The Structure of mRNA

A primary transcript is copied from a sequence of DNA that codes for a specific protein (Fig. 1).

• DNA unwinds
• Enzymes form phosphodiester bonds between ribonucleoside triphosphates
• mRNA is released .
 The primary transcript retains all of the information of the DNA sequence from which it was transcribed.

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.

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).

The universal Genetic Code was deciphered in the early 1960s.
It is a triplet code. In other words, each sequence of 3 mRNA nucleotides is a codon which corresponds to one amino acid.

Example:

 5'- A U G U U U C G U A C G U A A- 3'

  N- Met    -  Phe  -  Arg  -  Thr   -C

 Each codon - codes for a specific amino acid or serves as a stop codon. (Stop codons are: UAA, UAG, UGA
These three codons do not code for an amino acid. They signal as a stop signal for translation. )

 The Start codon is AUG.
AUG codes for the amino acid, methionine, so every protein initially begins its sequence with methionine regardless of whether it is a prokaryotic or eukaryotic protein.

 There are 3 possible reading frames on a message depending upon where you start. So punctuation is very important (AUG) codon critical (Fig. 4).

 Because there are 4 different nucleotides, and it is a triplet code--  there are --  4³=64 possible codon triplets for only 20 different amino acids. Therefore, most amino acids are specified by more than 1 codon-- in other words the genetic code is a degenerate code.

 An example, all of the following codons code for the amino acid arginine:

CGU
CGC
CGA
CGG

 The Genetic code also is highly conserved, the same in organisms as diverse as bacteria, plants, and man.

 There are as many different mRNAs as there are genes. Most proteins have between 100 and 1,000 amino acids so an mRNA must be at least between 300 and 3,000 nucleotide bases long.

Each mRNA possesses codons that are read by tRNAs.