Introduction to DNA MCQ - Practice Questions & Answers

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Introduction

Mendel gave us a basic understanding of inheritance patterns and the genetic basis of such patterns.
However, at the time of Mendel, the nature of those ‘factors’ regulating the pattern of inheritance was not clear.
Over the next hundred years, the nature of the putative genetic material was investigated culminating in the realisation that DNA – deoxyribonucleic acid – is the genetic material, at least for the majority of organisms.
Mendel’s ‘factors’ are called genes. They come in alternating pairs of dominant and recessive alleles.
Deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) are the two types of nucleic acids found in living systems.
DNA acts as the genetic material in most of the organisms.
RNA though it also acts as genetic material in some viruses, mostly functions as a messenger.
RNA has additional roles as well. It functions as an adapter, structural, and in some cases as a catalytic molecule.

Structure of DNA

DNA as an acidic substance present in the nucleus was first identified by Friedrich Meischer in 1869. He named it ‘Nuclein’.
In 1953, James Watson and Francis Crick, based on the X-ray diffraction data produced by Maurice Wilkins and Rosalind Franklin, proposed a very simple but famous Double Helix model for the structure of DNA.
DNA is a long polymer of deoxyribonucleotides.
The length of DNA is usually defined as the number of nucleotides (or a pair of nucleotide referred to as base pairs) present in it.
This also is the characteristic of an organism.
For example, a bacteriophage known as φ 174 has 5386 nucleotides,
Bacteriophage lambda has 48502 base pairs (bp), Escherichia coli has 4.6 × 106 bp, and haploid content of human DNA is 3.3 × 109 bp.
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 – Purine (Adenine and Guanine), and Pyrimidines (Cytosine, Uracil and Thymine).
Uracil is present in RNA at the place of Thymine.
A nitrogenous base is linked to the pentose sugar through an N-glycosidic linkage to form a nucleoside, such as adenosine or deoxyadenosine, guanosine or deoxyguanosine, cytidine or deoxycytidine and uridine or deoxythymidine.
A phosphate group is linked to 5' -OH of a nucleoside through phosphoester linkage
Two nucleotides are linked through 3'-5' phosphodiester linkage to form a dinucleotide.
More nucleotides can be joined in such a manner to form a polynucleotide chain.
A polymer thus formed has at one end a free phosphate moiety at 5' -end of a ribose sugar, which is referred to as 5’-end of a polynucleotide chain.
Similarly, at the other end of the polymer, the ribose has a free 3' -OH group which is referred to as 3'-end of the polynucleotide chain.
The backbone in a polynucleotide chain is formed due to sugar and phosphates.
The nitrogenous bases linked to the sugar moiety project from the backbone.