The ribosome, a structure composed of RNA and protein, constructs proteins based on the instructions provided by DNA; ribosomes may be free floating in cytoplasm or attached to form rough endoplasmic reticulum. It is here that the code from the DNA is interpreted and a specific sequence of amino acids, which are the building blocks of proteins, is produced. The following is a summary of the two main parts of this process in eukaryotes called transcription and translation. Ribonucleic acid (RNA) is an organic molecule that carries genetic messages out of the nucleus. It consists of a single strand of nucleotides. Transcription is the formation of messenger mRNA from the template DNA strand to be used to build proteins. Messenger RNA (mRNA) is an RNA molecule made from a DNA template and contains the complementary gene sequence—that is, the sequence opposite to the DNA strand. The mRNA leaves the nucleus, the organelle that houses DNA, and goes into the cytoplasm, the watery interior of the cell, where the ribosomes are located. Translation is the assembly of amino acids into a proteins in the ribosomes through the reading of mRNA by transfer RNA and the ribosome. Transfer RNA (tRNA) is the molecule that carries each amino acid to the strand of mRNA during translation of protein synthesis.
The concept that genetic information flows from DNA to RNA, and from RNA to proteins is called the central dogma of biology. While this idea has undergone much revision as new information becomes available, the general premise has remained the same. The original thinking was that there was "one gene for one protein," with the idea that each small piece of DNA would code for a particular protein. Research has shown that many proteins are the result of more than one chain of amino acids. Hemoglobin, the protein in blood that carries oxygen, is composed of four different proteins. In addition, modification can occur that allows multiple proteins to be produced from one gene sequence. These details do not alter the directional flow of genetic information from DNA to protein.
Types of RNA
Three Types of RNA
- mRNA: This is messenger RNA. It is the molecule made from a DNA template that contains the complementary gene sequence, that is, the sequence opposite to the DNA strand. It transcribes the DNA code in the nucleus and carries it out into the cytoplasm. It is a single-strand sequence complementary to the DNA gene sequence.
- tRNA: This is transfer RNA. It is the molecule that carries each amino acid to the strand of mRNA during translation of protein synthesis. It is a looped structure that recognizes a particular sequence of three nucleotides (basic structural genetic units) on the mRNA and binds to the specific amino acid coded by that mRNA sequence. This form of RNA then brings the amino acids to the ribosomes for production of the protein chain.
- rRNA: Ribosomal RNA (rRNA) is the RNA component of ribosomes that catalyzes peptide bond formation. Together with protein molecules, it forms a ribosome, which is a site of protein synthesis.
The structure of an RNA molecule is similar to that of a DNA molecule except for a few differences: RNA is single-stranded rather than double-stranded; it contains a ribose sugar rather than a deoxyribose sugar, and the base uracil is used in place of thymine.
Comparison of DNA and RNA
|Structure||double-stranded (forms a double helix)||single-stranded|
|Nitrogen-containing bases||A, T, G, C||A, U, G, C|