the SC increases as the invading strand first extends into the homologue and then is recaptured by the broken chromatid, forming double Holliday junctions. Investigators have been able to observe the process of SC formation with electron microscopy in meiocytes from the Allium plant Figure 6. Bridges approximately nanometers long begin to form between the paired homologues following the DSB.
Only a fraction of these bridges will mature into SC; moreover, not all Holliday junctions will mature into crossover sites. Gerton, J. Homologous chromosome interactions in meiosis: Diversity amidst conservation. Nature Reviews Genetics 6 , — doi Hassold, T. To err meiotically is human: The genesis of human aneuploidy. Nature Reviews Genetics 2 , — doi Lopez-Maury, L. Tuning gene expression to changing environments: From rapid responses to evolutionary adaptation.
Nature Reviews Genetics 9 , — doi Marston, A. Meiosis: Cell-cycle controls shuffle and deal. Nature Reviews Molecular Cell Biology 5 , — doi Page, S. Chromosome choreography: The meiotic ballet. This process is called cell division. One cell doubles by dividing into two. Two cells become four and so on. The diagram below shows cells dividing. Stem cells provide a pool of dividing cells that the body uses to restock damaged or old cells.
They have the potential to develop into different cell types in the body. When a stem cell multiplies, the resulting cells may remain as stem cells.
But under the right conditions, they become a type of cell with a more specialised function. For example a muscle cell, red blood cell or brain cell. Stem cells occur in the body in various places and stages during our lifetime. In the embryo, they give rise to all the different tissues and organs of the body. In adults each type of stem cell is usually only able to develop into a few specific types of cell.
For example, adult stem cells in the bone marrow, known as haematopoietic stem cells, usually only give rise to different types of blood cell. Scientists now believe that stem cells might play a role in the development of cancer. They think that some tumours develop from faulty stem cells. This has led to the idea of cancer stem cells, which scientists have now identified in a range of cancer types. The types include bowel, breast and prostate cancer as well as leukaemia.
When cells divide and grow they do this very precisely so that the new cells are exactly the same as the old ones. Each cell makes copies of all its genes. Then each cell splits into 2 with one set of genes in each new cell. During the process, there are lots of checks to make sure that everything has copied correctly. But sometimes mistakes happen, which can lead to cancer.
You can read about genes and cancer on the page about how cancer starts. Content not working due to cookie settings. View a transcript for the video about how healthy cells divide.
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