MOLECULAR STRUCTURE OF DNA AND RNA

INTRODUCTION

    Genetics is a branch of biology which elucidate about the heredity and the variation of

inherited characteristics. Heredity is the biological process which is responsible for passing

genetic information from one generation to another. Gene is the basic physical and functional

unit of heredity. Genes are made up of DNA. DNA is the hereditary material found in the

nucleus of eukaryotic cells (animal and plant) and the cytoplasm of prokaryotic cells (bacteria)

that determines the composition of the organism. DNA is found in the nucleus of every cell, and

it is exactly the same in each cell. There is another type of genetic material found in cells and

viruses known as ribonucleic acid (RNA).

MOLECULAR STRUCTURE OF DNA AND RNA

     In 1962, James Watson, Francis Crick, and Maurice Wilkins were awarded the Nobel

Prize in Medicine for their work in determining the structure of DNA. There are two types of

nucleic acids, deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). The building blocks of

nucleic acid are nucleotides, which are made up of three parts: a deoxyribose (5-carbon sugar), a

phosphate group, and a nitrogenous base. These units are covalently linked between the

phosphate groups of the fifth carbon of one nucleotide to the pentose sugar attached to the third

carbon of the second nucleotide. Series of these covalent linkages among nucleotide units form

the polymer nucleic acids. Nucleotides are attached together to form two long strands that spiral

to create a structure called a double helix. RNA molecules on the other hand are single stranded

and just have some regions where the strand twists. American biochemist, Phoebus Levene first

coined the term “nucleotide” in the 1900s, long before James Watson and Francis Crick

discovered the structure of the DNA.

The structure of nucleic acids can be envisioned to a ladder that is made up of alternating steps

that are symbolizing its three significant parts: pentose sugar, the phosphate group, and the

nitrogenous base. The two strands of DNA run in opposite directions. These strands are held

together by the hydrogen bond that is present between the two complementary bases. The strands

are helically twisted, where each strand forms a right-handed coil and ten 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.

RNA is a ribonucleic acid that helps in the synthesis of proteins in our body. This nucleic acid is

responsible for the production of new cells in the human body. It is usually obtained from the

DNA molecule. RNA resembles the same as that of DNA, the only difference being that it has a

single strand unlike the DNA which has two strands and it consists of an only single ribose sugar

molecule in it. Hence is the name Ribonucleic acid. RNA is also referred to as an enzyme as it

helps in the process of chemical reactions in the body.

FIG: STRUCTURE OF DNA AND RNA

PENTOSE SUGAR

 A pentose sugar is a five-carbon sugar that serves as the polymer backbone of DNA and

RNA. Each carbon atom of the sugar molecule are numbered as 1′, 2′, 3′, 4′, and 5′ (1′ is

read as “one prime”).

 A combination of a base and a sugar is called a nucleoside.

 Nucleotides are joined together by phosphodiester linkage between 5′ and 3′ carbon

atoms of pentose sugar.

 RNA has the ribose sugar while DNA has the deoxyribose. The two differ regarding the

functional group attached to the second carbon position. An -OH group can be found in

ribose while deoxyribose has only hydrogen instead -OH.

 The difference in the number of oxygen atoms in their structures serves as markers for

enzymes to easily distinguish them from each other.

 Also, this functional group difference makes the RNA relatively less stable than DNA.

Because of the -OH group, RNA is readily hydrolyzed at basic pH.

FIG: STRUCTURE OF PENTOSE SUGAR

PHOSPHATE GROUP

 Along with the pentose sugar, the phosphate group makes up the polymer backbone of

DNA and RNA. The group is attached to the fifth carbon in place of the hydroxyl group.

 The phosphate group determines the direction of the nucleic acids. The double-stranded

nature of the DNA can be attributed to the twisting of the polymer backbone. It is also

negatively charged and can easily bond with water molecules.

 Phosphate groups can be joined together to form phosphodiester bonds. Phosphate groups

can also be joined to other molecules, such as sugar.

 When phosphate is added to a nucleoside, the molecule is called a nucleotide.

 Nucleotides can have one, two, or three phosphate groups. Nucleotides with two or three

phosphate groups are good energy donors.

 Nucleotides are named according to the number of phosphates (mono-, di-, or tri-), the

type of sugar (deoxy- or not), and the type of base.

Nucleotides are named as follows:

 If the sugar is deoxyribose, the name of the nucleotide begins with "deoxy," as in

deoxyadenosine monophosphate (dAMP). If the sugar is ribose, this molecule would be

adenosine monophosphate (AMP).

 The name of the purine or pyrimidine base is modified to indicate that it is combined with

a sugar as follows: adenine becomes adenosine, cytosine becomes cytidine, guanine

becomes guanosine, uracil becomes uridine, and thymine becomes thymidine.

 The number of phosphate groups is indicated by the terms monophosphate, diphosphate,

or triphosphate.

FIG: PHOSPHATE GROUP

NITROGENOUS BASE

 Phoebus Levene discovered that the genetic material is made up of four smaller sub-units

distributed in equal quantities. These were later found to be the nitrogenous bases

 The nitrogenous bases, important components of nucleotides, are organic molecules and

are so named because they contain carbon and nitrogen.

 They are bases because they contain an amino group that has the potential of binding

extra hydrogen, and thus, decreases the hydrogen ion concentration in its environment,

making it more basic.

 Each nucleotide in DNA contains one of four possible nitrogenous bases: adenine (A),

guanine (G) cytosine (C), and thymine (T). RNA nucleotides also contain one of four

possible bases: adenine, guanine, cytosine, and uracil (U) rather than thymine.

 Adenine and guanine are classified as purines. The primary structure of a purine is two

carbon-nitrogen rings.

 Cytosine, thymine, and uracil are classified as pyrimidines which have a single carbonnitrogen

ring as their primary structure. Each of these basic carbon-nitrogen rings has

different functional groups attached to it.

 The purines on one strand of DNA form hydrogen bonds with the corresponding

pyrimidines on the opposite strand of DNA, and vice versa, to hold the two strands

together

 Adenine forms two hydrogen bonds with thymine in DNA and two hydrogen bonds with

uracil in RNA, while three hydrogen bonds are formed between guanine and cytosine.

 Within DNA molecules, this is their most important function and is known as base

pairing. Because hydrogen bonds are not as strong as covalent bonds, base pairings can

easily be separated, allowing for replication and transcription.

 The nitrogenous bases are simply known by their symbols A, T, G, C, and U. DNA

contains A, T, G, and C whereas RNA contains A, U, G, and C.

 The bases are part of the DNA and RNA that serve as the storage of information and

encodes for the phenotype, or the visible physical characteristic of an organism.

ADENINE

 Adenine belongs to the purines which are composed of a nitrogen group with six

members of nitrogen ring attached to a ring with five nitrogen units. In DNA,

adenine pairs with thymine whereas in RNA, it pairs with uracil.

 Chemical formula for Adenine: C5H5N5.

 When fused with ribose, adenine forms the nucleoside adenosine. On the other

hand, when fused with deoxyribose, it forms deoxyadenosine.

 This adenosine is an essential component of adenosine triphosphate ATP which is

a unit of energy and is utilized during metabolism.

FIG: STRUCTURE OF ADENINE

GUANINE

 Also a purine molecule, guanine is composed of a fused system of pyrimidine and

imidazole ring with double bond conjugates. This bicyclic molecule is planar due

to being unsaturated.

 Chemical formula for Guanine: C5H5N5O.

 As a nucleoside, guanine is called as guanosine.

 Guanine pairs with cytosine

FIG: STRUCTURE OF GUANINE

CYTOSINE

 It a pyrimidine base that pairs with guanine. When combined with ribose, cytosine

forms the nucleoside cytidine.

 This molecule can then be further phosphorylated to form phosphoric groups.

 Chemical formula for Cytosine: C4H5N3O.

Cytosine is very essential in cancer biology since its deamination alone is the

main cause in the formation of cancer like leukemia.

 Cytosine pair with Guanine

FIG: STRUCTURE OF CYTOSINE

THYMINE

 Derived from the hydrolysis of deoxyribonucleic acid via catalytic reduction,

thymine is a pyrimidine base that pairs with adenine in DNA.

 The bond is secured with a hydrogen bond that stabilizes the structure of the

nucleic acid.

 Chemical formula for Thymine: C5H6N2O2.

 In RNA, uracil replaces thymine at carbon five positions.

FIG: STRUCTURE OF THYMINE

URACIL

 Uracil is a pyrimidine that replaces thymine in RNA and therefore is the one that

pairs with adenine. Through methylation, uracil can be converted into thymine.

 Chemical formula for Uracil: C4H4N2O2.

 In the body, uracil is used to synthesize various enzymes which are necessary for

the correct functioning of cells.

FIG: STRUCTURE OF URACIL