View source for Biochemistry of DNA Replication and Repair Mechanisms

== Introduction ==

[[DNA replication]] is a fundamental biological process that occurs in all living organisms to copy their DNA. This process is "semiconservative" in nature, meaning that each strand in the DNA double helix acts as a template for the synthesis of a new, complementary strand. DNA replication is the basis for biological inheritance in all life forms. 

[[Image:Detail-27526.jpg|thumb|center|A close-up view of DNA strands during the replication process|class=only_on_mobile]]
[[Image:Detail-27527.jpg|thumb|center|A close-up view of DNA strands during the replication process|class=only_on_desktop]]

== Biochemistry of DNA Replication ==

DNA replication is a complex process involving several biochemical steps. The process begins with the unwinding of the double helix to expose the DNA bases in each strand of the DNA molecule. This unwinding is done by an enzyme called [[DNA helicase]]. 

Once the DNA strands are separated, another enzyme, [[DNA polymerase]], begins the synthesis of new DNA strands. DNA polymerase adds new free nucleotides to the exposed bases of the original DNA strand. The sequence of these nucleotides determines the genetic code, as they form genes that encode for proteins. 

DNA polymerase can only add nucleotides in one direction, from 5' to 3'. This means that one new strand, the leading strand, is synthesized continuously, while the other, the lagging strand, is synthesized in fragments, known as Okazaki fragments. These fragments are later joined together by the enzyme [[DNA ligase]] to form a complete DNA strand. 

[[Image:Detail-27528.jpg|thumb|center|A detailed view of the DNA polymerase enzyme adding nucleotides to a DNA strand|class=only_on_mobile]]
[[Image:Detail-27529.jpg|thumb|center|A detailed view of the DNA polymerase enzyme adding nucleotides to a DNA strand|class=only_on_desktop]]

== DNA Replication Mechanisms ==

There are three main mechanisms of DNA replication: semiconservative, conservative, and dispersive. 

=== Semiconservative Replication ===

In semiconservative replication, the two strands of the original DNA molecule separate, and each serves as a template for the synthesis of a new, complementary strand. This results in two DNA molecules, each composed of one old and one new strand. This is the most common form of DNA replication and is the one that occurs in cells.

=== Conservative Replication ===

In conservative replication, the two strands of the original DNA molecule stay together, and a completely new DNA molecule is synthesized from two new strands. This form of replication is not common in nature.

=== Dispersive Replication ===

In dispersive replication, both strands of the original DNA molecule serve as templates, but the replication is not continuous along the entire length of the molecule. Instead, replication occurs in sections, resulting in DNA molecules with alternating segments of old and new DNA.

[[Image:Detail-27530.jpg|thumb|center|Three diagrams showing the different mechanisms of DNA replication: semiconservative, conservative, and dispersive|class=only_on_mobile]]
[[Image:Detail-27531.jpg|thumb|center|Three diagrams showing the different mechanisms of DNA replication: semiconservative, conservative, and dispersive|class=only_on_desktop]]

== DNA Repair Mechanisms ==

DNA repair is a collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its genome. These mechanisms include direct repair, base excision repair, nucleotide excision repair, and mismatch repair.

=== Direct Repair ===

Direct repair is a mechanism that identifies and repairs damaged or mismatched bases without breaking the DNA backbone. This is done by specialized enzymes that can recognize and correct specific types of damage.

=== Base Excision Repair ===

Base excision repair (BER) is a mechanism that identifies and repairs small, non-helix-distorting base lesions in the genome. This is done by removing the damaged base, cleaving the DNA backbone, and replacing the damaged nucleotide.

=== Nucleotide Excision Repair ===

Nucleotide excision repair (NER) is a mechanism that identifies and repairs bulky, helix-distorting lesions in the genome. This is done by removing a segment of the DNA strand containing the lesion and replacing it with new DNA.

=== Mismatch Repair ===

Mismatch repair (MMR) is a mechanism that identifies and repairs erroneous insertion, deletion, and mis-incorporation of bases that can arise during DNA replication and recombination, as well as repairing some forms of DNA damage.

[[Image:Detail-27532.jpg|thumb|center|Four diagrams showing the different mechanisms of DNA repair: direct repair, base excision repair, nucleotide excision repair, and mismatch repair|class=only_on_mobile]]
[[Image:Detail-27533.jpg|thumb|center|Four diagrams showing the different mechanisms of DNA repair: direct repair, base excision repair, nucleotide excision repair, and mismatch repair|class=only_on_desktop]]

== See Also ==

* [[Genetic recombination]]
* [[DNA damage (naturally occurring)]]
* [[Mutagenesis]]
* [[DNA damage theory of aging]]

[[Category:Biochemistry]]
[[Category:Genetics]]
[[Category:Molecular biology]]
[[Category:random]]