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Task 2 (M2)MitosisMitosis is a process in which one cell divides into two new cells, that are genetically identical to each other. Mitosis is very important for our body, it helps us through the process of development and growth, by dividing cells and replacing them with old and dead cells.

This is necessary for us to be alive. There are 23 pairs of chromosomes inside the nucleus 2 copies of each, which makes it a diploid cells. At the end of the mitosis 2 daughter cells are produced that are genetically identical to each other. This process takes place inside all the somatic (body) cells.

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There are 5 stages of Mitosis• Prophase: first the chromosomes start to condense, this helps them pull apart in later stages. Then mitotic spindle starts to form. Spindle helps the chromosomes move during the process of mitosis, the spindle grows between centrosomes as they move apart. Soon nucleolus disappears from the nucleus, this indicates that the nucleus is almost ready to break down. Nucleus breaks down so condensed chromosomes can be released so they can attach to microtubules of the mitotic spindle. Microtubules can bind to chromosomes at the kinetochore. • Metaphase: now spindle has captured all the chromosomes and line them up in the middle of the cell ready divide.

All the chromosomes align at the metaphase plate. Now two kinetochores of each chromosome should be attached to spindle fibre from opposite spindle poles. At this point before moving to the next stage, the cell would perform a test to check that all the chromosomes are at the metaphase plate and correctly connected to the spindle fibre. This is called spindle checkpoint. This makes sure that the sister chromatids will separate evenly between two daughter cells.• Anaphase: in this phase sister chromatids are separated from each other and are pulled toward the opposite ends of the cell. Protein holding the sister chromatids breaks down allowing them to separate and move to the opposite ends of the cell.

Free Microtubules that are not attached to the chromosomes elongate and push apart, separating the poles and making the cell longer.• Telophase: now the cell is almost finish dividing, and starts to regain its normal shape and structure as the cytokinesis takes place. Mitotic Spindle is broken down into its building blocks. Now two new nuclei are formed one for each cell to store chromosome, the nuclear membrane also reappears, as the chromosomes go back to their stringy form. • Cytokinesis is the process of dividing cytoplasm to form two new cells. This takes place at end, either in anaphase or telophase depending on the cell, and finishes shortly after telophase.

Reference: https://www.khanacademy.org/science/biology/cellular-molecular-biology/mitosis/a/phases-of-mitosis Date:26/11/17       MeiosisMeiosis is a specialised type of cell devision that reduces the number of chromosomes in half, by creating four haploid cells, each genetically different from the original cell.

There are 5 stages of meiosis.• Prophase 1:Chromosomes are condensed tightly from lose stringy form. Each appears as a pair of chromatids. Centrioles starts to move to the opposite ends of the cell (poles), and starts to produce spindle fibre. Homologous chromosome pair up to produce a bivalent. Now the crossing over occurs so gene from chromosomes are reconnected in a chris cross pattern. This gives chromosomes unique combinations of alleles. • Metaphase 1: After the crossing over occurs, the spindle begins to capture the chromosomes and move the toward the centre of the metaphase plate.

Independent assortment of chromosome occurs. This gives new and unique combinations of chromosomes.• Anaphase 1: the chromosomes of the same homologous pairs are pulled apart and move apart to opposite ends of the cell. However, the sister chromatids of each chromosome remain attached to one another and don’t come apart.• Telophase 1: new nuclear envelops form, and chromosomes go back to the stringy form. Two haploid cells are produced as the product of this process.

However, chromosomes still made of 2 sister chromatids.Meiosis 2• Prophase 2: again chromosomes condense same as in prophase of mitosis and meiosis 1. Then nuclear envelope breaks  down. Spindle formation occurs however, at the right angle to the spindle of prophase 1.• Metaphase 2: chromosomes line up at the metaphase plate (at the equator). Independent assortment of chromatids occur.

Chromatids are no longer identical because of crossing over.• Anaphase 2: chromatids are pulled to the opposite poles. • Telophase 2: new nuclear envelope forms for each set of chromosomes, and chromosome decondense.• Cytokinesis: cytokinesis splits the chromosome sets into new cells, resulting in 4 haploid cells.Reference: https://www.khanacademy.org/science/biology/cellular-molecular-biology/meiosis/a/phases-of-meiosis Date: 26/11/17Task 4 (P4)Describe the behaviour of chromosome during: crossing over, independent assortment and random fusion of gametes. Explain how these leads to variation.

Crossing over: this occurs during prophase one of meiosis. Homologous pairs come together in the process of synapsis, where overlapping section chromosomes (chiasmata) are exchanged. This results in chromosomes that is neither maternal nor paternal, but still contains gene from both parents. This can lead to genetic variation because, chromosomes can have many points where it crosses over and gene is exchange. Therefore in each division different crossing over points can lead to genetic variation. Independent assortment: during anaphase of meiosis, homologous chromosomes are distributed randomly, separating and segregating from each other.

This results in gametes that have unique combination of chromosomes. Therefore making the genetically different.Random fusion of gametes: two gametes unite to produce an offspring. However, out of millions of gametes which two would fuse and produce an offspring. This obviously is matter of chance, but still is a source of variation.Reference: https://www.ck12.org/biology/Genetic-Variation/lesson/Genetic-Variation-Advanced-BIO-ADV/ date: 26/11/17Stages of mitosis task 2 (P2)


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