Fine structure of gene

fine structure of gene
university of mysore

1. “ FINE STRUCTURE OF GENE ”.

2. Inherited determinant of the phenotypic is called gene. In 1928 belling suggested that the chromomers ,which appear like a series of granules on the chromosomes, might be the genes. INTRODUCTION :

3. Gene control biochemical reaction by controlling the production of enzymes. Biochemical reaction in vivo occurs as a series discrete, stepwise reaction. A single enzyme specifically catalyzes each reaction. GENE CONCEPT

4. The genes of almost all organisms are DNA. This is shown by following observation The DNA content of all cells of a species of an organism is constant .It is halved in the gametes. Cells of similar hereditary origin have DNA of similar composition .The relative amounts of the four nucleotides are constant. Gene mutation can be explained on the basis of alteRNAtion of DNA structure. IN ALMOST ALL CASES GENES ARE DNA :

5. Most plant viruses and some animal viruses contain RNA as the genetic material In the reo group of viruses and in the wound tumour virus the RNA is present in the double stranded form. In the tobacco mosaic virus( tmv ), a tobacco virus ,influenza type a virus and bacterial viruses f2,r17 and ms2 the RNA is single stranded. THE GENES OF SOME VIRUSES ARE RNA :

6. The gene has a mutational property as evidence by the presence and occurrence of allele producing different phenotypes. The gene has a recombination property, as evidenced by the observation that it can separate itself from the other genes through crossing over PROPERTIES OF GENES :

7. STRUCTURE OF GENE

8. Length : 10 ̴ 200 bp Features : lack of conservation. Functions : pass through the ribosome Hold the first coding segment Promote the entry of mRNA into protein synthesis 5’UNTRANSLATED SEGMENT :

9. Length : 50 ̴ 600 bp Feature : variation in length AATAAA (ATTAAA) Function : threads through the ribosome’s Hold the mRNA stability Termination sites Indicate the site for attachment of the poly (A) tail 3’UNTRANSLATED SEGMENT :

10. ENHANCER : located at about thousands of bp from the gene PROMOTER : Basal promoter : TATA box , CAAT box or GC box, located within 20-30 bp from start point. Upstream promoter : located at about 100 bp upstream from start point

11. PROMOTER REGION : Initiate transcription Recognition point for RNA polymerase. Several hundred nucleotides in length immediately adjacent to the gene they regulate

12. TATA box : Common to almost all eukaryotic mRNA promoter Consensus sequence : TATAA/TA 20-30 bases upstream of the start point

13. Roles of TATA box: Bind RNA polymerase Initiate transcription Recognized and bound by a general initiation factor TFIID

14. Intron and exon : Lack of correspondence between the DNA and mRNA Intron : do not encode mRNA Exon : encode mRNA. Intron : Donot encode for mRNA Length : 50 ̴ 30000 bp (average : 200 – 400 bp ) Present in pre-mRNA only

15. Initiator and terminator : Initiator → exons + introns → terminator Length : 300 ̴ 1500 bp Initiator : ATG(CTG) Terminator : TGA , TAA OR TAG. CODING SEQUENCE :

16. Exon : Encode for mRNA Four types of exons : Initial exon . InteRNAl exon . Terminal exons . Single exon ( intronless genes)

17. Some genes encode for protein , others encode for tRNA , rRNA etc Genes code for mRNA , mRNA code for protein The sequence of genes encoding for protein directly determines their functions (amino acid sequence) Genetic code is highly conserved Some variation exist - mutations of DNA lead to loss of function of protein , some may have diseases. HOW DO GENES FUNCTION ?