DESCRIBE TRANSCRIPTION AND TRANSLATION IN DNA???????!!!!!!!?

Transcription :

1. INTITIATION : RNA polymerase binds to promoter region of DNA and initiates transcription process. As it moves along DNA unwinds and one of these 2 straands serve as DNA template for RNA synthesis. As a result complementary copy of RNA is constituted on template strand.

2. ELONGATION : Elongation of RNA chains takes place by means of core enzyme which move along with DNA template. After commencement of chain elongation singma factor of RNA polymerase dissociates from core enzyme and can be reused. RNA chain growth takes place in 5' -> 3' direction. It requires bivalent ion Mg2+ or Mn+.

3. TERMINATION : The mRNA thus separates from DNA template chain. It undergoes processing and passes into cytoplasm through pores in nuclear envelope. The spliy area of DNA forms hydrogen bonds and gets spirally coiled to assume the original form. It is accompained with Rho factor for succesful termination of RNA strand.

Synthesis of TRANSCRIPTION.

Sigma factor is weakly attached with other chains. The function of sigma subunit is to confer specify of RNA synthesis at promoter region. Once initiation begins, sigma facto gets dissociated. Rho factor helps in elongation and termination of RNA molecule.

Transcription in eukaryotes has to face exons and introns. Due to splicing (done by snRPS), INTRONS are removed. Now exons are joined in a definite pattern. HnRNA shows 2 process :

> Capping : Here an unusual nucleotide [methyl guanosine triphosphate] is added to 5'- end of HnRNA.

> Tailing : Here adenylated residues are added at 3'- end in template independent manner. This type of fully processed HnRNA is called mRNA.

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TRANSLATION refers to process of polymerisation of amino acids from a polypeptide chain. The order and sequence of amino acids are defined by bases in the mRNA. The amino acids are joined by a bond known as peptide bond.

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Mechanism of TRANSLATION :

1. Activation of Amino acids : This reaction is brought about by binding of an amino acid with ATP. The step requires enzyme called amino acyl RNA synthetases.

2. Transfer of amino acid to tRNA : The amino acid is transfered to tRNA. As a result enzyme and ATP are liberated.

3. Initiation of Polypeptide chain :Charged tRNA shifts to ribosome. Ribosome consists of structural RNAs and so different proteins. The information for sequence of amino acid is present in sequence of bases of mRNA. Inititiation of polypeptide chain in prokaryotes is always brought about by amino acid methionine which is regularly coded by codon AUG.

[ NOTE : mRNA has some additional sequences tat are not translated and are referred as untranslated regions ( UTR ). The UTRs are present at both 5' and 3' ends. They are required for efficient translation process. ]

4. Elongation : During this, complexes composed of amino acid, linked to tRNA subsequently bind to appropriate codon in mRNA forming the complementary bases with tRNA anticodon. Ribosomes moves aling mRNA towards it 3'end . This shift of ribosome along mRNA is called translocation. Hydrolysis of GTP provides energy. This shift releases of tRNA which is empty.

5. Termination : The termination of polypeptide is signalled by one of the 3 terminal triplets in mRNA. There are UAG, UAA and UGA. At the time of termination , terminal codon immediately follows in last amino acid codon. After this, polypeptide chain, tRNA and mRNA are released. The subunits of ribosomes get dissociated.

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