Varieties of RNA

Ribosomal RNA
Ribosomal RNA (rRNA) is, as its name suggests, the RNA that plays a crucial role in the creation of ribosomes. This kind of RNA is transcribed in the nucleolus (all other types are not) from DNA segments that are looped into structures called nucleolar organizer regions. Because rRNA is often in high demand in a cell, the data necessary for their creation is often repeated a number of times on the DNA strand. This repetition allows thousands of identical molecules to be transcribed simultaneously.

Once transcribed, rRNA can then go to work in the construction of ribosomes. In a cell, ribosomes are the only places where proteins can be synthesized. These are the structures that allow tRNA, mRNA, and amino acids to create polypeptide chains.

Messenger RNA
Messenger RNA (mRNA) is the structure that carries the genetic code instructions for how to create new proteins and enzymes. The major function of the DNA is to allow a cell to synthesize proteins, and it is the mRNA that provides the physical link between the DNA blueprints and the actual polypeptide chains themselves- the finished product.

Proteins are, in essence, a linear series of amino acids arranged in a particular order. There are twenty different amino acids, and from them an almost infinite number of different proteins can be created. To describe these proteins mRNA uses the genetic code. Every three nitrogenous bases in the molecule makes up a unit in this code called a codon, and each codon, then, represents a specific amino acid. Because there are four different nitrogenous bases and three bases to a codon, there are 64 (4x4x4=64) possible units in the genetic code system. With only twenty amino acids, then, many acids can be represented by more than one possible codon. These are called synonymous codons. For example, GGU, GGC, GGA, and GGG all represent the amino acid glycine.

mRNA Codon - Amino Acid Table - a chart of the 64 codons and what they stand for.

In addition to the codons representing amino acids, there are 3 that are reserved as stop codons. UAA, UAG, and UGA all signify the end of a complete gene. There is also a start codon, AUG. This unit, though, doubles as the code for methionine, depending upon its position in the strand. If the codon is found in the leading end of an mRNA strand it signals its start. Otherwise, it adds another amino acid to the chain being generated.

In eukaryotes mRNA is not created ready to go to work. Instead, it must first go through "posttranscriptional modification" to purify its information. Directly after being synthesized from a DNA strand, mRNA contains many noncoding sequences called introns that must be removed by enzymes. Once the introns have been removed, only the relevant portions (called exons) remain. The final mRNA is arranged as follows: First, there is a leading segment, called the "cap." After the cap comes the actual gene, or cistron. Finally, the end of the molecule is attached to many adenine bases. This is sometimes called the poly-A tail.

In prokaryotes mRNA is transcribed from the DNA in its finished form- no posttranscriptional modification is necessary. Also, prokaryotic mRNA lacks a cap and a poly-A tail. Finally, many of these strands contain more than one gene. This polycistronic mRNA allows for many different proteins to be created within the same small area, which is sometimes convenient for certain cellular processes.

Transfer RNA
Though ribosomes can be created with the help of rRNA, and mRNA can bring data to these structures concerning what proteins need to be created, ribosomes themselves cannot decode the genetic messages into polypeptide chains. For this task, a third type of ribonucleic acid is required, tRNA.

Each strand of tRNA is able to decode a specific portion of the mRNA code in the ribosome because of its three major qualities:

1. It connects to a specific amino acid through a charging enzyme
2. It binds to ribosomes
3. Finally, tRNA has an anticodon that allows it to connect to a specific codon in mRNA through Watson-Crick binding

As each of the codons in the mRNA strand being processed binds to a tRNA molecule, the amino acids connected to the transfer RNA are lined up and combined to form the final product: a protein molecule.