`text(Nucleosides:)` Nucleoside contains only two basic components of nucleic acids (a pentose sugar and a nitrogenous base). During their formation, 1- position of pyrimidine or 9 – position of purine moiety is linked to C1 of sugar (ribose or deoxyribose) by α - linkage. See fig.1.

General structure of a nucleoside
Depending upon the type of sugar present, nucleosides are of two types:

(i) Ribonucleosides and
(ii) Deoxyribonucleosides


`text(Nucleotides:)` Nucleotides contains all the three basic components of nucleic acids. Nucleotides are nucleoside monophosphates.

They are of two types depending upon the type of sugar – Ribonucleotides and Deoxyribonucleotides. Nucleotide may be represented as follows.,

See fig.2.

Primary structure and its double helix: Sequence in which four nitrogen bases are attached to the sugar phosphate backbone of a nucleotide chain is called primary structure. Watson and crick in 1953 proposed that DNA polymers form a duplex structure consisting of two strands of polynucleotide chain coiled around each other in the form of a double helix. Bases of one strand of DNA are paired with bases on the other strand by means of hydrogen bonding. According to Chargaff rule – Thymine and adenine can be joined by 2 hydrogen bonds (T = A) while cytosine and guanine can be joined by three hydrogen bonds. See fig.

Important functions are

(a) `text(Replication)`

Process by which a single DNA molecule produces two identical copies of itself is called replication. Replication of DNA is an enzyme catalyzed process. In this process, two strands of DNA helix unwind and each strand serves as a template or pattern for the synthesis of a new strand. Newly synthesized complementary strand is an exact copy of the original DNA. In this way hereditary characteristics are transmitted from one cell to another.

See fig.1.

`DNA undersettext(Transcription)->RNA undersettext(Translation)-> text(Protien)`

(b) `text(Protein Synthesis)`

`(i)` `text(Transcription)`

It is the process of synthesis of RNA (mRNA) by using DNA as template. This process is similar to replication process. Differ in following ways. In transcription, ribose nucleotide assemble along the uncoiled template instead of deoxyribose nucleotide and base uracil (U) is substituted for the base thymine (T). Synthesis of RNA or DNA always takes place in 5' - 3' direction. Process is catalyzed by an enzyme RNA polymerase. In this way DNA transfers its genetic code to mRNA.After synthesis, RNA detaches from DNA and moves from nucleus to the cytoplasm where it acts as template for protein synthesis. DNA returns to its double helix structure

`(ii)` `text(Translation)`

It is the process of synthesis of protein. This process is directed by mRNA in the cytoplasm of cell with the help of tRNA (transfer RNA) and ribosomal particles (RNA – protein complex).

See fig.2.

The process occurs with the attachment of mRNA to ribosome particle mRNA then gives the message of the DNA and dictates the specific amino acid sequence for the synthesis of protein. 4 bases in mRNA act in the form of triplets and each triplet acts as a code for a particular amino acid. This triplet is called codon. There may be more than one codon for same amino acid. E.g. amino acid methionine has code AUG while glycine has 4 codons GGU, GGC, GGA, GGG. These codon expressed in mRNA is read by tRNA carrying anticodon and is translated into an amino acid sequence. This process is repeated again and again thus proteins are synthesized. After completion, it is released from ribosome

See fig.3.

`text(Protein synthesis)` is a fast process and about 20 amino acids are added in one second. It may be noted that translation is always unidirectional but transcription can sometimes be reversed. (RNA is copies into DNA) This is called reverse transcription (occurs in Retroviruses).

Genetic Code

Segment of DNA is called gene and each triplet of nucleotides is called a codon that specifies one amino acid. This relationship between nucleotide triplets and amino acids is called genetic code. E.g. See fig.4.