Structure of DNA: Overview, DNA Types and Function

Deoxyribonucleic acid (DNA) is a molecule encoding the genetic instructions of life. DNA is a molecule composed of two polynucleotide chains that coil around each other to form a double helix carrying genetic instructions for development, functioning, growth and reproduction of all known organisms and many viruses. These instructions are found inside every cell and are passed down from parents to their offspring.

DNA was first identified in 1869 by the Swiss biologist, Johannes Friedrich Miescher while he was performing his research on white blood cells. Miescher used biochemical methods to isolate DNA — which he called Nuclein — from white blood cells and sperm, and determined that it was different from protein.

In 1952, chemist Rosalind Franklin, who was working in the lab of biophysicist Maurice Wilkins, used X-ray diffraction — a way of determining the structure of a molecule by the way X-rays bounce off it — to learn that DNA had a helical structure.In 1953, Wilkins showed the photo to biologists James Watson and Francis Crick.

Armed with the information that DNA was a double helix. Watson and Crick published a landmark 1953 paper in the journal Nature. In that paper, they proposed a model of DNA as we now know it: a double helical ladder with sugar-phosphate sides and rungs made up of A-T and G-C base pairs. They also suggested that, based on their proposed structure, DNA could be copied — and, therefore, passed on.

Proteins, lipids, complex carbohydrates, nucleic acids are the four major types of macromolecules that are essential for all known forms of life of which DNA is part.

Read Also: Bioinformatics

Keyterms: DNA, Nucleotides, Phosphate group, Sugar group, Nitrogen base, Adenine, Thymine, Guanine and Cytosine

Read Also: Class 12 DNA Packaging

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Structure of DNA

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DNA is made up of molecules called nucleotides. Each Nucleotide contains a Phosphate group, Sugar group and the four Nitrogen base. (Adenine (A), Thymine (T), Guanine (G) and Cytosine (C)).

Structure of DNA

Ladder like structure is known as the double- helix structure. double helix structure of a DNA molecule was discovered through experimental data by James Watson and Francis Crick. It was proved that DNA is responsible for storing the genetic information in living organisms.

The bases on one strand pair with the bases on another strand: Adenine pairs with Thymine and Guanine pairs with Cytosine.

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Chargaff’s Rule: A=T; C=G

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Erwin Chargaff, a biochemist, discovered that the number of nitrogenous bases in the DNA was present in equal quantities. The amount of A is equal to T, whereas the amount of C is equal to G.

In other words, the DNA of any cell from any organism should have a 1:1 ratio of purine and pyrimidine bases, this ratio plays an important role in structural formation of DNA.

The double-helix structure is formed when the nucleotides attach together to from the two longer strands that spiral to create the Double Helix with phosphate and sugar molecules at the sides, while the base pairs would be the Rungs.

Among the three components of DNA structure, sugar is the one which forms the backbone of the DNA molecule. All polynucleotides contain an alternating sugar-phosphate backbone. This backbone is formed when the 3' end (dark gray) of one nucleotide attaches to the 5' phosphate end (light gray) of an adjacent nucleotide by way of a phosphodiester bond. It is also called Deoxyribose. The nitrogenous bases form hydrogen bonds at the opposite strands making a ladder-like structure.

The two strands of DNA run parallel to each other and are held together by the hydrogen bond that is presented between the two complementary bases. These strands are helical and twisted, where each strand from a right handed coil while a single turn is made by the 10 nucleotides.

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.

DNA molecules are so long, that they can't fit into cells without the right packaging. To fit inside cells, DNA is coiled tightly to form structures called chromosomes. Each chromosome contains a single DNA molecule. Humans have 23 pairs of chromosomes, which are found inside each cell's nucleus.

Chargaff’s Rule

Read More: Molecular Basis of Inheritance

According to the U.S. National Library of Medicine (NLM), human DNA is made up of around 3 billion Base pairs of which 99% bases are same in all people.

There are 3 different types of DNA namely A, B and Z-DNA.

• A-DNA: It is a rare type of structural conformation that a DB=NA can adopt dehydrating conditions, it is a double stranded helical structure.
• B-DNA: It is most common predominate type of structural conformation, it occurs in B form under natural physiological conditions.
• Z-DNA: It is a left handed double helical conformation in which the diuble helix winds to the left in zig-zag pattern. It occurs under high salt concentration.

Read Also: Translation Protein Synthesis

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Replication

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Division is a very essential element of growth in organisms. During reproduction the Cells divide further into daughter cells, but when cells divide into daughter cells it is essential that each daughter cell have the same genetic information as their parent, for this the DNA must be replicated in its genome. Thus the double stranded structure of the DNA provides a way to this process of Replication. Here, the two strands separate and recreate a complementary DNA by an enzyme called DNA polymerase. They further find the correct base pairing into original strand and copy the antiparallel strands of double Helix. The old strand dictates the new strands base and thus a perfect copy of the DNA is formed.

Read Also: Translation