Structure and Functions of DNA

Microbial genetics :

Nucleic acid :

The term nucleic acid refers DNA and RNA. Deoxyribonucleic acid and Ribonucleic acid are the macromolecules composed of monomers called nucleotide. Therefore, DNA and RNA are also called polynucleotides. A molecule of nucleotide is composed of three components i.e. pentose sugar, nitrogenous base, phosphate.

Pentose sugar :

Sugar present in the DNA and RNA are composed of five carbon compounds such called pentose sugar. Sugar found in DNA is deoxyribose and those found in RNA is called ribose. Ribose and deoxyribose are structurally similar, a difference is that -OH group found in C'₂ of ribose is replaced by -H in the deoxyribose.

Nitrogenous bases :

There are two types of nitrogenous bases found in DNA and RNA i.e. purine and pyrimidine derivatives. Adenine and Guanine are purine derivatives whereas Uracil, Thymine and Cytosine are Pyrimidine derivatives. Uracil is found only in RNA whereas Thymine is found only in DNA. Adenine, Guanine, and Cytosine are found both in DNA and RNA.

Phosphoric acid:

Each nucleotide molecule consists of a phosphate group. In the DNA strand, the phosphate group alternate with deoxyribose and the successive nucleotides of DNA are linked through phosphate group bridges in which the 5' hydroxyl group of nucleotide by a phosphodiester linkage.

Nucleoside :

Nucleosides are composed of pentose sugar and nitrogenous bases. Nucleosides composed of ribose sugar are called ribonucleosides and those composed of deoxyribose sugar are called deoxynucleosides.

Nucleotides :

Nucleotides are nucleoside phosphate composed of the pentose sugar, nitrogenous bases, and phosphate. A phosphate group is attached to the C'₅ of pentose sugar by phosphate bond. Depending upon the no. of the phosphate group attached, nucleotides are of three types i.e. nucleoside monophosphate, diphosphate, and triphosphate. Nucleotides containing deoxyribose sugar are deoxyribonucleotides and those containing ribose sugar are ribonucleotides.

Structure of DNA :

Primary structure of DNA :

DNA is a polymer of deoxyribonucleotides. One molecule of deoxyribonucleotide contains deoxyribose sugar, nitrogenous bases, and phosphate. Nitrogenous bases are attached to the C₁ of the deoxyribose sugar by a β-glycosidic bond. In a single strand of DNA, each nucleotide is linked with two other nucleotides, one through C'₅and another through C'₃ by a phosphodiester bond. This process is repeated many times to give long polynucleotide molecule. But the two nucleotides which are situated at the extreme ends are linked with only one nucleotide. Hence in these nucleotides either C'₃or C'₅is, free end at which C'₅of the nucleotides free is called 5' end of the DNA and another end at which C'₃ is free is called 3' end of the DNA. The arrangement of the nucleotides and their sequence in a single strand of DNA is called its primary structure where alternating sugar and phosphate form backbone of the DNA and nitrogenous bases are regarded as a side chain.

Secondary structure of DNA :

Given by Watson and Crick in 1953

Some important features of Watson and Crick model of DNA include following points :include following points :

1. Two strands of DNA are coiled around a common axis in the form of the right-handed double helix. The coiling occurs in such a way that it forms two types of the grooves; one is major and another is the minor groove.

2. Two strands of the DNA run in opposite direction; one in 5'-3' and another in 3'-5'. This nature of DNA is called antiparallel.

3. Alternating phosphate and deoxyribose sugar i.e. on the periphery while nitrogenous bases lie in the centre of the helix. The two strands of DNA are joined together by H- bonds. Adenine of one strand is bonded with Thymine of other strands by double H-bond. Cytosine is bonded with Guanine by triple H-bond. Due to this H-bonding two strands of the DNA aren't identical but are complimentary. The overall structure of double-stranded DNA resembles winding staircase with the sugar and phosphate forming rolling and H-bonded base pair forming steps staircase.

4. The space in the centre of the helix is 10.85Å where complimentary base pairs are fitted.

5. The diameter of the DNA helix is 20Å. The length of each helix is 3.4Å. Each helix has 10 base pairs. Thus the distance between two nearest base pair is 3.4Å.

6. Double stranded DNA has characteristics base composition which was first noted by Erwin Charagaff and is called Charagaff rule.

Tertiary structure of DNA :

Double stranded DNA coils upon itself to form very small compact DNA. In the eukaryotic organism, DNA is bonded with histone proteins to form a chromosome. Various bond including hydrophobic interaction is involved in supercoiling of the DNA. Due to this supercoiling very large DNA is confined within the nucleus or in the cells.

Denaturation of DNA :

The two antiparallel strands of the DNA are held together by H-bonding between complementary bases. In some extent, the hydrophobic interaction between the adjacent bases also occurs. This is called base-stacking (The interaction between adjacent nucleotides in a single strand of DNA is called base-stacking). If a DNA solution is heated≥ 90 °C, two strands of DNA are separated by breaking the hydrogen bonding. This process is called denaturation of the DNA. Denatured DNA can be detected by measuring the observations at 260 nm wavelength using a spectrophotometer.

Renaturation of the DNA :

If the melted DNA is cooled, separated strand begin to reassociate, this process is called renaturation of DNA. Stable double stranded DNA is only formed if the two strands collide exactly to their complementary bases along the entire length of DNA. These processes are very difficult for the large and complex DNA. Thus, renaturation gives information about the complexity of the DNA.

Functions of DNA :

1. It is the carrier of genetic information from one generation to other.

2. It controls all the biological activities of cells.

3. It synthesizes RNA.

4. It can replicate to form new DNA molecules.