Test: Molecular Basis of Inheritance - NEET MCQ

In a DNA molecule, A-T rich parts melt before G-C rich parts because there are two H-bond between A and T whereas in between G and C, there are three H-bond.

The two chains of DNA have anti-parallel polarity this is because one chain has free phosphate moiety at 5'-end of the sugar and another chain has free phosphate moiety at 3'-end.

In DNA, the code letters are A, T, G, and C, which stand for the chemicals adenine, thymine, guanine, and cytosine, respectively. In base pairing, adenine always pairs with thymine, and guanine always pairs with cytosine.

The backbone of a DNA chain or strand is built up of alternate deoxyribose sugar and phosphoric acid groups. The phosphate group is connected to carbon 5' of the sugar residue of its own nucleotide and carbon 3' of the sugar residue of the next nucleotide by (3' → 5') phosphodiester bonds. The two DNA chains are antiparallel, that is, they run parallel but in opposite directions. In one chain the direction is 5' → 3' while in the opposite one, it is 3' → 5'.

The genetic material should be stable enough not to change with different stages of life cycle, age or with change in physiology of the organism. The DNA strands being complementary, if separated by heating come together only when appropriate conditions are provided. Further, 2'-OH group present at every nucleotide in RNA is a reactive group and makes RNA labile and easily degradable. RNA is also now known to be catalytic, hence reactive. Therefore, DNA chemically is less reactive and structurally more stable when compared to RNA. Therefore, among the two nucleic acids, DNA is a better genetic material.

Histones are basic proteins found in the eukaryotic chromosomes. These are rich in the amino acids lysine and arginine. Histonie proteins are basic proteins consisting of 5 types – H₁, H₂A, H₂B, H₃, H₄. DNA is coiled around it. It exists as octamers linked with H₁.

A chain of DNA has 140 base pairs, make  turns and twist around a histone octamer forming nucleosome. The core of
nucleosome consists of 4 histones H₂A, H₂B, H₃ and H₄.

Histones contain a large proportion of the positively charged (basic) amino acids, lysine and arginine in their structure. DNA is negatively charged due to the phosphate groups on its backbone.

DNA is the hereditary material found in the nucleus of eukaryotic cells and the cytoplasm of prokaryotic cells that determines the composition of the organism. There is another type of genetic material found in cells and viruses known as ribonucleic acid (RNA). DNA is double stranded as well as single stranded biomolecule.

When bacteria Streptococcus pneumoniae are grown on a culture plate, some produce smooth shiny colonies (S) while others produce rough colonies (R). This is because the S strain bacteria have a mucous (polysaccharide) coat, while R strain does not. Mice infected with the S strain (virulent) die from pneumonia infection but mice infected with the R strain do not develop pneumonia. In Griffith's experiment, some 'transforming principle', transferred from the heat-killed S strain, had enabled the R strain to synthesize a smooth polysaccharide coat and become virulent which must be due to the transfer of the genetic material. This is known as transformation.

Strand of DNA which has 3' → 5' polarity is called template strand or master strand or antisense, or (-) strand. The other strand which has a polarity of 5' → 3' is displaced during transcription. This non-template strand which does not take part in transcription is also called sense or coding strand or plus (+) strand because genetic code present in this strand is similar to genetic code (based on mRNA) except that uracil is replaced by thymine.

Transformation does not involve passive entry of DNA molecules through permeable cell walls and membranes. It does not occur ‘naturally’ in all species of bacteria, only in those species possessing the enzymatic machinery involved in the active uptake and recombination processes. Even in these species, all cells in a given population are not capable of active uptake of DNA. Only competent cells, which possess a so – called competence factor are capable of serving as recipients in transformation.

Paramecium aurelia has two strains : killer and sensitive. The killer strain is able to kill the sensitive strain protist by means of chemical paramecin, secreted by minute cytoplasmic particles called kappa particle. The sensitive strain protists do not have kappa particles. The sensitive strain can also become killer if it receives sufficient kappa particles.

Structural nonhistone chromosomal proteins are called scaffold proteins as they constitute the core axis of the chromosome. They contain very little lysine and arginine but instead possess abundant tyrosine and tryptophan types of amino acids.

Unlike DNA, RNA is usually singlestranded. Additionally, RNA contains ribose sugars rather than deoxyribose sugars, which makes RNA more unstable and more prone to degradation. RNA is synthesized from DNA by an enzyme known as RNA polymerase during a process called transcription. Viruses having RNA have shorter life span and mutate at faster rate.

The DNA polymerase works in the 3' → 5' direction thus continuously synthesising the strand having polarity 5' → 3'.

DNA replication is semiconservative, a type of replication in which one strand of the daughter duplex is derived from the parent while the other strand is formed new. This is carried out by the separation of two strands. The separated strands function as templates. The new strands built up over the templates of old strands will have complementray base pairs (A opposite T and G opposite C). The two daughter DNA molecules thus, formed will be carbon copies of the parent molecule but shall have one new strand and one old strand.

DNA-polymerase can polymerise nucleotides only in 5' → 3' direction on 3' → 5' strand because it adds them at the 3' direction on 3' → 5' strand because it adds them at the 3' end. Replication over the two templates proceeds in opposite directions. One strand with polarity 3' → 5' forms its complementary strand continuously because 3' end of latter is always open for elongation. It is called leading strand. Replication is discontinuous on the other template with polarity 5'→ 3' because only a short segment of DNA strand can be built in 5' → 3' direction due to exposure of a small stretch of template at one time. Short segments of replicated DNA are called Okazaki fragments. DNA strand built up of Okazaki fragments is called lagging strand. As one strand grows continuously while the other strand is formed discontinuously, DNA replication is semidiscontinuous.

Only one of the two strands of DNA possesses correct hereditary information. It is known as sense strand. Its complementary strand is called antisense strand. Antisense RNA that is made from the DNA strand that is complementary to the sense strand of the DNA.

The nitrogen bases of DNA are of two types namely (i) pyrimidines and (ii) purines. The nitrogen bases of the two chains formed complementary pairs with purine of one and pyrimidine of the other held together by hydrogen bonds. The two chains are antiparallel with 5'→ 3' orientation of one and 3'→5' orientation of the other.