STRUCTURE OF GENE and genetic code in animals ppt

Structure of gene and genetic code

It permits essentially the same complement of enzymes and other proteins to be specified by microorganisms varying widely in their DNA base composition.

Degeneracy also provides a mechanism of minimising mutational lethality.

1. Gene structure and genetic code

2. Shine-Dalgarno = RBS

3. Contdd ..

4. Eukaryotic gene

5. Kozak sequence= GCCGCCACC ATG G

6. Genetic code The genetic code is the set of rules by which information encoded within genetic material is translated into proteins by living cells Biological decoding is accomplished by the ribosome which links amino acid in an order specified by mRNA using tRNA molecules to carry amino acid sequence and to read the mRNA three nucleotides at a time.

7. DISCOVERY George Gamow postulated that the three letter code must be employed to encode the 20 amino acids and one stop codon used by living cells to built proteins. A code for 2 nucleotides would allow for only a maximum of 16 amino acids (4²) A code of three nucleotides could code for maximum of 4³ or 64 amino acids. George Gamow

8. The Crick ,Brenner et al experiment was first to demonstrate that codons consist of three DNA bases Marshall Nirenberg & Hienrich.J.Matthaei were the first to elucidate the nature of codon in 1961 at the National institute of Health SYDNEY BRENNER Marshall Nirenberg

9. CONTT ….. They used the cell free system to translate a poly uracil RNA sequence (UUUU….) & synthesised polypeptide consisted only amino acid phenylalanine Severo Ochoa’s lab demonstrated that poly adinine (AAAAA…) coded for poly lysine & poly cytocine (CCCC…) coded for poly proline Subsequent work by Har Gobind Khorana identified the rest of the genetic code Har Gobind Khorana

10. PROPERTIES OF GENETIC CODE The main features of the genetic code were worked during 1960 main properties of code are: The genetic code is composed of nucleotide triplets The genetic code is non overlapping The genetic code is comma free

11. CONTT… The genetic code is degenerative The genetic code is ordered The genetic code contains start & stop codons The genetic code is universal

12. SALIENT FEATURES OF THE CODE Sequence reading frame a codon is defined by the initial nucleotide from which translation starts Example : GGGAAACCC ,if read from the first position contains the codon GGG,AAA,CCC & if read from the second position contains the codons GGA & AAC & if read starts from third position GAA & AAC. Every sequence can thus be reading in three different frames each producing different amino acids sequence Double stranded DNA has six possible reading frames three in forward orientation on one strand & three reverse on forward orientation on opposite strand The actual frame in which a protein sequence is translated is defined by a start codon (AUG)

13. CONTT… Start & Stop codon AUG is used to initiate polypeptide chains. UAG,UAA,UGA- specify polypeptide chain termination OR STOP CODON. Degenerative code & ordered code All amino acids except methionine & tryptophan are specified by more than one codon Three amino acids leucine, serine and arginine each specified by six different amino acids The occurrence of more than one codon per amino acid is called degeneracy

14. The Genetic Code

15. Contt … DEGENRACY IS PRIMARILY OF TWO TYPES :

16. Importance of degeneracy It permits essentially the same complement of enzymes and other proteins to be specified by microorganisms varying widely in their DNA base composition. Degeneracy also provides a mechanism of minimising mutational lethality.

17. Contt … Genetic code is universal The genetic code is universal, that is, the same codon used (with few exceptions) in all living organisms, procaryotes and eukaryotes.

18. Exceptions of universility Mitochondrial genes When mitochondrial mRNA from animals or microorganisms is placed in a test tube with the cytosolic protein-synthesizing machinery (amino acids, enzymes, tRNAs , ribosomes ) it fails to be translated into a protein. The reason : These mitochondria use UGA to encode tryptophan ( Trp ) rather than as a chain terminator. When translated by cytosolic machinery, synthesis stops where Trp should have been inserted. In addition, most animal mitochondria use AUA for methionine not isoleucine and all vertebrate mitochondria use AGA and AGG as chain terminators. Yeast mitochondria assign all codons beginning with CU to threonine instead of leucine (which is still encoded by UUA and UUG as it is in cytosolic mRNA).

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