All Free Biology MCQs with Answers

• A. E.coli
• B. Diplococcus pneumoniae
• C. Salmonella
• D. Pasteurella pestis

Explanation: The transformation experiment was first conducted by Frederick Griffith in 1928 using the bacterium Streptococcus pneumoniae, also known by its older name, Diplococcus pneumoniae. His experiment demonstrated the phenomenon of bacterial transformation, where non-virulent bacteria could be changed into virulent ones by a "transforming principle" from heat-killed virulent bacteria. This laid the foundation for the eventual identification of DNA as the genetic material.

• A. All prokaryotes
• B. All prokaryotes and some eukaryotes
• C. All prokaryotes and all eukaryotes
• D. All prokaryotes and some protozoans

Explanation: The operon model was given by Jacob and Monod (1961) for the regulation of protein synthesis in prokaryotes only. In bacteria, the genes that contain the information for assembling the enzymes for a metabolic pathway are usually clustered together on the chromosome in a functional complex called an operon. Regulation of protein synthesis in eukaryotes is explained by the gene battery model given by Britten and Davidson.

• A. Proteins
• B. Lipid
• C. Carbohydrates
• D. Phospholipids

Explanation: DNA transcribes to form mRNA. Its function is to carry coded information from DNA for the synthesis of proteins. The RNA consists of a coding region called an exon and noncoding regions called introns. The exons are thus the functional part that codes for proteins.

• A. Co-­repressor binds with repressor
• B. Co­-repressor does not bind with repressor
• C. Co­-repressor binds with inducer
• D. cAMP have negative effect on lac operon

Explanation: The tryptophan operon (trp operon) in bacteria is repressible. Here, the repressor is inactive, and it becomes active as DNA DNA-binding protein only when complexed with a corepressor (tryptophan). In the absence of tryptophan, the operator site is open to binding by RNA polymerase, which transcribes the structural genes of the tryptophan operon, leading to the production of enzymes that synthesize tryptophan. When tryptophan becomes available, the enzymes of the tryptophan synthetic pathway are no longer needed, and tryptophan (co­repressor) repressor complex blocks transcription. The regulation of this operon is also a negative control.

• A. One cistron contains many genes
• B. One gene contains many cistrons
• C. One gene contains one cistron
• D. One gene contains no cistron

Explanation: A gene is a hereditary unit consisting of a sequence of DNA and occupying a specific position or locus within the genome. Gene activity ultimately affects the phenotype of the organism possessing the gene. Thus gene is a physical and functional unit of genetic information. A cistron is a unit of genetic function. In prokaryotes, there is a one gene-one enzyme correspondence. It means that in these organisms, genes and cistrons are equivalent.

• A. 3.4 nm
• B. 0.34 nm
• C. 20 nm
• D. 10 nm

Explanation: B-DNA is an antiparallel double helix. The double-strand or duplex is coiled plectonemically in a right-handed fashion around a common axis, like a rope staircase twisted in a spiral. The coiling produces alternate major and minor grooves. One turn of the spiral has a distance of 3.4 Å between two adjacent nucleotides.

• A. Both are polymer of nucleotides
• B. Both have similar pyrimidine
• C. Both have similar sugar
• D. Both are genetic material

Explanation: Deoxyribonucleic acid and ribonucleic acid are made up of several nucleotide monomers. Each nucleotide consists of a pentose sugar, a phosphate group, and nitrogenous bases. DNA has a deoxyribose sugar, whereas RNA has a ribose sugar. The bases in the DNA molecule are A, T, G, and C, whereas in RNA, thymine is absent and instead uracil is found.

• A. Bacteria do not reproduce sexually
• B. RNA sometime controls the production of DNA and proteins
• C. DNA is the genetic material
• D. Bacteria undergo binary fission

Explanation: Transformation was first discovered by Griffith (1928) in Pneumococcus (Streptococcus pneumoniae), which causes pneumonia. Griffith injected a group of mice with nonencapsulated, rough (R), pneumococci; a second group with heat­killed encapsulated pneumococci cells, and a third group a mixture consisting of a few living nonencapsulated, rough pneumococci derived from a type S culture, and heat­killed encapsulated cells (S type). Griffith observed that the mice in the first two groups were not infected, and the mice in the third group died within a few days. The mice of the third group should have survived, as the organisms that could kill them had been killed, and the cells of the R type were incapable of causing disease. However, the mice died, and living virulent encapsulated cells of the type S were recovered from their dead bodies. It was observed by Griffith that killed encapsulated pneumococci had liberated some substance which favoured non­capsulated cells (R type) to produce a capsular substance. This substance, in later experiments, was proved to be DNA. These experiments showed that DNA is the genetic material.

• A. Nucleolus
• B. Nucleus only
• C. Cytoplasm only
• D. None of these

Explanation: DNA is mainly found in the nucleus. It is associated with RNA and proteins to form compact chromosomes. But some amount of DNA is also found in chloroplasts and mitochondria. This DNA is called extra­chromosomal DNA.

• A. Linear DNA without histones
• B. Circular DNA without histones
• C. Linear DNA with histones
• D. Circular DNA with histones

Explanation: The genetic material of prokaryotes is circular and single-stranded DNA. It has no association with histones. The eukaryotic genetic material is linear and double-stranded DNA. It is associated with histone proteins to form a nucleosome unit.