DNA Structure

Nucleic acids are the organic materials present in all organisms in the form of DNA and RNA. These nucleic acids are formed by the combination of nitrogenous bases, sugar molecules and the phosphate groups that are linked by different bonds in a series of sequences.

The DNA structure defines the basic genetic makeup of our body. Some virus may have RNA as their genetic material, but RNA has different functions in most of the organisms.

What is DNA?

DNA (abbreviated as Deoxyribonucleic acid) is a molecule that is responsible for carrying and transmitting the hereditary materials or the genetic instructions from parents to offsprings and for the production of proteins. This is also true for viruses as most of these entities have either RNA or DNA as their genetic material.

DNA – deoxyribonucleic acid, is an organic compound of complex molecular structure found in all prokaryotic and eukaryotic cells and in many viruses. 

DNA is found in all living organisms and is mainly responsible for the inheritance of genetic information in all living beings. The cells of the human body possess the same DNA which is located in the nucleus of cells. Hence, it is described as nuclear DNA.

The DNA present in the mitochondria of the cell is termed as mitochondrial DNA which is inherited from the mother to the child. In humans, there are approximately 16,000 base pairs of mitochondrial DNA.

Also Read: Difference between gene and DNA

Who Discovered DNA?

The DNA was first recognized and identified by the Swiss biologist, Johannes Friedrich Miescher in 1869 during his research on white blood cells.

The double helix structure of a DNA molecule was later discovered through the experimental data by James Watson and Francis Crick.

Finally, it was proved that it is responsible for storing the genetic information of a human being.

DNA Structure

Structure of DNA

DNA Diagram showing the double-helical structure of DNA

The DNA structure is a long, double helix that resembles a ladder which is twisted at both the ends as shown in the above figure. It is a nucleic acid and all nucleic acids are made up of nucleotides.

The DNA molecule is composed of basic materials called nucleotides and each nucleotide is composed of three different components such as sugar, phosphate groups, and nitrogen bases. 

The basic building blocks of DNA are nucleotides, which composed of a carbon-sugar group, a phosphate group, and a nitrogen base. The sugar and phosphate groups link the nucleotides together to form each strand of DNA. Adenine (A), Thymine (T), Guanine (G)  and Cytosine (C) are four types of nitrogen bases.

These 4 Nitrogenous bases pair together in the following way: AT and CG. These base pairs are essential for the DNA’s double helix structure, which resembles a twisted ladder.

The order of the nitrogenous bases determines the genetic code or the DNA’s instructions.

DNA Structure

Components of DNA Structure

Among the 3 components of DNA structure, the sugar is the one which forms the backbone of the DNA molecule. It is also called as deoxyribose. The nitrogenous bases of the opposite strands form hydrogen bonds, forming a ladder-like structure.

Structure of DNA

DNA Structure Backbone

The DNA molecule consists of 4 nitrogen bases namely adenine (A), thymine (T), cytosine (C) and Guanine (G) which completely forms the structure of a nucleotide. The A and G from a pair of purines and the C and T from pyrimidines. Both purines and pyrimidines differ as they have the carbon rings either single or double.

The two strands of DNA run in opposite directions. The two strands are held together by the hydrogen bond that is present between the two. The strands are helically twisted, where each strand forms a right-handed coil and 10 nucleotides make up a single turn.

The pitch of each helix is 3.4 nm. Hence, the distance between two consecutive base pairs (i.e., hydrogen-bonded bases of the opposite strands) is 0.34 nm.

Structure of DNA

 

DNA’s are coiled up forming chromosomes and each chromosome has a single molecule of DNA in it. Overall, human beings have around twenty-three pairs of chromosomes in them. These chromosomes are found in the cells of the nucleus. DNA’s play an important role in the process of cell division.

Also Read: DNA Packaging

Chargaff’s Rule

Erwin Chargaff, a biochemist, discovered that the number of nitrogenous bases in the DNA was not present in equal quantities. The amount of A is equal to T, whereas the amount of C is equal to G.

A=T; C=G

DNA Replication

DNA replication takes place in the following steps:

Step 1: Initiation

The replication of DNA begins at a point known as the origin of replication. The two DNA strands are separated by the DNA helicase. This forms the replication fork.

Step 2: Elongation

DNA polymerase III reads the nucleotides on the template strand and makes a new strand by adding complementary nucleotides one after the other. For eg., if it reads an Adenine on the template strand, it will add a Thymine on the complementary strand.

While adding nucleotides to the lagging strand, gaps are formed between the strands. These gaps are known as Okazaki fragments. These gaps or nicks are sealed by ligase.

Step 3: Termination

The termination sequence present opposite to the origin of replication terminates the replication process. The Tus protein terminator sequence pauses the leading fork until the lagging strand arrives and induces termination.

Also Read: DNA Replication

Functions of DNA

DNA is the genetic material which car­ries all the hereditary information which are coded in the arrangement of its nitrogen bases. Genes are the small segments of DNA, consisting mostly of 250 – 2 million base pairs, depending on the gene. A gene code for a polypeptide molecule, where 3 nitrogenous bases stand for one amino acid.

Thus, the sequence of a gene can be used to make a polypeptide, which then forms a protein. As every organism contains many genes in their DNA, different types of proteins can be formed. Proteins are the main functional and structural molecules in most of the organisms. Apart from storing hereditary information, DNA is also involved in:

  • Replication process: Transferring the genetic information from one cell to its daughters and from one generation to the next
  • Equal distribution of DNA during the cell division
  • Mutations: The changes which occur during the DNA sequences
  • Transcription
  • Cellular Metabolism
  • DNA Fingerprinting
  • Gene Therapy.

Why is DNA called a Polynucleotide Molecule?

The DNA called a polynucleotide because the DNA molecule is composed of nucleotides -deoxyadenylate (A), deoxyguanylate (G), deoxycytidylate (C), and deoxythymidylate (thymidylate) (T) which are combined together to create long chains called a polynucleotide. As per the DNA structure, the DNA consists of two chains of the polynucleotide, each in the form of a spherical spiral.

Also Read: Genetic Material

Stay tuned with BYJU’S to learn more in detail about the DNA structure and functions. You can also download BYJU’S app for further reference on the discovery and replication of DNA.

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