codons

[jnkdna]

i have a question i need some help answering. it is " a strand of mRNA was found to have 53 codons but the protein produced from it contained only 51 amino acids. suggest 2 reasons for this difference."
is it because 1 codon is the start codon & the other is a stop codon?

[MrMistery]

Yes, but it is a little more complex than that, because start codons are transcripted. So it would appear that your protein should have 52 aminoacids. But, if this were true, every protein should start with methionin, which is false. The answer life devised is very simple: if the protein doesn't start with methionin, it is added anyway so that transcription can start and it is removed afterwards.
Thus, a 53 codon-long mRNA can give:
- a protein with 52 aminoacids, if it starts with methionin
- a protein with 51 aminoacids, if it starts with another aminoacid. A good example for this case(and you can tell it to your teacher to amaze him;) ) is insulin. Insulin contains 51 aminoacids, the first one being Glycine. for the determining of the structure of insulin Sanger was awarded with the Nobel prize in 1952

[MrMistery]

Please do not post, or triple post. Post your message only once. Because this message was posted 3 times, 2 of the messages were deleted.
These were replies to the other 2 posts:

probably.the first codon(usually AUG) codes for methionine which is later removed in post translational modification.stop codon is responsible for the termination of protein synthesis.UAA,UGA,UAG are the stop codons.

start and stopcodons are also present in proteins.

--> maybe there was a mutation that changed an nucleotide so that the stopcodon occured earlier

--> or: a wrong nucleotide was inserted by RNA-polymerase and the stopcodon occured earlier.

Please never post the message more than once in the future
@sdekivit
From what i know, stop codons are not transcripted. If they are, what aminoacid do they bring? They don't code for anything...

[victor]

Maybe the protein structure will be formed like this [primary structure]:
Met-Lys-Gly-[Blnk]-Thr-Trp-Pro.......etc.

[MrMistery]

Of yeah... I forgot to mention that posibility..