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Transcript modifications

5' methyl guanosine cap

  • Protects transcript from degredation.
  • Helps with initiation of translation.

After the initial pre-mRNA transcript has been created the first major modification is the addition of a 5' methyl guanosine cap. The addition of the 5' 7-MG cap is important for two reasons: the 5' caps are recognized by protein factors that initiate translation, and it also helps protect the transcript from nucleases. Nucleases are very common in the cell and because of this unprotected RNA has a very short half-life inside the cell. Nucleases are actually so common that working with RNA in the laboratory can be quite difficult because the samples have a tendency to disintegrate into useless bits.

Splicing

  • Splicing removes non-coding exons from transcript.
  • Alternative splicing allows for different combinations of exons from same pre-mRNA transcript (gene).
  • Some RNAs can self-splice.

Eukaryotic genes contain two types of transcribed regions: introns and exons. Exons are the regions of the genome that contain actual coding information. Introns are non-coding, meaning that intronic sequences are never translated to protein. Introns are never included in the final processed mRNA transcript. Splicing is the process of removing introns from the pre-mRNA transcript to produce an exon-only mRNA molecule, which is then shipped off for translation. Generally, eukaryotic mRNAs are considered to monogenic. Monogenic means that an RNA transcript contains exons from only one gene. However, up to one fourth of the transcripts in C. elegans have been show to be multi-genic (i.e. they contain exons from multiple genes).

A further complication of the splicing process is that mRNA can undergo alternative splicing. To illustrate this let's imagine a gene that has 3 exons and two introns. From this gene, three different final transcripts are possible. In all transcripts the two introns are going to be removed, but the cell can combine the exons however it wants as long as the original order is maintained. This means that for this example the possible mRNA transcripts include: Exon1-Exon2, Exon1-Exon3, and Exon1-Exon2-Exon3; however, Exon3-Exon1 is not possible because the exons are out of order.

An interesting side note is that some introns are capable of self-splicing, that is they can politely remove themselves without the intervention of any proteins. This is significant mainly because it is a significant counter example to the idea that RNA is an inert transcript and action is soley the domain of proteins. RNAs should really be viewed as having both enzymatic properties and abstract information-carrying ability. Because of this many people believe that RNA was the original genetic molecule and that DNA and proteins evolved later in the game.

Alternative splicing is a very important and powerful tool. To understand the benefit alternative splicing gives the cell we need to understand something about proteins. Proteins can be understood as containing modularized functional units. These functional units can be active sites on enzymes, large structural motifs such as beta-sheets or alpha-helices, or motifs that direct the eventual destination of expressed proteins. A good example of an alternatively spliced pre-mRNA transcript is the mouse IgM immuoglobulin transcript. IgM exists in two forms: excreted and membrane bound. These two forms of the protein differ in the only in the C-terminus: the secreted protein has a secreted terminus motif while the membrane-bound protein has a C-terminal membrane anchor region. Both products come from the same pre-mRNA, but alternative splicing includes either the terminal exon that creates the excreted form of IgM or the membrane-bound form of IgM.

3' poly-adenylation

  • Important for cellular transport.
  • Helps stabilize the transcript

The poly(A) tails are formed in a two step process: an endonulcease cleaves around 1000-2000 non-coding bases from the 3' end of the pre-mRNA transcript and then poly(A) polymerase adds 20-200 AMP molecules to the 3' end of the transcript. The poly(A) tail is important in the cellular transport of the mRNA transcript and, like the 5' cap, also helps to stabilize the mRNA transcript.

Questions & Answers

what is variations in raman spectra for nanomaterials
Jyoti Reply
I only see partial conversation and what's the question here!
Crow Reply
what about nanotechnology for water purification
RAW Reply
please someone correct me if I'm wrong but I think one can use nanoparticles, specially silver nanoparticles for water treatment.
Damian
yes that's correct
Professor
I think
Professor
what is the stm
Brian Reply
is there industrial application of fullrenes. What is the method to prepare fullrene on large scale.?
Rafiq
industrial application...? mmm I think on the medical side as drug carrier, but you should go deeper on your research, I may be wrong
Damian
How we are making nano material?
LITNING Reply
what is a peer
LITNING Reply
What is meant by 'nano scale'?
LITNING Reply
What is STMs full form?
LITNING
scanning tunneling microscope
Sahil
how nano science is used for hydrophobicity
Santosh
Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest. Rafiq
Rafiq
what is differents between GO and RGO?
Mahi
what is simplest way to understand the applications of nano robots used to detect the cancer affected cell of human body.? How this robot is carried to required site of body cell.? what will be the carrier material and how can be detected that correct delivery of drug is done Rafiq
Rafiq
what is Nano technology ?
Bob Reply
write examples of Nano molecule?
Bob
The nanotechnology is as new science, to scale nanometric
brayan
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Damian
Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
Renato
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
?
Kyle
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
Adin
why?
Adin
what school?
Kyle
biomolecules are e building blocks of every organics and inorganic materials.
Joe
anyone know any internet site where one can find nanotechnology papers?
Damian Reply
research.net
kanaga
sciencedirect big data base
Ernesto
Introduction about quantum dots in nanotechnology
Praveena Reply
what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
Bharti
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
Damian Reply
absolutely yes
Daniel
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
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Source:  OpenStax, Statistical machine learning for computational biology. OpenStax CNX. Oct 14, 2007 Download for free at http://cnx.org/content/col10455/1.2
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