DNA is made of two strands that are twisted around each other in a double helix.
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DNA in a double helix are antiparallel, meaning that they are oriented in opposite directions to each other.
One end is called the 5 prime, while the other is called the 3 prime end.
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Before you can replicate DNA you first need to unwind it and break the Hydrogen bonds between the nitrogenous bases.
To do this a special enzyme called Helicase untwist the double helix at locations called replication forks.
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As the two strands break apart a new enzyme DNA Polymerase III attaches new nucleotides to the original strands only in the 5’ to 3’ direction.
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DNA pol III cannot initiate the adding of nucleotides to the leading or lagging strand of DNA.
There first must be a primer on the strand to get it started.
A primer consist of RNA.
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An enzyme called primase adds on the primer to the DNA strands.
Only one primer is required for DNA pol III to begin synthesis of the leading strand.
For synthesis of the lagging strand, each Okazaki fragment must be primed separately.
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The leading strand
Along one template strand, DNA pol III can synthesis a complementary strand continuously only in the 5’ to 3’ direction.
The DNA strand made by this mechanism is called the leading strand.
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To elongate the other new strand of DNA in the 5’ to 3’ direction DNA pol III must work along the other template strand in the direction away from the replication fork.
The DNA strand synthesized in this direction is called the lagging strand.
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Once the DNA strands come apart DNA pol III attached to the lagging strand’s template and moves away from the replication fork, synthesizing a short segment of DNA.
These segments of the lagging strand are called Okazaki fragments.
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DNA ligase another enzyme, eventually joins all the Okazaki fragments together forming a single new DNA strand.
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Why do we get old and die?
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There is a small portion of the cell’s DNA that DNA polymerase cannot replicate or repair.
DNA pol III can only add on to the 3’ end only.
So on the lagging strand the very end of it cannot be replicated.
Because of this every time your DNA is replicated it gets shorter every time.
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Eukaryotic chromosomal DNA molecules have nucleotide sequences called telomers at their ends.
Telomers do not contain genes; instead, the DNA typically consist of multiple repetitions of on short nucleotide sequence.
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It has been proposed that shortening of telomeres is somehow connected to the aging process of certain tissues and even to the aging of the organism.