Botany MCQs for NEET — Practice Questions with Answers

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).

The group of nucleotides that specifies one amino acid is a code word or codon. The nucleotides of mRNA are arranged as a linear sequence of codons, each codon consisting of three successive nitrogenous bases. Three codons UAG, UAA and UGA are the chain stop or termination codons. They do not code for any of the amino acids.

In most organisms AUG codon is the start or initiation codon, ie, the polypeptide chain starts either with methionine or N-formylmethionine.

Leucine – UUA, UUG, CUU, CUC, CUA, CUG

Alanine – GUC, GCC, GCA, GCG

GUU – Valine

UCA – Serine

Haploid plants can be produced in large number by anther and ovary cultures. Haploids may be useful for isolation of mutants, since, even recessive mutant alleles will be expressed in the mutagen treated generation itself. Desirable mutants may be selected at the haploid level and their chromosome number may be doubled to obtain homozygous mutant lines in a single generation.

Telomeres are ends of chromosome, have repetitive DNA sequences and are stable and resistant to exonuclease digestion hence, essential for chromosome stability.

Transcription factor is molecular basis of organ differentiation.