Structure of gene and organization

Structure of gene and organization

Objectives

At the end of this lecture, students will be able to-

• Describe the structure of gene

• Explain the organization of genes in

– Prokaryotes

– Eukaryotes

Contents

• Gene structure

• Prokaryotic gene organization

• Eukaryotic gene organization

Genes

• Genetic information is stored in packages – genes

• Gene holds the information for the production of polypeptide sequence

• Information is in the form of series of bases along the DNA molecule

Gene structure 

• Stretch of DNA –series of bases alomg the DNA molecule

• Intergenic DNA -stretch of DNA lie between genes

• Sense codon/ coding sequence – hold important sequence of information

• Anti sense codon/ non-coding sequence – do not hold sequence information

• Sequence is read from 3’ to 5’ end

• Operons

– Cluster of genes in prokaryotes

– Do not occur in higher organisms

– Example- Lac operon

– Encodes for permease and allow the entry of lactose in the cell

– Also allows the entry of transacetylase & β- galactosidase

• Pseudogenes –Nonfunctional gene on a chromosome

• Tandem gene clusters – regions of genes which are tandemly repeated

• Exons - stretch of coding DNA

• Intervening sequences/ Introns – stretches interuppting the coding area

Prokaryotic gene organization 

• Operon consists of a short sequence of codon upstream to start codon (AUG) – Shine-Dalgarno sequence

• Binding site for ribosome, essential for efficient translation

• RNA polymerase enzyme

– Transcribes gene

– Consists of 5 distinct polypeptides and one subunit

– Holoenzyme - initiates the synthesis of RNA chain

RNA polymerase 

• σ factor –recognize and bind RNA polymerase to correct initiation sites

• Midpoints of two conserved sequences occur at about 10 and 35 nucleotide-pairs: -10 and -35 sequences

• Most common -10 sequence – Pribnow box (TATAAT)

• -35 sequence – Recognition sequence (TTAGACA)

Eukaryotic gene organization 

• mRNA must pass from nucleus to cytoplasm for translation

• Consists of RNA polymerase I, II, III

• RNA polymerase I –in nucleolus; synthesis of rRNA

• RNA polymerase IIand III- Nucleoplasm

• RNA polymerase III - transcribes the genes for smaller nuclear RNA, tRNA

• RNA polymerase II transcribes nuclear structural genes

• Responsible for pre-mRNA synthesis

• Binding site for polymerase II is located at about 25 and 75 nucleotide base pair

• Consensus sequence for -25 sequence (Hognes box or TATA box) is TATAAAA

• Consensus sequence for -75 sequence (CAAT box) is GGCCAATCT

• Continuous sequence of nucleotides (exons) interrupted by non-coding intervening sequence

• mRNA transcript with both introns and exons – heterogenous nuclear RNA

Summary

• Gene holds the information for the production of polypeptide sequence in the form of series of bases along the DNA molecule

• Prokaryotic gene consists of operons, RNA polymerase, Pribnow box and recognition sequence

• Eukaryotic gene consists of exons, introns, RNA polymerase I, II, III, TATA box and CAAT box