NEET Biology Notes Molecular Basis of Inheritance Genetic Material
It is that substance which controls the inheritance of traits from one generation to next. Living matter is made up of biochemicals. Therefore, genetic material should as be biochemical in nature.
DNA as a Genetic Material
The various indirect and direct evidences in support of DNA as a genetic material are as follows:
Friedrich Miescher in 1869 explained that nucleus possess deoxyribose nucleic acid (DNA) that has capacity to replicate to produce similar copies of it. It carry genetic information from one generation to next.
These are of two types:
- Transforming Principle
F Griffith (1928) conducted a series of experiments with Streptococcus pneumoniae, the bacterium causing pneumonia. He observed two strains of this bacterium; one forming smooth colonies with capsule (S-type) and the other forming rough colonies without capsule (R-type).
He concluded from his work that the R-strain bacteria had somehow been transformed by the heat-killed S-strain bacteria, which must be due to the transfer of the genetic material (transforming principle).
- DNA as Genetic Material
The unequivocal proof that DNA is the genetic material came from the experiments of Alfred Hershey and Martha Chase (1952). They worked with viruses that infect bacteria called bacteriophages.
They made two different preparations of the phage. In one, the DNA was made radioactive with 32 P and in the other, the protein coat was made radioactive with 35S. The phage containing radioactive DNA was radioactive that found in the bacterial cells indicating that the DNA is the genetic material.
Properties of Genetic Material
Genetic material must have following properties
- It should be able to produce its replica.
- It should be chemically and structurally stable.
- It should provide the scope for slow changes that are necessary for evolution.
- It should be able to express itself in the form of Mendelian characters.
Deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) are the two types of nucleic acids found in living systems. Out of these, DNA acts as the genetic material in most of the organisms.
It is a long polymer of deoxyribonucleotides. Each nucleotide has three components, i.e. a nitrogenous base, a pentose sugar (deoxyribose) and a phosphate group. There are two types of nitrogen bases, i.e. purines (adenine and guanine) and pyrimidines (cytosine, uracil and thymine).
Double-Helix Model of DNA
First of all, Watson and Francis Crick (1953) proposed a double-helix model of DNA based on the X-ray diffraction data, produced by Maurice Wilkins and Rosalind Franklin.
The important feature of this model are given below :
- It contains two polynucleotide chains. Its backbone constitutes sugar-phosphate and the bases project inside.
- The bases in the two strands are held together by hydrogen bonds forming base pairs. Adenine pairs with thymine through two hydrogen bonds and guanine with cytosine through three hydrogen bonds.
- The two chains have an anti-parallel polarity, i.e. one chain has a 5′ —> 3′ polarity, while the other has 3′ —> 5′ polarity.
- The two chains are coiled in a right-handed fashion and the pitch of the helix is 3.4 nm. There are about 10 base pairs in each turn with 0.34 nm between two base pairs.
- The plane of one base pair stacks over the other in the double helix.
- The diameter of DNA is 2 nm.
Packaging of DNA
In prokaryotes, such as E. coli, they do not have a well defined nucleus. The DNA is scattered throughout the cytoplasm. DNA (being negatively charged) is held with some basic proteins (that have positive charges) in a cytoplasmic region termed as nucleoid.
Iai^kàiÿotes, there is a set of positively charged, basiç proteins called histones found in nucleus. Histones are rich in the basic amino acid residues lysine and argine. Both the amino acid residues carry positive charges in their side chains. Histones are organised to form a unit of eight molecules called as histone octamer.
The negatively charged DNA is wrapped around the positively charged histone octamer to form a structure called nucleosome. A typical nucleosome contains 200 bp of DNA helix.
Nucleosome constitute the repeating unit of a structure in nucleus called chromatin. The nucleosomes in chromatin are seen as ‘beads-on-string’ structure, when viewed under electron microscope.
The chromatin fibres condense at metaphase stage of cell division to form chromosomes. The packaging of chromatin at higher level requires additional set of proteins called Non-Histone Chromosomal (NHC) proteins.In a nucleus, certain regions of the chromatin are loosely packed and they stain lighter than the other regions, these are called euchromatin.
The other regions are tightly packed and they stain darker and are called heterochromatin. Euchromatin is transcriptionally more active than heterochromatin.
RNA is a known to be genetic material in some viruses e.g. retroviruses. The essential life processes such as metabolism, translation, splicing, etc., have evolved around RNA, even before DNA has evolved as a genetic material.
RNA. used to act as a genetic material as well as a catalyst. The 2′ —OH group of ribonucleotides is a reactive group, that makes RNA act as a catalyst.
But, RNA being a catalyst is reactive and hence, unstable. Therefore, DNA has evolved from RNA with chemical modifications that make it more stable.
Structure of RNA
In RNA each molecule has three components as in DNA.
The sugar is ribose, which has an additional OH group on the 2′-position. The nucleosides and nucleotides are called ribo nucleosides and ribonucleotides, respectively. There are three types of RNA as mRNA, rRNA and fRNA.