Types of RNA
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Three general types of RNA exist: messenger, ribosomal, and transfer.
Messenger RNA (mRNA) is synthesized from a gene segment of DNA which ultimately contains the information on the primary sequence of amino acids in a protein to be synthesized. The genetic code as translated is for m-RNA not DNA. The messenger RNA carries the code into the cytoplasm where protein synthesis occurs.
Each gene (or distinct segment) on DNA contains instructions for making one specific protein with order of amino acids coded by the precise sequence of heterocyclic amines on the nucleotides. Since proteins have a variety of functions including those of enzymes mistakes in the primary sequence of amino acids in proteins may have lethal effects.
How can a polymeric nucleotide with only four different heterocyclic amines specify the sequence of 20 or more different amino acids? If each nucleotide coded for a single amino acid, then obviously only 4 of the 20 amino acids could be accommodated. If the nucleotides were used in groups of two, there are 16 different combinations possible which is still inadequate.
It has been determined that the genetic code is actually based upon triplets of nucleotides which provide 64 different codes using the 4 nucleotides. During the 1960's, a tremendous effort was devoted to proving that the code was read as triplets, and also to solving the genetic code. The genetic code was originally translated for the bacteria E. Coli, but its universality has since been established. The genetic code is "read" from a type of RNA called messenger RNA (mRNA). Each nucleotide triplet, called a codon, can be "read" and translated into an amino acid to be incorporated into a protein being synthesized. The genetic code is shown in Figure 7.
Several distinctive features of the genetic code are clearly evident. First, all of the 64 codons or triplets have a known function, with 61 coding for amino acids and the other 3 serving as a stop or termination signal for protein synthesis. Secondly, the code is degenerate, meaning that there are usually several codons for each amino acid. Only methionine and tryptophan have a single codon. More specifics on the importance of the degeneracy of the genetic code will be discussed in a later section.
In the cytoplasm, ribsomal RNA (rRNA) and protein combine to form a nucleoprotein called a ribosome. The ribosome serves as the site and carries the enzymes necessary for protein synthesis. In the graphic on the left, the ribosome is shown as made from two sub units, 50S and 30 S. There are about equal parts rRNA and protein. The far left graphic shows the complete ribosome with three tRNA attached.
The ribosome attaches itself to m-RNA and provides the stabilizing structure to hold all substances in position as the protein is synthesized. Several ribosomes may be attached to a single RNA at any time. In upper right corner is the 30S sub unit with mRNA and tRNA attached.
Note: The coordinates used in this display have only the alpha carbons of the proteins (CA) and the DNA backbone atoms.
Transfer RNA (tRNA) contains about 75 nucleotides, three of which are called anticodons, and one amino acid. The tRNA reads the code and carries the amino acid to be incorporated into the developing protein.
There are at least 20 different tRNA's - one for each amino acid. The basic structure of a tRNA is shown in the left graphic. Part of the tRNA doubles back upon itself to form several double helical sections. On one end, the amino acid, phenylalanine, is attached. On the opposite end, a specific base triplet, called the anticodon, is used to actually "read" the codons on the mRNA.
The tRNA "reads" the mRNA codon by using its own anticodon. The actual "reading" is done by matching the base pairs through hydrogen bonding following the base pairing principle. Each codon is "read" by various tRNA's until the appropriate match of the anticodon with the codon occurs. In this example, the tRNA anticodon (AAG) reads the codon (UUC) on the mRNA. The UUC codon codes for phenylalanine which is attached to the tRNA. Remember that the codons read from the mRNA make up the genetic code as read by humans.
|Quiz: Read and translate the codons on mRNA into the appropriate amino acids. |
G U A C G A A A A
|Quiz: Read and translate the codon on mRNA into the appropriate amino acid. |
AGA. What is the anticodon on the appropriate tRNA?
Charles Ophardt (Professor Emeritus, Elmhurst College); Virtual Chembook