DNA (Deoxyribonucleic Acid) - Types, Structure, Functions


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Prelims: General Science

Mains: Science and Technology- Developments and their Applications and Effects in Everyday Life. 

Deoxyribonucleic Acid (DNA) is the molecule that carries the genetic information for an organism's development and function. DNA is made up of two linked strands that wind around each other to form a twisted ladder shape known as a double helix. The majority of DNA is found in the cell nucleus (referred to as nuclear DNA), but a small amount can also be found in the mitochondria. DNA is essential in all living things for inheritance, protein-coding, and providing instructions for life and its processes.

About DNA

In the late 1800s, Swiss biochemist Frederich Miescher discovered DNA. However, it took nearly a century for scientists to figure out the structure of the DNA molecule and realize its significance in biology.

  • James Watson, Francis Crick, Maurice Wilkins, and Rosalind Franklin demonstrated the significance of DNA in 1953. These scientists discovered the double helix structure of DNA by studying X-ray diffraction patterns and building models.
    • This structure allows DNA to carry biological information from generation to generation.

Structure of DNA

Structure of DNA
Structure of DNA

DNA is a two-stranded molecule that looks twisted, giving it the distinctive shape of a double helix. Each strand has an alternating backbone of sugar (deoxyribose) and phosphate groups.

  • Each sugar has one of four bases attached to it: adenine (A), cytosine (C), guanine (G), or thymine (T).
    • Chemical bonds between the bases connect the two strands: A=T and C=G.
    • Human DNA consists of about 3 billion bases, and more than 99 per cent of those bases are the same in all people.
  • The sequence of the bases along DNA’s backbone encodes biological information, such as the instructions for making a protein or RNA molecule.
  • Each strand is a long sequence of nucleotides. These are the individual DNA units, and they are composed of:
    • A phosphate molecule
    • A sugar molecule called deoxyribose, containing five carbons
    • A nitrogen-containing region

Location of DNA

Location of DNA

Most DNA is located in the cell nucleus. The nucleus is often referred to as the "control centre of a cell" because it regulates functions such as metabolism, growth, and reproduction. The genome refers to an organism's entire DNA.

  • DNA is packed into chromosomes, which form genetic information to create genes.
  • Chromosomes are threadlike structures composed of protein and a single molecule of DNA that transport genomic information from cell to cell.
    • Humans have 23 pairs of chromosomes, for a total of 46 chromosomes.
Mitochondrial DNA
  • Apart from the nucleus, the mitochondria contain a small amount of DNA known as mtDNA. The mitochondria are often referred to as the "powerhouse of the cell" because they are responsible for releasing energy from food.

Types of DNA

Three major forms of DNA are double-stranded and connected by interactions between complementary base pairs. These are A-DNA, B-DNA, and Z-DNA. 

Types of DNA
  • A-DNA is a right-handed double helix composed of deoxyribonucleotides. The two strands of A-DNA are anti-parallel and not symmetrical.
    • The backbone of A-DNA is made up of sugar phosphates that are linked together indefinitely by phosphodiester bonds.
  • B-DNA has a right-handed double helix structure and is the most common type of DNA.
    • B-DNA has two strands that run in opposite directions. The structure is asymmetric, with major and minor grooves alternately present.
    • Sugar phosphates serve as the backbone of B-DNA.
    • B-DNA is narrower than A-DNA.
  • Z-DNA has a left-handed double helix structure.
    • The backbone's zigzag appearance distinguishes it from other forms of DNA. 
    • It can be observed in bacteria, eukaryotes, and viruses.

Functions of DNA

DNA contains the instructions required for an organism to develop, survive, and reproduce. To perform these functions, DNA sequences must be converted into messages that can be used to make proteins, which are the complex molecules that do the majority of the work in our bodies.

Protein Synthesis

Protein synthesis is the process by which cells make proteins. DNA carries the codes for proteins. However, the actual protein differs significantly from the DNA codes. The basic steps are as follows:

Steps of Protein Formation

- The process of copying genetic information from one strand of the DNA into RNA is termed as transcription.

- This molecule serves as a messenger, carrying information to other parts of the cell.


- In this step, the cell organelles called ribosomes act as translators by translating the messenger's code into the proper protein format or a chain of amino acids that form the building blocks of the protein.

- Each amino acid is created by combining three bases on the RNA.

Modification and FoldingThe third step is to modify, fold, and structure the final protein before delivering it to the required parts of the body.
Coding for Proteins

- Messengers read DNA and break it open into single-stranded polynucleotide chains, which are then copied into RNA.

- RNA forms opposite bases from those found in DNA. For example, G on the DNA forms C on the RNA strand.

- The bases combine in groups of three to form specific amino acids. There are a total of 20 such amino acids. These are also known as protein building blocks.

DNA Replication

DNA Replication

The biological process of producing two identical replicas of DNA from a single original DNA molecule is known as DNA replication. DNA replication aids in a variety of functions, from reproduction to cell, tissue, and body system maintenance and growth.

  • In this process, the tightly wound DNA strands unwind and literally unzip, leaving several bases without their partners on the other strand and remaining along the backbone of the molecule.
  • The bases are very particular about which base they will attach to, with adenine only pairing with thymine and guanine only pairing with cytosine.
  • Unpaired bases attach to these free bases, forming a new strand that is complementary to the original sequence.
  • The end result is a strand that is an exact match to the original before being unzipped. As a result, two new pairs of strands and two coiled DNA are formed.
  • Each new DNA contains one strand from the mother pair and one from the new one. The cell cycle would stop and cells would die if DNA was not replicated.
  • DNA replication can also be done in vitro (artificially, outside of a cell).

Stores Genetic Information

DNA is the chemical basis of heredity and can be considered a repository of genetic information. Over millions of years, DNA has been solely responsible for maintaining the identity of various species of organisms.

  • DNA carries the genetic information that determines our traits, such as eye colour, hair type, and susceptibility to certain diseases, in humans.


Question 1: With reference to the recent developments in science, which one of the following statements is not correct? (UPSC Prelims 2019)

  1. Functional chromosomes can be created by joining segments of DNA taken from cells of different species.
  2. Pieces of artificial functional DNA can be created in laboratories.
  3. A piece of DNA taken out from an animal cell can be made to replicate outside a living cell in a laboratory.
  4. Cells taken out from plants and animals can be made to undergo cell division in laboratory petri dishes.

Answer: (a)


Who discovered the DNA?

In the late 1860s, Swiss scientist Friedrich Miescher isolated DNA from human pus cells and is credited with discovering DNA.

What are the different types of DNA?

There are many different types of DNA, each with its unique structure. B-DNA is the most common type, but other types found in the genome include A-DNA, H-DNA, and Z-DNA.

What is DNA replication?

DNA replication is a process that occurs in cells when DNA copies itself. During cell division, this helps to ensure that each new cell has its complete genome.

What are the main functions of DNA?

DNA contains the instructions needed for an organism to develop, survive, and reproduce. To carry out these functions, DNA sequences must be converted into messages that can be used to produce proteins, which are the complex molecules that do most of the work in our bodies.

What is the structure of DNA?

DNA is a two-stranded molecule that appears twisted, giving it a unique shape as the double helix. Each strand has a backbone made of alternating sugar (deoxyribose) and phosphate groups. Attached to each sugar is one of four bases: adenine (A), cytosine (C), guanine (G), or thymine (T).