DNA and RNA: Structure, Function, and Types
Biological Function of DNA
DNA is the storehouse of genetic information and the molecule responsible for transmitting to offspring the necessary instructions to build all proteins in a living being. DNA has the ability to make copies of itself through a mechanism called replication, based on complementarity between the nitrogenous bases of the two strands of DNA.
DNA in Eukaryotes and Prokaryotes
In both cell types, DNA can take many forms and levels of complexity.
- In prokaryotes, there is a circular DNA molecule (closed ends) that is called the bacterial chromosome. Sometimes, smaller circular molecules are contained, called plasmids.
- In eukaryotes, DNA is found inside the nucleus, forming long linear molecules associated with basic proteins. Each DNA molecule forms a fiber, and all of these fibers constitute the chromatin. When the cell divides, each chromatin fiber is compacted and forms chromosomes. It is estimated that all the DNA contained in the 46 human chromosomes in a cell measures about 236 meters. Most of the DNA of eukaryotic cells is in the nucleus, but it is also found in mitochondria and chloroplasts.
- In viruses, DNA can take several forms. Viruses contain a single molecule that can be single-stranded or double-stranded and can occur in a linear or circular form.
RNA Structure
RNA is single-stranded, except in some viruses where it is double-stranded. There are some areas in single-stranded molecules, called hairpins, where the RNA folds back on itself. RNA takes a part of DNA as a template and performs the function of transmitting genetic information. There are several different types of RNA with the same chemical composition but different structures and functions.
DNA Structure
Primary Structure
DNA is a linear polymer composed of deoxyribonucleotides of adenine, guanine, cytosine, and thymine. As with proteins, DNA has different levels of structural complexity. It presents a primary structure, secondary structure, and sometimes tertiary structure. The DNA strand has two free ends: 5′, coupled with the phosphate group, and 3′, together with a hydroxyl. Nucleotide chains differ in size, composition, and base sequence, indicating the order in which A, T, C, and G are in the chain. The genetic information contained in a specific portion of DNA (what is known in molecular genetics as a gene) carries the information needed to join the amino acids in a certain order.
Secondary Structure: The Watson and Crick Model
The spatial structure of DNA was established in 1953 by Watson and Crick. They already knew that DNA was formed by a nucleotide structure with known dimensions, linked by phosphodiester bonds. They took the work on nitrogenous bases and X-ray diffraction as a basis for discovering the structure of DNA.
- The DNA molecule is long, stiff, and not folded, like proteins.
- In the molecule, structural details are repeated every 0.34 nm and every 3.4 nm.
- The purine base content is equal to the pyrimidine base content (the proportion of adenine equals that of thymine, and the proportion of cytosine equals that of guanine).
- DNA is a double helix of 2 nm in diameter, consisting of two strands coiled around an imaginary axis (its structure resembles that of a spiral staircase, where the steps are the nitrogenous base pairs and the railings are made of sugar and phosphoric acid).
- The winding is right-handed (clockwise) and plectonemic (to separate the two chains, it is necessary to unwind them).
- Each pair of bases is separated by 0.34 nm.
- The two chains are antiparallel (5′ to 3′ and 3′ to 5′) and complementary.