Chapter 12, Problem 18A

Summary Introduction

To explain:

Process of DNA replication with diagram.

Introduction:

DNA stands for Deoxyribonucleic Acid. DNA is a polymer of nucleotides. Nucleotides are molecules composed of a sugar deoxyribose, with a phosphate group and a nitrogenous base attached to it. These nucleotides are attached to each other in strands through phosphodiester bonds to form a ‘sugar-phosphate backbone’.

Explanation of Solution

DNA replication a process by which DNA makes copies of itself.

DNA replication occurs in three stages namely, Initiation, Elongation, and Termination.

Initiation: DNA synthesis is initiated at particular points within the DNA strand known as origins, which are particular specific coding regions. These origins are directed by initiator proteins that help in initiating replication process, forming a replication complex around the DNA origin. There are multiple origin sites, and when replication of DNA initiates, these sites are referred to as replication forks and appeared as Y shaped. The enzyme DNA Helicase unwinds the double helix DNA during replication process, and exposes each of the two strands of DNA as the template strands. DNA Helicase hydrolyze the ATP which is used to form the born between the bases, therefore it breaks the bondsresponsible for holding the two stands together.

A short piece of RNA known as RNA primer, bind to one of the strands of DNA called leading strand, and this acts as starting point for DNA synthesis.

Elongation: During the elongation process, DNA Polymerase binds to the leading strand of DNA and now it is able to start synthesizing the new DNA by pairing bases of template strands to their complementary bases. It is important to note that DNA polymerase is only able to extend the primer by adding free nucleotides to the 3’ end.

One of the template strands of DNA is read in a 3’ to 5’ direction, which means that the new strand will be formed in a 5’ to 3’ direction. This newly formed strand is referred to as the Leading Strand. Only one RNA primer is required to initiate DNA polymerase, during synthesis of leading strand as the process is occurring in a continuous manner. This is because DNA polymerase is able to extend the new DNA strand by reading the template 3' to 5', synthesizing in a 5' to 3' direction.

The other template strand or lagging strand is antiparallel, and is read in a 5’ to 3’ direction. As the helix unwinds, new RNA primers are added to the newly exposed bases on the lagging strand and DNA synthesis occurs in fragments, known as Okazaki fragments. The DNA synthesis of this strand also occurs in the 5' to 3' direction but many RNA primers are required during the synthesis of lagging strand.

The DNA synthesis of this strand also occurs in the 5' to 3' direction but many RNA primers are required during the synthesis of lagging strand.

Termination: The process of expanding the new DNA strands continues until there is either no more DNA template left to replicate.

Once DNA synthesis has completed, it is important that the newly synthesized strands are bound and stabilized.  The Okazaki fragments are then joined by the enzyme DNA ligase.

The replication process has ended. As the newly synthesized DNA has one new strand and one old strand, the DNA replication is referred to as semiconservative process.