Is it possible for any middle exons of a gene to start with ATG or end with TAG/TAA/TGA ? Shouldn't it behave as start or stop codon?
I think that TAG / TAA / TGA / ATG might be present at any position along with the start and end positions of a gene. But can it be present at start / stop of any middle exons?
Suppose a middle exon contains following nucleotide sequence.
Case 1 : ATGaacggattat
Case 2 : aataacggatTAG / aataacggatTGA / aataacggatTAA
Are the above cases possible? Please help and if possible provide me with examples/links of such exons/genes present.
Dear Arnav Garai,
Yes, it is quite possible that exons may start or end by ATG, TAG, TGA or TAA because the start or end of exon do not determine the coding sequence. As you know, the coding sequence is determined by sequence of mature mRNA formed after mRNA splicing. Please, have a look at the sequence of exons of phldb1 gene at this link - http://www.ensembl.org/Homo_sapiens/Gene/Sequence?db=core;g=ENSG00000019144;r=11:118606440-118658038 .
In this gene, you can clearly see TAG or TGA at beginning or end of exons. If you explore few more genes, you will find ATG also at beginning or end of exons..
Best regards,
Kshitiz
yes it is possible that start and stop codon may present within exons but if we see that very carefully then we will see that stop codon is not present within the same frame of the transcript (joining off all coding part of a nucleotide). If we proceed by reading three nucleotides (codon) at a time then the stop codon must not present in that frame.
E.g: ATGGCATGGATGA...
If you consider the above example then you can see the stop codon is not present in the same frame from where the coding region is started.
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