CHAPTER - 06 MOLECULAR BASIS OF INHERITANCE - Class 12 Biology

CHAPTER- 06 MOLECULAR BASIS OF INHERITANCE

DNA (Deoxyribonucleic Acid) and RNA (Ribonucleic Acid) are two types of nucleic acid found in living organisms. DNA acts as genetic material in most of the organisms. RNA also acts as genetic material in some organisms as in some viruses and acts as messenger. It functions as adapter, structural, and in some cases as a catalytic molecule.

The DNA - it is a long polymer of deoxyribonucleotides. A pair of nucleotide is also known as base pairs. Length of DNA is usually defined as number of nucleotides present in it. Escherichia coli have 4.6 x 106 bp and haploid content of human DNA is 3.3 × 109 bp.

Structure of Polynucleotide Chain

A nucleotide has three components – a nitrogenous base, a pentose sugar (ribose in case of RNA, and deoxyribose for DNA), and a phosphate group. There are two types of nitrogenous bases – Purines (Adenine and Guanine), and Pyrimidines (Cytosine, Uracil and Thymine).

 

Cytosine is common for both DNA and RNA and Thymine is present in DNA. Uracil is present in RNA at the place of Thymine

A polynucleotide chain 

A nitrogenous base is linked to pentose sugar with N-glycosidic linkage to form to form a nucleoside. When phosphate group is linked 5’-OH of a nucleoside through phosphoester linkage nucleotide is formed. Two nucleotides are linked through 3’-5’ phosphodiester linkage to form dinucleotide. More nucleotide joins together to form polynucleotide. 

In RNA, nucleotide residue has additional –OH group present at 2’-position in ribose and uracil is found at the place of Thymine. 

Structure differences

DNA	RNA
The sugarpresent in DNA is 2-deoxy-D - (- ) -ribose.	Thesugar present in RNAis D- (-)- ribose.
DNA contains cytosine and thymine as pyrimidine bases and guanine and adenine is purine bases.	RNA contains cytosine and uracilpyrimidine bones and guanine and adenine as purine bases
DNA has double strand α-helix structure.	RNA has a single stranded α-helix structure.
DNA molecules are very large their molecular mass may vary from 6✕ 106 - 16 ✕ 106 u	RNA molecules are comparatively much smaller with molecular mass ranging from 20,000 – 40,000

Functional differences

DNA	RNA
DNA has uniqueproperty of replication.	RNA usually does not replicate.
RNA controls the transmission of hereditary effects.	RNA controls the synthesis of proteins.

Double Helix Model for Structure of DNA-James Watson and Francis Crick, based on X-ray diffraction data produced by Wilkin and Rosalind proposed this model of DNA. 

The silent features of this model are-

a) DNA is made of two polynucleotide chains in which backbone is made up of sugarphosphate and bases projected inside it.

b) Two chains have anti-parallel polarity. One 5’à3’ and with 3’à5’. 

c) The bases in two strands are paired through H-bonds. Adenine and Thymine forms double hydrogen bond and Guanine and Cytosine forms triple hydrogen bonds. 

d) Two chains are coiled in right handed fashion. The pitch of helix is 3.4 nm and roughly 10 bp in each turn. 

e) The plane of one base pair stacks over the other in double helix to confer stability.

Francis Crick proposed the Central dogma in molecular biology, which states that the genetic information flows from DNA -----> RNA ------> Protein.

Packing of DNA helix-

In prokaryotes, well defined nucleus is absent and negatively charged DNA is combined with some positively charged proteins called nucleoids.

In eukaryotes, histones, positively charged protein organized to form 8 molecules unit called histone octomer. Negatively charged DNA is wrapped around the histone octomer to form nucleosome. . Histones are rich in the basic amino acid residues lysines and arginines. Both the amino acid residues carry positive charges in their side chains.

• Single nucleosome contains about 200 base pairs. Chromatin is the repeating unit of nucleosome.

• In nucleus, some region of chromatin are loosely packed (and stains light) and are referred to as euchromatin. The chromatin that is more densely packed and stains dark are called as Heterochromatin. Euchromatin is transcriptionally active chromatin, whereas heterochromatin is inactive.

The search for Genetic Material 

Transforming principle – Frederick Griffith in 1928 conducted experiment on bacteria Streptococcus pneumoniae (bacterium responsible for pneumonia). There are two types of strain of this bacteria, some produce smooth shiny colonies (S) and others produce rough colonies(R). Mice infected with the S strain (virulent) die from pneumonia infection but mice infected with the R strain do not develop pneumonia. 

S strain → Inject into mice → Mice die

 R strain → Inject into mice → Mice live

S strain (heat-killed) → Inject into mice → Mice live

S strain (heat-killed) + R strain (live) → Inject into mice → Mice die 

Griffith concluded that R strain bacteria have somehow transformed by heat killed S strain bacteria. Some transforming principles transferred from S strain to R strain and enabled the R strain to synthesise a smooth polysaccharide coat and become virulent. This must be due to the transfer of the genetic material. 

Biochemical Characterisation of Transforming Principle

• Oswald Avery, Colin MacLeod and Maclyn McCarty worked out to determine the biochemical nature of transforming principle of Griffith.
• They purified biochemicals (proteins, DNA, RNA, etc.) from the heat-killed S cells to see which ones could transform live R cells into S cells. They discovered that DNA alone from S bacteria caused R bacteria to become transformed.So, they concluded that DNA is the genetic material.

Experimental proof that DNA is the genetic material 

Alfred Hershey and Martha Chases (1952) worked with virus that infect bacteria called bacteriophages. 

• In one preparation, the protein part was made radioactive and in the other, nucleic acid (DNA) was made radioactive. These two phage preparations were allowed to infect the culture of E.coli. Soon after infection, before lysis of cells, the E.coli cells were gently agitated in a blender, to loosen the adhering phage particles and the culture was centrifuged.
• The heavier infected bacterial cells pelleted to the bottom and the lighter viral particles were present in the supernatant. It was found that when bacteriophage containing radioactive DNA was used to infect E.coli, the pellet contained radioactivity. 
• If bacteriophage containing radioactive protein coat was used to infect E.coli, the supernatant contained most of the radioactivity.

His experiment shows that protine does not enter the bacterial cell and only DNA is the genetic material.