All Free Biology MCQs with Answers

• A. DNA replication is occurring
• B. The DNA is condensed into a chromatin fibre
• C. The DNA double helix is exposed
• D. Transcription is occurring

Explanation: Histones help in the packaging of DNA. In eukaryotes, DNA packaging is carried out with the help of positively charged basic proteins called histones. Histones are of five types - H-1, H-2A, H-2B, H-3, and H-4. H1 is attached to the linker DNA. Histone contains a large proportion of the positively charged (basic) amino acids, lysine, and arginine in its structure. DNA is negatively charged due to the phosphate groups on its backbone. The result of these opposite charges is a strong attraction and, therefore, a high binding affinity between histones and DNA.

• A. Template strand
• B. Coding strand
• C. Alpha strand
• D. Anti-strand

Explanation: The template strand serves as the template for RNA synthesis (transcription). It is also known as the antisense strand. During transcription, an enzyme called RNA polymerase reads the template strand and synthesises a complementary RNA molecule. The resulting RNA molecule is complementary to the template strand, with the exception of using uracil (U) in place of thymine (T) in RNA. The strand of DNA on which RNA polymerase binds to catalyse transcription is called the template strand. It is also known as the master or antisense strand. It has a polarity of 3' ➡ 5'.

• A. UAA
• B. UAG
• C. AUG
• D. UGA

Explanation: Polypeptide synthesis is signaled by two initiator codons or start codons, i.e., AUG (methionine codon) and rarely by GUG (valine codon). The starter codon is "AUG." It serves as the initiation codon and codes for the amino acid methionine in eukaryotes and, in most cases, in prokaryotes. It also signals the beginning of protein synthesis or translation in the ribosome. The AUG codon is often referred to as the "start codon." The other codons mentioned, UAA, UAG, and UGA, are stop codons that signal the termination of protein synthesis.

• A. Heterocyclic nitrogenous bases
• B. Chargaff's rule
• C. Complementary base pairing
• D. 5' phosphoryl and 3' hydroxyl ends

Explanation: Chargaff's rules apply only to double-stranded DNA molecules. These do not apply to single-stranded DNA or RNA molecules. Chargaff's rules state that DNA helices contain equal molar ratios of A and T, G and C. This is because in a DNA molecule, complementary base pairing occurs between A and T, and C and G base pairs. This complementary base pairing is not possible in the case of a single-stranded RNA molecule. Thus, Chargaff's rules do not apply to RNA.

• A. Polysaccharide synthesis
• B. RNA and protein synthesis
• C. Lipid synthesis
• D. Nucleotide synthesis

Explanation: In certain development stages, the polytene chromosomes bear conspicuous swellings called chromosome puffs. The larger swellings are called Balbiani rings. In the region of a puff or Balbiani ring, the DNA strands uncoil, become active, and produce a number of copies of mRNA. The mRNAs may remain temporarily stored in the puff, and they may undergo transcription to form proteins. Thus, Balbiani rings are the sites of RNA and protein synthesis.

• A. Negative and repressible because the repressor protein prevents transcription
• B. Feedback inhibition occurs because an excess of β-galactosidase can switch off transcription
• C. Positive and inducible because it can be induced by lactose
• D. Negative and inducible because the repressor protein prevents transcription

Explanation: The control of expression of the lac operon is negative (as it is turned off normally) and inducible. An inducible operon is an operon that remains switched off normally but becomes operational in the presence of an inducer (lactose, actually allolactose, a metabolite of lactose, in the case of lac operon). The inducible operon generally functions in catabolic pathways. In the presence of an inducer, the repressor has a higher affinity for the inducer than for the operator gene. When lactose is added, a few lactose molecules are carried into the cell by the enzyme lactose permease, as a small amount of this enzyme is present in the cell even when the operon is not working. These few lactose molecules are converted into allolactose molecules, which act as an inducer and bind to the repressor (a product of the regulator gene). The repressor-­inducer complex fails to join with the operator gene, thus it is turned on.

• A. A-Transcription, B-Translation, C-Francis Crick
• B. A­-Translation, B-Extension, C ­-Rosalind Franklin
• C. A­-Transcription, B-­Replication, C-James Watson
• D. A­-Translation, B­-Transcription, C­-Ervin Chargaff

Explanation: The expression of the genetic material occurs normally through the production of proteins. This involves two consecutive steps. These are transcription and translation. The DNA codes for the production of messenger RNA (mRNA) during transcription. Messenger RNA carries coded information to ribosomes. The ribosomes read this information and use it for protein synthesis. This process is called translation. F.H.C. Crick described this unidirectional flow of information in 1958 as the 'central dogma of molecular biology'.

• A. Transacetylase
• B. Lactose permease and transacetylase
• C. β-galactosidase
• D. Lactose permease

Explanation: A nonsense mutation stops polypeptide synthesis due to the formation of a termination or non­sense codon. In the lac operon, the sequence of structural genes is Z, Y, and A, which respectively code for b-­galactosidase, lactose permease, and transacetylase. If the gene Y has a nonsense mutation, gene expression will stop at it, resulting in non­expression of both gene Y and successive gene A. Thus, only the β-galactosidase enzyme will be produced.

• A. Non­-overlapping
• B. Ambiguous
• C. Degeneracy
• D. Universal

Explanation: The genetic code is non-­ambiguous. A non­ ambiguous code means that there is no ambiguity about a particular code. One codon specifies only one amino acid and not any other. There are 64 codons. Out of 64, 3 are stop codons or nonsense codons, i.e., these do not code for any amino acid, and the rest of the 61 codes for one of the 20 amino acids. Neither of them codes for more than one amino acid, except for GUG, which normally codes for valine, but in certain conditions, it also codes for N-formyl methionine as the initiation codon.

• A. Looping
• B. Inducing
• C. Slicing
• D. Splicing

Explanation: Introns, which mainly occur in eukaryotes, are transcribed into messenger RNA (mRNA) but are then removed from the transcript before translation. In some cases, the removal of introns is an autocatalytic process (self-splicing) in which the RNA itself acts as an enzyme.