All Free Biology MCQs with Answers

• A. The proportion of adenine in relation to thymine varies with the organism
• B. There are two strands which run antiparallel­; one in the 5' ➡ 3' direction and the other in 3' ➡ 5' direction
• C. The total amount of purine nucleotides and pyrimidine nucleotides is not always equal
• D. There are two strands which run parallel in the 5' ➡ 3' direction

Explanation: A DNA molecule has two unbranched complementary strands, which are spirally coiled. The two chains are antiparallel, i.e., they run parallel but in opposite directions. One chain has the polarity 5' ➡ 3', whereas the other has 3' ➡ 5'. Both are held together by hydrogen bonds between their bases, i.e., A = T and G = C, and the amount of adenine is equal to thymine, and guanine equals cytosine. The base ratio A + T / G + C may vary from one species to another, but is constant for a given species. The purine and pyrimidines are always in equal amounts (A + G = T + C), but A + T is not necessarily equal to G+C.

• A. Guanine, Adenine-Purines
• B. Adenine, Thymine-Purines
• C. Thymine, Uracil-­Pyrimdines
• D. Uracil, Cytosine-Pyrimidines

Explanation: Adenine and Guanine are the purine bases, while Cytosine, Thymine and Uracil are pyrimidine bases.

• A. AUG, ACG -Start/methionine
• B. UUA, UCA -­Leucine
• C. GUU, GCU -­Alanine
• D. UAG, UGA -­Stop

Explanation: AUG codes for methionine and is the initiation or start codon that starts the synthesis of the polypeptide. UAA (ochre), UAG (amber), and UGA (opal) do not specify any amino acid, so they are called termination codons. CUU, CUC, CUA, and CUG code for leucine, whereas GCU, GCC, GCA, and GCG code for alanine.

• A. 2 nm
• B. 20 nm
• C. 0.34 nm
• D. 3.4 nm

Explanation: DNA is the largest macromolecule made of helically twisted two antiparallel polydeoxyribonucleotide strands held together by hydrogen bonds. The two strands of DNA are together called a DNA duplex. They have a diameter of 20Å. One turn of a spiral has a distance of 34Å. It contains 10 deoxyribonucleotides in each strand, so that the distance between two adjacent nucleotides is 3.4 Å.

• A. Beadle and Tatum
• B. R. Franklin
• C. Hershey and Chase
• D. A. Garrod

Explanation: In 1948, Beadle and Tatum proposed the one­ gene one­ enzyme hypothesis, which states that a gene controls the metabolic machinery of the organism through the synthesis of an enzyme. This laid the foundation of biochemical genetics. Beadle and Tatum were awarded the Nobel Prize in 1958. This one gene-one enzyme theory has now been changed to the one gene-one polypeptide hypothesis as proposed by Yanofsky. This hypothesis suggests that one gene synthesises one polypeptide and many polypeptides form one enzyme.

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

Explanation: Messenger RNA or mRNA has been named so because it carries the coded information from DNA for the synthesis of proteins. It carries the coded information in a number of base triplets called codons. It is transcribed onto DNA by the enzyme RNA polymerase. Hence, its base sequence is complementary to the DNA on which it has been synthesised. In eukaryotes, each gene transcribes its own mRNA; therefore, the number of mRNAs corresponds to the number of genes. rRNA is a type of RNA that forms structural and functional components of ribosomes. tRNA is a class of RNA having structures with triplet nucleotide sequences that are complementary to the triplet nucleotide coding sequences of mRNA. It binds with amino acids and transfers them to ribosomes.

• A. Only on the ribosomes present in cytoplasm
• B. Only on the ribosomes attached to the nuclear envelope and endoplasmic reticulum
• C. On the ribosomes present in the nucleolus as well as the cytoplasm
• D. On ribosomes present in the cytoplasm as well as mitochondria

Explanation: Protein synthesis is the process by which an mRNA is read by ribosomes to synthesise a polymer of amino acids known as a protein. The process is known as translation. The process of protein synthesis occurs in different organelles, such as mitochondria, chloroplasts, cytoplasm, and rough endoplasmic reticulum in animal cells. Protein synthesis is performed by structures known as ribosomes. Ribosomes are present in mitochondria, cytoplasm, endoplasmic reticulum (ER), etc.

• A. DNA polymerase
• B. RNA polymerase
• C. Reverse transcriptase
• D. DNA dependant RNA polymerase

Explanation: The answer to this question is reverse transcriptase. Reverse transcriptase is an enzyme that uses an RNA template to synthesise DNA. It is found in retroviruses, such as HIV, and is used to create a DNA copy of the viral RNA genome. Reverse transcriptase is also used in some molecular biology techniques, such as RT-PCR, to create cDNA from mRNA.The other options are incorrect:DNA polymerase synthesises DNA from a DNA template.RNA polymerase synthesises RNA from a DNA template.DNA-dependent RNA polymerase synthesises RNA from a DNA template.

• A. Lipase
• B. Protease
• C. Endonuclease
• D. Exonuclease

Explanation: The answer is endonuclease. Endonucleases are enzymes that hydrolyze internal phosphodiester bonds in a polynucleotide chain. They are used in many different cellular processes, such as DNA replication, DNA repair, and gene expression.The other options are incorrect because :Lipase hydrolyzes triglycerides into fatty acids and glycerol.Protease hydrolyzes proteins into peptides and amino acids.Exonuclease hydrolyzes phosphodiester bonds at the ends of a polynucleotide chain.

• A. AAAT box
• B. TATA box
• C. GGTT box
• D. CAAT box

Explanation: The DNA sequence that holoenzyme RNA polymerase binds to during transcription, and the DNA assumes a saddle-like structure at that point, is called the TATA box. The TATA box is a consensus sequence, meaning that it is a sequence of nucleotides that is found in the promoter region of many eukaryotic genes. It is located about 10 base pairs upstream of the transcription start site. The TATA box is important for transcription because it helps to position the RNA polymerase in the correct place to start transcription. It also helps to recruit other transcription factors that are necessary for transcription to occur.The other options are incorrect because:AAAT is not a consensus sequence for the promoter region of eukaryotic genes.GGTT is not a consensus sequence for the promoter region of eukaryotic genes.CAAT is a consensus sequence for the CAAT box, which is another important promoter element, but it is not the sequence that RNA polymerase binds to to initiate transcription.