Gene structure

explains the structure and features of the prokaryotic and eukaryotic gene

1. Fine Structure of Gene By, Kayeen Vadakkan Assistant Professor, Department of Biotechnology St. Mary's College Thrissur, Kerala , India

2. Objectives To understand the history of gene To learn structure of prokaryotic gene To delineate eukaryotic gene To study salient features and characteristics of gene Molecular Biology, Kayeen Vadakkan, Department of Biotechnology, St. Mary's College, Thrissur

3. Prelude Double stranded structure of DNA 5’ and 3’ concept Molecular Biology, Kayeen Vadakkan, Department of Biotechnology, St. Mary's College, Thrissur

4. Introduction and History of gene Definition A gene is a specific sequence of DNA containing genetic information required to make a specific protein. Types of Gene based on organism Prokaryotic gene ( which is seen in prokaryotes, example : Bacteria, Cyanobacteria) Eukaryotic gene (Which is seen in higher organisms such as Plants, Animals) Types of gene based on activity 1. House keeping genes ( genes which are always active ) 2. Specific genes. (Those genes which are getting active only during some special condition) Molecular Biology, Kayeen Vadakkan, Department of Biotechnology, St. Mary's College, Thrissur

5. Types of gene based oh behavior. 1. Basic genes : These are the fundamental genes that bring about expression of particular character . 2. Lethal genes: These bring about the death their possessor. 3. Multiple gene : When two or more pairs of independent genes act together to produce a single phenotypic trait . 4. Cumulative gene: Some genes have additive effects on the action of other genes. These are called cumulative genes. 5. Pleiotropic genes: The genes which produce changes in more than one character is called pleiotropic gene. 6. Modifying gene: The gene which cannot produce a character by itself but interacts with other to produce a modified effect is called modifier gene. 7. Inhibitory gene: The gene which suppresses or inhibits the expression of another gene is called inhibitory gene Introduction and History of gene (Continues) Molecular Biology, Kayeen Vadakkan, Department of Biotechnology, St. Mary's College, Thrissur

6. Introduction and History of gene (continues) Molecular Biology, Kayeen Vadakkan, Department of Biotechnology, St. Mary's College, Thrissur The classical principles of genetics were deduced by Gregor Mendel in 1865 on the basis of breeding experiments with peas. He assumed that each trait is determined by a pair of inherited ‘factors’ which are now called gene. I n 1909 Wilhelm Johannsen coined the term ‘GENE ’. Figure 1: Concept of Gene

7. P rokaryotic gene structure Molecular Biology, Kayeen Vadakkan, Department of Biotechnology, St. Mary's College, Thrissur Figure 2: Prokaryotic gene structure

8. Prokaryotic gene structure (Continues) Prokaryotic Gene is composed of three regions: 1.Promoter region 2.RNA coding sequence 3.Terminator region Prokaryotic gene is continues and uninterested where there is no introns present The region 5’ of the promoter sequence is called upstream sequence and the region 3’ of the terminator sequence is called downstream sequence . Molecular Biology, Kayeen Vadakkan, Department of Biotechnology, St. Mary's College, Thrissur

9. Prokaryotic gene structure (Continues) Promoter region: This is situated on upstream of the sequence that codes for RNA. This is the site that interact RNA polymerase before RNA synthesis (Transcription). Promoter region provides the location and direction to initiate transcription Figure 3 : prokaryotic promoter region Molecular Biology, Kayeen Vadakkan, Department of Biotechnology, St. Mary's College, Thrissur

10. Prokaryotic gene structure (Continues) Figure 4: Prokaryotic promoter region At -10 there is a sequence TATAAT or PRIBNOW BOX. At -35 another consensus sequence TTGACA These two are the most important promoter elements recognized by transcription factors . Molecular Biology, Kayeen Vadakkan, Department of Biotechnology, St. Mary's College, Thrissur

11. Prokaryotic gene structure (Continues) RNA coding sequence: The DNA sequence that will become copied into an RNA molecule (RNA transcript). Starts with an initiator codon and ends with termination codon No introns (uninterrupted). Collinear to its mRNA. Any nucleotide present on the left is denoted by (-)symbol and the region is called upstream element . E.g. -10,-20,-35 etc. Any sequence to the right of the start is downstream elements and numbered as +10,+35 etc .). Terminator region: The region that signal the RNA polymerase to stop transcription from DNA template. Transcription termination occur through Rho dependent or Rho independent manner . Molecular Biology, Kayeen Vadakkan, Department of Biotechnology, St. Mary's College, Thrissur

12. Eukaryotic gene structure Eukaryotic gene are complex structures compared that prokaryotic gene. They are composed of following regions Exons Introns Promoter sequences Terminator sequences Upstream sequences Downstream sequences Enhancers and silencers(upstream or downstream) Signals (Upstream sequence signal for addition of cap. Downstream sequences signal for addition of poly A tail .) Molecular Biology, Kayeen Vadakkan, Department of Biotechnology, St. Mary's College, Thrissur

13. Eukaryotic gene structure (Continues) Figure 5: Eukaryotic gene structure Molecular Biology, Kayeen Vadakkan, Department of Biotechnology, St. Mary's College, Thrissur

14. Exons C oding sequence, transcribed and translated . Coding for amino acids in the polypeptide chain . Vary in number ,sequence and length. A gene starts and ends with exons.(5’ to 3’). Some exon includes untranslated(UTR)region . Introns C oding sequences are separated by noncoding sequences called introns. They are removed when the primary transcript is processed to give the mature RNA All introns share the base sequence GT in the 5’end and AG in the 3’end. Introns were 1st discovered in 1977 independently by Phillip Sharp and Richard Roberts. Eukaryotic gene structure (Continues) Molecular Biology, Kayeen Vadakkan, Department of Biotechnology, St. Mary's College, Thrissur

15. Eukaryotic gene structure (Continues) Significance of Introns Introns don't specify the synthesis of proteins but have other important cellular activities. Many introns encodes RNA’s that are major regulators of gene expression. Contain regulatory sequences that control trancription and mRNA processing. Introns allow exons to be joined in different combinations(alternative splicing), resulting in the synthesis of different proteins from the same gene. Important role in evolution by facilitating recombination between exons of different genes(exon shuffling). Molecular Biology, Kayeen Vadakkan, Department of Biotechnology, St. Mary's College, Thrissur

16. Eukaryotic gene structure (Continues) Promoters A promoter is a regulatory region of DNA located upstream controlling gene expression. 1 . Core promoter – transcription start site(- 34) Binding site for RNA polymerase and it is a general transcription factor binding sites. 2. Proximal promoter -contain. primary regulatory element. These together are responsible for binding of RNA polymerase II which is responsible for transcription. Molecular Biology, Kayeen Vadakkan, Department of Biotechnology, St. Mary's College, Thrissur

17. Eukaryotic gene structure (Continues) Upstream (5’end) 5’UTR serve several functions including mRNA transport and initiation of translation. Signal for addition of cap(7 methyl guanisine ) to the 5’end of the mRNA. The cap facilitates the initiation of translation. Stabilization of mRNA. Downstream (3’end) 3’UTR serves to add mRNA stability and attachment site for poly-A-tail. The translation termination codon TAA. AATAA sequence signal for addition of poly A tail. Molecular Biology, Kayeen Vadakkan, Department of Biotechnology, St. Mary's College, Thrissur

18. Eukaryotic gene structure (Continues) Terminator R ecognized by RNA polymerase as a signal to stop transcription Enhancer E nhances the transcription of a gene upto few thousand bp upstream. Silencers R educe or shut down the expression of a near by gene. Molecular Biology, Kayeen Vadakkan, Department of Biotechnology, St. Mary's College, Thrissur

19. Number of genes in each organism is more than the number of chromosomes; hence several genes are located on each chromosome. The genes are arranged in a single linear order like beads on a string. Each gene occupies specific position called locus. If the position of gene changes, character changes. Genes can be transmitted from parent to off springs. Genes may exist in several alternate formed called alleles. Genes are capable of combined together or can be replicated during a cell division . Genes may undergo for sudden changes in position and composition called mutation. Genes are capable of self duplication producing their own exact copies. Salient features of gene Molecular Biology, Kayeen Vadakkan, Department of Biotechnology, St. Mary's College, Thrissur

20. REFRENCES The Cell (fifth edition) by Geoffrey M. Cooper and Robert E. Hausman . Genes IX by Benjamin Lewin .