NEET Practice Questions (MCQs) with Answers & Solutions

Nucleosome is sub-microscopic sub-unit of chromatin which is formed by wrapping of DNA over a core of histone proteins. The term was coined by Oudet et. al, (1975). It is oblate structure with a length of 10 nm and a thickness of 5-5.7 nm. Its core is called nu-body. The latter is formed of four pairs of histone molecules. H₂A, H₂B, H₃ and H₄. DNA makes 1.75 turns over the

In prokaryotes, hypothesis was given in 1961 to explain the protein synthesis regulation. This hypothesis was given by F Jacob and J Monod and for this awarded Nobel Prize in 1965, the hypothesis was known by the name of Operon Model.

The operator gene is the segment of DNA, which exercise a control over transcriptions. In the absence of lactose, the repressor binds with the operator gene.

The genetic code is degenerate, ie, a given amino acid can be specified by more than one codons.

The genetic code inside the cell medium is said to be non-ambiguous because a particular codon always codes the same amino acid. But in certain rare cases the genetic code is found to be ambiguous, ie, some codons code for different amino acids under different conditions. The genetic code is almost universal.

Britten and his coworkers (1966-68) have demonstrated that many vertebrate DNAs reassociate, especially if broken into smaller pieces. This observation gave rise to the hypothesis that certain short sequences of bases are repeated hundred or more times in such DNAs. Such DNAs have been called repetitive or satellite DNAs. All eukaryotes except yeast contain satellite DNA. Satellite DNA regions like VNTR/RFLP are the basis of DNA fingerprinting (forensic science).

The point mutations involve alterations in the structure of gene by altering the structure of DNA. Point mutations are of two types – (i) base pair substitution and (ii) frameshift substitution.

Insertion is the addition of one or more nitrogenous bases to a nucleotide chain.

Duplication is the presence of one block of genes more than once in a haploid component.

Deletion is the removal of one or more nitrogenous bases from a nucleotide chain.

In some eukaryotes, genes consist of coding nucleotids sequences, which are separated from each other by blocks of non-coding sequences. The coding sequences here are called exons and non-coding sequences are called introns. The primary transcript from a typical eukaryotic gene contains introns as well as exons. The introns are removed from this primary transcript by a process called RNA splicing.

Semi-conservative replication of DNA was first demonstrated in E. coli. According to the semi-conservative mode proposed by Watson and Crick, each strand of the two double helices formed would have one old and one new strand. The semi-conservative nature of DNA replication was proved by the experiment of Meselson and Stahl (1958).

The general features of genetic code are:-

(i) The genetic code is written in linear form, using the ribonucleotide bases that compose mRNA molecule as letters.

(ii) Each word of codon consists of three letters, i.e, the codon is triplet.

(iii) The genetic code inside the cell medium is said to be non-ambiguous.

(iv) The code is degenerate, i.e, a given amino acid can be Specified by more than one codons.

(v) The codon contains ‘start’ and ‘stop’ signals.

(vi) The code is said to be commaless.

(vii) The code is non-overlapping.

In DNA molecule the adjacent deoxy ribonucleotides are joined in a chain by phosphodiester bridges or bonds, which link the 5’ carbon of deoxyribose of one mononucleotide unit with 3’ carbon of deoxyribose of next mononucleotide unit. According to Watson and Crick DNA molecule consists of two such polynucleotide chains wrapped helically around each other, with the sugar phosphate chain on the outside and purine and pyrimidine on the inside of helix. The two strands run antiparallely, ie, one strand has phosphodiester linkage in 5’ → 3’ direction while other strand has phosphodiester linkage in 5’ → 3’ direction. Chargaff (1950) suggested that despite wide compositional variations exhibited by different types of DNA the total amount of purines equaled the total amount of pyrimidines (A + G = T + C).