DNA consists of nucleotides linked by phosphodiester bond, each composed of desossirobosio (carbohydrate pentose), a nitrogenous base and a phosphate group. Nucleotides are 4 and the only differences are in the nitrogenous base.
The nucleotides can be divided into pyrimidines: cytosine (C) and thymine (T), purines, adenine (A) and guanine (G). In DNA, the total number of purines equals the number of purines (
rule Chargaff
), since the adenine binds with thymine (AT) with two H bonds with guanine and cytosine (GC) with 3 ties H. This pairing of complementary bases.
The structure of the DNA double helix (two spirals cylindrical polynucleotide chains side by side). The direction of rotation is clockwise
and antiparallel strands run 
: in the same direction, but with the opposite direction. nitrogenous bases are addressed within the molecule that binds with H bond with nucleotide nitrogenous base of the opposite chain.
The direction of the polynucleotide chain is from 5 'to 3'. These numbers refer to the position of the deoxyribose C that links the phosphate group.
As DNA replication? DNA replication is semiconservative
, part of the original DNA (1 / 2) is retained in the replication.
replication takes place in two stages. 1) The DNA is carried out, 2) is added to the complementary nucleotide.
A protein complex called replication complex
is committed to synthesize new DNA. The DNA moves, the complex of replication is stationary. At first enzyme involved DNA helicase that separates the two chains of H bonds by forming
replication forks. The
DNA polymerase involved in adding nucleotides, but do not start from nothing, they need a primer of RNA (primer
), synthesized by the enzyme primase . Eventually Enzino DNA topoisomerase newly synthesized DNA that separates the two are intertwined.
The two DNA strands are developed in a different way: all the work is done in only one way in which a filament that early is oriented in the right direction and nucleotides are added without interruption. The other, called delayed , completes a piece at a time with these nucleotide segments
Okazaki fragments that begin with a primer. The DNA polymerase I removes the primer and replaces them with DNA itself, between an Okazaki fragment and the other holes are completed by DNA ligase
. The last part of the DNA obtained from lagging strand ends with a segment that Can not be combined, and then is cut, the end of the DNA is a sequence of nucleotides, such telomeres, which serve for this.
As the DNA is repaired?
errors during DNA replication and the presence of environmental coupling can cause errors in DNA.
There are three types of repairs: - the mechanism proofreader
acts during replication and DNA polymerase is the rejection of an incorrect nucleotide.
- the repairman disappaiamenti
mechanism acting after the completion of the newly synthesized DNA is not yet defined. - the excision repair mechanism consists of enzymes that constantly monitor the DNA to search for Disappear occurred to environmental factors.
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