DNA Structure

Introduction

DNA is an extremely tiny structure that is closely packed inside every cell giving constant instructions on how to operate an individual’s body.

Deoxyribonucleic Acid (DNA) is the genetic molecule responsible for transmitting the characteristics of parents to children. Present inside the nucleus of the cell, DNA determines the structure, function and development of living beings.

Most DNA is found encapsulated inside the cell nucleus called nuclear DNA while a comparatively lesser amount of it is present in the mitochondria of the cell called mitochondrial DNA.

The genetic information is stored in the form of four nitrogenous bases – adenine (A), Guanine (G), Cytosine (C) and Thymine (T) in the form of codes. Refer to figure 1.1. These four are called base pairs in which Adenine (A) always pairs with Thymine (T) and Cytosine (C) always pairs with Guanine (G). The sequence in which these A-T and C-G pairs run along the DNA determines almost every characteristic of an individual – hair colour, eye colour, height, etc.

These base pairs (A-T and C-G) attach to sugar on one side and a phosphate molecule on the other. The bases, sugar molecules and phosphate molecules together form the nucleotide, a single DNA strand. The base pairs club together forming like the rungs of the ladder and they get twisted to form the double-helix structure and the phosphate molecules attach to the lateral side to form elongated bands of vertical sidearms on each side. Nucleotides combine together to form nucleic acid or DNA.

Location

DNA molecules are tightly wound as coils inside a chromosome which is present in the nucleus of a cell. These coils are wound around a histone, a protein molecule that acts as the support system for DNA to coil around. 99.9% of DNA is exactly the same in all humans while the tiny per cent accounts for the diversity.

Size and structure

The size of a DNA chain varies between 2.2 – 2.6 nm wide and a single nucleotide measures about 0.33 nm in length. (1 nanometer is 1*10^-9 m). If you look more closely, the four bases (A, T, C and G) are nitrogenous compounds which are connected to one another by hydrogen bonds.

Adenine and Thymine are combined together with two hydrogen bonds while Cytosine and Guanine are paired together with three hydrogen bonds.

Adenine and Guanine are known as purines – two carbon-nitrogen bonds are found in each of their two rings. Thymine and Cytosine are known as pyrimidines – it has only one ring and a carbon-nitrogen bond attaches to it. The purines always bond with pyrimidines (A-T and C-G).

Each strand of DNA combines with the other strand to form a double helix structure as the purines and pyrimidines are held together by hydrogen bonds that link them together.

Types of DNA

Based on the coiling pattern, structure, sequence and the number of strands, DNA is classified into three main types among others – A-DNA, B-DNA and Z-DNA.

Characteristics	A-DNA	B-DNA	Z-DNA
Helical Coiling	Right-handed	Right-handed	Left-handed
Groove description	Backbones that are far away give rise to major groove; minor groove occurs when they are closer	Major groove is deep and wide; narrow and deep minor groove	Major groove is narrow and deep; wide and shallow minor groove
Condition for formation	Low humidity and high salt concentration	High humidity with low salt & ion concentration	High salt and presence of cations
Base pairs per turn	per turn has 11 base pairs	per turn has 10 base pairs	per turn has 12 base pairs
Angle of twist per repeating unit	20 degree	36 degree	60 degree
Occurrence	Common and similar to B-DNA	Common	Comparatively less common
Length per turn	3.4 nm	3.4 nm; width – 2 nm	4.56 nm

DNA Replication

Our DNA constantly replicates itself and each cell in our body is loaded with every information required to construct a person from top to toe. During replication, this double helix structure separates to form single open strands of DNA.

Thus, it could allow an enzyme called DNA polymerase to access the strands individually.

As the DNA polymerase runs down the length of a single-stranded DNA, it identifies the sequence in which the bases are attached and is used as a template for further replication of the bases. When the DNA polymerase identifies a T base on a single strand it attaches an A to it. when it identifies a C base it attaches a G base to it. Thus, two different double helix structures of DNA are produced simultaneously – an old strand and a new strand. The old strand retains the information of the parental characteristics while the new strand gives rise to diversity or inclusion of individual characteristics of the organism.

Only with the help of an electron microscope, these structures can be seen which look translucent, even then. There are still lots of experiments going on about DNA which pave the way to better healthcare and curing complex diseases.

FAQs

Q1. What are nitrogen bases?

The genetic information is stored in four nitrogen bases – adenine (A), Thymine (T), Cytosine (C) and Guanine (G) in the form of codes.

Q2. What are purines and pyrimidines?

Adenine and Guanine are known as purines – two carbon-nitrogen bonds are found in each of their two rings. Thymine and Cytosine are known as pyrimidines – it has only one ring and a carbon-nitrogen bond attaches to it. The purines always bond with pyrimidines (A-T and C-G).

Q3. How two single DNA strands attach to one another?

Every single strand is attached to the other strand in the opposite direction. Every single DNA strand has a 5’ end and a 3’ end. While one end of the strand starts with 5’ (5 prime) and ends with 3’, it will attach to the other strand which starts with 3’ and ends with 5’. Every double strand of DNA forms in this way and this type of attachment to each other is, hence, antiparallel.

Q4. What are the major types of DNA?

Based on the coiling pattern, structure, sequence and the number of strands, DNA is classified into three main types among others – A-DNA, Z-DNA and B-DNA.

Q5. What is DNA replication?

As the DNA polymerase runs down the length of a single-stranded DNA, it identifies the sequence in which the bases are attached and is used as a template for further replication of the bases. When the DNA polymerase identifies a T base on a single strand it attaches an A to it. when it identifies a C base it attaches a G base to it. Thus, two different double helix structures of DNA are produced simultaneously – an old strand and a new strand.