AMINO ACIDS ARE SUBUNITS OF PROTEINS

Amino acids are small organic molecules,generally colourless crystalline solids which are soluble in water but insoluble in organic solvents.Amino acids are formed of C,H,O & N.Some amino acids may contain sulphur(S).There are over 2000 amino acids out of which only 20 amino acids and their derivatives occur in proteins.Amino acids found in proteins are called proteins amino acids whereas others are called non-protein amino acids.


Each amino acid exhibits one definig property viz. they all posess a carboxylic acid group and an amino group both linked to their alpha-carbon atom.Each amino acid also has a side chain atched to its alpha-carbon.The identity of this side chain is what distinguishes one amino acid from the other.

Amino group occurs in alpha-posotion or carbon next to terminal carboxylic group.Therefore,protein amino acids are also called alpha-amino acids.

In the cell where pH is close to 7, free amino acids exist in their ionised form but when they are incorporated into a polypeptide chain , the charges on amino and carboxylic groups disappear.This ionised structure is termed as zwitter ion.

Cells use amino acids to build proteins—polymers made of amino acids, which are joined head-to-tail in a long chain that folds up into a three dimensional structure that is unique to each type of protein.The covalent bond between two adjacent amino acids in a protein chain is called a peptide bond; the chain of amino acids is also known as a polypeptide. Peptide bonds are formed by condensation reactions that link one amino acid to the next. Regardless of the specific amino acids from which it is made, the polypeptide always has an amino (NH2) group at one end—its N-terminus—and a carboxyl (COOH) group at its other end—its C-terminus). 

 Amino acids in a protein are held together by peptide bonds. the four amino acids shown are linked together by three peptide bonds, one of which is highlighted in yellow. One of the amino acids, glutamic acid, is shaded in gray. the amino acid side chains are shown in pink. The two ends of a polypeptide chain are chemically distinct. One end, the N-terminus, is capped by an amino

group, and the other, the c-terminus, ends in a carboxyl group. The sequence of amino acids in a protein is abbreviated using either a three-letter or a one-letter code, and the sequence is always read from the N-terminus .

Twenty types of amino acids are commonly found in proteins, each with a different side chain attached to the α-carbon atom .The same 20 amino acids are found in all proteins, whether they hail from bacteria, plants, or animals. How this precise set of 20 amino acids came to be chosen is one of the mysteries surrounding the evolution of life; there is no obvious chemical reason why other amino acids could not have served just as well. But once the selection had been locked into place, it could not be changed, as too much chemistry had evolved to exploit it. Switching the types of amino acids used by cells would require a living creature to retool its entire metabolism to cope with the new building blocks.

THE STRUCTURE OF DNA

Well before biologists understood the structure of DNA, they had recognized that inherited traits and the genes that determine them were associated with the chromosomes. Chromosomes were discovered in the nineteenth century as threadlike structures in the nucleus of eukaryotic cells that become visible as the cells begin to divide.As biochemical analysis became possible, researchers learned that chromosomes contain both DNA and protein.But which of these components encoded the organism’s genetic information was not clear.

DNA carries the hereditary information of the cell and the protein components of chromosomes function largely to package and control the enormously long DNA molecules. Biologists in the 1940s had difficulty accepting DNA as the genetic material because of the apparent simplicity of its chemistry .DNA, after all, is simply a long polymer composed of only four types of nucleotide subunits, which are chemically very similar to one another.

In 1950s, DNA was examined by X-ray diffraction analysis,a technique for determining the three-dimensional atomic structure of a molecule .The early results indicated that DNA is composed of two strands wound into a helix. The observation that DNA is double-stranded was of crucial significance. This structure immediately suggested how DNA could encode the instructions necessary for life, and how these instructions could be copied and passed along when cells divide.

 A DNA molecule Consists of Two Complementary Chains of Nucleotides

A molecule of deoxyribonucleic acid (DNA) consists of two long polynucleotide chains. Each chain, or strand, is composed of four types of nucleotide subunits, and the two strands are held together by hydrogen bonds between the base portions of the nucleotides.Each nucleotide is composed of a sugar– phosphate covalently linked to a nitrogenous base.

The nucleotides are covalently linked together into polynucleotide chains, with a sugar– phosphate backbone from which the  nitrogenous bases extend.

DNA molecule  is composed of two polynucleotide chains held together by hydrogen bonds between the paired bases. The arrows on the DNA strands indicate the polarities of the two strands, which run antiparallel to each other in the DNA molecule.

The two polynucleotide chains in the DNA double helix are held together by hydrogen-bonding between the bases on the different strands. All the bases are therefore on the inside of the double helix, with the sugar–phosphate backbones on the outside The bases do not pair at random, however: A always pair with T, and G always pairs with C  In each case, a bulkier two-ring base is paired with a single-ring base (a pyrimidine). Each purine–pyrimidine pair is called a base pair, and this complementary base-pairing enables the base pairs to be packed in the energetically most favorable  arrangement in the interior of the double helix. In this arrangement, each base pair has a similar width, thus holding the sugar–phosphate backbones an equal distance apart along the DNA molecule.The members of each base pair can fit together within the double helix because the two strands of the helix run antiparallel to each other—that is, they are oriented with opposite polarities.The antiparallel sugar–phosphate strands then twist around each other to form a double helix containing 10 base pairs per helical turn. This twisting also contributes to the energetically favorable conformation of the DNA double helix.

A consequence of the base-pairing requirements is that each strand of  a DNA double helix contains a sequence of nucleotides that is exactly complementary to the nucleotide sequence of its partner strand—an A always matches a T on the opposite strand, and a C always matches a G. This complementarity is of crucial importance when it comes to both copying and repairing the DNA.