2006-09-19 20:51:44 · 5 answers · asked by Anonymous in Science & Mathematics ➔ Biology
I have got two answers, thank you so much. But it seems I should add some details about the question. many mRNA have untranslated regions (UTR) besides their capping and poly(A) tail fraction. it is said that the UTR between the stop codon and the poly(A) tail has something to do with mRNA stability. I would like to confirm this opinion and like to know if any other role(s) played here. thank you.
2006-09-19 22:05:21 · update #1
I’ll only refer to eukaryotic mRNAs as most prokaryotic mRNAs have short lifetimes. The answers so far are mixing up a few processes. Alternative splicing can generate different protein coding mRNAs but those mRNAs will still have 3’ UTRs. PolyA tails (~250 adenines long) are put on after a polyA signal but the distance between the stop codon and the start of the polyA tail can be quite long. The average 3’ UTR in humans is about 0.6 kb and can be much longer. The polyA tail is not considered part of the 3' UTR.
The structure and composition of the 3' UTR is another way to regulate gene expression. Some sequences (e.g. AU Rich Element) can enhance or shorten the time that the mRNA hangs around in the cell. Other sequences can act to regulate translation of the message. There can be binding sites for miRNAs (microRNAs) in the 3’ UTR that will block translation of the mRNA.
2006-09-20 06:50:24 · answer #1 · answered by Slackenerny 4 · 1⤊ 0⤋
As described in the answers above, the Poly A tails in the UTR of mRNAs protect them from the action of nucleases. There can also be other reasons.
In some cases, one mRNA can produce more than one proteins.
During translation of some mRNAs, frameshift occurs due to 'hiccups' in the ribosome. This leads to a change in the reading frame of the mRNA and consequently a polypepdide with a different sequence of aminoacids is produced.
This means, If there is a stop codon in the 100th place, and this frameshift occurs when the 90th nucleotide is being translated, then the stop codon in the 100th place will be missed and translation will proceed until the next stop codon is encountered.
The product of such a process will have a similar aminoacid sequence until the location where frameshift occured. The remaining aminoacids (the ones coded by the new reading frame after 90th Base) will be different.
Therefore, the UTR may be present to facilitate such a process.
2006-09-20 05:58:24 · answer #2 · answered by mad g 2 · 1⤊ 0⤋
Ak R is basically correct.
In eukaryotes encoded DNA is transcribed into RNA ready for translation to take place. A series of enzymes (polymerases) copy the selected DNA sequence, and produce a length of hnRNA (precursor RNA). Before this can be made into mRNA, it must be modified. A cap is added to the start of the sequence to enable it to leave the nucleus, and a polyadedine tail is added to the end. This tail has several functions; the tail stimulates correct splicing (cutting up) of the mRNA, as Ak R says, it prevents endonuclease activity, but it is also required for translation to occur because it enables a ring to be formed. This ring is important because eukaryotic mRNA is translated in a ring configeration. So if there was no sequence after the stop codon (which only indicates to the ribosome where to stop adding amino acids) translation will not happen.
I hope that explains it for you. If you would like more info, or help with the explanation, you can e-mail through my info page
Additional:
I tried to reply to your e-mail but it wouldn't let me e-mail you...so
Re: thanks and further discussion
Also i should add that in bacteria, negative translational control can take place in the 3' UTR, although in most cases it is in thr 5' UTR.
Hope this helps in some way. If you have access to a good library, ie a clincal library, look for "Molecular Biology of the Cell", Published by Garland Science, and written by Bruce Alberts et al. It will explain things better than me!
BB
Could you let me know if you read this. Else i will try and e-mail you with it again
2006-09-20 05:19:39 · answer #3 · answered by Bacteria Boy 4 · 1⤊ 0⤋
The part of the mRNA sequence after the stop codon is just a long tail of adenine molecules, called a poly adenine tail. This tail along with the methyl guanosine head is attached to the mRNA after splicing (removal of junk regions) both are put on to prevent cellular enzymes (RNA ligases) from 'eating away' parts of the mRNA
2006-09-20 03:57:23 · answer #4 · answered by Anonymous · 2⤊ 0⤋
To put it simply mRNA sequences after a stop codon are offcuts from from the mutations that formed the sequence during the course of it`s evolution.
2006-09-20 04:04:36 · answer #5 · answered by LenV 2 · 1⤊ 1⤋