DNA Model: Structure, Types and More

Do you share some characteristic features from your parents? Do all organisms have the same type of DNA? DNA is the genetic material of an organism, and a part of DNA called gene is the unit of heredity. The DNA Model is helically twisted in a right-handed fashion that carries the genetic instructions for the functioning, development, growth and reproduction of all known organisms and many viruses.

Because they are made up of smaller monomeric units called nucleotides, the two DNA strands are known as polynucleotides. Each nucleotide is made up of a sugar termed deoxyribose, a phosphate group, and one of four nitrogen-containing nucleobases (cytosine [C], guanine [G], adenine [A], or thymine [T]).

This article covers the structure of DNA, types of DNA, etc. To know more about the DNA Model, scroll down the article.

What is DNA?

DNA (deoxyribonucleic acid) is the largest macromolecule or biopolymer that is made up of small monomeric units called nucleotides joined by phosphodiester bonds. DNA carries the genetic instructions for the functioning, development, growth and reproduction of all known organisms and many viruses. Only a few viruses like retroviruses and viroids have RNA as genetic material.

Fig: DNA Model

Structure of Nucleotides

i. The basic unit of DNA is a nucleotide that has three components – a nitrogenous base, a pentose sugar (deoxyribose) and a phosphate group.
ii. Nitrogen Bases: There are two types of nitrogenous bases as follows:
(a) Purines: Heterocyclic, \(9\)–membered double-ring structure with \({\rm{N}}\) at position \(1,3,7\) and \(9,\) e.g., Adenine \(\left( A \right)\) and Guanine \(\left( G \right).\)

Fig: Nucleotide Structure

(b) Pyrimidines: Heterocyclic, \(6\) – member single–ring structure with \({\rm{N}}\) at \(1\) and \(3\) positions, e.g., Cytosine \(\left( C \right),\) Thymine and Uracil. Cytosine is common in both and DNA and RNA ; thymine is present in DNA and uracil is present in RNA at the place of thymine.
iii. Pentose Sugar: The sugar has \(5\) carbon atoms, and thus it is a pentose. The nucleic acids are of two types depending on the pentose sugar. Those containing Beta-ribose are called ribonucleic acid RNA and those containing Beta-\(2’\)-deoxyribose (ribose with an oxygen atom removed from carbon \(2\)) are called deoxyribonucleic acids (DNA).
iv. Phosphoric Acid: This gives nucleic acids their acid character.

Fig: Structure of Ribose and Deoxyribose

vi. A polynucleotide chain shows the following types of linkage or bond in its components:
(a) \({\rm{N}}\) – glycosidic linkage: A nitrogenous base is linked to the pentose sugar through a \({\rm{N}}\) – glycosidic linkage to form a nucleoside. Purine nucleosides have \(1′ – 9\) glycosidic linkage (carbon \(1’\) of sugar \(9\) positions of \(A/G.\)) Pyrimidine nucleosides have \(1 – 1\) linkage, i.e., sugar carbon \(1’\) and \(1\) position of \(T/C.\)
(b) Phosphoester linkage: When a phosphate group is linked to \(5′ – {\rm{OH}}\) of a nucleoside through phosphoester linkage, a corresponding nucleotide is formed. Two nucleotides are linked through \(3′ – 5’\) phosphodiester linkage to form a dinucleotide.
vii. The process is repeated up to several million times to make a polynucleotide. An unbranched sugar-phosphate backbone is thus formed.

Fig: Structure of Purines and Pyrimidines

What is Chargaff’s Rule?

i. In \(1950,\) Erwin Chargaff formulated important generalizations about DNA structure; these generalizations are called Chargaff’s rules in his honour.
ii. It is applicable only for double-stranded DNA.
iii. The purines and pyrimidines are always in equal amounts, i.e., \(A + G = T + C.\)
iv. The amount of Adenine is always equal to that of thymine, and the amount of Guanine is always equal to that of Cytosine, i.e., \(A = T\) and \(G = C.\) However, the amount of \(A + T\) is not necessarily equal to \(G = C.\)
v. Ratio \(\frac{{A + G}}{{T + C}} = \frac{{{\rm{Purines}}}}{{{\rm{Pyrimidines}}}}\; = 1\)
vi. The base ratio \(\frac{{A + T}}{{G + C}} = \;{\rm{constant\;for\;species}}.\) This ratio can be used to identify the source of DNA and can help in classification. For humans, its value is \(1.52,\) while for E.coli, it is \(0.92.\)
vii. The deoxyribose sugar and phosphate components occur in equal proportions.

X-ray Crystallographic Structure

I. X-ray Crystallography: Maurice Wilkins and Rosalind Franklin obtained very fine X-ray diffraction pictures of DNA.
II. It was suggested that the structure of DNA was sort of helix with \(3.4\,\mathop {\rm{A}}\limits^{\rm{o}}\) periodicity. But they had not proposed a definitive model for DNA.

Fig: X-ray Crystallographic Structure of B-DNA

Structure and Features of DNA Model

i. In \(1953,\) James D Watson and Francis H C Crick proposed a double-helical model of DNA molecules. They were awarded the Nobel prize in \(1962\) along with Maurice Wilkins, who had produced a large body of crystallographic data supporting the model.
ii. Watson and Crick showed that the DNA consists of two polynucleotide chains which are built like a spiral case or twisted ladder-like.
iii. Each chain forms a right-handed helical spiral, and two chains coil around each other to form a double helix.
iv. The chains run in the opposite direction that is antiparallel, i.e., one strand runs from the \(5′ \to 3’\) direction and the other runs from the \(3′ \to 5’\) direction.
v. The backbone is constituted by sugar-phosphate, and the bases project inside.
vi. The two polynucleotide strands are held together by hydrogen bonds between purines and pyrimidines.
vii. The bases are \(H\)-bonded with each other based on Chargaff’s rule, i.e., Adenine binds to Thymine by \(2\) \(H\)-bonds, while Cytosine binds to Guanine by \(3\) \(H\)-bonds.
viii. The plane of one base pair stacks over the other in a double helix. This, in addition to \(H\)-bonds, confer stability to the helical structure.
ix. The chains are complementary to each other since for every Adenine in one chain, there will be thymine in the other same way, for every Guanine, there will be a cytosine in the other and so on.
x. One complete turn is called pitch or gyre.
xi. Each turn of the double helix or the pitch of the helix is \(3.4\,{\rm{nm}}\) \(\left( {34\,\mathop {\rm{A}}\limits^{\rm{o}} } \right).\)
xii. It has roughly \(10\) base pairs in each turn. The distance between two adjacent base pairs is approximately equal to \(0.34\,{\rm{nm}}\left( {3.4\,\,\mathop {\rm{A}}\limits^{\rm{o}} } \right).\)
xiii. The diameter of the double helix is \(2\,{\rm{nm}}\left( {20\,\,\mathop {\rm{A}}\limits^{\rm{o}} } \right).\)
xiv. Sugar and phosphate form the backbone of the helix, while bases are aligned towards the axis.
xv. The distance between two bases present in opposite strands is \(1.1\,{\rm{nm}}\left( {11\,\,\mathop {\rm{A}}\limits^{\rm{o}} } \right).\)
xvi. It has two external grooves, a deep, wide one called a major groove which is about \(2.2\,{\rm{nm}}\left( {22\,\,\mathop {\rm{A}}\limits^{\rm{o}} } \right).\) and a shallow, narrow one called a minor groove which is about \(1.2\,{\rm{nm}}\left( {12\,\,\mathop {\rm{A}}\limits^{\rm{o}} } \right).\)
xvii. These grooves are responsible for binding with proteins and enzymes to regulate functions of DNA and provide stability to the structure of DNA.

Fig: Structure of DNA Model

Properties of DNA

i. DNA helices can be right-handed or left-handed. But the \(B\) – conformation of DNA having the right-handed helices is the most stable and common type.
ii. Denaturation of DNA: If a DNA molecule is exposed to a high temperature or titration with an acid or an alkali, exposure to urea, amides and similar solutes, extremes of pH. etc., the two strands unwind and separate by the breakdown of the hydrogen bonds between the base pairs. This process is called denaturation or melting.
iii. The temperature at which the two different strands separate completely is called melting temperature, i.e., represented as Tm and the melting temperature is specific for each specific sequence.
iv. Renaturation of DNA: This is also known as annealing. When the temperature and pH return to optimum biological level, the unwound strand of DNA rewinds and gives back the double-stranded DNA.
v. The sample of DNA having a higher melting point must have more \(C-G\) content because the \(C-G\) pair has \(3\) hydrogen bonds in it.

What are the Types of DNA?

Most of the DNA is in the basic Watson-Crick model, simply called B-DNA or \(B\)-form DNA.

In certain conditions, different forms of DNAs are found and are like A-DNA, Z-DNA, C-DNA, D-DNA, E-DNA. This deviation in forms is based on their structural diversity.

A-DNA

i. A-DNA is a right-handed double helix made up of deoxyribonucleotides, and it appears only when the relative humidity of the environment is less than \(75\%.\)
ii. It has \(11\,{\rm{bp}}\) per turn.
iii. It has a major groove, which is narrower and deeper than the B-DNA form.
iv. Minor groove is broader and shallower.
v. A-DNA helix is \(23\,\mathop {\rm{A}}\limits^{\rm{o}}\) in diameter, and the pitch is \(2.86\,{\rm{nm}}{\rm{.}}\)

B-DNA

i. B-DNA contains right-handed helical coiling.
ii. It is a biologically important form of DNA that is commonly and naturally found.
iii. B-DNA has \(10\,{\rm{bp}}\) per turn.
iv. B-DNA has a wide major groove and a narrow minor groove.
v. The B-DNA helix is about \(20\,\mathop {\rm{A}}\limits^{\rm{o}}\) in diameter.
vi. Pitch is \(3.4\,{\rm{nm}}.\)

Z-DNA

i. Z-DNA is a left-handed type.
ii. It is called Z-DNA because of its zig-zag structure.
iii. Helix is \(18\,\mathop {\rm{A}}\limits^{\rm{o}}\) in diameter, and it has \(12\,{\rm{bp}}\) (\(6\) dimers) per turn.
iv. The major groove is flat, and the minor groove is deep.
v. Pitch is about \(4.5\,{\rm{nm}}\)

C-DNA

i. It is a right-handed form of DNA.
ii. The helix is \(18.9\,\mathop {\rm{A}}\limits^{\rm{o}}\) in diameter, and it has \(9\,{\rm{bp}}\) per turn.

D-DNA

This is a rare variant with \(8\) base pairs per helical turn, formed in a structure devoid of Guanine.

E-DNA

It is extended or eccentric DNA.

 Fig: Types of DNA

Functions of DNA

DNA has a very important role as genetic material in most living organisms. It helps in carrying genetic information from cell to cell and from generation to generation.
Thus, some of the functions of DNA are as follows:
i. It helps in the storage of genetic information.
ii. It controls all the metabolic reactions of cells through RNAs and RNA directed synthesis of proteins.
iii. DNA gives rise to RNAs through the process of transcription.
iv. DNA is packed into chromosomes with the help of histone proteins. This is essential for the equitable distribution of DNA during cell division.

Summary

Genetic material is composed of genes, which are units of heredity. It passes from one generation to the next generation. DNA is the largest macromolecule that is composed of small monomeric units called nucleotides joined by phosphodiester bonds. Each nucleotide is made up of a pentose sugar, a phosphate group and a nitrogenous base. Through this article, we understood the characteristic features of DNA and the structural importance of the DNA Model. We also got to know about Watson and Crick’s double-helical structure of DNA and Chargaff’s rule that says the purines and pyrimidines are always in equal amounts, i.e., \(A + G = T + C.\)

Frequently Asked Questions

We have provided some frequently asked questions about DNA Model here:

Q.1. Who has given the model of DNA?
Ans: In 1953, James D Watson and Francis H.C. Crick proposed a double-helical model of DNA molecules. 

Q.2. What are the 3 functions of DNA?
Ans: The three functions of DNA are as follows:
1. It helps in the storage of genetic information.
2. It controls all the metabolic reactions of cells through RNAs and RNA directed synthesis of proteins.
3. DNA gives rise to RNAs through the process of transcription.

Q.3. How does B-DNA differ from Z-DNA?
Ans: B-DNA contains right-handed helical coiling, and Z-DNA is left-handed with a zig-zag structure. B-DNA has a wide major groove and narrow minor groove, whereas Z-DNA has a flat major groove and minor groove is deep.

Q.4. What is DNA and what shape is it?
Ans: DNA (deoxyribonucleic acid) is a type of macromolecule known as nucleic acid. It is shaped like a twisted double helix and is composed of long strands of alternating sugars and phosphate groups, along with nitrogenous bases.

Q.5. What are the 3 types of DNA?
Ans: The three types of DNA are A-DNA, B-DNA, and Z-DNA.