What is DNA?

DNA, or deoxyribonucleic acid, is a long polymeric nucleic acid molecule discovered in the late 1930s. It is a polymer; a long chain-like molecule made up of several monomers connected in a sequence. It possesses certain characteristics that qualify it as a genetic component. Certain organisms have different types of nucleic acids as their genetic material - DNA or RNA.

DNA Structure 

Nucleotides

Monomers of DNA: The nucleotide is the fundamental unit of a DNA molecule. They can be found in ell as either nucleic acid components or on their own. Sugar, nitrogenous base, and phosphoric acid are three distinct components that make up these complex molecules. This makeup is true for both DNA and RNA, whereas proteins have only amino acids with functional groups.

Sugar component

The sugar in the nucleotide is a pentose with five carbon atoms and is known as 2'deoxyribose. Straight-chain or Fischer structure and ring or Haworth structure are the two types of pentose sugar. In a nucleotide, it is the ring shape of 2' deoxyribose. The sugar represents a modified ribose in which the hydroxyl group attached to carbon atom 2' has been replaced with a hydrogen group. RNA has ribose sugar.

Nitrogenous bases

These are complex single or double ring structures connected to the sugar's 1' carbon. Any of four different nitrogenous bases can occupy this role in DNA. They are double-ring purines that form adenine and guanine. Thymine and cytosine are pyrimidines with only one ring. RNA whereas has uracil as one of the pyrimidines.

Phosphoric acid

A nucleoside is a sugar and base molecule. Phosphoric acid is added to a nucleoside at the sugar's 5' carbon leading to the formation of nucleoside monophosphate, nucleoside diphosphate, and nucleoside triphosphate. Alpha, beta, and gamma are the three types of phosphates, with alpha being the one that is directly attached to the sugar.

DNA is a long chain of nucleotides, whereas proteins are long chains of amino acids. The protein cannot serve as genetic material as proteins cannot replicate. Proteins are not structurally and chemically stable always. Proteins form the end products, which will code for genes. Proteins can easily be denatured with various exposures. Different proteins have different stabilizing factors, which make proteins inefficient to be the genetic material. RNA is not the genetic material due to RNA's less stability on hydrolysis of a hydroxyl group.

Nucleotides

Phosphodiester bonds link it: Individual nucleotides are joined together to form a polymer, which gives DNA its structure. A polynucleotide is a name for this form of polymer. Three nucleotides make up the structure of a trinucleotide. The alpha-phosphate group attached to one nucleotide's 5' carbon is joined to the 3' carbon of the next nucleotide to form a nucleotide monomer. The phosphodiester bond is the connection between nucleotides in polynucleotides. 'Phospho' indicates the presence of a phosphorus atom, while 'diester' indicates the presence of two ester bonds in a single linkage (C-O-P).

Polynucleotides

It has chemically distinct ends: polynucleotide ends differ from one another. The 5'P terminal is one end, where the triphosphate group attached to the 5' carbon has not taken part in the phosphodiester bond. The 3'OH terminal, where the unreacted group is 3' Hydroxyl, is on the other end. This distinction indicates whether a polynucleotide's direction is 5' to 3' or 3' to 5'.The direction is very important to understand in molecular genetics.

It can be of any duration or in any order: The number of nucleotides that can be joined together to create a DNA polynucleotide is unlimited. Any base could appear in any series.

Double Helix

In 1953, James Watson and Francis Crick discovered the double helix, which influenced every aspect of molecular genetics. They were able to deduce the exact structure of DNA. Watson and Crick used model-building to solve the issue of considering DNA as genetic material. They created a scale model of how DNA could be observed using established knowledge and chemistry rules. The final model took into account the works and results of two other scientists named Erwin Chargaff and Rosalind Franklin. Proving DNA as genetic material, nor RNA or protein, was one of the greatest researches.

Chargaff Base ratio for Correct Structure by McCarty, Avery, and MacLeod

The discovery by Avery, McCarty, and MacLeod that DNA is the transformation theory inspired another scientist, Chargaff. Chargaff and colleagues E. Vischer and S. Zamenhof used sensitive paper chromatographic techniques to assess the exact quantities of four nitrogenous bases in DNA samples from various species. The number of adenine residues equals the number of thymine, and the number of guanines equals the number of cytosines, resulting in A=T and C=G. A+G = T+C was also reported.

X-ray Diffraction Analysis Indicates that DNA is a Helical Molecule

X-rays were used to bombard the crystallized DNA fiber. The molecule's 3D structure (DNA) represents the X-Ray laser. Rosalind Franklin was the one who photographed X-ray diffraction using DNA crystals. DNA was discovered to be the helical structure with two periodicities of 3.4 Angstrom and 34 Angstrom.

Conclusions by Watson and Crick

They discovered that DNA is a double helix. The number of polynucleotides in DNA was discovered to be two, indicating that DNA is double-stranded. On the inside of the helix, nitrogenous bases were stacked, with a sugar-phosphate backbone on the outside. Hydrogen bonds connect the bases of two polynucleotides. Since adenine was always found paired to thymine and guanine was always found paired to cytosine, the Chargaff rule made sense. There are three hydrogen bonds between G and C, and between A and T, there are two hydrogen bonds. The helix's two strands are anti-parallel. Right-handed DNA has been discovered, which means the spiral of DNA is on the right-hand side. The pitch of DNA is 34 Angstrom making the space between two base pairs is 3.4 Angstrom. The DNA diameter is 20 Angstrom in diameter. Some organisms have double-stranded DNA while some organisms have single-stranded DNA; organisms might have single-stranded RNA or double-stranded RNA as their genetic material, like viruses.

What is Base Pairing in DNA?

In molecular genetics, base pairing is the most significant function. The rule states that A pairs with T and G pairs with C; but, due to the large sizes needed to fit in the helix. As a result, the two polynucleotides are mutually beneficial. In molecular genetics, complementary base pairing is crucial. A form, B form, and Z form are the three types of DNA that exist. They differ due to the amount of water present in the DNA solution from which the crystals develop. The complementary base-pairing was universally observed in all organisms, whether bacterium, virus, higher plants, or animals. This base-pairing forms the basis of the central dogma in which transcription and translation take place. Transcription is the production of an RNA copy from a DNA sequence, which usually makes mRNA. The translation is converting this mRNA into protein which will code for specific genes. Proteins form an integrated part of the body system – coding genes and making enzymes are one of them.

Griffith's Experiment

Frederick Griffith experimented to prove that bacterium can transfer genetic information through transformation. In the experiment, Two strains of pneumococcal bacterium that infects mice were used in the study: type 3S smooth and virulent and type 2R rough and non-virulent. Another bacterium, the 3S strain, created a polysaccharide capsule to shield itself from the host's immune system, resulting in the host's death. However, since the 2R strain lacked a defensive capsule, the host immune system could defeat it. The heat was used to destroy the bacterium from the 3S strain, and the remaining bacterium was added to the 2R strain. The mice were not affected by any of them, but their combination killed them. Griffith came up with a transformation theory that turned a lethal 2R strain into a lethal 3S strain. This experiment concluded that something could move through the healing process and was picked up by the 2R strain, bringing it back to life. Thus the presence of the transformation principle was proved by their experiment. Avery, MacLeod, and McCarty verified the results. The other experiment was Hershey and Chase experiment, which proved genetic material to be the nucleic acid – DNA, using radioactive materials.

Common Mistakes

• The numbering of carbon is not 1,2,3,4,5,6, but it is 1’,2’,3’,4’,5', and 6', called prime. The carbons in sugar are distinguished from the carbon and nitrogen in nitrogenous bases using this prime.
• The study of DNA, RNA, and proteins is important to study the genetic makeup of a bacterium or virus.
• Nucleic acids include single-stranded, and double-stranded DNA and RNA, different in different organisms like viruses have single-stranded/double-stranded RNA while bacterium has double-stranded DNA.

Context and Applications

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for

• Bachelors. in Biology
• Bachelors in Microbiology
• Bachelors in Microbiology
• Masters in Microbial Biotechnology
• Masters in Molecular genetics

Related Concepts

• RNA
• Prokaryotic genome
• Eukaryotic genome
• Nucleotides