Nucleic Acid (Bio Molecules)

Nucleic Acid

·        These are longchain polymers with a high molecular mass.

·        Nucleic acids are the polymers of nucleotides which in turn consist of a base, a pentose sugar and phosphate moiety.

·        Nucleic acids are responsible for the transfer of characters from parents to offsprings.

·        There are two types of nucleic acids — DNA and RNA. DNA contains a five carbon sugar molecule called 2-deoxyribose whereas RNA contains ribose. Both DNA and RNA contain adenine, guanine and cytosine.

·        The fourth base is thymine in DNA and uracil in RNA.

·        The structure of DNA is a double strand whereas RNA is a single strand molecule.

·        DNA is the chemical basis of heredity and have the coded message for proteins to be synthesised in the cell.

·        There are three types of RNA — mRNA, rRNA and tRNA which actually carry out the protein synthesis in the cell.


Components of nucleic acids

 Complete hydrolysis of DNA (or RNA) yields a pentose sugar, phosphoric acid and nitrogen containing heterocyclic compounds (called bases). In DNA molecules, the sugar moiety is β-D-2-deoxyribose whereas in RNA molecule, it is β-D-ribose.




DNA contains four bases viz. adenine (A), guanine (G), cytosine (C) and thymine (T). RNA also contains four bases, the first three bases are same as in DNA but the fourth one is uracil (U).




Structure of Nucleic Acids

In the structure of nucleic acids, nucleosides and nucleotides play important roles. Here is a description of their structures:

1.     Nucleoside: A nucleoside is formed when a base is attached to the 1' position of a sugar molecule. The sugar molecule can be either deoxyribose (in DNA) or ribose (in RNA). The base can be adenine (A), thymine (T), cytosine (C), guanine (G), or uracil (U). Nucleosides are named based on the sugar and base present, such as adenosine, thymidine, cytidine, guanosine, and uridine. In nucleosides, the sugar carbons are labeled as 1', 2', 3', etc., to distinguish them from the base.

2.     Nucleotide: A nucleotide is formed when a nucleoside is linked to a phosphate group at the 5' position of the sugar molecule. The phosphate group is attached to the 5' carbon atom of the sugar, forming a phosphodiester linkage. Nucleotides are the building blocks of nucleic acids. They are named after the nucleoside they contain, followed by "monophosphate" to indicate the presence of a phosphate group. For example, adenosine monophosphate (AMP), guanosine monophosphate (GMP), and cytidine monophosphate (CMP) are examples of nucleotides.

3.     Dinucleotide: Dinucleotides are formed when two nucleotides are joined together by a phosphodiester linkage between the 5' carbon of one nucleotide and the 3' carbon of the other nucleotide. This linkage creates a chain-like structure, with the sugar-phosphate backbone and the attached bases. Dinucleotides play important roles in the structure and function of nucleic acids.

The sequential arrangement of nucleotides in DNA and RNA forms the primary structure of nucleic acids. This linear arrangement of nucleotides contains the genetic information necessary for various cellular processes. The specific order of nucleotides determines the genetic code, which directs the synthesis of proteins and other cellular functions.

Overall, nucleosides and nucleotides are essential components of nucleic acids, with nucleotides serving as the building blocks and carrying the genetic information required for life processes.


Nucleoside vs Nucleotide

nucleoside consists of a nitrogenous base covalently attached to a sugar (ribose or deoxyribose) but without the phosphate group. A nucleotide consists of a nitrogenous base, a sugar (ribose or deoxyribose) and one to three phosphate groups.

Nucleoside = Sugar + Base
Nucleotide = Sugar + Base + Phosphate


Fig.  Structure of (a) a nucleoside and (b) a nucleotide





Chemical Composition

Sugar + Base. A nucleoside consists of a nitrogenous base covalently attached to a sugar (ribose or deoxyribose) but without the phosphate group. When phosphate group of nucleotide is removed by hydrolysis, the structure remaining is nucleoside.

Sugar + Base + Phosphate. A nucleotide consists of a nitrogenous base, a sugar (ribose or deoxyribose) and one to three phosphate groups.

Relevance in medicine

Several nucleoside analogues are used as antiviral or anticancer agents.

Malfunctioning nucleotides are one of the main causes of all cancers known of today.


Examples of nucleosides include cytidine, uridine, adenosine, guanosine, thymidine and inosine.

Nucleotides follow the same names as nucleosides, but with the indication of phosphate groups. For example, 5'-uridine monophosphate.


Biological function of nucleic acid

a.  The DNA never leaves its place of origin but uses the RNA to act as an intermediate to communicate with the rest of the cell.

b.  This intermediate mRNA enters the nucleus of the cell during the synthesis of proteins, and bonds with one of the DNA strands.

c. The sequence of nitrogen bases in DNA is oppositeNucleic acids, mainly DNA and RNA, play an essential role in the bodies of living organisms. The functions performed by these are as follows:

d.   Nucleic acids help synthesize proteins in the body.

e.  RNA is an especially important factor in the manufacturing of proteins.

f. to that of RNA, as they are complementary. This process is called transcription.

g. For instance, if an RNA strand reads UUCCGGAA, then the DNA strand would read AAGGCCTT.

h.Furthermore, the messenger RNA helps to transmit the code from the nucleus of the cell to the ribosomes. These are then assembled to form proteins.

i. On reaching the ribosomes, the mRNA does not immediately set out to form proteins.

j.  In this step, the transferred RNA, also called the tRNA attaches itself to the mRNA, to translate the information carried by the mRNA into a readable form.

k. This is done with opposite base pairings and in sets of three, also called codons.

l. Thus each three-letter set can be a possible codon, encoding vital instructions and information.

m. This correspondence is also known as the genetic code. It is present uniformly throughout all living organisms.

n. The loss of nucleic acids, or DNA in cells, can be the cause for mutation and a variety of other diseases.

o. DNA is a vital part of the fingerprinting method employed by forensic experts. Often used in matters of paternal disputes as well as criminal cases, the study of DNA is among the most flourishing fields of research, including evolution, anthropology, natural history, and epidemiology.



Primary structure of DNA:
The sequence in which the four nitrogen bases are attached to the sugar phosphate backbone of a DNA strand is called the primary structure of DNA.

Secondary structure of DNA:
James Watson and Francis Crick determined the double helix nature of DNA.

DNA forms a secondary structure in the form of a double-stranded helix, like a twisted ladder. The two strands of DNA are held together by hydrogen bonds between specific pairs of bases. Adenosine always pairs with thymine. Cytosine always pairs with guanine. The strands are said to be complementary to each other. The complementary strands run in opposite directions. 2hydrogen bonds are formed when adenine pairs up with thymine. When a guanine base pairs with cytosine, 3 hydrogen bonds form between the bases.



It is usually a single strand of ribonucleotides and take up right handed helical conformation.


There are three important types of RNA in living systems:

· Messenger RNA carries genetic information from DNA to ribosomes   for the synthesis of proteins.

·  Transfer RNA brings amino acids to the messenger RNA.

·   Ribosomal RNA is the RNA component of the ribosome where proteins are synthesised in the cell.