Second genetic code overlapping and split genes

HNGU PATAN.
m.sc sem 3 botany
cbo - 503

1. Second genetic code Overlapping and Split genes

2. CONTENTS Introduction Second genetic code overlapping genes split genes

3. Introduction In prokaryotes, the coding sequences of genes are continuous. i.e, uninterrupted with a very few exceptions. In addition, the genes are organised into groups, each such groups forms a single transcription unit. i.e, operon Generally, the different genes occur as discrete unit separate from each other , but some prokaryotic genes may be overlapping.

4. Eukaryotic genes present some interesting organisations. Such as split genes, Overlapping genes , pseudogenes etc.

5. Second Genetic Code An imprecise term that sometimes refers to the nature of the amino acid residues of a protien which determine its secondary and tertiary structure, and sometimes to the features of tRNA molecule that make it recognizable by one amino acid synthase but not by others. During 1988-89, this problem of interaction of specific tRNAs with corresponding amino acy 1 RNA synthetase enzymes (aaRSs) for aminoacylation of tRNAs (transfer of amino acid to tRNA) was described as the second genetic code.

6. Although we generally say that individual codons are meant for specific amino acids, we should recognize the fact that in reality, codons do not specify amino acids, instead they specify tRNA molecules, which in turm specify amino acids with the help of enzymes called ‘ amino acyl RNA synthetases’ (aaRSs). X-ray crystallographic structures of several ‘tRNA-synthetase complexes’ representing intermediates in the formation of ‘amino-acyl tRNAs’ have been studied. the most important of these is a complex between ‘glutamy I-tRNA synthetase’ with tRNAgln (tRNA for glutamine) and ATP.

7. It was shown that there are multiple points of contact all along the inner side of the L-shaped structure of tRNA, which help in recognition of tRNA by synthetase enzyme and in the formation of linkage between tRNA and amino acid .

8. Second Half of the Genetic code The ‘second Half of the genetic code’ refers to the of a ‘protein folding’ problem or to the rules, which govern the formation of a three dimensional structure from the primary structure (amino acid sequence ) of a protien. This problem also received attention of molecular biologists during 1980s and 1990s, when the phrase ‘the second half of Genetic code’ was popular. The problem continues to be important and is now more often described as ‘protein folding’ problem.

9. Importants of Second genetic code Second genetic code led to the burgeoning field of genetic engineering in which the genetic material of cells is altered to tailor make proteins, such as those used as medicines to treat diabetics and heart diseases. The newly deciphered code plays a key role in a later step in protein synthesis. The newly revealed code helps explain a crucial aspect of protein synthesis : the attraction of building blocks, amino acids , to genetic materials inside the cell.

10. Overlapping Genes An overlapping gene is a gene whose expressible nucleotide sequence partially overlaps with the expressible nucleotide sequence of another in this way, a nucleotide sequence may make a contribution to the function of one or more gene products. Bacteriophage X174 contains a single stranded DNA approximately 5400 nucleotides in length.the genome of X174 consists of nine cistrons.

11. From the information about proteins coded, an estimate could be made of the number of nucleotides required. This estimated of number of nucleotides exceeds 6000 which is much higher than the actual number of nucleotides presented i.e., 5400. Therefore,it was difficult to explain how these proteins could by synthesized from a DNA Segment which is not long enough to code for the required number of amino acids.

12. On detailed study of the system,it was discovered that sequences in the segment could be utilized by two different cistrons coding for different proteins. Such overlapping of cistrons will be theoretically possible if the two cistrons have to function at different times and their nucleotide sequence are translated in two different reading frames. In 1976, Barrell and his co- workers discovered that in X174 , having nine cistrons ( A, B,C, D, E, J ,F,G,H) , cistron E is presented between D and J and that the cistron E overlaps cistron D.

13. Overlapping genes

14. It could be shown that amber Mutations in cistron E lie within the cistrons D and these amber mutati- ons do not influence the translation of cistron D into its proteins. Similarly some other nonsense mutations for cistrons E also tie in Cistrons D suggesting that the cistr- ons D and E overlap in the DNA se- quences and that the cistron D

15. and E are translated in two different reading frames so that amber codon in mRNA of one cistron will not be read as termination codon during the translation of mRNA of the other cistron.

16. Split Genes Genes with interrupted sequence of nucleotides are referred to as split genes. Usually a gene has a continuous sequence of nucleotides. In other words, there is no interruption in the nucleotides sequence of a gene.such nucleotide sequence codes for a particular single polypeptide chain.

17. However, it was were observed that the sequence of nucleotides was not contains in case of some genes,the sequences of nucleotides were interrupted by intervening sequences. Split genes were independently discovered by Richard J. Roberts and Philip A. Sharp in 1977,for which they shared the 1993 Nobel prize in Physiology or Medicine. The first observations of interrupted genes, i.e, genes in which there are noncoding intron sequences between the coding exon sequences ,were made in animal viruses in 1977.

18. Split Genes have two types of sequences: 1 ) Normal sequences. ( exons) 2 ) Interrupted sequence ( introns) 1) Normal sequence: This represents the sequence of nucleotides which are included in the mRNA is translated from DNA of split gene.these sequences code for a particular polypeptides chain and are known as exons.

19. Split Genes

20. 2) Interrupted sequence ( introns) : The intervening or interrupted sequences of split gene are known as introns. These sequences do not code for any peptide chain. Moreover, interrupted sequences are not included into mRNA which is transcribed from DNA of split genes.

21. Important features of split genes. Each interrupted gene begin's with an exon and ends with an exon. The exons occur in the same precise order in the mRNA in which they occur in the gene. The same interrupted gene organisation is consistently presented in all the tissues of organisms. most introns are blocked in all reading frames. i.e, termination codons occur frequently in their three reading frames. Therefore, most introns fo not seem to have coding functions.

22. Significance of split genes The significance of split organisation of eukaryotic genes is not clear. In some cases, different exons of a gene code for different active regions of the protein molecule e.g.in the case of antibodies. Thus, it has been suggested that introns are relics of evolutionary processes that brought together different ancestral genes to form new larger genes. It is also possible that some introns have been introduced within certain exons during evolution.

23. Introns may also provide for increased recombination rates between exons of a gene and thus may be of some significance in genetic variation. Introns are known to code for enzymes involved in the processing of hn RNA. ( heterogeneous RNA).

24. References Fundamentals of Biochemistry, By :- Dr. J.l. Jain Dr. Sunjay Jain Nitin jain Genetics By: P. K. Gupta WWW. Google.com https://www.genescript.com>secondary gene...