Structure of gene and organization

Structure of gene and


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

• Describe the structure of gene

• Explain the organization of genes in

– Prokaryotes

– Eukaryotes


• Gene structure

• Prokaryotic gene organization

• Eukaryotic gene organization


• 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


• 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

– Also allows the entry of transacetylase & β-

• 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

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

• RNA polymerase enzyme

– Transcribes gene

– Consists of 5 distinct polypeptides and one subunit

– Holoenzyme – initiates the synthesis of RNA chain


• 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)

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

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

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


• 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