RNA
Ribonucleic acid, or RNA, is mainly involved in the process of protein synthesis under the direction of DNA. RNA is usually single-stranded and is made of ribonucleotides that are linked by phosphodiester bonds. A ribonucleotide in the RNA chain contains ribose (the pentose sugar), one of the four nitrogenous bases (A, U, G, and C), and the phosphate group.
Types of RNA
There are four major types of RNA:
- messenger RNA (mRNA),
- ribosomal RNA (rRNA),
- transfer RNA (tRNA), and
- microRNA (miRNA).
Messenger RNA (mRNA)
The first, mRNA, carries the message from DNA, which controls all of the cellular activities in a cell. If a cell requires a certain protein to be synthesized, the gene for this product is turned on. Then, the messenger RNA is synthesized in the nucleus. The RNA base sequence is complementary to the coding sequence of the DNA from which it has been copied. However, in RNA, the base T is absent and U is present instead. If the DNA strand has a sequence AATTGCGC, the sequence of the complementary RNA is UUAACGCG. In the cytoplasm, the mRNA interacts with ribosomes and other cellular machinery (see image below).
A ribosome has two parts: a large subunit and a small subunit. The mRNA sits in between the two subunits. A tRNA molecule recognizes a codon on the mRNA, binds to it by complementary base pairing, and adds the correct amino acid to the growing peptide chain. Image Attribution: OpenStax Biology
Ribosomal RNA (rRNA)
The mRNA is read in sets of three bases which we refer to as codons. Basically, each codon codes for a single amino acid. In this way, the mRNA is read and the protein product is made. Ribosomal RNA (rRNA) is a major constituent of ribosomes on which the mRNA binds. The rRNA ensures the proper alignment of the mRNA and the ribosomes. Furthermore, the rRNA of the ribosome also has an enzymatic activity (peptidyl transferase) and catalyzes the formation of the peptide bonds between two aligned amino acids.
Transfer RNA (tRNA)
Transfer RNA (tRNA) is one of the smallest of the four types of RNA. It is usually 70–90 nucleotides long. In addition, it carries the correct amino acid to the site of protein synthesis. As a matter of fact, it is the base pairing between the tRNA and mRNA that allows for the insertion of the correct amino acid in the polypeptide chain.
microRNA (miRNA)
microRNAs are the smallest RNA molecules. Their role involves the regulation of gene expression by interfering with the expression of certain mRNA messages.
Features of DNA and RNA
The table below summarizes features of DNA and RNA.
| Features of DNA and RNA | ||
|---|---|---|
| DNA | RNA | |
| Function | Carries genetic information | Involved in protein synthesis |
| Location | Remains in the nucleus | Leaves the nucleus |
| Structure | Double helix | Usually single-stranded |
| Sugar | Deoxyribose | Ribose |
| Pyrimidines | Cytosine, thymine | Cytosine, uracil |
| Purines | Adenine, guanine | Adenine, guanine |
Even though the RNA is single stranded, most RNA types show extensive intramolecular base pairing between complementary sequences, creating a predictable three-dimensional structure essential for their function.
Transcription and Translation
As you have learned, information flow in an organism takes place from DNA to RNA to protein. DNA dictates the structure of mRNA in a process known as transcription, and RNA dictates the structure of protein in a process known as translation. This is known as the Central Dogma of Life, which holds true for all organisms. However, exceptions to the rule occur in connection with viral infections.
Video Summarizing Nucleic Acids
This short video below by Ricochet Science describes the structure and function of nucleic acids.