Deoxyribonucleic Acid (DNA) Genetic Material


Introduction to DNA

Download PDF
Bookmark added to your notes.
View Notes

The nucleic acids in living organisms are of two types. They are deoxyribonucleic acid and ribonucleic acid. The DNA is the genetic material in almost all living organisms except some viruses. In RNA viruses, RNA is the genetic material of a cell. The RNA carries the genetic information thus, it functions as a messenger. It also functions as an adaptor that picks up amino acids Sometimes, it also carries catalytic molecules. 

The deoxyribonucleotides make up the DNA the genetic material. It is acidic in nature and is present in the nucleus of the cell. Mischer identified the DNA cell or the DNA in 1869 and he named it nuclein. After that, Altmann found out that they are acidic in nature therefore he named them nucleic acids. The number of nucleic acids helps in defining the length of the DNA. These nucleic acids are known as the base pairs. It is a characteristic of an organism. We will learn about the location in the cell of DNA and what is genetic material and more about DNA. 

Structure of the Polynucleotide Chain

The DNA as genetic material is found in living organisms. The nucleotide is the basic unit of DNA. The nucleotides are composed of three units that are a nitrogenous base, a pentose sugar and a phosphate group. Pentose sugar is deoxyribose in nature. Two types of nitrogenous bases are present. They are:

  • Purines: They are heterocyclic in nature. They have a double ring structure. The double ring is 9-membered in nature. The nitrogen is present at positions 1,3,7 and 9. Examples of purines are Adenine and Guanine. 

  • Pyrimidines: It is also heterocyclic in nature. It is a 6-membered ring structure. It is a single ring structure. The nitrogen is present at 1 and 3 positions. Cytosine, Thymine and Uracil are pyrimidines. In both DNA and RNA, cytosine is common whereas thymine is present in DNA and Uracil is present in RNA at the place of thymine. 

The polynucleotide linkage shows two types of linkages, they are N-glycosidic linkage and Phosphodiester linkage. In N-glycosidic linkage, the nitrogenous base is linked to the pentose sugar with the help of an N-glycosidic linkage which then forms the nucleoside. In phosphodiester linkage, the phosphate group is linked to the 5’-OH of the nucleoside. This takes place with the help of phosphodiester linkage. By this, a nucleotide is formed. The polymer that is formed by this has a free phosphate moiety at the 5’-end of the sugar. Similarly, the other end has a 3’-OH free end. Sugars and phosphates form the backbone of the polynucleotide chain. The backbone is formed by the nitrogenous base that is linked to the sugar moiety. 

Derivation of the DNA Structure

A question arises that “DNA is found in what part of the cell?” The DNA as genetic material is found in the nucleus of the cell. There were two methods that were followed for the derivation of the DNA structure. They are:

  • X-Ray Crystallography: Wilkins and Franklin did these studies. It was obtained from a very fine X-diffraction of the DNA. From these studies, they suggested that the structure of DNA is sort of a helix. But they were not able to produce a definitive model for the DNA. 

  • Erwin Chargaff’s Rule: The studies were made on the basis of the base composition of the DNA. Some generalizations on the double-stranded DNA were put forward. The purines and the pyrimidines are produced in equal amounts. Adenine purine is equimolar to thymine pyrimidine. 

Watson-Crick Model

The Double-helix model which is the most famous model of DNA was proposed by Watson and Crick. The main thing about their model was the base pairing that is present between the two strands of the polynucleotide DNA. This base pairing is a very unique property of the polynucleotide chains. The base pairs are complementary to each other. This means that if we know the base pairing of one strand then we can formulate the base pairing of the other strand also. 

Adenine is always present complementary to thymine and Guanine is always present complementary to cytosine. The two chains of the DNA are always present or run in an antiparallel pattern to each other. The pairing between the two strands is done with the help of hydrogen bonds. Due to this, a purine always comes opposite to pyrimidine. Due to this, there is always a uniform distance between the two strands. The helical chain is twisted in a right-handed fashion. 

[Image will be Uploaded Soon]

Functions of the DNA

The Functions of the Dna the Genetic Material are:

  • It carries hereditary information or it works as a tool for carrying the genetic material for carrying the genetic information.

  • It helps in carrying out the variations that occur at the time of meiosis.

  • It is useful in DNA fingerprinting technique.

  • The sudden mutations are present in the DNA.

  • They help in controlling the mutations. 

  • The process of DNA replication is carried out with the help of DNA.

Packaging of DNA Helix

The prokaryotes do not have a defined nucleus but still, their DNA is not present in scattered form in the cell. It is found in the cytoplasm and is present in the supercoiled stage. No-histone basic proteins help in maintaining the coils. Protein polyamines have a positive charge and this helps in maintaining the coils. Nucleoid is the name given to the supercoiled structure. In eukaryotes, the coiling is done with the help of positively charged histone proteins. These histone proteins are rich in basic amino acid residues. The basic amino acid residues are lysines and arginines. Five types of proteins are present in it out of which four of them are present in pairs and they make octamer structures. Two types of chromatin are present that is:

  • Heterochromatin: This region is darkly stained in nature. The chromatin material in it is densely packed. This is inactive transcriptionally. 

  • Euchromatin: This region is lightly stained in nature. It is transcriptionally active and has loosely packed chromatin. 

FAQ (Frequently Asked Questions)

1. How Many Types of DNA are There?

Answer: There are four types of DNA. They are:

  • A-Type: In this type of DNA eleven base pairs are present in each turn. The rotation of this DNA is right-handed in nature.

  • B-Type: In this type of DNA, ten base pairs are present in each turn. This DNA also has right-handed rotation. 

  • C-Type: In this type of DNA, 9.33 base pairs are present per turn. This DNA also has right-handed rotation. 

  • Z-Type: In this DNA, twelve base pairs are present per turn. This DNA has left-handed rotation. 

2. Where is DNA found in a Cell and What are the Properties of Genetic Material?

Answer: The DNA is found in the nucleus of the cell. To be genetic material, it should be stable chemically and structurally. It should be easily able to generate its replica. A scope of slow mutation should always be present in it. It should be able to express itself in Mendelian characteristics.