Nucleic Acids

Bio-moleculesare compounds composed of

repeating units of their building blocks i.e. monomers.

There are four major classes of bio-molecules found in the cells and tissues, among them include; lipids, polysaccharide, proteins and nucleic acids.

Nucleic acidsare high molecular compounds

composed of mono-nucleotides linked to chains by 3`, 5` phosphodiester linkages.

Nucleic acids are of 2 types: 1. DNA (the building blocks of DNA are

deoxyribonucleotides).2. RNA (the building blocks of RNA are

ribonucleotides). 

Nucleotidesare compounds consisting of a

nucleoside combined with a phosphate group and forming the basic constituent (building blocks) of DNA and RNA.

In other word, nucleotides are made up of a nitrogenous base, a pentose sugar (a five carbon sugar) and a phosphate group.

The nucleotides are of 2 types:1. Ribonucleotide, which contain ribose.2. Deoxyribonucleotide, which contain 2`

deoxyribose.

Nucleosidesare compounds consisting of nitrogenous

bases (purine or pyrimidine) linked to pentose sugar usually ribose or deoxyribose by N-glycosidic linkages.

The nucleosides are N-glycosides in which the sugar C1 atom is linked to the N9 of purine, or to the N1 of pyrimidine.

There are 2 different types of nucleosides:1. deoxyribonucleosides containing 2-

deoxyribose and2. ribonucleosides containing ribose.

Pentose sugar Structure of nucleotides

N-basesnitrogenous bases of nucleic acids are

of 2 types: Purine bases. Pyrimidine bases.Major purine bases are Adinine (A),

and Guanine (G).Major pyrimidine bases are Cytosine

(C), Uracil (U) and Thymine (T).DNA contains: A, G, C and T. Whereas

RNA contains A, G, C and U

GENERAL CHARACTERIZATION OF NUCLEIC ACIDS

CONCENTRATION

Varies depending upon the functional state of the cells. Ex. spermatozoa: DNA 60%, muscle cells: DNA 0.2%. DNA is present in high conc. where there is high protein synthesis on dry matter basis.

LOCATION

DNA

Prokaryotes: DNA located in the special region of the cytoplasm, nucleoid or bound to the bacterial membrane. A small position of DNA is located in plasmids.

Eukaryotes: DNA is distributed between the nucleus 97-99% (where it forms part of chromosomes and nucleolus) and extra nuclear organelles (mitochondria).

RNA

• Distributed uniformly over the cell. Hence distribution indicates that function is more dynamic & extensive.

Molecular mass

• DNA, Varies with biological complexity at the cell.

• RNA less than DNA

Structure and Level of Organization of Nucleic Acid 

Nucleic acids posses' primary, secondary and tertiary levels of organization.

DNA because of it is high molecular mass posses all the 3 regular structures, whereas RNA does not exhibit regular 1, 2, 3 structures.

Primary structure

Of DNA & RNA is a linear polynucleotide chain made up of mononucleotides, linked together by 3`, 5`phosphodiester linkages.

The buildup of both DNA & RNA primary structures follows the same principle.

I.e. each pentose 3`-OH group of MNP is linked covalently to pentose 5`-OH group of the neighboring, MNP. Hence the name derived as 3`, 5` phosphdiester linkage.

Linear chains of DNA & RNA, hence have 2 ends called as 3` (3 prime) and the 5` (5 prime) end.

The 5` end always containing phosphate group (NTP) and 3` end, contain a hydroxyl group (OH group). Thus the chains are polar and directed either

5` 3` or 3` 5`.

The genetic text of DNA composed of code words called codons, or code triplets, which are linear sequences of 3 adjacent nucleotides.

The sites of DNA chain containing information on the primary structure of RNA(s) are refer as Structural Genes. The ordering of nucleotides in RNA is the same as in the DNA region i.e. being replicated.

THE PRIMARY STRUCTURE DETERMINES HIGHER LEVELS (SECONDARY & TERTIARY) OF THE NUCLEIC ACID ORGANIZATION.NOTE: IF STRUCTURAL GENE – STRUCTURAL PROTEIN.IF REGULAR GENE – REGULAR PROTEIN.

Secondary and Tertiary structures of DNA

Chargaff (1949) established a number of important relationships concerning the contents of individual bases in DNA these relationships help us in classifying the sec. structure of DNA.

Chargaff Rule 1. Total number of purine (A+G) is equal

to total number of pyrimidine (C+T). I.e. A + G / C + T = 12. A = T or A/T = 1.3.G = C or G/C =1.4. The numbers of (A + T) and (G + C) are

only variables, i.e. A + T ± G + C.■If (A + T) > (G + C) then DNA is referred as (AT- type).■If (G + C) > (A + T) then DNA is referred as (GC- type).

WATSON AND CRICK FINALLY PROPOSED DNA AS A DOUBLE HELIX STRUCTURE(SECONDARY STRUCTURE)

Characterization of DNA double helix

The DNA double helix is formed via a special paring of a base of one polypeptide chain with a base of the other polypeptide chain, A with T, and G with C.

The first pairing through 2-H bonds and the second pairing through 3-H bonding.

i.e. A = T , C = G.Such a much correspondence is called as

complimentary. Due to their complementary, the two chains run opposite to one another,

i.e. antiparallel.One in 5` 3`, the other in 3` 5` direction..

DNA secondary structure

The sugar-POA is directed outwards, where as base pairing protrudes into the interior of the helix.One full turn of helix contains 10 nucleotides.In native chromosomal DNA, the double helix deviates from the perfect pattern in the chain regions referred as Palindrome.In palindromes the ordering of nucleotides along the chain from right to left and from left to right is the same (repetitive, non informative segments).for example 5' A C C T A G G T 3' 3' T G G A T C C A 5'

Tertiary structure of DNA (super coiling) Twisted double helical structure in space is known as tertiary structure.