Types and Structure of Ribonucleic Acid
Edited by Jen Moreau, Marisha
Ribonucleic acid (RNA) is a polymeric molecule and like the DNA (deoxyribonucleic acid), is composed of nucleotides. RNA with a chain length that ranges from 10 to more than 1000 nucleotides, serve various biological functions in an organism's cell. These functions include coding, decoding, carrying genetic information, regulation, and expression of genes in living organisms. A nucleotide is the building block of ribonucleic acid; consisting of three sub-units namely, nitrogenous base, a five-carbon sugar, and a phosphate group.
While proteins are widely known to be responsible for catalysis in cells, some RNA also play crucial roles as catalysts in biological reactions. For example, ribozyme catalyzes chemical reactions in a similar fashion as protein enzymes.
There are three main characteristics that distinguish RNA from other types of nucleic acids:
- 1RNA is a single-stranded molecule, however, it can form a double-stranded molecule through complementary pairing.An example of an RNA complementary pairing is HIV viral RNA. HIV viral RNA is normally single-stranded but is converted to a double-stranded RNA by the enzyme 'reverse transcriptase' once the virus enters the host cell.Advertisement
- 2The sugar on the RNA molecule contains a ribose sugar on its backbone.This means that ribose has a hydroxyl group (OH) on the 2' carbon and deoxyribose does not. Deoxy stands for the lack of one oxygen atom that is present in ribose sugar. This makes deoxyribose more stable than ribose.
- 3There are two types of nitrogenous bases in any ribonucleic acid, namely -- purine and pyrimidine.Purines are composed of adenine (A) and guanine (G), while pyrimidines are made up of cytosine (C), and uracil which is written as (You). Sometimes thymine (T) will be present along with uracil depending on the type of ribonucleic acid. Consequently, the nitrogenous bases in RNA are adenine, guanine, uracil, and cytosine.
Comparison with DNA
DNA (Deoxyribonucleic acid) is another type of nucleic acid which carries the genetic instruction in all living organisms. The genetic information includes growth, development, reproduction, and functioning of an organism. Like an RNA molecule, DNA is also composed of nucleotides. However, slight differences exist between the nucleotides of the two, as well as in the arrangement of the molecules. Although these differences may appear subtle, they contribute towards significant functional differences in the two molecules.
The chemical structure of RNA differs from DNA in the following ways:
- 1DNA is a double-stranded helical molecule, as compared to RNA which is usually single stranded.Occasionally, RNA can be paired to form a double strand.
- 2In the nitrogenous base of DNA, thymine is paired with adenine while in RNA, the thymine is replaced by uracil.
- 3The DNA molecule contains a deoxyribose sugar as opposed to the ribose sugar found in RNA.
Synthesis of RNA
The synthesis of RNA is called transcription. The transcription is a process of transcribing DNA nucleotide sequence information into RNA sequence information. The synthesis is catalyzed by an enzyme called RNA polymerase. During the initiation of transcription, RNA polymerase unwinds a short stretch of double-helical DNA to produce a single-stranded DNA template from which it takes instructions. The enzyme then progresses in the 3′ to 5′ direction to synthesize a complementary RNA molecule.
There are different classes of RNA namely:
- 1messenger RNA (mRNA)
- 2Ribosomal RNA (rRNA)
- 3Transfer RNA (tRNA)
- 4Small interfering RNA (siRNA)
- 5Micro RNA (miRNA)
- 6Small nuclear RNA (snRNA)Advertisement
Messenger RNA (mRNA) carries information about the protein to be synthesized to the ribosome, hence it is termed as the messenger. The ribosome is found in both prokaryotes and eukaryotes and serves as the site of protein synthesis. Ribosomes link amino acids together in the order specified by the mRNA, thus an mRNA can be called as the architect in protein synthesis, while ribosome is the constructor.
Ribosomal ribonucleic acid (rRNA) is made up of two components, protein, and RNA. rRNA binds tRNA and other accessory molecules for synthesis. The RNA component of the ribosome has a catalytic function like an enzyme, hence it is called the ribozyme.
Transfer RNA (tRNA) is another class of ribonucleic acid and it acts as the middleman between mRNA and rRNA in the synthesis of protein. tRNA has two arms, with one of the arms attached to amino acids, while the other binds to the mRNA. Amino acids are carried by tRNA and connected by a peptide bond during the process of translation, to produce a protein.
MicroRNA (miRNA) is a non-coding RNA of 20-22 nucleotide length, that plays an important role in gene silencing. miRNA binds to mRNA, thus silencing the gene. The application of miRNA is being explored in biotechnology to silence unwanted genes in cancer therapy. The failure of miRNA to efficiently silence unwanted genes often result in the uncontrolled synthesis of the protein, which is a hallmark for cancer. Hence, efforts are on to increase the activity of miRNA in cells for stricter post-transcriptional regulation of gene expression.
Small interfering RNA (siRNA) are double-stranded RNA and contain about 19 nucleotides. siRNA, along with miRNA bind to mRNA and promotes gene silencing. While miRNA converts mRNA to 'processing bodies' which are then either stored or destroyed, the siRNA on the other hand always destroys the mRNA.
Small nuclear RNA (snRNA) is involved in the process of gene splicing. siRNA is often found in the intron region of genes encoding proteins that are involved in the ribosome synthesis. The average length of snRNA is 150 nucleotides. However, the specific mechanism by which snRNA regulates RNA modification is still unknown.
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Categories : Biology
Recent edits by: Jen Moreau