Prokaryotic DNA

Microbial Genetics:

Prokaryotic DNA:

DNA is a polymer of deoxyribonucleotide or simply deoxynucleotides. It is composed of monomeric units namely deoxyadenylate(DAMP), deoxyguanylate(DGMP), deoxycydylate(DCMP) and deoxythymidylate(DTMP).

A DNA is a macromolecule formed by several thousand units or monomers. These monomers are called Nucleotides: A nucleotide is formed of one molecule of the deoxyribose sugar, one molecule of phosphoric acid and one of the four nitrogenous bases. Watson and Crick have proposed the double helical structure of DNA molecule for which they were awarded the Nobel prize. DNA molecule consists the following constituents.

Nitrogenous bases: There are two types of nitrogen bases, they are purines and pyrimidines. The purine is a 9 member ring structure whereas the pyrimidine is the 6 member ring structure.Purine of DNA is Adenine(A) and Guanine(G) whereas the pyrimidine of DNA is thymine(T) and Cytosine(C).

Pentose sugar: The pentose sugar present in the DNA is deoxyribose sugar.

Phosphate group: Phosphate group in DNA is present in the form of phosphate acid.

Structure of DNA:

Primary structure of DNA: DNA is the polymer of a deoxyribonucleotide. Each nucleotide is composed of a phosphate group, nitrogen base, and deoxyribose sugar. Each nucleotide is added to the 3 hydroxyl group of the pre-existing chain by the phosphodiester bond. The primary structure of DNA consists of single strands DNA which is found in some viruses such as parvovirus.

The secondary structure of DNA: The double helical structure proposed by Watson and crick is the secondary structure of DNA. The illumination of DNA structure is considered as a milestone in the era of modern biology. The structure of DNA double helix is comparable to a twisted ladder. The salient feature of Watson and Crick model of DNA are described below;

The strands are made up of alternate bands of deoxyribose sugar and phosphate molecules. They are joined by the phosphodiester bond.Each deoxyribose sugar in the strands has one N-base horizontally attached to it at carbon. The four N-bases can occur in any possible sequences along the length of a strand. The N-base +deoxyribose sugar + phosphate together form one unit or deoxyribonucleotide in the strand.Such many deoxyribonucleotides are linked with each other in a linear fashion, therefore, the resulting strand is described as the polynucleotide strand and DNA molecule as polynucleotide molecule.

The two polynucleotide strands are held together by hydrogen bonds specific pairs of purines and pyrimidines. Purines of one polynucleotide chain pair with the pyrimidine of the other i.e Adenine with thymine and guanine with cytosine. A and T are held together by two hydrogen bonds and G and C are held together by three bonds.

The DNA strands are antiparallel i.e. one strands run in the 5’→3’ direction while the other in 3’→5’ direction.

The width (or diameter) of a double helix is 20 A.

Each turn of the helix is 34A with 10 pairs of nucleotides. Each pair placed at a distance of about 34 A. Each strand of DNA has a hydrophilic deoxyribose phosphate backbone on the outside of the molecule while the hydrophobic bases are stacked inside.

The two polynucleotides chains are not identical but complementary to each other due to base pairing.

The two strands are held together by hydrogen bonds formed by complementary base pairs. A and T are connected by two hydrogen bonds whereas G and C are connected by three hydrogen bonds.The content of adenine equals to that’s of thymine and guanine equals to that of cytosine.

The genetic information resides on one of the two strands known as template strand or sense strand.

Circular DNA molecule:Circular DNA molecules are formed found in almost all prokaryotes. Eg; bacteria. The molecule has two complementary strands which form a covalently closed circular DNA.This DNA is supercoiled and highly folded. This is because the diameter of a bacterial cell. (E Coli) is about 1-2 microns, while the total length of the circular DNA is about 1100 microns. In several groups of small bacteria and viruses, the circular DNA is single stranded. It becomes double stranded only during replication.

Functions:

• DNA is a genetic material, hence it carries all the hereditary information from one generation to another generation.
• DNA has unique properties of formation of carbon copies. This is essential for transfer of genetic information.
• DNA gives rise to RNA through transcription process.
• DNA play a boy role in protein synthesis.
• Any change in the sequence of nitrogen bases due to addition or deletion causes mutation.
• The tertiary structure of DNA: Tertiary structure of DNA deals with the arrangement of DNA in the chromosome. The cellular DNA is highly compact implying a high degree of structure organization. The bacteria E. coil has about 4600 kb of DNA in its chromosomes. When stretched the chromosomes will be than 1mm,about 500 times larger than E Coli call itself. To occupy such length DNA in the cell. DNA must be packed into a more tightly compacted form. The compacted form of DNA is called tertiary structure of DNA. The process called supercoiling helps the DNA to remain in the compact form. Supercoiling is a state in which double-stranded DNA molecules are further twisted.

Functions:

• Hereditary character
• Individuality
• Carry genetic information
• Evolution (emerge of new species)

Characters:

• Viscosity higher than water
• Denaturation (heat and other chemicals)
• Renaturation (cooling)
• Abosorbance→260nm

References

Arvind, Keshari K. and Kamal K Adhikari. A Textbook of Biology. Vidyarthi Pustak Bhander.

Michael J.Pleczar JR, Chan E.C.S. and Noel R. Krieg. Microbiology. Tata Mc GrawHill, 1993.

Powar. and Daginawala. General Microbiology.

Rangaswami and Bagyaraj D.J. Agricultural Microbiology.