Primary Transcript - Introns(intervening sequences must be...

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Primary Transcript A primary transcript is copied from a sequence of DNA that codes for a specific protein ( Fig. 1 ). o DNA unwinds o Enzymes form phosphodiester bonds between ribonucleoside triphosphates o mRNA is released . The primary transcript retains all of the information of the DNA sequence from which it was transcribed. Messenger RNA (mRNA) An mRNA is an RNA that is translated into protein. In prokaryotic cells a primary transcript is used directly as an mRNA (often times before it is even completely transcribed). In eukaryotic cells a primary transcript is processed before being exported from the nucleus as an mRNA: A 5'CAP of 7-methyl guanosine is added ( Fig.2 ). A poly(A) tail is added to the 3' end of the transcript.
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Unformatted text preview: Introns (intervening sequences) must be cut from the transcript by a process known as RNA splicing ( Fig. 3 ). mRNAs are very short-lived compared to DNA. In prokaryotes they are only around for a few minutes. Continuous synthesis of protein requires a continuous synthesis of mRNA. This helps the prokaryotic cell respond quickly to a fluctuating envirnoment and fluctuating needs. In eukaryotic cells the mRNA are stable for 4-24 hrs. The mRNA of prokaryotic cells is polycistronic (one transcript can code for several different proteins) ( Fig. 5 ). The mRNA of eukaryotic cells is monocistronic (each transcript only encodes a single protein) ( Fig. 1 )....
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