Flow of genetic information

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