DNA Replication |Definition, Steps, & Diagram

By | October 3, 2019

DNA Replication

DNA Replication Definition:

For the growth of an individual, cell division is a necessary part. When the act of cell division occurs, the DNA must be replicated. During cell division, the DNA successfully copied in the daughter cells. Many enzymes take place for this act. The DNA has to be inherited and copied in two daughter cells. This phenomenon required during meiosis for the process of producing gametes. The DNA replication must occur accurately during cell division because any mistake in this act can be passing onto the next developing generation. The cells require copying their DNA quickly and with lesser error in it. The increase in error can raise the risk of diseases such as cancer.

Introduction of DNA Replication

The Basic Idea: DNA replication is a process in which the DNA divides into two same copies during cell division. The DNA copied accurately in the daughter cells. In DNA replication, the genetic information is duplicated to produce two identical copies of the genome of an individual. The process of DNA replication occurs during the Synthesis Phase, or S phase of the cycle of a cell, before the process of mitosis or meiosis. During the process of DNA replication each of the two strands which makes the double helix work as a template from which new strands are copied.

DNA Replication Steps:

Replication of the DNA depends on the pairing of the bases between the two strands of the DNA. The two strands determined by the location of the chemical bonds in the DNA backbone. During the process of cell division, a cell can be replicated the ‘leading strand’ as a single unit, but it must be replicate the ‘lagging strand’ in small pieces.

There are three basic steps that take place during the process of DNA replication.

  • Initial:

The process of DNA replication starts from a location on the double helix called ‘oriC’ from which specific initiator proteins bind and the trigger unwinding. The enzymes named ‘helicases’ work to unwind the double helix by breaking the hydrogen bonds between complementary base pairs, and other proteins keep a single strand by rejoining. The proteins named as ‘topoisomerase’ surround the unzipping strand and relax the twisting.

  • Elongation:

The cell creates a short sequence of the RNA known as primers which provide the starting point of the elongation. With the primer, a new strand of DNA grows one base at a time. The existing strand of DNA is a template for the new strand. The ‘lagging strand’ works to unwind in small sections which DNA polymerase replicates in the leading direction. The small fragments occur in the result of the lagging strand. These fragments terminate in an RNA primer which removes subsequently so enzymes can stitch the fragments into elongating strand.

  • Termination:

After the process of elongation completed, two new double helices replaced by the original helix. During the process of termination, the last primer sequence removed from the end of the lagging strand. This part of the lagging strand is the ‘telomere section’ which contains a repeating non-coding sequence of bases. In the end, enzymes named as ‘nucleases’ proofread the new double helix structure and remove the mispaired bases. Then, DNA polymerase fills the gaps created by excised bases.

DNA Replication Enzymes

DNA Polymerase:

The DNA polymerase is an enzyme that catalyzes the linking of the 3’ hydroxyl group of the end nucleotide to the 5’ phosphate of the nucleotide to be added. The DNA polymerase is working for catalyzing the synthesis of polydeoxyribonucleotides from the mono-deoxyribonucleoside triphosphates (dNTPs), which performs the most fundamental function in DNA replication, repair, and some other cases.

DNA Replication Process

Starting DNA Replication:

In the process of DNA replication, DNA made a copy of itself during the cell division. In the first step of DNA replication, ‘unzip’ the double helix of DNA molecule. Secondly, the enzyme named as ‘helicase’ breaks the hydrogen bonds by holding the complementary bases of DNA together. In last, one of the strands is oriented in the 3’ to 5’ direction; this is the “leading strand”. While the other strand is oriented in the 5’ to 3’ direction; this is the “lagging strand”. As a result, two different strands replicated differently.

Primers and Primase:

A short nucleic acid sequence is a “primer” which provides a starting point for DNA synthesis. In living organisms, the primers are short strands of RNA.

The primers must have to synthesize by an enzyme named as “primase”. It is a type of RNA polymerase, the process of DNA replication occurs. The synthesis of the primer occurs for the enzymes that synthesis the DNA, these are known as DNA polymerases.

Leading and Lagging Strands:

The ‘leading strand’ is the parent strand of DNA that runs in 3’ to 5’ direction toward the fork, and it is able to be replicated by DNA polymerase continuously.

The other strand used in DNA replication is ‘lagging strand’ which is parent strand runs in 5’ to 3’ direction toward the fork, and it is able to be replicated by DNA polymerase discontinuously.

The difference between both strands is continuously and discontinuously replication.

The Maintenance and Cleanup Crew:

The cleanup crew has responsibilities of cleaning, stocking and supplying facility areas. It performs and documenting the inspection and maintenance activities.

DNA Replication in Eukaryotes:

DNA replication in eukaryotes is different than bacterial replication by primase consisting of DNA polymerase and two smaller proteins create RNA primer and initiator DNA, and two different DNA polymerases synthesize the lagging and leading strands. The mechanism of DNA replication in eukaryotes is similar to DNA replication in prokaryotic. Although, eukaryotes DNA replication needs some special consideration due to differences in DNA size, a unique linear DNA end structure known as ‘telomeres’.

Diagram of DNA Replication

DNA Replication

DNA Replication in Prokaryotes:

DNA replication in prokaryotes is formed when an enzyme named helicase separates the DNA strands at the origin of replication. The DNA becomes highly coiled ahead of the fork of replication. The ‘topoisomerase’ breaks DNA’s phosphate backbone ahead of the replication fork.

DNA Replication

Mode of DNA Replication:

After the discovery of the double helix structure of the DNA, one big question concerned DNA replication. The structure of DNA double helix gives a hint about how copying takes place. It seemed like the two complementary strands of the helix might separate during the replication, each works as a template in the construction of a new matching strand.

The Three Models for DNA Replication:

There were three models that had been purposed by the scientific community for DNA replication; for the structure of DNA. These three models are:

  • Semi-conservation Replication:

In the semi-conservation replication model, two strands of DNA unwind from each other. Each of strands serves as a template for the synthesis of a new complementary strand. The result is in two molecules of DNA with one original and one new strand.

  • Conservative Replication:

In conservative replication model, the result of DNA replication is one molecule that consists of both original DNA strands, and another molecule which consist of two new strands.

  • Dispersive Replication:

In the dispersive replication model, the result of DNA replication is two molecules of DNA which are a mixture of ‘hybrids’ of parental and daughter DNA. Each strand is a patchwork of original and new DNA.

DNA Replication

The Meselson-Stahl Experiment:

Meselson and Stahl purposed an experiment on DNA replication by using E. coli bacteria as a model system.

They start by growing E. coli in medium containing a heavy isotope of nitrogen, 15N. By growing 15N on medium, the bacteria took up nitrogen and synthesize new biological molecules, including the DNA. After growing many generations in the 15N medium, the nitrogenous bases of the DNA bacteria were labeled with heavy nitrogen 15N. The bacteria then switched to medium ha ‘light’ 14N isotope and allow growing for various generations. DNA has made up 14N because this had only nitrogen available for DNA synthesize.

Meselson and Stahl studied how E. coil cells divided, so they were able to collect small samples from each generation. Then, they measured the density of 15N and 14N DNA’s using “density gradient centrifugation”.

The result of this method is the separation of molecules such as DNA into bands by spinning them with high speeds, when another molecule is present such as cesium chloride, which forms density gradient from top to bottom of the spinning tube. The Density Gradient Centrifugation permit very small differences like between 15N and 14N labeled DNA to be detected.

DNA Replication

Where DNA Replication does occur?

DNA replication occurs in the nucleus of the eukaryotes and the cytoplasm of prokaryotes. No matter where DNA occurs, the basic process of DNA is the same in both eukaryotes and prokaryotes. The structure of DNA borrows itself easily to DNA replication. Each side of the double helix in DNA runs in an anti-parallel (opposite) direction.

Summary of DNA Replication in E. coli:

In E. coli, the process of DNA replication regulated accurately to ensure that the daughter cells inherit the copy of the genomic DNA. The process regulates the initiation and elongation had characterized. Completion for this process requires several proteins associated with repairing double-strand breaks, it occurs independently of homologous recombination and targeted by some bacterial viruses for inactivation, during the transition to lytic replication.