Explain how DNA is replicated and why this is important in biology.

When cells divide by cytokinesis the nucleus also has to divide so that each new cell contains a nucleus with a complete copy of all of the genetic information. DNA replication therefore needs to ensure that all of the 3 billion base pairs in the human genome are copied correctly each time the nucleus divides, this is done through semi conservative replication.

DNA is made up of 4 bases, Adenine, Thymine, Guanine and Cytosine that make up the genetic code. These make up polynucleotide chains that run in antiparallel making up the DNA double helix. When DNA replication occurs the DNA double helix is unzipped by an enzyme called helicase, which exposes the bases on each strand. These 2 strands can then be used as templates, meaning the new DNA molecules will be made up of 1 of the template strands and 1 newly synthesised strand. 

The new strands are synthesised by the enzyme DNA polymerase, however this enzyme can only work in a 5' to 3' direction and so on the stand named the leading strand (which runs from 5' to 3') the DNA polymerase can create a full new DNA strand by incorporating nucleotides that are complemetary to the template strand, however, the other strand (the lag strand) runs 3' to 5'. This means that the DNA polmerase has to create short fragments of the newly synthesised strand as the DNA is continually unzipped by the helicase enzyme, these fragments are known as okazaki fragments that can be joined together using a third and final enzyme- DNA ligase. This therefore results in 2 new identical DNA helixes.