Nucleic acid

Every living cell contains nucleoproteins, substances made up of proteins combined with biopolymers of another kind the nucleic acids.

Nucleic acids are the long chain bio-polymers with complex structure. They serve as transmitters of genetic information. There are two types of nucleic acids they are Deoxyribonucleic acid (DNA) and Ribonucleic acid (RNA). Nucleic acids are built up by monomeric units called nucleotides. They contain a polyphosphate ester chain.

Composition

Nucleic acids are made up of nucleotides. Nucleotides contain a sugar and a heterocyclic nitrogenous base along with phosphoric acid. They are held by 3' and 5' phosphate bridges. Nitrogenous bases are found in nucleotides which are of two types. They are - Purines and Pyrimidines.

Nucleic acids are of two types. They are DNA and RNA. In higher cells, DNA is localised mainly in the nucleus within the chromosome. A small amount of DNA is present in the cytoplasm also it is contained in the mitochondria and chloroplasts.

Structure of DNA

DNA molecule is a double helical structure made of deoxyribo nucleotides. RNA is also present in nucleus as well as cytoplasm. DNA is the major source of genetic information which is copied into RNA molecules (transcription). The sequence of nucleotides contains the 'code' for specific amino acid sequences. Proteins are then synthesized in a process involving translation of RNA. This double helical structure of DNA was proposed by Watson and Crick. Hence, it is known as Watson and Crick model of DNA.

E chargoff found that the base composition in DNA varied from one species to other species but in all cases the amount of adenine was equal to that of thymine (A=T)and the amounts of cytosine and guanine were also found to be equal (G=C) therefore the total amount of purines was equal to the total amounts of pyrimidines (A+G=C+T) However the AT/CG ratio varies between species for e.g., this ratio is 1.5 in man while in E coli it is 0.93.

Watson and Crick model of DNA

The DNA double helix structure is similar to a twisted ladder.

Salient features of Watson and Crick model

• It is a double helix with two right handed helical polydeoxy ribonucleotide strands twisted around the same central axis.

• The two strands are antiparallel. The phosphodiester linkages of one of these strands runs in 5' to 3' direction while the other strand runs in 3' to 5' direction. The bases are stacked inside the helix in planes perpendicular to the helical axis.

• These two strands are held together by hydrogen bonds. In addition to hydrogen bonds, other forces e.g., hydrophobic interactions between stacked bases are also responsible for stability and maintenance of double helix.

• Adenine always pairs with thymine while guanine always pairs with cytosine.

• A-T pair has 2 hydrogen bonds while G-C pair has 3 hydrogen bonds. Hence, G C is more stronger than A=T.

• The content of adenine is equal to the content of thymine and the content of guanine is equal to the content of cytosine. This is Chargaff's rule, which is proved by the complementary base pairing in DNA structure.

• The genetic information is present only on one strand known as template strand.

• The double helix structure contains major and minor grooves in which proteins interact with DNA.

The diameter of double helix is 2nm. The double helical structure repeats at intervals of 3.4 nm (one completer turn) which corresponds to 10 base pairs.

a) The B-conformation of DNA having the right handed helices is the most stable

b) On heating the two strands of DNA separate from each other (known as melting) and on cooling these again hybridize (annealing) The temperature at which the two strands separately completely is known as its melting temperature (T_m) which is specific for each specific sequence.

Watson and Crick model of DNA

Different forms of DNA

Double helical structure exists in six different forms. They are A-DNA, B-DNA, C-DNA, D-DNA, E-DNA and Z-DNA. Among these only 3 forms of DNA are important. They are B-DNA, A-DNA and Z-DNA.

B-DNA

This is nothing but the double helical structure described by Watson and Crick. It has 10 base pairs in each turn.

A-DNA

This is also a right handed helix. It has 11 base pairs per turn.

Z-DNA

This is a left handed helix. It has 12 base pairs per turn. The strands in this form move in a 'zig-zag' manner and hence it is called as Z-DNA.

Structure of RNA

RNA molecule is a single stranded structure made of ribonueleotides. RNA contains the pyrimidine uracil instead of thymine in DNA. RNA is of 3 types, based on the cellular composition.

Biological functions

Nucleic acids are responsible for many biological functions. Nucleic acids control heredity on a molecular level. The important functions performed by nucleic acids are replication and protein synthesis. The DNA is the hereditary information of the organism which is coded in the form of a sequence of bases along the polynucleotide chain.

The DNA preserves this information and uses it by 3 processes called

a) replication

b) transcription

c) translation

These concepts constitute the central dogma of molecular biology and were summarized by Francis Crick in the following diagram.

Translation is unidirectional but transcription can sometimes be reversed i.e., RNA is copied into DNA. This reverse transcription occurs during life cycle of some retroviruses.

Thus we can infer that base sequence in DNA indirectly contents the sequence of amino acids in the protein. Since protein molecule can contain a maximum of 20 different types of amino acids, it is like a large sentence written in a language of 20 letters, but the hereditary message is written in a language of only 4 letters: it is written in a code with each word of 3 letters (triplet codon) standing for a particular amino acid.

Replication

This is a process in which a single DNA molecule produces two identical daughter molecules of itself. This replication is a semi conservative process. The parent DNA has two complementary strands, which unwinds by breaking hydrogen bonds. The separated strand acts as templates for the synthesis of new strands. As a result two identical copies of DNA are produced. These daughter molecules contain one parental strand and one new strand. Hence, this is known as semi conservative replication.

DNA replication follows the base paining rules by which A pairs with T and G pairs with C. Thus each daughter molecular is an exact replication of the parent molecule.

The replication of DNA occurs in 5' to 3' direction. The DNA synthesis differs in both strands that is lagging strand and leading strand. In leading strand the synthesis of DNA is continuous where as in lagging strand it is discontinuous. These discontinuous pieces are called as okazaki fragments. They are joined to form continuous strand by using DNA ligase.