Nucleic Acids/types of Nucleic Acids// DNA//RNA

 

                           NUCLEIC ACIDS

PART 1- DNA (DEOXYRIBONUCLEIC ACID)

The nucleic acid found in microorganism (e.g. Viruses, Bacteria), plants and animals carry the genetic informations.

A nucleic acid is the polynucleotide that is polymer of nucleotides.

Nucleic acid are mainly found in Nucleus and therefore called Nucleic acid.

Nucleic acids are responsible for the transmission of characters from parents to offspring.

F. Miescher, in 1868 first time isolated nucleic acid from WBCs (White Blood Corpuscles).

Nucleic acids are two types-

A.   DNA (Deoxyribonucleic Acid)

B.   RNA( Ribonucleic Acid)

A.  DNA (Deoxyribonucleic Acid)-

DNA is found in all plants, animals, prokaryotes and some viruses.

In eukaryotes, DNA is present inside the Nucleus but can also occur in  Mitochondria and Chloroplast whereas in prokaryotes it is dispersed in Cytoplasm.

DNA contains all information which is required by the cell for its working, in the form of genetic codes.

DNA is a polymer of nucleotide. The nucleotides of DNA are Deoxyribonuleotides.

Each deoxyribonucleotide consists of Pentose sugar (deoxyribose), a phosphate group and a nitrogenous base.

Four types of nitrogenous bases can occur in DNA. These are- Adenine (A), Guanine (G), Cytosine (C) and Thymine (T).

Adenine and Guanine are purine bases while Cytosine and Thymine are pyrimidine bases.

Structure of DNA (Watson And Crick Model)-

J.D.Watson and F.H.C. Crick in 1953 proposed a double helix model for DNA molecules.

According to this model, the DNA molecule consist of two strands, which run in opposite direction (that is one in 5’to 3’ direction, while other in 3’to 5’ direction). So these are Antiparallel.

Both strands are held together by weak hydrogen bonds which are formed between complementary nitrogenous bases (A with T and G with C) of the two chains.

Adenine combines with Thymine by two hydrogen bonds (A=T) and Guanine combines with Cytosine by three hydrogen bonds (G≡C).

Due to formation of hydrogen bonds between the two chains the entire structure becomes twisted so it called double helix.

Double helical structure of DNA is intimately related to its functions such as Replication, transcription.

The sequence of nucleotides in DNA is highly specific as it contains genetic information.

The length of one complete turn of double helix is 34Å (3.4nm).

The turning of double helix results in the formation of deep and wide groove called Major groove.

The distance between two strands forms a Minor groove.

One turn of double helix at every 34 Å distance includes 10 nucleotides.

The distance between two successive base pairs is 3.4 Å (0.34 nm).

The diameter of the helix is 2nm.

                                            Double helical structure of DNA

 

Different forms of DNA-

DNA has different forms of double helix which vary from in spacing between nucleotides and number of nucleotides per turn, rotation per base pair, vertical rise per base pair base pair and helical diameter.

These different forms of DNA are-

i.                    B-Form of DNA (B-DNA)-

The structure of B-form of DNA has been proposed by Watson and Crick.

B-DNA is present in every cell.

It has 10 base pair per turn and diameter is 20nm.

It is rightly handedly coiled (clockwise rotating).

ii.                 A-Form of DNA (A-DNA)-

It is dehydrated form of DNA, which occur in high salt concentration.

 A-DNA has 11 base pair per turn and its diameter is 2.3nm.

A-DNA is also right handedly coiled.

iii.              C- Form of DNA (C- DNA)-

C- DNA is also called complementary DNA.

As compared to A-DNA and B-DNA, in C-DNA the number of base pair turn is less.

iv.              D-Form of DNA ( D-DNA)-

The D- Form of DNA is found rarely as extreme variants.

D-DNA has 8 base pair per turn of helix.

v.                 Z-form of DNA (Z-DNA)-

The Z-DNA has left handed helix.

In Z-DNA polynucleotide chain has a zigzag path.

Z-DNA has 12 base pairs per turn.

Functions of DNA-

·       DNA carries genetic information, which is transferred from one generation to the next.

·       DNA is the blue print of all informations, which are necessary for the proper functioning of the cell and organism.

·       It is also responsible for the formation of RNA.

·       DNA is the only molecule that can replicates that is it can make its own copies by itself.