Genetics and Genomics

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Composition of an mRNA


Eukaryotic mRNAs undergo several steps of processing before they are translated. A 5' cap is added as well as a 3' poly A tail. Perhaps most strikingly, introns are removed so that only the coding sequence remains. Depending on the gene this removal can be quite extensive.

When examining eukaryotic mRNAs and chromosomal DNA, you make the following observations and conclusions:

Comparing the processed mRNAs of a well-conserved gene from two species you find nearly identical sequences. However, when comparing the gene sequences you find shorter sequences that are very similar in both species, both in sequence and length. These are interspersed by longer sequences showing very little similarity in either sequence or length.

The shorter sequences found in both mRNA and DNA are probably

while the longer sequences are probably


You do not find the differences in the processed mRNAs as the sequences which differ between the genes were


The longer sequences not only vary in sequence but also in length. This is most likely because