Cell Replication:


Interphase -
Stage G1: cell growth
Stage S: DNA replicates
Stage G2: preparation for mitosis

Mitosis - division of nucleus to form 2 genetically identical daughter nuclei

Cytokinesis - division of cytoplasm to form 2 daughter cells

http://www.johnkyrk.com/mitosis.html


Tumors(cancers) are the result of uncontrolled cell divion, these can occur in any organ or tissue.

Interphase is an active period in the life of cell when many metabolic reactions occur, including protein synthesis, DNA replication and an increase in the number of mitochondria and/or chloroplasts.





Prophase
During prophase, the chromatins that were untidy during interphase begin t shorten and become more organized, which then start to form actual chromosomes. The centrioles that were once bundled together begin to separate to opposite sides of the cells. Also, the nuclear membrane and nucleolus disappear.

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Metaphase
The centrioles then create spindles which attach to the chromosomes centromeres, which are currently near the metaplate of the cell.

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Anaphase
The spindles begin to shorten during this stage, splitting the chromosomes in half and creating sister chromotids. Each sister chromatid is an identical copy of the other chromatid that it split from.

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Telophase
The last phase of mitosis is telophase, which is when the spindle fibers disappear and a nuclear envelope develops around each new set of chromatids. Then telophase is over and the last thing to happen is cytokenisis, when the cell pinches and splits itself into two cells.

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Here is a video that sums up mitosis






The cell division process that produces new cells for growth, tissue repair, embryonic development, asexual reproduction and the general replacement of older cells is called mitosis. In this process, a somatic cell divides into two complete new cells that are identical to the original one. Human somatic cells go through the 6 phases mentioned above.





DNA REPLICATION
1) DNA Replication starts with the breaking of hydrogen bonds between bases of the two antiparallel strands. The unwounding of the two strands is the starting point. Helicase is the enzyme that splits the two strands. The initiation point where the splitting starts is called "origin of replication", replication is bidirectional.The structure that is created is known as "Replication Fork".


2) RNA Primase attracts RNA nucleotides which bind to the DNA nucleotides of the 3'-5' strand due to the hydrogen bonds between the bases. RNA nucleotides are the primers (starters) for the binding of DNA nucleotides.

3) 5'-3' template- is called leading strand because DNA Polymerase 3 can "read" the template and continuously adds nucleotides (complementary to the nucleotides of the template, for example Adenine opposite to Thymine etc). The 3'-5' template is harder to "read" by DNA Polymerase 3. The replication of this template is complicated and the new strand is called lagging strand. In the lagging strand the RNA Primase adds more RNA Primers.The gap between two RNA primers is called filled with DNA caled "Okazaki Fragments".


4) In the lagging strand the DNA Polymerase I reads the fragments and removes the RNA Primers. The gaps are closed with the action of DNA Polymerase (adds complementary nucleotides to the gaps) and DNA Ligase (adds phosphate in the remaining gaps of the phosphate - sugar backbone).

5)The last step of DNA Replication is the Termination. This process happens when the DNA Polymerase reaches to an end of the strands. We can easily understand that in the last section of the lagging strand, when the RNA primer is removed, it is not possible for the DNA Polymerase to seal the gap (because there is no primer). So, the
end of the parental strand where the last primer binds isn't replicated. These ends of linear (chromosomal) DNA consists of noncoding DNA that contains repeat sequences and are called telomeres. As a result, a part of the telomere is removed in every cycle of DNA Replication.

6) The DNA Replication is not completed before a mechanism of repair fixes possible errors caused during the replication. Enzymes like nucleases remove the wrong nucleotides and the DNA Polymerase fills the gaps.



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