What is semi-conservative replication?

Answer 1

It is highly important for DNA to be able to replicate itself so that chromosomes can be copied to give the exact same genetic code to every new cell that is made. It is the base pairing rules that allow DNA to replicate.

DNA replication takes part before cell division during mitosis and meiosis. It occurs in the Interphase stage of division. In this phase, the chromosomes become long, unravelled threads of chromatin making them visible under the microscope.

Replication happens in a series of different steps, each controlled by enzymes, with ATP supplying energy.

Two identical DNA molecules result from the replication process, with one original strand and one new strand. This process is called semi-conservative replication.

It is important to understand that semi-conservative replication is a complex, enzyme-controlled reaction. To understand just how complex DNA replication can be, it must be stated that deoxyribose is a five-carbon sugar. Chemists number the carbon atoms, 1 to 5. The phosphate group bonds C3 from one sugar to C5 of the net sugar, giving an antiparallel structure.

Because of this antiparellel structure, it influences DNA replication and becomes a complex process. The main steps in semi-conservative replication are outlined here:

1. The enzyme helicase unwinds the DNA helix and the base pairs are exposed.

2. The enzyme polymerase joins new bases, known as nucleotides, to the existing strand.

3. DNA polymerase, however, cannot begin adding bases directly to a new strand from scratch. A primer molecule containing an RNA of about 10 nucleotides is needed to start the replication. The RNA primer is removed later on when the process is finished.

4. The only way DNA polymerase can work is by bonding nucleotides directly down the entire length of the 3' - 5' strang. This is referred to as the leading strand in replication.

5. The other strand (running 5' - 3' ) must be copied in short segments of around 1000 bases, called Ohazaki fragments. This is known as the lagging strand in DNA replication.

6. The enzyme ligase bonds the Ohazaki fragments to form one continuous strand of DNA.

7. To ensure replication is as fast as possible, the DNA helix is opened at many sites along the entire molecule for simultaneous replication. Each opening becomes a replication fork allowing replication to proceed in both directions from the fork. This technique makes the DNA replication very fast and efficient.

Answer 2

Types of Semiconservation

1. DNA gyrase - makes the DNA spin, Break a DNA strand, Break sugar phosphate bond, spins strand one time

2. DNA polymerase - matches new nucleotides to the existing strand, connects s-phosphate of the next nucleotide, New DNA is made in ozaki fragments

3. DNA ligase - joins okazaki fragments or the next nucleotide