Question

Speculate as to why eukaryotes have either a) much more complex promoter and enhancer regions than prokaryotes or b) intron exon structure while prokaryotes do not.

Please answer a and b if possible

Answer 1

Ans.(a) Actually eukaryotes complex structure/regions of promoter and enhancer are due to more complex gene expression in eukaryotes than prokaryotes.

In eukaryotes the process of transcription and translation are physically separated because the hnRNA(heterogeneous nuclear RNA) requires further processing before translation to proteins.

In eukaryotes there are atleast three RNA polymerase in nucleus other than those in organelles for different type of RNA transcribing but this complexity is not found in prokaryotes.

While in prokaryotes the transcription and translation occurs in same compartment( as ther is no separation of cytosol and nucleus in prokaryotes),there is no need of further processing of RNA to become active and there is no gap in transcription and translation and can be coupled or translation can start much before the mRNA is fully transcribed.

1. Promoter contains specific DNA sequences for protein recognition, they by binding to promoter  regions and assists  RNA polymerase.

Promoter elements have mainly two regions:-

a) Core promoter: Mininimum portion essential for proper transcription initiation.

b)Proximal promoter: Sequence at upstream of gene which contain primary regulatory elements.

In prokaryotes the promoter consists of mainly two short sequences at -10 and -35 positions upstream from the transcription start site.The sequence -10 also called pribnow box have six nucleotides i.e TATAAT .It starts transcription in prokaryotes.Other sequence -35 have six nucleotides i.e TTGACA .It's presence allows a very high transcription rate in prokaryotes.

2. In eukaryotes promoters typically lie upstream of gene and can have regulatory elements of many kilobases and are located away from the transcriptional start site.

Many eukaryotic promoters have TATA box (sequence TATAAA) which lies close to the transcriptional start site (often within 50 bases) which binds a TATA binding protein which assists in the formation of the RNA polymerase transcriptional complex.

In eukaryotes enhancer region are binding sequence and when DNA binding protein binds to an enhancer ,the shape of DNA changes,this allows the interaction between the activators bound to the enhancers and transcription proteins bound to the promoter region and RNA polymerase to occur.

In eukaryotes the transcriptional complex can cause the DNA to bend back on itself which allows for placement of regulatory sequences/enhancers far from the actual site of transcription.

Enhancer is DNA sequence (50 - 1500 bp)which enhances transcription.

Prokaryotes have less complex enhancers for them and the regulatory proteins that bind to the bacterial enhancer must contact RNA polymerase to activate transcription and it occurs either by DNA looping or tracking of the enhancer binding protein along the DNA.

Further presence of enhancers in eukaryotes allows the promoter sites to initiate transcription  in remote location on the DNA .

Ans.(2) Prokaryotes do not have introns and exons because they have transcription coupled to translation and thus they don not have time for that,since intron splicing will stop coupling. Presence of introns and exons in eukaryotes make the translation more complex than in prokaryotes.Eukaryotes have exon and intron in DNA which is important as random mutation may occur at any part of DNA and if it occurs in intron,this will not pass on to offspring as eukaryotes undergo further processing of hnRNA(unique to eukaryotes) , process of splicing in which introns are removed and exons are joined in a definite order.

Actually introns are non functional sequneces of DNA intervening the exons( functional sequence of DNA).The presence of introns in eukaryotes is reminiscent of antiquity.