Which of the following build(s) new strands of DNA?

A) The origins of replication

B) DNA polymerases

C) Parental DNA

D) The leading strand

E) The lagging strand

Diagram showing process of DNA replication
Diagram showing the process of DNA replication

The correct answer is B) DNA polymerases.

The genetic code is present on our DNA molecules which occur along with histone proteins to make up the chromatin material of the cells. To ensure that enough genetic material is present for cell division to occur, the DNA has to be copied.

This process occurs in the S stage of the cell cycle before mitosis occurs. The first step involves an initiator protein which starts to unwind the DNA helix.

Helicase enzyme then starts to work and catalyzes the breaking of the hydrogen bonds between the complementary nitrogen bases.

The strands separate at what is known as the origin of replication, and replication proceeds at the replication fork. Primase then makes a sequence known as a primer which starts the process off.

DNA polymerase enzymes carry nucleotides in towards the DNA strand and help the strand to elongate and extend in the 3’ direction.

The leading strand is where the nucleotides are added continuously, while the lagging strand is where they are added in pieces at each of several primer molecules.

Eventually, these Ozarki fragments on the lagging strand are joined together by the enzyme ligase. At the end of the replication, hydrogen bonds form between complementary bases, and the structure twists into a helix.

Chromosomes

Chromosomes consist of several strands of DNA molecules. In eukaryotic cells, chromosomes consist of more than nucleic acid. In fact, there are special proteins called histones which attach at various points along the strands of DNA.

These histones are important in enabling the DNA to be tightly packed into the chromatin material. The individual chromosomes themselves do not become evident until the chromatin changes form by the modification of various chemical groups on the histones.

Having the correct chromosome number is essential and a fetus will often not survive if there is an abnormal number of chromosomes, or in some cases, the fetus may be viable by the child will be born with developmental problems.

This means that the amount of DNA in a cell has to be correct which will only occur if the process of replication works properly.

DNA

The DNA molecule is where our genetic code is stored. This is the information that has to be copied before a cell divides. This is to ensure that there will be enough genetic material for the cells that are formed at the end of cell division.

DNA is copied during the S stage of interphase of the cell cycle. During interphase, the genetic material is also checked for any signs of damage and it is either repaired or the cell is programmed to self-destruct (apoptosis).

The basic unit of DNA is a nucleotide that consists of a phosphate group bonded to a sugar called deoxyribose. This sugar is then bonded to a nitrogen base, which is either a purine or a pyrimidine.

Purines include adenine and guanine, and these have double rings. Pyrimidines include thymine and cytosine, which have only a single ring structure.

There are base-pairing rules in which thymine can only bond with adenine and cytosine with guanine. These complementary bases pair up and become attached by hydrogen bonds. These types of bonds are relatively weak which is important when DNA replication is needed.

A DNA molecule consists of two strands of polynucleotides that are attached by the hydrogen bonds between the nitrogenous bases. In addition, the entire structure also twists to form a helical shape.

The DNA replication process

The process starts with the helical structure unwinding, which is achieved by an initiator protein. Then the strands need to separate which is accomplished by the enzyme DNA helicase which starts off the process at a specific region on the DNA strand that is known as the replication origin.

Each strand becomes a template for the formation of a new strand of DNA. The copying of the genetic material also means that the chromatin material doubles since the DNA make up the chromosomes along with histone proteins (in eukaryotic cells).

The unwinding is made possible by this helicase enzyme actually breaking the hydrogen bonds that are present between corresponding nitrogen bases. In this way, the two strands of DNA are able to separate and the strands separate from the replication origin.

The replication process begins from the 5’ end of the DNA strand, and it then progresses towards the 3’ end of the molecule. It begins at the replication origins and results in what is known as a replication fork.

Primase and polymerases

Other enzymes, including primase and polymerase, are also important in the process of replication. The enzyme primase is responsible for assembling a primer. The primer is a series of nucleotides that are put together to form a short sequence.

The DNA polymerase enzymes are then involved in actually building the new DNA strand in each case. It does this by collecting and carrying free nucleotides in, to where the template strand is. Then it adds these nucleotides to the strand extending it in the direction of the 3’ end.

The polymerase enzymes add nucleotides continuously to one strand of DNA, which becomes known as the leading strand.

The enzymes add nucleotides in a non-continuous manner to the other strand, which is known as the lagging strand.  In the case of the lagging DNA strand, there are several primers present acting as initiation sites for the process of DNA replication.

These discontinuous sections on the lagging strand are known as Ozarki fragments. Gaps between the fragments are eventually filled by action of the enzyme ligase.

At the end of replication the DNA strands bond at complementary nitrogen bases following base-pairing rules. The entire structure then twists to form a double helix. At the end of the process two completely new double helices of DNA have been formed.

References

  1. C Rye, R Wise, V Jurukovski, J DeSaix, J Choi, Y Avissar (2017). Biology. Houston: USA, Rice University.
  2. Editors of Encyclopedia Britannica (2018). DNA. Retrieved from Encyclopedia Britannica.
  3. Editors of Encyclopedia Britannica (2018). Polymerase chain reaction. Retrieved from Encyclopedia Britannica.
  4. PH Raven, RF Evert, SE Eichhorn (1987). Biology of Plants. New York: USA, Worth Publishers.
  5. RL Dorit, WF Walker, RD Barnes (1991).  Zoology. Philadelphia: USA, Saunders College Publishing.

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