All Free Biology MCQs with Answers

• A. Conservative
• B. Semi conservative
• C. Dispersive
• D. Semi-dispersive

Explanation: The model of DNA replication in which parental DNA would become completely dispersed, and each strand of all the daughter molecules would be a mixture of old and new DNA, is called the Dispersive model of DNA replication. In this model, the original DNA is fragmented into pieces, and each new DNA strand is a patchwork of segments from both the old and new DNA.

• A. Rapid cell division
• B. Lack of nutrition
• C. Fast mutation
• D. Lack of oxygen

Explanation: Cancerous cells are the cells that undergo rapid cell division. These cells are destroyed by X-ray radiation. During cell division, the DNA double helix opens up and undergoes various other processes. Such processes are disrupted when exposed to radiation, and the cancerous cells die selectively when irradiated.

• A. RNA virus converts its RNA to DNA
• B. There are no host cells present
• C. Nutrients are scarce
• D. Spikes are forming in the new virus

Explanation: Reverse transcriptase is an enzyme that allows the conversion of RNA into DNA. It is commonly found in retroviruses, such as HIV, where it plays a crucial role in the replication of the viral genome. By converting the viral RNA into DNA, the virus is able to integrate its genetic material into the host cell's DNA and hijack the host's cellular machinery to produce new virus particles. Reverse transcriptase is also used in molecular biology research, such as in the polymerase chain reaction (PCR) technique.

• A. Vinca rosea
• B. Vicia faba
• C. Drosophila melanogaster
• D. E. coli

Explanation: Taylor conducted his experiment on treated root tip cells of Vicia faba and treated them with radioactive thymidine (3H) to label the molecule of DNA, and then they grew them in the normal unlabelled medium, with colchicine, to see the semi-conservative nature of the chromosome replication. Semi-conservative mode of replication produces two copies of DNA, each copy containing one original strand and one new strand of DNA.

• A. The lagging strand towards the replication fork
• B. The leading strand away from replication fork
• C. The lagging strand away from the replication fork
• D. The leading strand towards the replication fork

Explanation: A lagging strand is a replicated strand of DNA that is formed in short segments called Okazaki fragments. Its growth is discontinuous. The direction of growth of the lagging strand is 3' ➡ 5' though in each Okazaki fragment, it is 5' ➡ 3'.

• A. tRNA
• B. mRNA
• C. miRNA
• D. rRNA

Explanation: The most abundant RNA molecule in a cell is typically ribosomal RNA (rRNA). Ribosomal RNA is a type of RNA that plays a crucial role in the ribosome, which is the cellular machinery responsible for protein synthesis.

• A. Muton
• B. Cistron
• C. Operon
• D. Recon

Explanation: Cistron (or gene) is a length of DNA that contains the information for coding a specific polypeptide chain or a functional RNA molecule (i.e. transfer RNA or ribosomal RNA). Hence, the cistron is a unit of function. Currently, such a gene is called a structural gene. A "cistron" is an older term that was used in early molecular biology to describe a segment of DNA that encodes a single gene. It is essentially equivalent to what we now commonly refer to as a "gene."

• A. 5S rRNA
• B. 18S rRNA
• C. 23S rRNA
• D. 5.8S rRNA

Explanation: The 23s rRNA is a structural RNA present in the ribosomes and a ribozyme that catalyses the formation of peptide bonds during translation in bacteria (prokaryotes).

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

Explanation: Chargaff's rules are applicable only to double-stranded DNA molecules. These are not applicable 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 are not applicable to RNA.

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