Editing DNA replication (section)

== DNA structure ==
[[File:DNA Structure+Key+Labelled.pn NoBB.png|thumb|right|upright=1.55|The structure of the DNA [[double helix]] (type [[B-DNA]]). The [[atom]]s in the structure are color-coded by [[Chemical element|element]], and the detailed structures of two [[base pair]]s are shown in the bottom right.]]
DNA is a double-stranded structure, with both strands coiled together to form the characteristic [[double helix]]. Each single strand of DNA is a chain of four types of [[nucleotide]]s. Nucleotides in DNA contain a [[deoxyribose]] sugar, a [[phosphate]], and a [[nucleobase]]. The four types of [[nucleotide]] correspond to the four [[nucleobase]]s [[adenine]], [[cytosine]], [[guanine]], and [[thymine]], commonly abbreviated as A, C, G, and T. Adenine and guanine are [[purine]]<ref>{{Cite journal |vauthors=Friedberg EC |date=January 2003 |title=DNA damage and repair |journal=Nature |volume=421 |issue=6921 |pages=436–440 |bibcode=2003Natur.421..436F |doi=10.1038/nature01408 |pmid=12540918 |doi-access=free}}</ref> bases, while cytosine and thymine are [[pyrimidine]]s. These nucleotides form [[phosphodiester bonds]], creating the phosphate-deoxyribose backbone of the DNA double helix with the nucleobases pointing inward (i.e., toward the opposing strand). Nucleobases are matched between strands through [[hydrogen bonding|hydrogen bonds]] to form [[base pair]]s. Adenine pairs with thymine (two hydrogen bonds), and guanine pairs with cytosine (three [[hydrogen bonds]]).<ref>{{Cite web |date=2023-03-13 |title=Base Pair: Definition, Rules In DNA And RNA |url=https://www.funbiology.com/base-pair-definition-rules-in-dna-and-rna/ |access-date=2023-08-04 |language=en-US |vauthors=Sabhadiya A}}</ref>

[[Directionality (molecular biology)|DNA strands have a directionality]], and the different ends of a single strand are called the "3′ (three-prime) end" and the "5′ (five-prime) end". By convention, if the base sequence of a single strand of DNA is given, the left end of the sequence is the 5′ end, while the right end of the sequence is the 3′ end. The strands of the double helix are anti-parallel, with one being 5′ to 3′, and the opposite strand 3′ to 5′. These terms refer to the carbon atom in deoxyribose to which the next phosphate in the chain attaches. Directionality has consequences in DNA synthesis, because DNA polymerase can synthesize DNA in only one direction by adding nucleotides to the 3′ end of a DNA strand.{{cn|date=November 2024}}

The pairing of complementary bases in DNA (through [[hydrogen bonding]]) means that the information contained within each strand is redundant.  Phosphodiester (intra-strand) bonds are stronger than hydrogen (inter-strand) bonds. The actual job of the phosphodiester bonds is where in DNA polymers connect the 5' carbon atom of one nucleotide to the 3' carbon atom of another nucleotide, while the hydrogen bonds stabilize DNA double helices across the helix axis but not in the direction of the axis.<ref>{{Cite web |title=DNA function & structure (with diagram) (article) |url=https://www.khanacademy.org/test-prep/mcat/biomolecules/dna/a/dna-structure-and-function |access-date=2020-12-10 |website=Khan Academy |language=en}}</ref> This makes it possible to separate the strands from one another. The nucleotides on a single strand can therefore be used to reconstruct nucleotides on a newly synthesized partner strand.<ref>{{Cite book |title=Molecular Biology of the Cell |vauthors=Alberts B, Johnson A, Lewis J, Raff M, Roberts K, Walter P |date=2002 |publisher=Garland Science |isbn=0-8153-3218-1 |edition=4th |pages=238–240}}</ref>