Recently, you read about nucleic acids and their general structures. Now we present a nucleic acid called DNA, which stands for deoxyribonucleic acid. DNA is present in every cell of every living creature and carries that individual's unique genetic information. DNA plays an important role in the development of each organism, so let us take a closer look at this molecule.
This topic will help you to understand more deeply what DNA is and introduce DNA structure, properties, and functions.
Structure and properties
As a nucleic acid, DNA is a polymer consisting of repeating subunits called nucleotides. DNA consists of nucleosides made with deoxyribose as the sugar component and one of four nitrogenous bases: guanine (G), cytosine (C), adenine (A) and thymine (T). Following base-pairing rules (A-T, G-C), two strings of nucleotides connected along the sugar-phosphate backbone form a double-stranded helix structure with two antiparallel chains. The ends of the chains are numbered for orientation: the end with a phosphate group is called the 5'-end, and the end with the hydroxyl group is called the 3'-end.
DNA can take on different conformations. The dominant form is B-DNA, which is a right-handed helix. In each turn of this coil, there are about 10 pairs of nucleotides stretching 3.4 nm in length. If you picture the chain as a ribbon with two sides, one side of the chain would have a smaller space between the backbones and the other would have a wider space. This is because the chains are not perfectly aligned, and we call these sides the "major groove" (wider) and "minor groove" (narrower). The major groove has more room for other molecules to interact with the chain, so DNA-associated proteins generally interact with regions of the major groove. In certain conditions, other forms of DNA like A-DNA or Z-DNA can be found in a cell. A-DNA is a compact right-handed helix, each coil of which contains 11 base pairs. Z-DNA has a left-handed, zigzag structure. These conformations are quite interesting to know more about.
Size of DNA
All bases, with rare exceptions, are paired. Such pairs are taken as a unit of measurement and are designated "bp" (from the English base pair). To describe the large scale of most DNA strands, we use:
- kilobase or kb — a thousand base pairs;
- megabase or Mb — a million base pairs;
- Gigabase or Gb — a billion base pairs.
Biodiversity and the phenomenon of supercoiling
DNA can be presented in different forms. For instance, eukaryotic DNA is long and linear. It is wound around histone proteins to form a compact structure called chromatin, which allows every cell to store its large amount of genetic information. Chromatin is much more stable than unwound DNA. Histones are not only used for DNA packing, but also for DNA regulation. If a strand of DNA is weakly twisted, genes on that strand may be more accessible to be activated more frequently. The opposite is true for tightly wound strands. In prokaryotes, DNA is presented as closed circular molecules. A long DNA contained in a central area of a cell is called a nucleoid DNA. Many prokaryotes also have short DNAs called plasmids that are distinct from nucleoid DNA and may afford various genetic benefits in specific environments.
Interestingly, some circular DNAs and protein-capped linear DNAs are capable of supercoiling. Supercoiling is the phenomenon of additional helicalization that DNA undergoes for compaction and regulation. Supercoiling can occur in different directions. When two DNA strands wind around each other into a helix, it is called a twist. When the axis of a double helix coils in on itself, it is called a writhe. Right-handed coils are assigned negative numbers (negative supercoiling), and left-handed coils are assigned positive numbers (positive supercoiling). The picture below shows variations of plasmid supercoiling.
DNA functions
DNA is the main carrier of genetic information. Every instruction for a cell to function is recorded in DNA. DNA plays an important role in protein synthesis. It is like a cookbook holding all the "recipes" to create different proteins.
DNA is also how genetic information is shared by organisms, typically from parent to offspring. During cell division, DNA is replicated and two identical copies are produced. One is kept in the original cell and one goes to the newly made cell. Thus cells store identical copies of DNA, identical "recipes."
Because it contains the instructions for the life of each cell, DNA is generally highly conserved each time it is copied. However, even in such a conservative structure, mistakes can be found. These mistakes are called mutations. Parents' DNA copies with their mutations take part in the exchange of genetic material during sexual reproduction or chromosomal crossover. After crossing over, children's cells inherit parents' information, including beneficial and non-beneficial mutations. This process is schematically shown in the figure below. The inheritance of mutations is one of the mechanisms of evolution.
Conclusion
- DNA is a nucleic acid. It consists of a phosphoric acid residue, a deoxyribose, and a nitrogenous base (guanine (G), cytosine (C), adenine (A) or thymine (T));
- DNA is a double-strand helix structure and has more than one conformation (B-DNA, A-DNA, Z-DNA);
- DNA is linear in eukaryotic cells and circular in prokaryotic cells. In addition, prokaryotes often have short DNA chains called plasmids. Only closed or protein-capped structures are capable of supercoiling;
- DNA plays an important role in storing and transferring genetic information