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DNA stands for Deoxyribonucleic Acid. It is a self replicating substance and is the most important part of chromosomes. DNA is the carrier of genetic information in all living organisms. This genetic information plays a vital role in the growth, functioning and reproduction of living organisms.

Structure of DNA
DNA contains four basic building blocks viz. Adenine(A), Cytosine(C), Guanine(G) and Thymine(T). It consists of two long polymers called as nucleotides that are arranged in a double helix structure. The double helix structure is stabilized with hydrogen bonds.

DNA structure | Image source: wikimedia.org

Did you know: DNA was first discovered by Friedrich Miescher in 1869. Its double helix structure was first discovered by Francis Crick and James Watson.

What is the purpose of DNA replication?

First let us understand what is the purpose of DNA replication. DNA replication is the process where in identical replicas are produced of the original DNA molecule. For a living organism to grow or reproduce, a cells must first replicate its DNA. DNA replication occurs naturally in all living organisms. The double helix structure of the DNA makes it easy for the cell to replicate its DNA. The replication process can be summarized as follows:

1. Each strand of the double helix runs in opposite direction. This twisted double helix structure is first unzipped and unbound. A replication fork is formed.
2. Each separated strand now acts as a template for replicating a new partner strand
3. Nucleotides of the DNA are matched to make the new partner strands into the new replicated double helix structures.
When does DNA replication occur?

The cell cycle consists of 4 phases that are listed here:

  • G0 phase (or resting phase)
  • G1 phase (or Gap 1)
  • S phase (or the Synthesis phase)
  • G2 phase (or Gap 2)
  • M phase (or the Mitosis phase) - the Mitosis phase consists of two processes viz. Karyokinesis and cytokinesis

Note: G1, S and G2 are collectively known as the interphase.

This cycle is also known as the cell division cycle and it represents the series of events that take place when a cell divides itself to produce two daughter cells. The important event of replication of DNA which is a must for a successful cell division takes place during the S phase or the synthesis phase.

The Gap 1 checkpoint makes sure that all factors are ready for the DNA replication to take place during the S phase. During the S phase the amount of DNA in the cell doubles though there is still only one cell. The cell continues to grow during Gap 2.

The cell division is completed during the Mitosis phase which is a highly complex phase. It in turn consists of four subphases namely prophase, metaphase, anaphase and telophase. This phase is followed by cytokinesis where the nuclear division takes place and two daughter cells are produced that are identical to each other and also identical to the original mother cell.

Why is DNA replication important?
It is important for a cell to divide itself so that the organism can grow and continue life. DNA serves as the blueprint that must be passed on in its exact form to the daughter cells so that they can perform their function.

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