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 II
and 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