Flow of genetic
information
Objectives
At the end of this
lecture, student will be able to
• Explain the concept of ‘central dogma of molecular
biology’
• Describe the process of prokaryotic gene expression
• Describe the process of eukaryotic gene expression
Content
• Flow of genetic information
• Prokaryotic gene expression
• Eukaryotic gene expression
The flow of
genetic information
The central
dogma of molecular biology
• Unidirectional flow of genetic information
• DNA makes RNA and RNA makes proteins
• DNA is transcribed to RNA molecule (messenger RNA)
• RNA is translated into a protein sequence according to
genetic code
The “Central Dogma”
• Flow of genetic information in a cell
• DNA replication – 2 daughter DNA molecules formed by
duplication of information in DNA molecule
• In retroviruses, single stranded RNA is converted to DNA
by enzyme reverse transcriptase
• RNA replication – RNA genome is copied directly into RNA
without intermediary use of DNA
Eg. – Corona viruses and hepatitis C virus
Flow of
genetic information
Prokaryotic
gene expression
• RNA polymerase – responsible for transcription
• Promoter
– Contains conserved sequence
– Required for specific binding of RNA polymerase
– Required for initiation of transcription
– Extends up to 40bp
• Termination
– Self complementary sequence at 3’ end of gene
– Forms hairpin structure in RNA acts as terminators
– Hairpin has high G-C content giving high stability and
casing polymerase to pause
• Operons
– Unit of prokaryote gene expressor
– Coordinately regulated structural gene
– Controls elements that are recognised by regulatory gene
products
– Lac operon consists of regulatory gene, operator gene, 3
structural genes, a promoter next to the operon gene – RNA polymerase binds
• RNA polymerase transcribes a gene from the promoter
• Resulting mRNA contains regions coding for multiple
proteins (polycistronic)
• Coding sequence of mRNA are translated into proteins
(ribosomes)
• Ribosomes binds to ribosome binding site (RBS)
• Translate message into aminoacid sequence from start codon
(AUG) to stop codon
Eukaryotic
gene expression
• Transcription in nucleus and translation in cytoplasm
• Pre messenger RNA (pre-mRNA) synthesized by one of the
three polymerase II
• It encodes only one protein
• RNA polymerase II is responsible for transcribing all
protein coding gene, small nuclear RNA genes and sequence encoding micro RNA
• RNA formed is modifies in cytoplasm by capping at 5’ end
poly A tail at 3’ end
• Addition of g7 methyl guanosine residue – 5’ cap
• Mature 3’ end is generated by cleavage followed by
addition of poly A by enzyme poly A polymerase
• Modification increases stability
• Protein encoding sequences are interrupted by introns
• Introns in pre-mRNA are removed
• Exons- continuous protein coding sequence
• Process is called splicing
Chemical modification
during transcription
• Directed by RNA-protein complex known as snRNPs (Small
nuclear ribonucleoproteins)
• Mature mRNA exported from nucleus to cytoplasm
• Translated to proteins on ribosome
Summary
• DNA is copied to make RNA and RNA is template for protein
synthesis
• mRNA is copied from DNA and then used repeatedly for
protein synthesis
• Unidirectional flow of genetic information of genetic
information from DNA through RNA to protein
• Prokaryotic gene expression involves transcription of a
single gene or a operon starts at promoter, ends at terminator and produces
monocistronic or polycistronic mRNA which further getting translated to
proteins
• Monocistronic mRNA are transcribed from a gene, initiated
at a promoter
• Pre mRNA is capped at 5’ end and has a poly (A) tail added
to the 3’ end
• Introns are removed by splicing before mature mRNA is
exported from the nucleus to be translated to ribosome in the cytoplasm