Nucleic Acids

 

 

  Nucleic acids are extremely complex molecules produced by living cells and viruses, to pass on hereditary characteristics from one generation to the next, and to trigger the manufacture of
specific proteins. The name nucleic acids comes from their initial isolation from the nucleic of living cells. However, certain nucleic acids are found not in the cell nucleus but in cell cytoplasm.
Nucleic acid molecules are very large chains of repeating nucleotide units linked in many sequences. Thus, nucleic acids are high polymers with very high molecular weights. A nucleotide is a molecular unit or a nucleic acid

 • a phosphate group.
• a pentose sugar (ribose or deoxyribose).
• a nitrogen base (purine or pyrimidine).
A nucleoside is a compound consisting of 2 subunits – a pentose sugar and a nitrogen base. It is a precursor of a nucleotide. A summary of nucleic acid formation is:
• Pentose sugar + Nitrogen base = Nucleoside
• Nucleoside + Phosphate group = Nucleotide
• Nucleotide + Nucleotide + Nucleotide = Nucleic acid.

 TYPE OF NUCLEIC ACIDS

There are two types of nucleic acids – DNA (Dexyribonucleic acid) and RNA (Ribonucleic acid); both are chemical relatives that are universally present in all living cells and they form the chemical
basis of life. Both DNA and RNA contain the purines – Adenine (A) and Guanine (G) and the pyrimidine cytosine (C). The second kind of pyrimidine in DNA is Thymine (T) where as it is Uracil (U) in RNA. Therefore a unique pyrimidine distinguishes DNA from RNA.

 DNA

The DNA is a polymer made up of repeating units of mononucleotides carrying the genetic material of all cellular organisms and most viruses. DNA carries the information needed to direct protein synthesis ad replication. Protein synthesis is the production of the proteins needed by the cell or virus for its activities and development. Replication is the process of which DNA copies itself for each descendant cell or virus, passing on the information needed for protein synthesis. In most cellular organisms, DNA is organised on chromosomes located in the nucleus of the cell.

 DNA STRUCTURE

The DNA is a spiral ladder with the nucleotides forming the side pieces and the steps composed of a combination of purine and pyrimidine which join the deoxyribose sugars in the side
pieces to hold them together. The purines are of two types Adenine (A) and Guanine (G), the pyrimidines too are of two types Cytosine (C) and Thymine (T). In forming the steps of the ladder G
may join C, or C may join G, and T may join A, or A may join T, usually by hydrogen bond. With the pairings G–C, C–G, A–T or T– A occurring throughout the length of the DNA molecule. The
combination of the sugar molecule, phosphate group and a nitrogenous base completes the basic structure of a nucleotide. With purine linked to a pyrimidine precisely adenine (A) always
pairing with thymine (T) and guanine (G) pairing with cytosine (C). A mirror image of the nucleotide is added to produce a double nucleotide chain which will twist to produce the α – helix.

 RNA

Ribonucleic acid (RNA) - In cellular organisms is the molecule that directs the middle steps of protein production and the genetic material of certain viruses. In cellular organisms the
DNA, carries the information that determines protein structure. But DNA cannot act alone and relies upon RNA to transfer this crucial information (translate) during protein synthesis – production of the
proteins needed by the cell for its activities and development. In RNA viruses, the RNA directs two processes – protein synthesis (production of the virus’s protein coat) and replication (the process by which RNA copies itself).

 RNA STRUCTURE

The structure of RNA is similar to that of DNA and it is composed of a single string of ribonucleotides, each of which is composed of
• a pentose sugar (ribose sugar)
• a phosphate group
• a nitrogenous base (one of the two bases – adenine, guanine, uracil and cytosine)
These components are joined together in the same manner as in DNA molecule. But RNA differs chemically from DNA by being single stranded, having a D-ribose sugar instead of
Deoxyribose sugar and having uracil as nitrogenous base instead of thymine. These nitrogenous bases can occur in any sequence.

 TYPES OF RNA

There are three types of RNA classified based on their molecular size. The smallest type of RNA is called transfer – RNA (tRNA) which carries amino acids to the ribosomes for
incorporation into a protein. Each amino acid has different classes of tRNA that read the codes of mRNA, therefore involved in protein synthesis. The tRNA receives information from mRNA, through pairing of their bases and accordingly selects particular amino
acids and pass to the ribosome.

The second type of RNA is the ribosomal – RNA (rRNA), thisis larger than tRNA and composes the ribosomes in the cytoplasm, the specialised structures that are the sites of protein synthesis.

Transfer – RNA are the most abundant type of RNA and they coordinate the sequential coupling of tRNA molecules to the series of mRNA codons.

The largest type of RNA is the messenger – RNA (mRNA). Messenger – RNA is a strand of RNA that is complementary to the DNA sequence for a gene and carries the genetic blueprint copied
from the sequence of bases in a cell’s DNA. This blue print specifies the sequence of amino acids in a protein. All the types of RNA are formed as needed, using specific sections of the cell’s
DNA as template.