DNA Structure and Genetic Information
DNA as a Carrier of Genetic Information
In the early twentieth century, it was accepted that genes were on chromosomes and that they were the carriers of genetic information. However, the evidence that genes are made of DNA (not protein) and its acceptance in the scientific community did not take place until 1950.
Chemical Composition of Nucleic Acids
There are two types of nucleic acids: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Both are long polymers of nucleotides. They consist of a nitrogenous base, a sugar, and phosphoric acid.
Nitrogenous Bases
There are four different nitrogenous bases in nucleic acids:
- In DNA: adenine (A), guanine (G), cytosine (C), and thymine (T).
- In RNA: adenine (A), guanine (G), cytosine (C), and uracil (U).
The nitrogenous bases are basic molecules that contain nitrogen. Adenine and guanine are purine bases, a structure consisting of two cycles, similar to purine. Cytosine, thymine, and uracil are pyrimidine bases, consisting of a single cycle similar to pyrimidine.
Pentose Sugars
The carbohydrate is a pentose, and will vary according to whether the nucleic acid is DNA or RNA.
- In DNA, it is deoxyribose.
- In RNA, it is ribose.
The binding of nitrogen 1 of a base and carbon 1 of a pentose by an N-glycosidic bond forms a nucleoside. They are named by adding the ending “-osine” to the name of the purine base and “-idine” to the pyrimidine bases. When a nucleoside is attached to phosphoric acid through the hydroxyl group on carbon 5′ of the pentose by a phosphoric ester bond, it forms a nucleotide.
Nucleic Acids as Polynucleotides
Nucleic acids are polynucleotides. The nucleotides are joined together through the phosphate radical located in the 5′ carbon of a nucleotide, and the hydroxyl radical and carbon 3′ of the next. Therefore, it is bound together by a phosphodiester bond, formed in the 5′ → 3′ direction.
Structure of DNA
It is a very long, unbranched chain composed of only four types of nucleotides. In DNA, there are three structural levels. Also, to get the DNA to fit in the cell nucleus, it needs to be packaged, associating with proteins.
Primary Structure of DNA
The nucleotide sequence of a single string or thread. It distinguishes a pentose-phosphate backbone and a sequence of nitrogenous bases. The sequence of nucleotides in this long linear molecule, not its simple composition, is what enables it to store information that is so diverse: the so-called biological or genetic information.
Secondary Structure of DNA
The arrangement in space of two polynucleotide strands or chains of a double helix, with the nitrogenous bases encountered in the interior, forming hydrogen bonds, and the pentose-phosphate backbone outside the helix. The method of X-ray diffraction by Franklin and Wilkins allowed the observation of the structure of DNA as a fiber with a diameter of 20 Å, in which some units were repeated every 3.4 Å, having one more repeat every 34 Å.
Watson and Crick’s Double Helix Model
DNA is composed of two polynucleotide chains that are antiparallel, i.e., have the links 5′ → 3′ oriented in a different sense, complementary, and rolled over each other as a double helix or plectonemic. Hydrogen bonds between the bases and hydrophobic interactions between the rings of the same stabilize the arrangement of the bases within the helix. Pentose and phosphate are abroad. Due to the ability of ionization of phosphates, nucleic acids are acidic and are defined as polyanions. Due to the complementarity of bases, this model allowed us to explain how this molecule is able to copy and provide exact replicas to the daughter cells. The double helix is very stable, but if heated above 65oC, the two strands are separated and DNA denaturation occurs. Lowering the temperature takes place in the renaturation of the double helix, which is what allows the hybridization of two complementary strands, even being of different DNA.
Tertiary Structure or Supercoiled DNA
DNA has a tertiary structure that consists of a 20 Å fiber that is twisted on itself, forming a kind of superhelix. This arrangement is called supercoiled DNA and is due to the action of enzymes called topoisomerase-II. This curl provides stability to the molecule and reduces its length.