Ribonucleic Acid (RNA)

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Ribonucleic Acid (RNA)

Ribonucleic acid (RNA), like deoxyribonucleic acid (DNA), is composed of nucleic acids that are found in the nucleus of plants and animals. Nucleic acids consist of high–molecular–weight macromolecules, which are made up of hundreds or thousands of smaller single unit molecules called nucleotides, all bound together. These molecules are the storehouse and delivery system of genetic traits and represent an organism’s instruction manual for its protein–comprised manufacturing system. RNA, unlike DNA, is also found in parts of the cell other than the nucleus. In fact, the majority of the RNA is present in the cytoplasm in various forms. Nuclear RNA is comprised of single stranded sequences (DNA is double stranded) and has a lower molecular weight than DNA.

Each nucleotide molecule consists of a sugar group, a phosphate group, and an amino (nitrogen containing) group. The main difference between RNA and DNA is that in RNA the sugar is ribose (a five carbon sugar), while in DNA the sugar is deoxyribose. The prefix deoxy means that one oxygen atom is missing from the ribose. RNA is built from the same nucleotides as DNA just as proteins are built up from amino acids. There are only four bases that makeup RNA: adenine, cytosine, guanine, and uracil (A, C, G, and U, respectively). DNA contains thymine (T) instead of U. Structurally, the backbone consists of alternating sugar and phosphate parts, while the amino groups stick out like branches from the backbone. This coiled backbone in RNA if stretched out, would resemble a stretched out slinky.

The discovery of RNA

Knowledge of the chemistry of a living cell nucleus is thought to have begun in 1869, when the Swiss biochemist Friedrich Miescher (1844–1895) separated the nucleus from the other parts of the cell and isolated phosphorus–containing substances that we

now call nucleic acids, the molecular substrate of the genetic code. It was later found that there were two kinds of nucleic acids, according to the bases that were identified. One type of nucleic acid was obtained from animalglands and later called DNA, while the other type, obtained from yeast cells, was called RNA. It was not until the 1940s that biochemists realized that both DNA and RNA are present in all living cells, whether plant or animal. Although DNA is present only in the nucleus of the cell, RNA is found in both the nucleus as well as the cytoplasm.

Many key discoveries lead to the identification of the source, structure and function of an organism’s genetic material. In 1950, American biochemist Erwin Chargaff (1929–1992) determined that the arrangement of nitrogenous bases in DNA was variable, however, the specific bases seemed to occur in a one–to– one ratio (now known as complementary base pairing). In 1953, British James D. Watson (1928–) and American Francis H. C. Crick (1916–2004) deciphered the molecular structure of DNA using research from their own lab as well as vital results obtained from colleagues. They determined the structure of DNA to be a double helix with two long molecular threads or strands, twisted around each other. American chemist Marshall Nirenberg (1927–) was later credited with translating the code of life and was awarded the Nobel Prize in 1968. He demonstrated that RNA could be translated into protein. Initially, it was thought that there was only one kind of RNA, but other types of RNA with specialized functions have since been discovered.

The role of RNA in gene expression

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 will 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 a 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 (non-coding 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 to 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.

KEY TERMS

Cytoplasm— All the protoplasm in a living cell that is located outside of the nucleus, as distinguished from nucleoplasm, which is the protoplasm in the nucleus.

Gene— A specific sequence of amines, or bases, on a DNA molecule. The sequence is a code for the production of a specific kind of protein or RNA molecule, and therefore for a specific inherited characteristic.

Nucleus— The part of a living cell that is enclosed within a membrane and that contains all the genetic information in the form of DNA.

Protoplasm— The thick, semi–fluid, semi–transparent substance that is the basic living matter in all plant and animal cells.