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|Chapter Six: DNA, RNA, and Protein Synthesis|
There are three types of RNA: messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA). mRNA is used to carry the genetic code from the nucleus to the ribosomes in the endoplasmic reticulum, tRNA bonds to amino acids and it used in the synthesis of proteins, and rRNA is a component of ribosomes.
mRNA is by far the longest of the RNA molecules, since its function is to transport significant portions of the DNA code to the ribosomes. Before entering the cytoplasm on its way to the ribosomes, an mRNA molecule is modified somewhat. First, a 7-methylguanosine "head" is added which serves to help attach the mRNA to a ribosome during protein synthesis. Also, a so-called poly-A "tail" consisting of about 200 adenosine residues is attached to the end of the mRNA molecule. Research suggests that this tail protects the mRNA molecule from being destroyed by enzymes in the cytoplasm.
When the code on the mRNA is translated into a sequence of amino acids in order to form a protein molecule, tRNA serves as the molecule which transfers the amino acid to the ribosome when the codon requires it. On one end of the tRNA molecule is an area to which a specific amino acid attaches. On the other end is a sequence of three base pairs called the anticodon, the complement of the codon on the mRNA which codes for a certain protein. In the following section about a process called translation, the importance of this will become clear.
The third type of RNA is rRNA. The small subunit of a ribosome contains an rRNA strand 1542 nucleotides long, and the larger subunit contains two strands of rRNA, one 2904 nucleotides in length and the other 120. It is still not known how the rRNA and proteins in the ribosome fit together.
When scientists first began to tackle the problem of solving the genetic code, it was known that there are about 20 significant amino acids found in proteins. If each base coded for a particular amino acid, then only four amino acids would be possible. If instead two bases were required to code for a single amino acid, only 16 amino acids would be possible. Therefore, scientists hypothesized that three bases in sequence coded for one amino acid, since this would allow for the existence of 64 amino acids, well more than the 20 needed. This has since been proven to be true and each of the triplets has been termed a codon.
Through a great deal of experimentation, scientists were able to determine which codons on the mRNA molecule code for which amino acids. Most amino acids have several codons associated with them. The chart below shows all of the different amino acids and their codons.
|Amino acid||Associated codon(s)|
|Alanine||GCA, GCC, GCG, GCU|
|Arginine||AGA, AGG, CGA, CGC, CGG, CGU|
|Asparagine||AAC, AAU, GAC, GAU|
|Glutamic acid||GAA, GAG|
|Glycine||GGA, GGC, GGG, GGU|
|Isoleucine||AUA, AUC, AUU|
|Leucine||UUA, UUG, CUA, CUC, CUG, CUU|
|Proline||CCA, CCC, CCG, CCU|
|Serine||AGC, AGU, UCA, UCC, UCG, UCU|
|Threonine||ACA, ACC, ACG, ACU|
|Valine||GUA, GUC, GUG, GUU|
|"Stop" codon||UAA, UAG, UGA|