Gene Expression and Regulation
Short Questions and Answers
One Mark Questions with Answers
1. Initiation codon of protein synthesis is
Answer: (a) AUG
2. One gene one enzyme hypothesis was performed by
(a) Watson and Crick
(b) Jacob and Monod
(c) Beadle and Tatum
(d) Tatum and Leaderberg
Answer: (c) Beadle and Tatum
3. The one which is capable of self replication is
Answer: (d) DNA
4. Synthesis of DNA over RNA template occurs in
(c) Reo virus
(d) Rous sarcoma virus
Answer: (d) Rous sarcoma virus
5. Enzyme needed for transcription is
(b) RNA ase
(c) DNA polymerase
(d) RNA polymerase
Answer: (d) RNA polymerase
6. Khorana deciphered the triplet codon of
(a) tyrosine and tryptophan
(b) cysteine and valine
(c) serine and isoleucine
(d) none of these
Answer: (b) cysteine and valine
7. Site for RNA that binds to mRNA is
(a) 5′ end
(b) 3′ end
Answer: (c) anticodon
8. Process of translation is
(a) RNA synthesis
(b) DNA synthesis
(c) protein synthesis
(d) ribosome synthesis
Answer: (c) protein synthesis
9. Who proved that DNA is genetic material?
(b) Boveri and Sutton
(c) Hershey and Chase
(d) none of these
Answer: (c) Hershey and Chase
10. Protein helping in opening of double helix in form of replication fork is
(b) DNA gyrase
(c) DNA ligase
(d) DNA polymerase
Answer: (b) DNA gyrase
11. Double hydrogen bonds occur in DNA between
(a) uracil and thymine
(b) thymine and cytosine
(c) adenine and guanine
(d) adenine and thymine
Answer: (d) adenine and thymine
12. Hydrogen bonds between cytosine and guanine are
Answer: (b) 3
Two Marks Questions with Answers
1. What do you understand by the antiparallel arrangement of DNA strands?
Answer: The two DNA chains are antiparallel that is they run parallel but in opposite directions. In one chain the direction 5′-3′ while in the opposite one it is 3′-5′. The two chains are held together by hydrogen bonds between their bases.
2. Why replication is not continuous on the DNA template with 5′-3′ direction?
Answer: Replication is not continuous on the other template because only a short segment of DNA strand can be built in 5′-3′ direction due to exposure of a small stretch of template at one time.
3. If DNA is damaged due to mutations, can it be repaired by DNA polymerase? How?
Answer: There is a separate repair mechanism for any damage caused due to mutation, UV exposure or mismatching that escapes proofreading mechanism. A nick or break is caused by an endonuclease near the region of repair. DNA polymerase 1 removes the mismatched or wrong nucleotides if present and synthesizes a correct replacement by using the intact strand as template. The newly formed segment is sealed by DNA ligase.
4. Differentiate between bacterial transduction and bacterial transformation.
Answer: Transduction requires a vector while transformation does not require vector. Transduction involves bringing genes from a living host while transformation involves absorption of gene from nonliving medium. Transduction can occur any time while receptivity for transformation is present for a brief period at the end of active growth. Transduction does not require calcium chloride while calcium chloride or some other chemicals with similar property is required in transformation.
5. Differentiate between cancer cells and normal cells.
Answer: Cancer cells show indefinite growth cells. Cancer cells are derived from normal cells through transformation, so the normal limits of growth are bypassed. Cancer cells show metastasis. It means, they invaded normal tissue and start a new chain of transformation even away from the original site.
Three Marks Questions with Answers
1. What is gene expression?
Answer: Gene expression is a mechanism at the molecular level by which a gene is able to express itself in the phenotype of an organism. The mechanism of gene expression involves biochemical genetics. It consists of synthesis of specific RNAs, polypeptides, structural proteins, proteinaceous biochemicals or enzymes which control the structure or functioning of specific traits.
2. What do you mean by transduction?
Answer: It is the transfer of foreign genes by means of viruses. Transduction was first discovered by Zinder and his teacher Leaderberg in Salmonella typhimurium. The process also occurs in E coli and a number of other hosts. A virus may pickup gene of the host in place of its own gene during its multiplication in the host cell. Such a virus is never virulent. It passes over the gene of the previous host to the new host. Transducing viruses may carry the same genes or different genes.
3. Differentiate between lysogenic phase and lytic phase.
Answer: In lysogenic phase the viral genome or its complementary DNA gets integrated with the host DNA. It is called pro phase or provirus. The viral genome does not integrate with the host DNA. The host DNA is not hydrolysed during lysogenic phase while the host DNA is often hydrolysed in lytic phase. In lysogenic phase the virus is non virulent whereas the virus is virulent in lytic phase. In lysogenic phase the host cell does not get lysed while in lytic phase the host cell undergoes lysis.
4. Explain in brief the lytic cycle.
Answer: It is the reproductive cycle of virulent phages, e.g., T4 bacteriophage. The phage attaches itself to the host cell, example, E. coli, through its tail fibres. The fibres bend and bring the tip of tail in contact with the host cell wall. The tip of the tail produces a hole in the bacterial cell wall by means of enzyme lysozyme. The tail sheath contracts and injects the viral genome into the host cell. After entering the host cell, the viral DNA transcribes some early mRNAs to form some enzymes over the host ribosomes. Some of these are nucleases. They degrade host DNA and mRNAs. Parental viral DNA functions as a template and replicates repeatedly with the help of bacteria, nucleotides. Simultaneously, host machinery is used by phage genes to synthesise proteins for viral lysozyme, internal proteins and capsid proteins. Different components combine to form new viruses or phage particles. The host cell ruptures by means of lysozyme releasing the phage particles.
5. What is central dogma?
Answer: Central dogma is the flow of information from DNA to mRNA (transcription) and then decoding the information present in mRNA in the formation of polypeptide chain or protein (translation). In other words the four letter language of DNA is transcribed into four letter language of mRNA which is then translated into 20 letter language of protein. The concept of central dogma was advanced by Crick in 1958. It proposes unidirectional flow of information from DNA to RNA and then to protein Commoner propounded a circular flow of Information. Temin and Baltimore reported that retroviruses operate a central dogma reverse. RNA of these viruses first synthesizes DNA through reverse transcription or teminism. DNA then transfers information to RNA which take part in translation of coded information to form polypeptide.