A German biochemist discovered in the late nineteenth century that nucleic acids, which are long-chain polymers of nucleotides, are composed of sugar, phosphoric acid, and numerous nitrogen-containing bases. Later, it was discovered that nucleic acid’s sugar can be either ribose or deoxyribose, resulting in two forms: RNA and DNA. Oswald Avery demonstrated in 1943 that DNA conveys genetic information. He even argued that DNA itself may be the gene. In the 1940s, DNA was largely regarded as the genetic molecule, but at the time, most people believed that the gene would be composed of protein. To be certain, scientists needed to determine the structure of this molecule and comprehend how it worked.
Linus Pauling discovered in 1948 that numerous proteins have the structure of an alpha helix, which is spiralled like a spring coil. Erwin Chargaff, a biochemist, discovered in 1950 that the order of nitrogen bases in DNA varied greatly, but certain bases consistently occurred in a 1:1 ratio. These discoveries were crucial for the subsequent description of DNA.
In the early 1950s, a race was on to discover DNA. Francis Crick, a graduate student at Cambridge University, and James Watson, a research fellow born in 1928, were inspired by Pauling’s findings. At the same time, Maurice Wilkins (born in 1916) and Rosalind Franklin were investigating DNA at King’s College in London. The goal of the Cambridge team was to construct physical models in order to limit down the possibilities and create an accurate representation of the molecule. The King’s group employed an experimental method, examining DNA x-ray diffraction pictures in particular.
In 1951, Watson attended a talk by Franklin regarding her previous work. She discovered that DNA may exist in two different forms, depending on the relative humidity of the air. This allowed her to conclude that the phosphate portion of the molecule was on the exterior. Watson returned to Cambridge with a hazy recollection of the information delivered by Franklin, although being highly critical of her lecture style and attitude. Watson and Crick developed a flawed model using this information. It prompted their unit’s leader to instruct them to cease DNA research. However, the subject kept resurfacing.
Franklin’s x-ray diffractions revealed that the “wet” version of DNA (at the higher humidity) possessed all of the hallmarks of a helix. She suspected that all DNA was helical, but she did not wish to declare this until she had adequate evidence on the other type as well. Wilkins was annoyed. In January 1953, he presumably showed Watson Franklin’s results without her knowledge or approval. Later, Crick said, “I’m afraid we always had — let’s say a condescending attitude toward her.”
Watson and Crick proposed that the molecule was composed of two chains of nucleotides, each in a helix as Franklin had discovered, but with one chain ascending and the other chain descending. In the summer of 1952, Crick had just learnt of Chargaff’s results regarding base pairs. This was introduced to the model so that base pairs might interlock in the midst of the double helix to maintain a constant spacing between the chains.
Watson and Crick demonstrated that each DNA strand serves as a template for the other. During cell division, the two strands divide and a new “other half” is created on each strand, identical to the previous one. This allows DNA to replicate without altering its structure, with the exception of rare mutations.
The structure fitted the experimental data so precisely that it was nearly immediately accepted. The discovery of DNA has been termed the greatest significant biological achievement of the last century, and the field it opened may constitute the frontier of science for the next century. Franklin had passed away by the time Watson, Crick, and Wilkins were awarded the Nobel Prize in physiology/medicine in 1962. The Nobel Prize is only awarded to living recipients and cannot be split between more than three winners.
Watson and Crick model of DNA
Watson and Crick exhibited the structure of DNA after examining the manuscript of Linus Pauling and Corey. Linus Pauling and Corey presented the unsuccessful 3D structure of nucleic acid in 1953. Then, (in early 1953), Watson and Crick postulated a double-helical structure for DNA by combining physical and chemical property data. The main characteristics of the DNA model developed by Watson and Crick include:
Physical Properties of DNA
- The Watson and Crick model describes DNA as a double-stranded helix composed of two polynucleotide chains. The two polynucleotide chains are spirally or helically twisted, giving them the appearance of a twisted ladder.
- Both polynucleotide strands of DNA have opposite polarity, indicating that they will run in antiparallel directions, i.e., one in the 5′-3′ direction and the other in the 3′-5′ direction.
- 20 is the diameter of a double-stranded DNA helix.
- Internuclear distance or distance between two nucleotides is 3,4. After one complete revolution, the DNA helix measures 34 in length and possesses 10 base pairs per turn.
- The DNA is twisted in a “Clockwise” or “Right-handed” way.
- The rotation of DNA results in the creation of vast grooves, known as “major groove.” The space between the two strands forms a narrow depression, hence the term “minor groove.” The DNA coiling results in the creation of main and minor grooves, which also serve as sites for DNA-binding proteins.
Chemical Properties of DNA
- Polynucleotide chains include four nucleotide bases, including adenine, guanine, cytosine, and thymine. Two purine bases with a single ring structure are adenine and guanine. Two pyrimidine bases with a double-ring shape are cytosine and thymine.
- The nitrogenous bases form a “Complementary base pairing” that connects the two strands. Therefore, a purine base will couple with a pyrimidine base in a complementary manner, with ‘Adenine’ pairing with ‘Thymine’ and ‘Guanine’ pairing with ‘Cytosine’.
- The nucleotide bases in the polynucleotide strands of DNA will form a strong hydrogen bond with one another.
- Adenine pairs complementarily with thymine via two hydrogen bonds, whereas guanine pairs complementarily with cytosine via three hydrogen bonds.
- The DNA nucleotide base composition follows Chargaff’s rule, in which the total of purines and pyrimidines is equal. The base composition of A + G = T + C conforms to Chargaff’s criterion, whereas the base composition of A + T does not equal G + C.
- The three major components of DNA polynucleotide strands are nitrogenous bases, deoxyribose sugar, and a phosphate group.
- The backbone of DNA is composed of sugar-phosphates. The “Phosphodiester bond” holds the two polynucleotide strands of DNA to the sugar-phosphate backbone. Consequently, the phosphodiester bond between sugar and phosphates and the hydrogen bond between nitrogenous bases contribute to the “DNA Stability.”
Watson-Crick Model of DNA Summary
- Important characteristics of the Watson-Crick model or DNA double helix model are as follows:
- The DNA molecule is composed of two polynucleotide chains or strands that are spirally wound around one another and coiled around a common axis to form a right-handed double-helix.
- The two strands are antiparallel, meaning they are running in opposite directions, with the 3′ end of one chain facing the 5′ end of the other.
- The sugar-phosphate backbones remain on the outside, while the purine and pyrimidine bases comprise the helix’s centre.
- Hydrogen bonds between the purine and pyrimidine bases of the opposing strands hold the two strands together.
- Adenine (A) always forms two hydrogen bonds with thymine (T), while guanine (G) always forms three hydrogen bonds with cytosine (C). This complementarity is referred to as the fundamental pairing rule. Consequently, the two stands are complementary.
- The base sequence throughout a polynucleotide chain is changeable, and the genetic information is carried by a specific sequence of bases.
- According to Chargaff’s principles (E.E. Chargff, 1950), A = T and G = C; as a corollary, purines (A+G) = ∑ pyrimidines (C+T); similarly, (A+C) = (G+T). It also states that the ratio of (A + T) to (G + C) within a species is constant (range 0.4 to 1.9).
- DNA has a diameter of 20nm or 20. Along the axis, adjacent bases are separated by 0.34 nm or 3.4. The length of a complete turn of helix is 3.4 nm or 34, which corresponds to 10 base pairs per turn.
- The DNA helix has a shallow groove known as the minor groove (~1.2nm) and a deep groove known as the major groove (~.2nm).
Biological Importance of DNA
1. Hereditary material
- The genetic information contained in the nucleotide sequence of DNA aids in the production of certain proteins or polypeptides and is transmitted to daughter cells or progeny.
- Consequently, DNA is known as the molecular blueprint or the thread of life.
2. Autocatalytic role DNA
- During the S phase of the cell cycle, DNA replicates (self-duplication). During the process, each DNA strand of a double helix might serve as a template for daughter strand synthesis.
3. Hetero-catalytic role
- During transcription, any one DNA strand serves as a template for RNA production. This is known as the heterocatalytic function of RNA.
- During meiosis, DNA undergoes recombination and the occasional mutation (changes in nucleotide sequences), which generates diversity in the population and ultimately leads to evolution.
5. DNA controls cellular metabolism, growth, and differentiation.
6. DNA finger printing (=DNA typing or profiling)
- Each individual possesses mini-satellites or VNTRs, which are small nucleotide repeats (Variable Number of Tandem Repeats).
- VNTRs are vary between individuals and provide the basis of DNA fingerprinting. This method is used to identify criminals, determine paternity, and verify immigrants, among other purposes.
7. Recombinant DNA technology (Genetic engineering):
- Recombinant DNA is the result of the artificial cleaving and rejoining of DNA sequences from two or more organisms.
- Utilized for the manufacture of genetically modified organisms (GMOs), genetically modified foods (GMFs), vaccines, hormones, enzymes, clones, etc. It is also used for the construction of probes (short polynucleotide chains attached to a radioactive or fluorescent marker) for the diagnosis of diseases and the treatment of inherited diseases by replacing a faulty gene with a normal gene (gene therapy), as well as for the formation of clones and other purposes.