Cell and DNA Replication

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2.5.1 Outline the stages in the cell cycle, including interphase, mitosis and cytokinesis.
The stages that are present in the cell cycle are Interphase, mitosis and cytokinesis.Interphase where the cell is preparing for mitosis, and contains within it three stages, G1, G2, and the S phase, also known as the synthesis phase. In the G1 phase, the cell begins to grow and reproduce the proteins and the cytoplasmic organelles. These organelles include the mitochondria and the endoplasmic reticulum. In the G2 phase, the cell experiences growth as it prepares to divide. The S phase is the phase where DNA replication actually occurs, and the chromosomes are copied. Since new DNA is synthesised, it is called the synthesis phase. Mitosis is where the cell actually physically divides itself, after everything in the cell has been replicated. It is facilitated by the centrioles, who move to either side of the cell, attach the chromosomes with spindles, or micro tubules, and pull them apart. Two separate daughter cells are then formed with the copied chromosomes. Cytokinesis is the part of mitosis when the two cells are seperated. they are moved apart by the process of cytokinesis.

2.5.2 State that tumors (cancers) are the result of uncontrolled cell division and that these can occur in any organ of tissue.
Tumors, or cancerous materials, are caused by uncontrolled cell division, where cells divide without being able tobe stopped. These cells amass in the body as large lumps called tumors, and can cause harmful blockages throughout the body. Ususally, when too many cells are replicated, they are kept from becoming a problem by apoptosis, which is where a cell dies to prevent too many of itself being made. If there are too many, apoptosis will occur, and it is basically a cell self destructing to prevent harmful consequences. When this apoptosis does not occur, cancerous cell division can occur because it is not regulated.


2.5.3 --
Interphase is an active period in the life of a cell when many metabolic reactions occur, including protein synthesis, DNA replication and an increase in the number of mitochondria and/or chloroplast. In Interphase, the nuclear envelope is intact and none of the chromosomes are visible.

2.5.4 --
The events that take place in the phase of prophase is that the chromosomes condense and become visible. Bipolar spindles also develop. In late prophase, the nuclear envelope dissolves and chromosomes begin to migrate to an equatorial plane and are seen to contain two chromatids. In metaphase, chromosomes become fully condensed and are located at the metaphase plate, also known as the equator. In Anaphase, each centromere splits and the two chromatids of each chromosome are pulled to opposite poles. In Telophase, the chromosomes reach each of the poles and start to decondense, the nuclear membranes reform, and the cytoplasmstarts to divide.


2.5.5
During Mitosis, chromatin in the nucleus is uncoiled snd each chromosome replicates. This forms sister chromatids that separate into chromoses and move to opposite poles of the cell. So that a copy of each chromosome that was replicated at first is now at each end of the cell. After cytokynesis, the two cells generated are now genetically identical.

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2.5.6 Growth, embryonic development, tissue repair, and asexual reproduction all involve mitosis. As we grow, our body duplicates cells of our body by going through mitosis. Just as embryonic cells, like the one in the below video, is going through mitosis. Asexual Reproduction is nothing else but mitosis.



3.4.2- Since the nitrogenous bases of DNA can only pair with complementary bases, this allows for the old base sequence to be preserved and built again. The nitrogenous bases pair with their complements, for example- Adenine with Thymine and Cytosine with Guanine. The original base sequence will be preserved because each base only pairs with the its opposite, so when the DNA separates and replicates, it doesnt change at all.
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3.4.1 DNA replication starts with the protein helicase. Two helicase molecules start at one point, the "origin of replication" and move outward away from each other, "unzipping" the two strands of DNA as it goes. This forms a bubble of unzipped DNA that grows steadily larger. In eukaryotic cells, there are actually many of these bubbles being created in one DNA at once to make the process go faster. When these bubbles meet, they fuse together. The ends of the bubbles are called replication forks, and a protein called topoisomerase relieves the strain there that is caused by the helicase untwisting the strands.

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The separated DNA consists of a leading strand being replicated from 5' to 3', and a lagging strand being replicated from the 3' to 5'. On both strands, single stranded binding proteins attach to keep the strands from twisting back into a double helix. On the leading strand, RNA primase lays down one RNA primer and the DNA polymerase III lays down DNA continuously from there. The primer, which is 5-10 linked nucleotides of RNA, must be there because DNA pol III is not physically able to initiate the synthesis of new nucleotides. It can only continue it once it is started, by the RNA primer for instance.

The lagging strand therefore requires RNA primer too, but imany RNA primers must be created rather than just one. This is because DNA pol III must go in the 5'-3' still, so instead of just chilling in the replication fork it must run backwards to an RNA primer, synthesize (in that same backwards direction) until it hits the next primer, falls off, go forwards to the next RNA primer in the other direction, synthesize again until it hits that first RNA primer, and so on. The fragments that the DNA pol III is creating are called okazaki fragments after the Japanese scientist who discovered them. For each of these okazaki fragments, there must be 1 RNA primer laid out.


The direction of the new DNA synthesis (and some other stuff) are shown below.

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Once the okazaki fragments are laid down, DNA pol I comes along and replaces all of the RNA primer with DNA. This basically means that all of the u's are replaced with t's. However, DNa pol I cannot join the new DNA replacements to the adjacent end of the next okazaki fragments. DNA ligase, shown above as the pretty yellow dot, serves this function.

Another random note, the leading strand and lagging strands are actually different on each side of the bubble. For example, if the leadings strand is on the top at the left end, it will be on the bottom on the right end.

3.4.3-State that DNA replication is semi-conservative.
​The 3 models of DNA replication are shown below:


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Semi-conservative replication is now generally accepted to be the correct model. As I explained in concept 3.1.4, DNA replication involves the splitting of DNA into 2 strands, and each of those strands having their complementary based nucleotides synthesized by DNA pol III. Therefore, if the original DNA is yellow and the copied DNA is blue as shown above, each new DNA would have one yellow strand and one blue strand. In more scientific terms, the resulting 2 DNA after the process of DNA replication are each made up of one strand of the original DNA and one strand of the newly synthesized DNA.