What is the significance of mitosis and meiosis

Meiosis has various timescales in different organisms, which can be affected by several factors including temperature and environment of the organism, and the amount of nuclear DNA. The process lasts 6 hours in yeast but can last more than 40 years in human females, due to a developmental hold at prophase I, until ovulation. What is an example of a disease caused by an error in this process?

Uncontrolled mitosis occurs in cancer, where either genes that stop cell division tumour suppressors are switched off, or genes that encourage cell division oncogenes are overactive. Errors in meiosis can lead to the wrong number of chromosomes ending up in germ cells, this is called aneuploidy.

This can trigger miscarriage, but is occasionally tolerated. Another example is Klinefelter syndrome , where XY males have an additional X chromosome. Mitosis is the Greek word for thread , after the thread-like chromosomes that can be seen under the microscope in dye-stained cells during cell division.

This refers to the outcome of meiosis, where the genetic information in each new cell is halved. Oskar Hertwig described the fusion of egg and sperm in the transparent sea urchin egg in In mitosis, prophase, metaphase, anaphase and telophase occur once.

Meiotic prophase I is much longer that mitotic prophase. This is where chromosomes exchange sections of DNA. This is important for generating genetic diversity but is also crucial mechanically to hold homologous chromosomes together. Mitotic prophase is much shorter that meiotic prophase I. There is no crossing over in mitosis.

In meiotic metaphase I pairs of homologous chromosomes line up along the metaphase plate. The way in which the homologous pairs are oriented randomly with respect to the cell poles is referred to as the law of independent assortment and ensures a random and independent distribution of chromosomes to the daughter cells of meiosis I and ultimately to the haploid gametes at the end of meiosis II.

In anaphase of meiosis I cohesin at the centromeres of the chromosomes is not cleaved and it therefore continues to hold sister chromatids together as the homologous chromosomes are segregated to opposite cell poles. In anaphase of mitosis and meiosis II , cohesin protein holding the centromeres of the sister chromatids together is cleaved , allowing the sister chromatids to segregate to opposite poles of the cell , at which point they are called chromosomes.

In meiosis, cytokinesis must occur twice : once after telophase I and again, after telophase II. In mitosis, cytokinesis does not always occur , some cells divide and are multinucleate , like muscle cells. Are uncondensed but are still organised. The entire genome is replicated to create two identical semi-conserved copies of each chromosome.

Align along the metaphase plate, the midpoint between the two centrosomes. Sister chromatids are joined at the centromere by proteins that form a structure called a kinetochore. Cohesin is cleaved at the centromere of chromosomes, resulting in sister chromatids being pulled to opposite poles of the cell.

Chromosomes have returned to their interphase structure. This is a topic of much research, but it seems as though each chromosome occupies its own territory within the nucleus. Metaphase II. Figure 5: During metaphase II, the chromosomes align along the cell's equatorial plate. The events of metaphase II are similar to those of mitotic metaphase — in both processes, the chromosomes line up along the cell's equatorial plate, also called the metaphase plate, in preparation for their eventual separation Figure 5.

Anaphase II. Figure 6: Anaphase II involves separation of the sister chromatids. During anaphase II , microtubules from each spindle attach to each sister chromatid at the kinetochore. The sister chromatids then separate, and the microtubules pull them to opposite poles of the cell. As in mitosis, each chromatid is now considered a separate chromosome Figure 6.

This means that the cells that result from meiosis II will have the same number of chromosomes as the "parent" cells that entered meiosis II.

Telophase II. Figure 7: Telophase II results in the production of four daughter cells. Finally, in telophase II , nuclear membranes reform around the newly separated chromosomes, which relax and fade from view.

As soon as the cytoplasm divides, meiosis is complete. There are now four daughter cells — two from each of the two cells that entered meiosis II — and each daughter cell has half the normal number of chromosomes Figure 7. Each also contains new mixtures of genes within its chromosomes, thanks to recombination during meiosis I. Why is meiosis important? More about meiosis.

Genes are packaged differently in mitosis and meiosis — but what is the effect of this difference? What else can go wrong with chromosomes in meiosis? Meiosis is important because it ensures that all organisms produced via sexual reproduction contain the correct number of chromosomes. Meiosis also produces genetic variation by way of the process of recombination.

Later, this variation is increased even further when two gametes unite during fertilization, thereby creating offspring with unique combinations of DNA. This constant mixing of parental DNA in sexual reproduction helps fuel the incredible diversity of life on Earth.

Watch this video for a summary of meiosis. Key Questions How did sexual reproduction evolve? What happens when meiosis goes wrong? Key Concepts chromosome meiosis haploid diploid recombination. Topic rooms within Genetics Close. No topic rooms are there. Browse Visually.

Other Topic Rooms Genetics. Meiotic cell division. Genetically modified gm food. Meiosis ppt. Related Books Free with a 30 day trial from Scribd. Related Audiobooks Free with a 30 day trial from Scribd. Elizabeth Howell. PreciousAngel Sarma. Parth Dabi. Benny Paul. Iceyvil Daarol. Jeremy Nemo. Chalifa Savira. Apple Grace Hidalgo. Khalid Roy. Misbah Khan. Sapna Sapna. In fact, recombination leads to an overall increase in the number of units that assort independently, and this increases variation.

While in mitosis, genes are generally transferred faithfully from one cellular generation to the next; in meiosis and subsequent sexual reproduction , genes get mixed up. Sexual reproduction actually expands the variety created by meiosis, because it combines the different varieties of parental genotypes.

Thus, because of independent assortment, recombination, and sexual reproduction, there are trillions of possible genotypes in the human species. During cell division, chromosomes sometimes disappear.

This occurs when there is some aberration in the centromere , and spindle fibers cannot attach to the chromosome to segregate it to distal poles of the cell.

Consequently, the lost chromosome never properly groups with others into a new nuclear envelope , and it is left in the cytoplasm , where it will not be transcribed. Also, chromosomes don't always separate equally into daughter cells. This sometimes happens in mitosis, when sister chromatids fail to separate during anaphase. One daughter cell thus ends up with more chromosomes in its nucleus than the other. Likewise, abnormal separation can occur in meiosis when homologous pairs fail to separate during anaphase I.

This also results in daughter cells with different numbers of chromosomes. The phenomenon of unequal separation in meiosis is called nondisjunction. If nondisjunction causes a missing chromosome in a haploid gamete, the diploid zygote it forms with another gamete will contain only one copy of that chromosome from the other parent, a condition known as monosomy.

Conversely, if nondisjunction causes a homologous pair to travel together into the same gamete, the resulting zygote will have three copies, a condition known as trisomy Figure 3. The term " aneuploidy " applies to any of these conditions that cause an unexpected chromosome number in a daughter cell. Aneuploidy can also occur in humans. For instance, the underlying causes of Klinefelter's syndrome and Turner's syndrome are errors in sex chromosome number, and Down syndrome is caused by trisomy of chromosome However, the severity of phenotypic abnormalities can vary among different types of aneuploidy.

In addition, aneuploidy is rarely transferred to subsequent generations, because this condition impairs the production of gametes. Overall, the inheritance of odd chromosome number arises from errors in segregation during chromosome replication. Often, it is these very exceptions or modifications of expected patterns in mitosis and meiosis that enrich our understanding of how the transfer of chromosomes is regulated from one generation to the next.

Belling, J. On the attachment of non-homologous chromosomes at the reduction division in certain chromosome daturas. Proceedings of the National Academy of Sciences 12 , 7—11 Farmer, J. On the maiotic phase reduction divisions in animals and plants. Quarterly Journal of Microscopical Science 48 , — Gilbert, S. Hirsch, J. Behavior genetics and individuality understood. Science 42 , — doi Uniqueness, diversity, similarity, repeatability, and heritability.

International Journal of Comparative Psychology 17 , — Paweletz, N. Walther Flemming: Pioneer of mitosis research. Nature Reviews Molecular Cell Biology 2 , 72—75 doi Chromosome Theory and the Castle and Morgan Debate. Discovery and Types of Genetic Linkage. Genetics and Statistical Analysis. Thomas Hunt Morgan and Sex Linkage. Developing the Chromosome Theory.