DNA Genetic Material: Hershey and Chase, Watson-Crick Model

There are two kinds of nucleic acids in living organisms known as deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). DNA is the genetic material in almost all living organisms with the exception of certain viruses. RNA is the genetic material of a cell in RNA viruses. It also serves as an adaptor, picking up amino acids. As it transports genetic information, RNA serves as a messenger. It may also transport catalytic molecules at times. In this article, we will have a look at DNA, derivation of its structure, model of Watson-Crick, and related experiments of Hershey and Chase.

Keyterms: DNA, RNA, genetic material, genes, cell, viruses, Living organisms, nucleic acids, nucleotide, Deoxyribonucleic acid, Ribonucleic acid

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What is DNA?

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Living organisms have deoxyribonucleic acid and ribonucleic acid. Except for some viruses, DNA is the genetic material in almost all living organisms. 

DNA is made up of deoxyribonucleotides, which are the building blocks of genetic material. It has an acidic nature and is found in the nucleus of the cell. The number of nucleic acids generally referred to as base pairs, contributes to the length of the DNA. It is an organism's characteristic. In 1869, Mischer discovered the DNA cell or DNA and named it nuclein. Altmann discovered that they are acidic in nature, and hence named them nucleic acids. 

DNA

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Structure of Polynucleotide Chain

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DNA is found in living organisms as genetic material. The nucleotide is the basic unit of DNA construction. A nucleotide molecule is composed of the following three components: 

• Nitrogenous Base, 
• Pentose sugar, and 
• Phosphate group. 

Structure of Polynucleotide Chain

In nature, pentose sugar is deoxyribose. There are two kinds of nitrogenous bases:

• Purines are heterocyclic compounds. They have a two-ring structure and the double ring has nine members. The nitrogen can be found at 1, 3, 7, and 9 positions. Purines include adenine and guanine.
• Pyrimidines are also heterocyclic compounds. It consists of a 6-membered single-ring structure. The nitrogen is present at positions 1 and 3. Pyrimidines include cytosine, thymine, and uracil. Cytosine is common in both DNA and RNA, whereas Uracil is present in RNA and thymine is present in DNA instead of uracil.

There are two types of polynucleotide linkages: 

• N-glycosidic linkage: The nitrogenous base is linked to the pentose sugar via an N-glycosidic linkage, resulting in the formation of the nucleoside.
• Phosphodiester linkage: The phosphate group is linked to the nucleoside's 5'-OH in phosphodiester linkage. This is accomplished through the use of phosphodiester linkage. A nucleotide is formed as a result of this. This results in a polymer with a free phosphate moiety at the 5'-end of the sugar. The other end has a 3'-OH free end as well. The polynucleotide chain's backbone is made up of sugars and phosphates. The nitrogenous base contributes to the formation of the backbone that is linked to the sugar moiety.

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Experiments of Hershey and Chase

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Griffith's experiment and the subsequent experiments to discover hereditary material in organisms are well known. Based on Griffith's experiment, Avery and his colleagues isolated DNA and demonstrated that DNA is the genetic material. However, it was not widely accepted until Hershey and Chase published their experimental results.

Alfred Hershey and Martha Chase set out in 1952 to discover the genetic material in organisms. Their experiments proved beyond a shadow of a doubt that DNA is the genetic material. Bacteriophages (viruses that attack bacteria) were central to Hershey and Chase's experiment.

The virus does not have its own reproduction mechanism and must rely on a host to do so. Their genetic material is transferred to the host cell once they attach to it. Bacteria serve as the host for bacteriophages. Bacteriophages manipulate infected bacteria so that bacterial cells begin to replicate viral genetic material. Hershey and Chase conducted an experiment to determine whether the genetic material that entered the bacteria was protein or DNA.

Experiment: Set A and B of viruses were cultured in a medium of radioactive sulfur and radioactive phosphorus respectively. It was observed that the first set had radioactive DNA but not radioactive protein as protein is not a phosphorus-based compound while DNA is. Set B had radioactive protein but not radioactive DNA. Then the viruses were left to infect E.Coli bacteria through centrifugation and blending.

Conclusion: Viruses having radioactive DNA transferred their DNA but the one having protein didn’t do so. Therefore, it was concluded that DNA is the genetic material.

Experiments of Hershey and Chase

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Functions of DNA 

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The DNA, or genetic material, serves the following purposes:

• It carries hereditary information or serves as a tool for transporting genetic material and genetic information.
• The DNA contains unexpected mutations.
• It aids in carrying out the variations that occur during meiosis.
• It can be used in the DNA fingerprinting technique.
• They aid in the control of mutations.
• The replication of DNA is carried out with the assistance of DNA.

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Derivation of DNA structure

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DNA is present in the nucleus of the cell that eventually serves as genetic material. For the derivation of the DNA structure, two approaches were used that were:

• Erwin Chargaff's Rule: The studies were conducted based on the DNA base composition. Purines and pyrimidines are produced in equal quantities. Some generalizations about double-stranded DNA have been proposed. Thymine pyrimidine is equimolar to adenine purine.

Purines and pyrimidines

• Wilkins and Franklin conducted these studies on X-Ray Crystallography. These studies suggested that the structure of DNA is similar to a helix. It was derived from very fine X-ray diffraction of DNA. However, they were unable to create a definitive DNA model.

X-Ray Crystallography

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Model of Watson-Crick

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Watson and Crick proposed the most famous DNA model, the double-helix model. The most important aspect of their model was the base pairing that exists between the two strands of polynucleotide DNA. This base pairing is a very distinct feature of polynucleotide chains. The base pairs are mutually beneficial. This means that if we know the base pairing of one strand, we can also determine the base pairing of the other.

Adenine is always present in the presence of thymine, and guanine is always present in the presence of cytosine. The two DNA chains are always present or run in an antiparallel pattern to one another. The pairing of the two strands is accomplished through the use of hydrogen bonds. As a result, a purine is always found opposite a pyrimidine. Therefore, the distance between the two strands is always uniform. The helical chain is twisted in the right hand.

Model of Watson-Crick

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DNA Helix Packaging

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Prokaryotes lack a distinct nucleus, but their DNA is not dispersed throughout the cell. It is usually found in the cytoplasm and is present in the supercoiled stage. No-histone basic proteins aid in coil maintenance. Protein polyamines have a positive charge, which aids in coil maintenance.

The supercoiled structure is known as a nucleoid. Coiling in eukaryotes is accomplished through the use of positively charged histone proteins. The basic amino acid residues in these histone proteins are abundant. Lysines and arginines are the basic amino acid residues. It contains five different types of proteins, four of which are present in pairs and form octamer structures. The two types of chromatin present are mentioned below:

• Euchromatin: In nature, this region is lightly stained. It has loosely packed chromatin and is transcriptionally active.
• Heterochromatin: In nature, this region is darkly stained. It contains a densely packed chromatin material and is transcriptionally inactive.

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Things to Remember

• There are two kinds of nucleic acids in living organisms known as deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). 
• DNA is the genetic material in almost all living organisms with the exception of certain viruses.
• The nucleotide is the basic unit of DNA construction.
• A nucleotide is composed of Nitrogenous Base, Pentose sugar, and Phosphate group.
• DNA carries hereditary information or serves as a tool for transporting genetic material and genetic information.
• Watson and Crick proposed the most famous DNA model, the double-helix model.
• Prokaryotes lack a distinct nucleus, but their DNA is not dispersed throughout the cell. 

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Previous Year Questions

1. Histone octamer contains… [JKCET 2011]
2. Heterogeneous nuclear RNA is converted to… [AMUEEE 2014]
3. Who gave semi-conservative mode of DNA replication… [JKCET 2014]
4. Adjacent nucleotides are joined by a… [AMUEEE 2015]
5. The successive nucleotides of RNA are….[JIPMER 2001]
6. When the centromere of a chromosome is located…
7. The back bone of RNA consists of…
8. When the centromere of a chromosome is…
9. Purines found both in DNA and RNA...[NEET 2019]
10. Ratio of complementary genes is… [NEET 2001]
11. Which form of RNA has a structure resembling clover leaf… [NEET 2004]
12. Which RNA is short lived… [COMEDK UGET 2006]
13. Who discovered transduction in bacteria?… [COMEDK UGET 2011]
14. VNTR stands for… [COMEDK UGET 2006]
15. Human genome project was officially started in… [COMEDK UGET 2011]
16. Franklin Conrat demonstrated that RNA is the genetic material of… [COMEDK UGET 2007]
17. DNA replication is semi-conservative because... [COMEDK UGET 2008]
18. DNA packaging in eukaryotes is done by formation of… [CUCET 2011]
19. A terminator codon which is called amber is… [COMEDK UGET 2011]
20. Clover leaf model of tRNA was suggested by… [JKCET 2010]