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VTU eNotes On Genetic Engineering & Applications (Bio-Technology)

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Publisher VTU eLearning
Author: Panel Of Experts
Number of Pages 119
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Unit 1 Introduction Genetic code Definition Genetic code can be defined as the relationship between sequence of DNA and the sequence of corresponding protein. -Genetic code is universal. The coded message is the same for all living organism. Breaking of genetic code shows that nucleic acid is a information molecule and genetic information is stored in the form of nucleotide triplet, known as triplet codon. - There are 64 codons due to possible permutation of four bases is 43 64. each of these codons has a specific meaning in protein synthesis i.e. 61 codons represent amino acids, where as 3 codons specifies for termination of protein synthesis. -The length of codon portion of the gene depends on the length of message to be translated. For ex, DNA contain gene sequence of 1500 nucleotide may contain 500 codons. There fore the sequence of triplets determining sequence of amino acids in a protein. - Genetic code is degenerate.i.e. in the universal genetic code system many amino acids are specified by more than one codon.for ex. Two triplets codons UUU UUC code for phenyl alanine, while serine amino acid is coded by six triplet codon namely UCU, UCC, UCA, UCG, ACU, AGC. - The initiation signal for the synthesis for the protein is the AUG codon. The AUG codon executes two functions in bacteria. When AUG is present in the beginning of the mRNA sequence, codes for N-formyl methyionine f-met and present any other region codes for normal methinine. In eukaryotes, initiation codon is AUG codes for methionine. - There are three termination codons present on mRNA. They are UAG,UAA and UGA for termination of polypeptide synthesis. After binding of ribosome to termination codon, completed polypeptide chain is released due to hydrolytic cleavage. Genetic elements that control expression of genes. 1. Promoters Prokaryotes of prokaryotes
Among prokaryotes, promoters of E. coli have been studied in detail Bacterial promoter region is about 40 bases long which include common sequences of about 6 bases which is
located upstream from the site at which RNA synthesis begins. This region is called Pribnow box. The pribnow box centers around the -10 position. The -10 sequences is hexamer, its consences sequences are TATAA. All the promoters contains sequence similar to TATAA at -10 consensus sequence. Similarly, another consensus sequence is TGTTGACA,centers around -35 i.e. about 35 base pair before the site of transcription. This regions is critical for the rapid initiation of transcription for most bacterial genes. The presence of essential nucleotide in all promoters is said to be conserved sequence. There are four conserve sequences in bacterial promoter. Promoters of Eukaryotes Several promoters have been identified among eukaryotes. In eukaryotes TATA box is situated in the upstream direction far away from the site of RNA synthesis. In addition to TATA box, some other sequences situated even further i.e. between -40 and 110 are called CAAT box and GC box. The core promoter surrounds the starting point extending from -45 to 20. These are sufficient for transcription to initiate rich in GC base pairs. Promoters are present both in up stream, and down stream direction. There are three types of promoters for RNA polymerase III of which two are internal. Type III also cleaves in a site specific manner, but differ from type I and type II by requirements. Type III RE requires the same additional factors as type I
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