Dna Replication Is Possible Biology Essay

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These proteins work together to bind the individual strands in the DNA double stranded helix and aid the helicases in opening it up into single strands. They are particularly useful in stabilizing the unwounded single-stranded conformation.

It relieves the strain of unwinding by DNA helicase.

It helps the DNA Polymerase III from getting dissociated from the DNA parent strand.

It relieves the DNA from its super coiled nature.

It causes the length of a telomeric DNA to increase by adding repetitive nucleotide sequences to the ends of eukaryotic chromosomes

There are 3 stages of DNA replication

Initiation

Elongation

Termination

INITIATION

It is the first stage and starting point of DNA replication. It has the following aspects and enzymes involved in it.

REPLICATION ORIGINS

Initiation always starts with replication origin. In order to begin the DNA replication the double stranded DNA helix must be opened. The site where first replication process occurs is called as replication origins.

UNWINDING OF THE DOUBLE HELIX OF DNA

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DNA helicases are used to unwind the DNA into two single strands. The hydrogen bonds between the complementary nucleotide bases break down. As a result the coiled double helix structure changes to two single DNA strands.

REMOVAL OF TORSIONAL STRAIN

Topoisomerase is a protein complex that removes the torsional stain from the uncoiled DNA after it's unwinding.

STABALIZATION OF DNA SINGLE STRANDS

Single- strand DNA binding proteins along with helicase acts on DNA strands and keep them separated and stabilized as the newly unwound DNA strands might have ability to twist again forming hydrogen bonds with complementary nucleotide bases.

REPLICATION BUBBLE

After such enzymes action replication bubble is created. It forms at multiple sites along the length of DNA. It catalyzes the replication process.

REPLICATION FORK

The open structure of DNA is often referred as replication fork. It is asymmetrical as two strands run anti-parallel directions. It forms as a result of helicase action as it breaks the hydrogen bonds between complementary nucleotide base pairs.

SYNTHESIS OF PRIMER

RNA polymerase enzyme or Primase is responsible for the formation of Primer. Primer is a short nucleotide structure containing 10 to 12 base pairs.

Primer is significant as the DNA Polymerase Enzyme which is responsible for the elongation a new DNA strand can only add nucleotides to the complementary stand but is unable to synthesize.

The primer serves as a new DNA strand containing ribose instead of deoxyribose. After elongation the Primer is removed and is replaced with DNA nucleotides by DNA Polymerase I.

ELONGATION

It is the process in which the DNA strand is synthesized from the template DNA by the action of DNA polymerase enzymes.

SYNTHESIS OF GROWING STRANDS

It involves the addition of nucleotides one by one in a sequence as specified by the original template strand. DNA is always synthesized in the 5' to 3' direction this means that nucleotides are added only to the 3' end of the growing strand. The 5' - phosphate group of the new nucleotide binds to the 3' - OH group of the last nucleotide of the growing strand.

The elongation differs for 5'-3' and 3'-5' template strands of DNA.

DNA POLYMERASE III ACTION

Since Polymerase III can only synthesize new strands in 5' to 3' direction the two strands replicate under slightly different mechanisms. The polymerase III add nucleotides to the new DNA strand and elongates it in single direction that is from 5'-3' direction.

LEADING STRAND SYNTHESIS

The leading strands require fewer steps and therefore it synthesizes quickly. The daughter strand complementary to the template strand 5'-3' proceeds in 3'-5' direction and is known as leading strand. This daughter strand is known as leading strand as the DNA Polymerase III, identifies the template and add continuously the complementary nucleotides.

LAGGING STRAND SYNTHESIS

The daughter strand complementary to template strand 3'-5' is known as lagging strand. This strand grows in discontinuous manner away from the replication fork. The DNA Polymerase delta is unable to read the template strand therefore DNA polymerase epsilon reads the template.

STEPS IN THE FORMATION OF LAGGING STRAND

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Formation of a section RNA primer by the action of RNA Primase.

Replacing the RNA primer with DNA with the help of DNA Polymerase I as it reads the fragments of RNA nd removes the Primer

For the more formation of double helix the helix must continue to unwind.

OKAZAKI FRAGMENT

The lagging strand grows discontinuously in the form of series of short segments that become connected later. These fragments are called as Okazaki fragments. Each Okazaki fragment is created by DNA Polymerase III in 5' to 3' direction. The Okazaki fragment synthesis begins from the replication fork and moves away from it.

Okazaki fragments are about 100-200 nucleotides long in eukaryotes.

Length of Okazaki fragment in prokaryotes is about 1000-2000 nucleotides.

The DNA ligase attaches the Okazaki fragments in the lagging strand in this way the daughter strand completely synthesize.

TERMINATION

It is the last stage of DNA replication. It is the process in which the DNA replication ceases at Replication Fork .The two new DNA double helix structures are created. As it is Semi-conservative Replication the DNA double helix formed as a result of it consist of one parent DNA Strand and one daughter DNA strand. The daughter strands both leading and lagging twist around their respective DNA parental template strands and separates. Termination occurs as two replication forks meet each other at the opposite end of parental chromosomes.

Following steps occur during termination:

TELOMERES FORMATION

In the last section of lagging strand when the RNA primer is removed, it is not possible for the DNA Polymerase to seal the gap as there is no primer available. So the end of the parental strand where the last primer binds isn't replicated. Because these ends of linear chromosomal DNA consists of non-coding DNA that contains repeat sequences and are called telomeres. As a result, a part of the telomere is removed in every cycle of DNA Replication. 

REPAIRING

The DNA Replication is not completed before a mechanism of repair fixes possible errors caused during the replication. Enzymes like nucleases remove the wrong nucleotides and the DNA Polymerase fills the gaps.