What is the significance of the first and last codons of an mRNA transcript?

Question

Can anyone help me with this

 

Answer 1

I looked in Wiki and there is a description and proper nomenclature for the Start and Stop.

http://en.wikipedia.org/wiki/MRNA

 

cap

"The 5' cap is a modified guanine nucleotide added to the "front" (5' end) of the pre-mRNA using a 5'-5'-triphosphate linkage. This modification is critical for recognition and proper attachment of mRNA to the ribosome,"

 

coding regions

"Coding regions are composed of codons, which are decoded and translated (in eukaryotes usually into one and in prokaryotes usually into several) into proteins by the ribosome. Coding regions begin with the start codon and end with a stop codon. In general, the start codon is an AUG triplet and the stop codon is UAA, UAG, or UGA. ..."

 

tail

"The 3' poly(A) tail is a long sequence of adenine nucleotides (often several hundred) added to the 3' end of the pre-mRNA. This tail promotes export from the nucleus and translation, and protects the mRNA from degradation."

Answer 2

The start codon (usually AUG) tells where to begin making proteins and start the translation process.  The stop codon ends the translation process.  When there is a mutation (frameshift, point, silent mutation, etc.) it can change the location of the start and/or stop codons.  This can cause significant changes in the organism or no visible change at all.  For example, this can result in the production of a different amino acid and protein than in the original DNA blueprint.  Not all mutations are bad, but some can cause significant harm.  Typically, because there may be several codes for the same amino acid (such as Alanine), the damage of the change in the start and/or stop code may be minimal.  

Answer 3

Hi Sherry,

 

Start codon is usually AUG, it encodes for methionine  and begins translation of mRNA by RNA polymerase. It is the signal that polymerase can start translating the transcript/mRNA to synthesize the protein. Stop codons UAA, UAG, or UGA signal/tell the polymerase that job is over, it is time to stop!