How does DNA replication work during cell division?

  

    DNA (Deoxyribo Nucleic Acid) is a molecule made up of two strands present in a double helix shape. It is the functional base, for all living things. It is present inside the nucleus of every cell. Each strand of the DNA is made up of millions of nucleotides. Each nucleotide has a phosphate group, a five-carbon sugar, and a nitrogenous base. DNA consists of four nitrogenous bases namely Adenine (A), Thymine (T), Guanine (G), and Cytosine(C). The pairing of nitrogenous bases present in both the strands takes place in such a way, that always Adenine combines with Thymine and Guanine combines with Cytosine. As shown in the figure (looking from left to right) the nitrogenous base (for example Adenine) is always present inside the strand. These nitrogenous bases are always connected to the 1' (one prime Carbon) of the sugar. Here the carbon sugar is missing a hydroxyl group in the 2’ (two prime) carbon. Hence the overall molecule is called as Deoxyribo Nucleic acid. As moving, the 5’ carbon of the sugar is attached to the phosphate group and the other end of the phosphate group is attached to the 3’carbon end of another sugar. This arrangement shows, that one strand runs from 5’end to 3’end, called the lagging strand and the other strand runs from 3’ end to 5’ end, called the leading strand. This continuous combination of the sugar and phosphate group forms the backbone of each strand in the DNA.

Credit (Molecular Structure of DNA): By Madprime (talk · contribs) - Own workiThe source code of this SVG is valid. This vector image was created with Inkscape ., CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=1848174

 
      In order for various functions to occur in our body, cell division must occur. Through this, one cell becomes multiple cells. This process is carried out by mitosis and meiosis. Mitosis is the process of cell division that occurs on somatic cells (non-sex cells); whereas meiosis is used for the cell division of gamete cells (sex cells). In both the cell division process, there is a step, in which the DNA is replicated. This DNA replication process is carried out by dozens of enzymes, to correctly copy the old DNA without any errors. Let’s look at the detailed process of DNA replication.
  
   At first, an enzyme called helicase, separates the DNA to be replicated from its double helix shape to an individual strand. This creates a replication fork. During unwinding, strain occurs in the strand, as two strands are twisted around each other. So to avoid the strain on the DNA, an enzyme called topoisomerase cuts the DNA molecule, twists it, and again fixes it. This reduces the strain in the DNA molecule while separating. This enzyme will always be ahead of the replication fork. Once the DNA is separated, it looks like two individual separate strands.

Helicase and Topoisomerase split the DNA strands.

 So another two new separate individual strands must be combined with the old strands to form two DNA molecules. To add the strands, an enzyme called primase kick-starts the replication by adding RNA primers to the existing strand. These RNA primers are 5-10 nucleotides long. After the nucleotides have been added an enzyme called DNA polymerase III attaches to the RNA primer and adds DNA bases to the leading strand. The direction of adding bases will always be from 3’ to 5’.

Primase adds RNA Primer.

DNA Polymerase III adds DNA bases.

    But in the lagging strand, the nucleotides run from 5’ to 3’ end. So adding a new strand to the lagging strand must be different.  So DNA polymerase III adds DNA bases from 3’ to 5’ as small chunks called Okazaki fragments. Each fragment requires its own RNA primer. After the new strand has been completed, the enzyme DNA polymerase I replace all the RNA primer with DNA bases. The new strand formed in the lagging strand is not continuous, as it is broken upon at regular intervals. So at last, an enzyme called ligase seals up the broken fragments and makes a continuous whole new strand.

Primase adds RNA Primer.

DNA Polymerase III adds DNA bases.

DNA Polymerase I converts RNA bases to DNA bases and Ligase seals the Okazaki fragments. 

    Thus by this impressive operation of these enzymes, an exact copy of DNA is produced. Each DNA consists of one old strand and one new strand. This is how DNA replication occurs. Now the cell can be divided, having its own DNA molecule inside the nucleus.