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DNA Synthesis
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"Hi! I'm glad you guys found me. I've been busy copying the chromosomes, so you came at the right time. Let me show you what I do.
"When we make new chromosomes, its very, very important that the new chromosomes are exactly like the old ones. Otherwise, you might have your nose coming out of your ear!"Making a new chromosome is like making a new zipper by using the old zipper as a model. A zipper is a little simpler than a chromosome because a zipper only has one kind of tooth. But, the overall operation would be the same.
"What the DNA replication team does is very much like that. While a zipper has only one kind of tooth, DNA has four; A(Adenine), T(thymine), C(Cytosine) and G(Guanine).called as nitrogenous bases. Lucky for me, they know where they belong.
"The first thing the DNA replication team does is unwind (un-zip) a small section of the old chromosome and pull the two strands apart.
where an A is on one strand, a T is on the other? where a C is on one strand, a G is on the other? Notice also, that C and T are smaller than G and A. C and G fit together and A and T fit together. Because they fit together only in these pairs, the free nucleotides float in and line up in the proper order all by themselves! This helps us make exact copies of the DNA.
"When we are done, each old strand will have a new strand wound up with it. A will always be across from T! C will always be across from G!
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Structure
The structure of deoxyribonucleic acid (DNA) was discovered by James Watson and Francis Crick in 1953. DNA was determined to be a right handed double helix based on x-ray crystallographic data provided to Watson and Crick by Maurice Wilkins and Rosalind Franklin. DNA is composed of repeating subunits called nucleotides. Nucleotides are further composed of a phosphate group, a sugar, and a nitrogenous base. Four different bases are commonly found in DNA: adenine (A), guanine (G), cytosine (C), and thymine (T). In their common structural configurations, A and T form two hydrogen bonds while C and G form three hydrogen bonds. Because of the specificity of base pairing, the two strands of DNA are said to be complementary. This characteristic makes DNA unique and capable of transmitting genetic information. |
..3 cheers to watson&crick |
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Copying the right way
Before it was known how DNA was replicated there were two basic models for DNA replication :Conservative dispersive and semiconservative replications. In conservative replication, the two strands of DNA do not unwind as a brand new copy is produced and the old molecule remains intact.In dispersive replicationthe DNA strand would unwind to produce the new strand and both would have new and old strands interdispersed along each strand. In semiconservative replication, however, the DNA strands unwind and each strand serves as a template for the replication. So both of the new copies of DNA have half of the old DNA and half of newly synthesized DNA. It was later found in 1957 by Meselson and Stahl that DNA followed the semiconservative model.
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..the right one |
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A slightly Complex Process
Many enzymes are necessary to successfully replicate DNA. First topoisomerase is used to unwind the DNA strand. By cutting one strand at a portion called the origin of replication the tension in the supercoil is released. Next helicase is used to prevent the strand from recoiling. DNA polymerase III then travels across the strand, from the five carbon (5', or the fifth carbon on the phosphate group) to the third carbon (3', or the third carbon on the phosphate group). DNA polymerase III places the complementary nucleotide on the strand. So adenine would be put opposite of thymine (and the reverse), and guanine would be opposite of cytosine (and the reverse). DNA polymerase also needs a primer to begin attaching the nucleotides on the strand. Primase acts as the primer for DNA polymerase. Primase attaches a small RNA primer so that DNA polymerase can begin working. The RNA primer is removed by RNase H and DNA polymerase I fills the spot. Ligase is then used to reattach the strands with their new nucleotides.
Throughout the entire process single-strand binding proteins are used to keep the DNA stable so that DNA polymerase can work.
It is important to realize that DNA polymerase can only work in the 5' to 3' directions. The reason behind this is because 3' is more stable than 5' when attaching a new nucleotides. If DNA polymerase ran in the other direction then there is the risk that the phosphate group could break off.
So when DNA polymerase is working it only goes in the 5' to 3' direction. However there are two sides, and one runs in the 5' to 3' direction, but the other runs from 3' to 5'. So while DNA polymerase can work continuously in the 5' to 3' direction it has to work in spurts on the 3' to 5' direction, which are called Okazaki fragments. The 5' to 3' is called the leading strand while the other is called the lagging strand.
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...the process as such
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