Ribonucleic Acid (RNA)

DNA contains all the necessary information to pass on inherited characteristics to the next generation. It represents an alphabet, just like the alphabet used to read words in English textbooks. The genetic alphabet, which is comprised of only four letters, produces proteins instead of words based on the specific DNA sequence. These sequences of word–like instructions dictate which specific proteins must be manufactured in order to create a specific trait such as brown or green eyes in a human, a muscle cell in the legs of a lizard, or a brain cell in an elephant. RNA serves as an intermediate molecule that translates the instructions from DNA into protein.

During the initiation of gene expression, the DNA double helix unwinds to produce two separate strands with their amines sticking out from the backbones. These strands of DNA then serve as an exposed pattern that can bind to complementary base pairs made up of RNA. The complementary base pairing is the same as DNA (A binds to T and C binds to G, vice versa) except that when RNA base pairs with DNA, the A in a DNA strand with bind to U instead of T to create the RNA strand.

RNA plays an important role in each step in gene expression. In the first, the DNA molecule containing a gene is transcribed into RNA. In the next step, these instructions, in the form of messenger RNA (mRNA), exit the nucleus into the cytoplasm. In the last step, the RNA is translated into protein by matching the correct amino acid with its cognate RNA codon (three base pair) sequence. Various unique RNA molecules play a role in these processes. The RNA molecule is transcribed from DNA by an enzyme called RNA polymerase. DNA is replicated or copied by different enzyme called DNA polymerase. RNA polymerase differs from DNA polymerase in that it pairs U with A. The transcribed RNA molecule undergoes extensive processing such as splicing out the introns (noncoding regions that separate exons) so that only the exons (regions that code for protein) remain. Additionally, its structure is stabilized by a long tail consisting of repeated A bases, called a polyadenylation tail that prevents the molecule from being degraded by proteins in the cytoplasm called RNases. mRNA is the processed form of RNA and represents a form of RNA that can be delivered from the nucleus to the cytoplasm. Once in the cytoplasm, the mRNA attaches to the ribosome, a particle that is 10–20 nanometers in size and is made up of both protein and RNA. The RNA in the ribosome is called ribosomal RNA (rRNA). Specific amino acids are then matched to the appropriate corresponding mRNA sequence, or codon, by another type of RNA called transfer RNA (tRNA). The tRNA transfers specific amino acids to the mRNA on the ribosomes during protein synthesis.

RNA, therefore, represents a group of molecules that form various structures with unique functions that are critical for both transcription and translation.