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  Chemistry of nucleic acids, Biology tutorial

Introduction:

The Chemistry of nucleic acids is essentially about what nucleic acids are, chemical groups which constitute them, linkages in them and other chemical characteristics comprising their nomenclature and their structures.

Definition of nucleic acids:

Nucleic acids are macromolecules which are found in cells and are liable for storage and transmission of genetic information. Chemically, nucleic acids are polymers of nucleotides attached together by phosphodiester linkages (bonds). Nucleic acids are separated into 2; Ribonucleic acid (RNA) that is single stranded containing Uracil (U), Adenine (A), Cytosine (C) and Guanine (G) ribonucleotides and deoxyribonucleic acid that is double stranded having Adenine, Thymine (T), Cytosine and Guanine deoxyribonucleotides.

Features of nucleic acids:

Linkage:

Components of nucleic acids are nucleotides attached by phosphodiester linkage to one another.

Component:

Based on the kind of sugar in nucleic acids, nucleic acids can either be deoxyribonucleic acids in which case they have dexyribose sugar or ribonucleic acids when sugar is a ribose one.

Denaturation:

Double stranded DNA or RNA can be denatured. DNA is extremely viscous at pH 7.0 and room temperature (250C). When such solution is subjected to extreme pH or temperature above 800C, its viscosity decreases sharply indicating that DNA has undergone physical change. Denaturation of doubles helical DNA is called melting of double helical DNA as is related with disruption of hydrogen bonds between paired bases in DNA. When temperature is utilized to denature DNA, temperature midpoint in transition is known as melting temperature (tm ) Renaturation of DNA is rapid one-step process that occurs when temperature and pH is returned to range in which DNA is stable.

UV Absorption:

Conjugated system of purines and pyrimidines resulted in marked absorption in ultraviolet (UV) region of light spectrum with Absorption maxima near 260nm. As proteins have much weaker absorption in this region, spectrum properties of nucleic acids have been helpful in locating and evaluating the substances in cells and tissues.

When the double stranded DNA is modified or denatured by change in temperature or pH, close interaction between stacked bases in nucleic acid is also affected as there is decrease in hydrogen bonding keeping both strands of DNA together. This procedure is related with marked increase in absorption near 260nm region, this event is known as hyperchromic effect. Denatured form of DNA can thus be detected by monitoring absorption of UV light. Deoxyribonucleic Acids (DNA), when isolated, may be circular or linear, that is having two ends. Though, DNA exists in the extremely condensed form within chromosome. While length of DNA in largest human chromosome may be 8cm, it is condensed in the mitotic chromosome whose length is only 5 nm. In prokaryotes (bacteria or blue-green algae) DNA is not related with considerable amount of protein whereas in eukaryotes, DNA is found in cytoplasmic organelles like chloroplasts, mitochondria and chromosomes of nucleic. Nuclear DNA of plant and animal cell is related with basic proteins, histones, that are noncovalently bound to DNA by ionic linkages. A DNA histone complex is known as chromatin.Ribonucleic acids(RNA) though are not related with these proteins (histones).

Size of Nucleic Acids:

DNA and RNA molecules are long and unbranched. RNA molecules though have capacity of forming secondary and tertiary structures. Their size can be defined in three kinds of unit (length, number of base pairs and mass) therefore 1nm (10-4cm) of DNA contains 3,000 bases. Length of DNA molecules having 300 to 300,000 base pairs can be estimated directly by electron microscope. DNA was initially thought to be no longer than (5x10-4cm) until it was found that it is very sensitive to hydrodynamic shear. When care it taken to avoid shear, significant longer DNA molecules can be isolated. Sizes of DNA can be resolved by using molecular-seiving effect of porous agarose gels and by sedimentation in the centrifugal field.

Kinds of nucleic acids:

There are three different kinds of DNA: The A form, B form and Z form. B form also called as B-DNA is the most stable. There are three major kinds of RNA, namely ( mRNA) messenger RNA, (tRNA) transfer RNA and rRNA (Ribosomal RNA) though more recently other RNA molecules do exist.

Messenger RNA (mRNA):

This comprise of approx 5-10% of total RNA in cell. Generally single stranded and their base sequence is generally complementary with that of DNA. mRNA is intimately involved in transcription and translation of information coded by DNA for protein synthesis.

Ribosomal RNA (rRNA):

These are found related with large number of proteins in the ordered complex. Ribosomal RNA has the helical structure resulting from folding back of single stranded polymer, but may exist in numerous conformation ribosomal RNA comprise about 74-80% of total RNA in a cell.

Transfer RNA (tRNA):

The tRNA includes approx 15% of total cellular DNA. They are comparatively small nucleic acids and range in length from 65-110 nucleotides the pairing of bases in DNA helix.

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