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What is DNA

DNA stands for Deoxyribonucleic Acid. It is the genetic or hereditary material or molecule or genome that is present in the form of chromosomes in the nucleus of the cells. However, some DNA is also found in the Mitochondria, a cell organelle present in the cell.

DNA contains or carries genetic code or instructions or information for growth, reproduction, development and traits or features or characteristics from organisms to their offspring. It is present in every cell of the body of an organism such as every cell of the human body.

Who discovered DNA?

Swiss biologist, Johannes Friedrich Miescher identified DNA for the first time in 1869 while he was doing research on white blood cells. For many years, scientists were not able to find out more about DNA and about its importance. Later, in 1953, James Watson, Francis Crick, were able to figure out the double helix structure of DNA that could carry genetic information.

Types of DNA

In the cells of eukaryotic organisms like plants, animals, apart from the nucleus, a small amount of DNA is also present in the cell organelle such as in mitochondria. So, based on the location, DNA can be of the following two types:

  • Nuclear DNA: As the name suggests, it is the DNA that is located in the nucleus of the eukaryotic cells. It is linear in shape and has open ends and there are 46 chromosomes (23 pairs) in the nucleus. So, this DNA is diploid, as it is inherited from two parents. Its mutation rate is less than 0.3%.
  • Mitochondrial DNA: As the name suggests, this type of DNA is located in the mitochondria. This DNA is a closed, circular in structure and it is haploid as it comes only from one parent (mother) or passed from mother to offspring. Its mutation rate is higher than nuclear DNA.

Forms of DNA I Structural Conformations of DNA

DNA is a long polymer of nucleotides. It can assume different types of structural conformations based on various factors such as salt concentration, hydration level, DNA sequence, presence of chemically modified bases, etc.

The DNA as revealed by the classic Watson - Crick Model is known as B-DNA or B-form DNA. Under certain conditions, DNA can attain various other forms or conformations, some of which are described below:

The common structural conformations of DNA are described below:

What is DNA

1) A-DNA

It is a rare type of DNA conformation that is adopted by DNA during dehydration conditions. Although it also has a double-stranded helical structure like B-DNA, it is shorter and has a compact structural organization. Structural features of A-DNA are given below:

  • A-DNA is formed or arises from B-DNA under dehydrating conditions.
  • The deoxyribose sugar is in the C3' endoconformation in A-form.
  • The base pairs are displaced away from the central axis and lie closer to the major groove. So, it has a ribbon-like helix with a wider cylindrical core.
  • It is wider and flatter than B-DNA.
  • It also has a right-handed helix with a helix diameter of 23 Angstrom (2.3 nm).
  • It is shorter than B-DNA due to the smaller vertical rise per base pair (2.56 Angstrom).
  • It has 11 base pairs per turn.
  • The adjacent base pairs are located at a distance of 2.9 Angstrom.

2) B-DNA

This form of DNA is the Watson-Crick double helix model of DNA that most people are familiar with. It is the most common DNA conformation or form in which DNA occurs under the natural physiological conditions such as salt concentration and pH in the cell. Some of the important structural features of B-DNA are given below:

  • DNA is mostly found in B-DNA conformation or form in a cell.
  • The deoxyribose sugar is in the C2' endoconformation in B-form.
  • It also has a right handed-helix.
  • Bases pairs are almost centred over the helical axis which means they are located at the core and the sugar phosphate backbone is located at the peripheral portion of the helix
  • All base pairs have same width
  • Around 10 base pair per turn
  • Helix is around 0 nm or 20 Angstroms in diameter.
  • The adjacent base pairs are located at a distance of 3.4 Angstrom.
  • The major groove of B-DNA is wide and deep, whereas, the minor groove is narrow and deep.
  • The glycosidic bond conformation in B-DNA is in anti-form.

3) Z-DNA

It is formed when the DNA has an alternating purine-pyrimidine sequence such as GCGCGC. Besides this, in this sequence the G and C nucleotides have different conformations that gives it a zig-zag pattern. The G nucleotide has sugar in the C3' endoconformation like A-DNA and unlike B-DNA, whereas the guanine base is in the synconformation due to this reason the Guanine is located over the sugar ring unlike normal anticonformation as found in A and B form of DNA.

Some of the major structural features of Z-form DNA are follows:

  • It has a left-handed double helical DNA conformation or shape.
  • The helix winds to the left in a zig-zag pattern or the sugar-phosphate backbone has zig-zag pattern.
  • Z-DNA was discovered by Andres Wang and Alexander Rich.
  • Its helical diameter is 18 Angstrom.
  • The distance between each base pair or helical rise per base pair is 4 Angstrom.
  • Each helical turn contains 12 base pairs.
  • The major groove is flat, whereas the minor groove is narrow and deep.
  • The glycosidic bond conformation is anti for pyrimidines and syn for purines.

Functions of DNA:

  • It stores the complete genetic information that is needed to form proteins and RNA of each organism. It provides the instructions or information for the synthesis of all cellular body proteins such as structural proteins and enzymes.
  • It specifies the identity of an organism.
  • It decides the activities and functions and behaviour of an organism such as gestation period, birth, maturity, aging and death.
  • It replicates to form daughter DNA and thus transfers one cope of genes to the daughter cell during cell division. Thus, genetic material is transferred from generation to generation.

Next TopicDNA Structure

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