If you were looking at an mRNA and saw the codon AUG what wo

If you were looking at an mRNA and saw the codon AUG, what would you conclude about it? What does it mean to that the genetic code is redundant, but not ambiguous? The genetic code is nearly universal, meaning the same RNA codon that designates tryptophan in humans, designates tryptophan in bacteria. a. What has this knowledge allowed us to conclude about the code? b. What has this knowledge allowed us to do with genes Transcription is the DNA-directed synthesis of RNA: a closer look Describe the general model of transcription. Include in your answer the steps of transcription and the key elements of each step. Compare and contrast transcription between prokaryotes and eukaryotes What makes RNA polymerase start transcribing in a gene at the right place on the DNA of a prokaryotic cell? What makes RNA polymerase start transcribing in a gene at the right place on the DNA of a eukaryotic cell?

Solution

A)Transcription is the first step in gene expression. It involves copying a gene\'s DNA sequence to make an RNA molecule.Transcription can be divided into four distinct stages:
Template recognition
Initiation
Elongation
Termination
Initiation:RNA polymerase binds to a sequence of DNA called the promoter,found near the beginning of a gene.Each gene has its own promoter.Once bound,RNA polymerase separates the DNA strands,providing the single-stranded template needed for transcription.
Elongation:One strand of DNA,the template strand,acts as a template for RNA polymerase.As it reads this template one base at a time,the polymerase builds an RNA molecule out of complementary nucleotides,making a chain that grows from 5\' to 3\'.The RNA transcript carries the same information as the non-template (coding) strand of DNA,but it contains the base uracil instead of thymine .
Termination:Sequences called terminators signal that the RNA transcript is complete.Once they are transcribed,they cause the transcript to be released from the RNA polymerase.

B)Prokaryotes do not have an organized nucleus,so the nuclear materials or DNA is in the cytoplasm.Therefore,the transcription occurs in the cytoplasm and all the precursors needed for the transcription are found in the cytoplasm.Prokaryotic transcription requires the RNA polymerase enzyme in order for the transcription to be successfully completed.The enzyme binds to the sigma factor and the promoter region,and then initiate the transcription by completing the holoenzyme.In prokaryotes,DNA is not bound to histones.Thus,the transcription initiates directly.This could be advantageous when prokaryotes have overlapping genes.Transcription starts at the promoter region and elongate through the coding region and ends when the RNA polymerase reads the termination signal.There are two types of termination signals,Rho-dependent and independant.Transcribed mRNA will be completely translated during the transcription,and no post-transcription processing will be undergoing most of the time.Transcriptionl unit has one or more genes.Transcription and translation are nearly simultaneous.

Eukaryotic transcription is more complex than eukaryotic transcription and occurs inside the nucleus.Unlike prokaryotes,eukaryotes contain five types of RNA polymerases according to the need of transcription and contain 10-17 subunits.The DNA detaches from histone proteins and unwinds near the promoter region.RNA polymerase and other transcription factors including enhancers will be bound to the promoter region.Transcription starts at the transcription initiation site and goes up to the transcription terminationsignal.Unlike prokaryotes,the transcript is very long and goes through extensive processing.Newly formed mRNA is called pre-mRNA.This is processed by slicing out the non-coding region,and coding regions will be joined back together.This is called the mature mRNA,and it is ready to be translated.Transcriptional unit has only one gene.RNA polymerase I,II nad III synthesises rRNA,mRNA and tRNA respectively.RNAs are released and processed in the nucleus.

c)To begin transcribing a gene,RNA polymerase binds to the DNA of the gene at a region called the promoter.Basically,the promoter tells the polymerase where to sit down on the DNA and begin transcribing.In bacteria,RNA polymerase core enzyme consists of five subunits-2 ALPHA subunits,1 BETA subunit,1 BETA\' subunit,and 1 SIGMA subunit.In bacteria,there is one general RNA transcription factor that is sigma.RNA polymerase core enzyme binds to the bacterial general transcription factor sigma to form RNA polymerase holoenzyme and then binds to a promoter.

d)In eukaryotes,the main RNA polymerase does not attach directly to promoters like bacterial RNA polymerase.Instead,helper proteins called basal transcription factors bind to the promoter first,helping the RNA polymerase in cells get hold on the DNA.Many eukaryotic promoters have a sequence called a TATA box.It is recognized by one of the general transcription factors,allowing other transcription factors and eventually RNA polymerase to bind.The functions of the bacterial general transcription factor sigma are performed by multiple general transcription factors that work together in eukaryotes.In eukaryotes,in RNA polymerase II-dependent transcription,there are six general transcription factors-TFIIA,TFIIB,TFIID,TFIIE,TFIIF,and TFIIH.