Nucleic acids ( Biomolecules )

Nucleic acids

Nucleic acids are polynucleotides that are, long chain like molecules composed of a series of nearly identical building block called nucleotides. The nucleic acids were isolated by F - Meischer ( 1868 ) from the nuclei of pus cells.

Structures of Nucleotide.

It is composed of following three components;

1. Pentose sugar; Two types of nucleic acids i.e. RNA contains ribose sugar and DNA contains deoxyribose sugar.
2. Nitrogen base; They are divided into four parts,
   Adenine; abbreviated 'A', has a two ring structure, so that makes it a purine. When it's in DNA, it pairs up with thymine. When it's in RNA, it pairs up with uracil. It makes up part of the energy molecule ATP and electron carriers FAD and NAD that are used in cellular respiration.
   Thymine; abbreviated ' T', is a pyrimidine which has a one ring structure and only present in DNA where it pairs with adenine.
   Uracil; abbreviated'U ', found in RNA and its bonds with adenine.
   Guanine; abbreviated 'G ' found both in DNA and RNA and it bonds with cytosine. It is a purine.
   Cytosine; abbreviated ' C ', found both in DNA and RNAand bonds with guanine. It has one ring so it's a pyrimidine.
   The complementary bases are Adenine and Thymine, Cytosine and Guanine which are attached by two hydrogen bonds and three hydrogen bonds respectively.
3. Phosphoric acid; Contains phosphate group that combines two nucleotides by a phosphodiester bond.

Formation of nucleotides.

A nucleotide is formed by the union of a sugar, a base and a phosphoric acid that are joined together by phospho- diester bond to make a long chain of nucleotides.
Nucleotide = a sugar + a phosphate molecule
or
= a nucleoside + a phosphate molecule

Here, the nucleoside is formed by a combination of a sugar and a base.

Nucleoside = a sugar + a base ( no phosphate molecule )

Types of Nucleic Acids

Deoxyribonucleic Acid ( DNA )

DNA or deoxyribonucleic acid is a macromolecule made up of nucleotide which is the hereditary material in humans and almost all other organisms. It is found in the nucleus, mitochondria, and plastids. D.S. Watson and F.H.C. Crick (1953 ) have proposed the possible model for the DNA molecule. A DNA contains following characteristics.

• It consists of two parallel chains of polynucleotides forming a double helical structure.
• Both the strands are spirally coiled and are anti- parallel ( i.e. 3' - 5' direction and the other 5'- 3 ' direction )
• The strands are joined together by weak hydrogen bonds.
• The distance between two strands is 20A° and two base pairs are 3.4A°.There are 10 base pairs on a complete turn ( 34A° ).
• DNA molecule contains 28% adenine, 24% guanine, 20% cytosine and 28% thymine.
• The nucleotide in a helix is joined together by phosphodiester bonds.
• Purine base and pyrimidine base are attached together by hydrogen bonds. ( A= T ) and ( C = G).

Functions

• It carries genetic information from one generation to other.
• It replicates itself when dividing.
• Controls all the biological activities of cells.
• Synthesizes RNA. ( DNA→ RNA ).

Ribonucleic Acid ( RNA )

It is found in the nucleolus, cytoplasm, and on the membranes of ribosomes. It also acts as the hereditary material in some viruses and helps in protein synthesis.

Structure;

• It is also macromolecule but smaller than DNA.
• Single-stranded formed of a polynucleotide chain.
• Formed of large no. of nucleotide arranged in a linear sequence and connected together by 3' - 5' phosphodiester bonds.
• Nucleotides of RNA are called ribonucleotides.
• It's nitrogen bases are adenine, Guanine, Cytosine, and Uracil.

Types of RNA
There are three types of RNA which are as follows;

1. Messenger RNA ( mRNA ); mRNA transcribes the genetic code from DNA into a form that can be read and used to make proteins. mRNA carries genetic information from the nucleus to the cytoplasm of a cell.
   
   
2. RibosomalRNA ( rRNA ); rRNA is located in the cytoplasm of a cell, where ribosomes are found. rRNA directs the translation of mRNA into proteins.
   
   
3. Transfer RNA ( tRNA ); Like rRNA, tRNA is located in the cellular cytoplasm and is involved in protein synthesis. Transfer of RNA brings or transfers amino acids to the ribosome that correspond to each three-nucleotide codon of rRNA. The amino acids then can be joined together and processed to make polypeptides and proteins.

Functions

• It helps in protein synthesis.
• Carries hereditary material in some virus.

Reference

• Chemistry.about.com.
• http://chemwiki.ucdavis.edu/