Structure of DNA

Introduction

All living things are made up of molecules that carry genetic instructions. The chemical name for that molecule is DNA except virus, because  some viruses have RNA genetic material. DNA or Deoxyribonucleic acid can be described as the genetic material or central information storage system in humans and other living organisms.The molecular structure of DNA consists of two strands that wrap around one another and form a shape known as the double helix. Each of these strands is made from alternating phosphate and sugar groups. 

Discovery of DNA 

• DNA was identified in 1869 by Fredrich Mischer. However, due to technical limitations, the structure of DNA was unknown for decades. The double helix model for the structure of DNA was propounded in 1953 by James Watson and Francis Crick

• James Watson and Francis Crick used the X-ray diffraction data produced by Maurice Watkins and Rosalind Franklin to propose their model. One of the key elements of their models was the proposition the two strands of the polynucleotide chains were paired at the base

• The base pairing of the polynucleotide chains confers that the strands are complementary to one another. Therefore, the sequence of one strand can be predicted if the sequence of the basis of another strand is known

Structure of DNA 

• The molecular structure of DNA comprises three components- a pentose sugar (deoxyribose for DNA), a phosphate group, and a nitrogenous base

• Nitrogenous bases are of two types- Pyrimidines (uracil, thymine, cytosine) and Purines (guanine and adenine)

• The Double-helix structure of DNA is made from two polynucleotide chains. The backbone of these chains is made from sugar-phosphate, and the bases of these strands project inwards

• The hydrogen bond (H-bonds), forming base pairs (bp), pairs the bases of the two strands. Thymine from the opposite strand forms two hydrogen bonds with adenine and vice-versa

• Since cytosine with three H-bonds bonds with guanine, a pyrimidine always comes opposite a purine. Uniform distance between the two strands of helix is formed because of this

• The stability of a helical structure is conferred by the stacking of the plane of one base pair over the other in a double helix in addition to H-bonds

• A prokaryotic DNA is circular in shape and is found freely moving within the cytoplasm. Within the cytoplasm, the prokaryotic DNA, which is not bound by proteins, is present in the nucleoid. Prokaryotic DNA also contains extrachromosomal plasmids

• Eukaryotic DNA does not contain plasmids and is bound to histone proteins. The eukaryotic DNA is linear in shape and is also contained within a nucleus

Types of DNA 

There are three major types of DNA. These are B-Form DNA, A-Form DNA and Z-Form DNA.

1. B-Form DNA- In a B-Form DNA, there are two strands of DNA. Both of the strands are in a right-hand helix which is bound around the same axis. 

2. A-Form DNA- A-Form DNA occurs only in a dehydrated sample of DNA. The strands in an A-Form DNA are in a right-handed double helix. This type of DNA is shorter and fatter in comparison to B-Form DNA. 

3. Z-Form DNA- While Z-Form DNA is longer and thinner than B-Form DNA, however, it does have antiparallel strands as B-Form DNA. Z-Form DNA has a left-handed double helix structure. 

DNA Functions 

DNA contains and stores the instructions for making proteins. The DNA’s function is to store genetic information that assists an organism in its functioning, development and reproduction. 

DNA Replication 

During cell division, the process by which DNA makes a copy of itself is known as DNA replication. DNA replication is an important process because the two new daughter cells formed after a cell divides must contain the same genetic information as the parent cell.

Primary and Secondary Structure of DNA

Primary Structure of DNA

• The primary structure of DNA is defined by the order or sequence of nucleotides 

• Nucleotides are also known as the building blocks of nucleic acids. Nucleotides are made of a nitrogenous base and a sugar-phosphate backbone. The nitrogenous bases such as thymine, uracil, cytosine and adenine have different bonding properties. However, thymine and uracil have similar structures and properties, which allows them to fulfill similar roles in DNA and RNA

• The nitrogenous bases are strung together with a sugar-phosphate backbone. A single molecule is formed when each nucleotide’s sugar links with another nucleotide’s phosphate. DNA takes the shape of a double helix because the nucleotides are strung together at an angle at which the phosphate-sugar bond transforms a string into a helix

• The double-helical shape and the sequence of letters in a DNA strand are part of its primary structure

Secondary structure of DNA 

• The secondary structure of DNA describes the formation of a double helix consisting of two polynucleotide chains wound around each other

• A double helix consists of two grooves, a major and a minor groove. The functional groups exposed on the bases of grooves interact with proteins and drugs

• The two polynucleotide chains run parallel to one another, and the orientation of the helix is usually right-handed

• Watson and Crick’s double helix model provided the basis to explain the mechanism with which cells can divide into two functioning daughter cells, how proteins are built to perform certain functions, and how genetic data is passed to the newer generations 

• The four aspects in which the structural forms of DNA may be different in- 

1. The handedness (right or left) of the coil. 

2. The size difference between major and minor grooves. 

3. The number of base pairs or complementary bases per turn. 

4. And the length of the helix’s turn. 

• Unlike the primary structure of DNA, the secondary structure of RNA contains just a single polynucleotide

Conclusion 

James Watson and Francis Crick used the X-ray diffraction data produced by Maurice Watkins and Rosalind Franklin to propose their model. One of the key elements of their models was the proposition the two strands of the polynucleotide chains were paired at the base. The base pairing of the polynucleotide chains confers that the strands are complementary to one another. Therefore, the sequence of one strand can be predicted if the sequence of the basis of another strand is known.