Everything you need to know about molecular biology. Some of the most frequently asked questions are as follows:-
Q.1. What is wobble hypothesis? What is the importance of wobble and degeneracy?
Ans: This hypothesis accounts for the observed pattern of degeneracy in the third base of a codon. According to this hypothesis the third base can undergo with the corresponding first base in the anticodon. The importance of wobble and degeneracy of the genetic code is that cell does not have to synthesize a different tRNA for each of the 61 sense codons. A simple example is that only two different tRNA anticodons are needed to recognize four different glycine codons.
Q.2. What is Shine-Dalgarno sequence? In which groups of microorganisms it is found?
Ans: The Shine-Dalgarno sequence is located before the start sequence of mRNA. This nucleotide sequence allows the mRNA to align with the 30S ribosomal subunit of the bacterial cell. It is about 7 bases upstream earlier) towards the 5 ‘-P end of the AUG start codon on the mRNA and is a polypurine consensus sequence AGGAGG and is referred to as Shine-Dalgarno sequence (Fig. 33.7). It is found in bacterial and archaeal cells.
Q.3. What do the codons UGA, UAA and UAG mean in normal translation?
Ans: In normal translation they mean for “Stop” codons.
Q.4. Why is genetic code said to be degenerate?
Ans: It is because more than one codon can code for the same amino acid.
Q.5. How many termination codons or nonsense codons are there?
Ans: There are only three termination codons also, called “nonsense codons” as they do not code for any amino acid.
Q.6. The codon AGG normally codes for argine but in altered translation it codes for stop. Where does it occur?
Ans: It occurs in human mitochondria.
Q.7. What is odd in the studies made in the mitochondrial DNA (double-stranded circular DNA genome) as one proceeds from lower to higher eukaryotes?
Ans: While one proceeds from lower to higher eukaryotes, the odd thing in the mitochondrial genome is that it gets smaller, e.g., the yeast mitochondrial DNA is five times larger than that of the human mitochondria.
Q.8. What has enabled a given tRNA that sometimes it specifically recognizes several codons?
Ans: The wobble in the base pair at the 5′ end on the anticodon enables the given tRNA to recognize several codons.
Q.9. All newly synthesized bacterial proteins start (initiate) with formylmethionine (may be abbreviated as fmet). How is formyl group removed from the fmet polypeptide in bacteria?
Ans: The formyl group is often removed from formyl-methionine by the enzyme deformylase leaving behind the methionine as the first amino acid in the polypeptide chain.
Q.10. What are genes? Define.
Ans: A gene may be defined as a sequence of nucleotides which specifies a particular polypeptide chain or RNA sequence or that regulates the expression of other genes. The genes which code for proteins are referred to as structural genes or cistrons while the other genes bearing regulatory function are called regulatory genes. The regulatory genes work to control the expression of structural genes. The structural and regulatory genes collectively constitute the genotype which determines the phenotype, i.e., observable structural and functional characteristics.
Q.11. Define an operon.
Ans: A DNA sequence which codes for one or more structural genes (polypeptides) of related function and the DNA sequence which regulates the expression.
Q.12. What controls induction and repression?
Ans: The regulatory genes that produce a regulator protein controls induction (i.e., causing increase in rate of synthesis of an enzyme), and repression (i.e., blockage of gene expression).
Q.13. What is the lac operon?
Ans: The lac operon stands for lactose operon, an operon which contains genes specifying proteins involved in the utilization of (3 -galactosides such as lactose. The lac operon occurs in Escherichia coli at ca. (= about) 8 minutes on the chromosome map. It has the structural: promoter-operator lac Zr-lac Y-lac A. The lac Z gene encodes P-galactosidase, lac Y encodes (3 -galactoside permease and lac A encodes thiogalactoside transacetylase (Fig. 33.8).
Q.14. What is catabolite repression?
Ans: The catabolite repression is the repression of transcription of genes coding for certain inducible enzyme systems by glucose or other readily utilizable carbon sources.
Q.15. What are positive regulators (activators) and negative regulators (repressors)? Describe.
Ans: Bacteria possess many enzymes whose rate of synthesis depends on the availability of external food molecules. These external molecules called inducers and co-repressors usually determine the rate of synthesis of enzymes by controlling the synthesis of their mRNA templates. Inducers and co-repressors act by binding to regulatory proteins referred to as activators and repressors.
Activators are positive regulators because their presence is required for the regulated enzyme to be made while repressors act as negative regulators because their regulatory activity is to prevent the synthesis of proteins. Thus, when lactose is absent the lac operon (lactose operon) repressor prevents synthesis of enzymes that metabolize lactose. However, upon binding an inducer (a molecule related to lactose) the repressor loses this Ability and permits the production of enzymes. The arabinose operon C protein which is an activator causes making of arabinose enzymes on binding to the inducer arabinose.
Q.16. What is palindromic sequence of DNA?
Ans: Literally speaking palindrome is a word that reads the same backward and forward. The palindromic sequence is a region of a nucleic acid that contains a pair of inverted repeat sequences. In a double stranded molecule of DNA such a region shows two-fold rotational (dyad) symmetry or hypemated dyad symmetry if the two IR sequences are separated by another sequence. A double-stranded palindromic sequence can adopt either of two possible formations:
1. A linear structure with inter-strand hydrogen bonding, e.g.,
5’… TCCACATGTOGA… 3′
3… AGGTGTACACCT… 5′
2. A cruciform structure in which of two strands each forms hairpins by intra-strand hydrogen bonding.
Q.17. Who discovered that X-rays induce mutations?
Ans: Hermann Muller and L.J. Stadler discovered independently in 1927 that X-rays induce mutations.
Q.18. What is cistron?
Ans: A cistron is a gene as defined in terms of the CIS-TRANS TEST, i.e., in a diploid cell or merozygote either of two homologous sequences in a genetic nucleic acid in which two mutations in trans fail to exhibit complete complementation. A cistron may also be defined as the functional unit of genetic inheritance a segment of genetic nucleic acid which codes for a specific polypeptide chain. The term cistron has also been used as a synonym for gene.
Q.19. What is recon?
Ans: The term recon was coined by S. Benzer. According to him it is a unit of genetic subdivision beyond which recombination does not occur.
Q.20. Define a mutator or mutator gene.
Ans: A mutator gene or mutator is designated as must within which certain mutations cause an increase in the spontaneous mutation rate in other genes, e.g., Escherichia coli mutations in the gene encoding the e subunit of DNA polymerase III (dna Q and mut D alleles) can result in extremely high levels of spontaneous mutations. The mutant alleles may differ from the wild type, e.g., by only one or two amino acid changes in the ε subunit. The mutation may lead to reduced accuracy in the polymerising (nucleotide selection) activity or in the proof reading activity of the enzyme. Some other mutator genes in E.Coli include in the mismatch repair system.
Q.21. What are split genes? Describe.
Ans: The split genes may be defined as the genes that are coded for, by noncontiguous segments of the DNA so that the mRNA and the DNA for the protein product of that gene are not colinear. These are the genes with intervening nucleotide sequences not involved in coding for the gene product.
The split genes have also been regarded as interrupted genes. A structural gene encoding a protein, rRNA or tRNA that contain one too many intervening sequences (introns) that although represented in primary RNA transcript of the gene are absent from the mature RNA molecule (mRNA, tRNA), therefore, do not contribute to the structure of the gene product.
Thus mutation of the RNA transcript of a split gene must involve a process of splicing to delete the intron and join together the remaining sequences called exons. In the case of mRNA the sequence of exons includes the coding sequence of the gene as well as noncoding leader and/ or trailor sequences. Introns themselves are usually noncoding. Most of the nuclear structural genes in higher eucaryotes are split genes.
Q.22. What are overlapping genes?
Ans: The overlapping genes are two or more genes in which part or complete gene is co-extensive with part of another. The genes may be translated in different reading frames or in the same reading frame with different start and/or stop points or different splicing patterns. The phenomenon of overlapping genes maximizes the coding capacity of a genome and can also provide a means for the regulation of expression of genes.
Q.23. The history of DNA world is written in gene sequences. Justify this statement.
Ans: The evolution of the organisms from a common ancestor is represented by a branched pathway known as geological tree (also known as phylogenetic or evolutionary tree). The branching pattern of tree is calculated using the principle of parsimony (based on economy) to determine the minimum number of genetic changes required to derive the sequence of the gene in each organism from a common ancestor. It is sometimes reasonable to assume that highly conserved protein like globin and cytochorme c can be used as a molecular clock to measure how long the species have been diverging from each other.