Introduction to DNA
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.
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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.
FAQs on Deoxyribonucleic Acid (DNA) Genetic Material
1. What is Deoxyribonucleic Acid (DNA)?
Deoxyribonucleic Acid, or DNA, is the primary hereditary molecule found in almost all living organisms. It is a long polymer that carries the genetic instructions used in the growth, development, functioning, and reproduction of life. These instructions are stored in the form of a code made up of four chemical bases.
2. What are the three basic components of a DNA nucleotide?
Each nucleotide in a DNA strand is composed of three fundamental units:
- A phosphate group, which forms the backbone of the DNA strand.
- A pentose sugar, specifically deoxyribose, to which the other two components are attached.
- A nitrogenous base, which is the part that holds the genetic code. The four bases are Adenine (A), Guanine (G), Cytosine (C), and Thymine (T).
3. What is the double helix structure of DNA?
The double helix is the structure formed by two DNA strands coiled around each other. This model, proposed by Watson and Crick, shows that the two strands run in opposite (antiparallel) directions. The outer edges consist of a sugar-phosphate backbone, while the inner 'rungs' are formed by pairs of nitrogenous bases connected by hydrogen bonds: Adenine (A) always pairs with Thymine (T), and Guanine (G) always pairs with Cytosine (C).
4. Where is DNA located inside eukaryotic and prokaryotic cells?
The location of DNA differs between cell types. In eukaryotic cells (like those in plants and animals), the majority of DNA is contained within a membrane-bound nucleus. In contrast, prokaryotic cells (like bacteria) lack a nucleus, so their DNA is found in the cytoplasm in a region called the nucleoid.
5. What are the primary functions of DNA?
DNA has several critical functions in a cell:
- Storing Genetic Information: It holds the complete blueprint required to build and maintain an organism.
- Replication: It can make exact copies of itself, ensuring genetic information is passed down during cell division.
- Gene Expression: It provides the code for creating proteins and functional RNA molecules through the processes of transcription and translation.
- Mutation and Evolution: It allows for changes (mutations) in the genetic code, which is the basis for variation and evolution.
6. Why is DNA considered a more stable genetic material than RNA?
DNA is chemically and structurally more stable than RNA, making it better suited for long-term storage of genetic information. This is mainly due to two reasons: the deoxyribose sugar in DNA lacks a reactive hydroxyl group that is present in RNA's ribose sugar, making it less prone to degradation. Additionally, the use of Thymine instead of Uracil in DNA allows for more efficient detection and repair of mutations.
7. How is the extremely long DNA molecule packaged to fit inside a tiny nucleus?
DNA undergoes a highly organised process of compaction. In eukaryotes, the negatively charged DNA strand wraps around a group of positively charged proteins called histones. This DNA-histone complex is called a nucleosome, which looks like 'beads on a string'. These nucleosomes are further coiled and condensed into a more compact structure called chromatin, which then organises into chromosomes during cell division.
8. What is the significance of Chargaff's rules in discovering the structure of DNA?
Erwin Chargaff's rules were a critical piece of evidence for determining DNA's structure. He discovered that in any DNA sample, the amount of Adenine (A) was equal to the amount of Thymine (T), and the amount of Guanine (G) was equal to Cytosine (C). This A=T and G=C finding strongly suggested a specific and consistent base-pairing mechanism, which became a foundational principle for the Watson and Crick double helix model.
9. What is the difference between euchromatin and heterochromatin?
Euchromatin and heterochromatin are two forms of chromatin that differ in their structure and activity. Euchromatin is a loosely packed form of chromatin that is rich in genes and is often under active transcription. In contrast, heterochromatin is a tightly packed form of chromatin that is transcriptionally inactive and contains fewer genes.
10. What are the key properties that a molecule must have to serve as genetic material?
To serve as genetic material, a molecule must exhibit four key properties as per the CBSE syllabus:
- It must be able to generate its own replica (Replication).
- It must be chemically and structurally stable to preserve information.
- It must provide the scope for slow changes (mutation) that are required for evolution.
- It must be able to express itself in the form of 'Mendelian Characters' through transcription.
