DNA Replication

DNA Replication is a crucial step in the cell cycle- all DNA must be copied so that a full set of DNA can be passed to the daughter cell when the cell undergoes mitosis.  

DNA replication is semiconservative- each strand of the double helix acts as a template to form a new strand of DNA. New DNA is made by enzymes called DNA polymerases. The two new strands are called the leading and lagging strands- the leading strand is formed towards the direction of the split, and the lagging strand is formed in pieces, moving away from the direction of the split.

Other enzymes involved in replication of DNA:

DNA primase - primes the DNA for replication, getting it started by adding a primer section for the DNA polymerase to attach to. Like adding water to an empty pump before suction can be drawn.

Topoisomerase -  this enzyme prevents the DNA double helix ahead of the replication fork from getting too tightly wound as the DNA is opened up. It does this by making temporary nicks in the helix to relieve tension and then seals them back up to prevent permanent damage.

DNA ligase- seals up any nicks that remain after the primers have been replaced.

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To sum the whole process up:

The double helix is unwound and split down the middle, with the two strands each acting as templates for the new strands of DNA. Topoisomerase works ahead of the replication fork to prevent the DNA from coiling too tightly.

DNA polymerase attaches free nucleotides to the split strands, matching base pairs as it travels along the template strand. Adenine gets attached to thymine, and guanine gets attached to cytosine. The leading strand is constructed in the direction of the split (towards the helicase) and the lagging strand is constructed away from the split (away from helicase) in small sections called Okazaki fragments. Ligase comes in and seals up the breaks between fragments.