Questions: Molecular Basis of Inheritance

Q1 Describe the structure of a polynucleotide chain.

Answer (English): A nucleotide has a nitrogenous base, a pentose sugar and a phosphate group. The sugar is deoxyribose in DNA and ribose in RNA. A base is attached to the 1 prime carbon of the sugar by an N-glycosidic bond, forming a nucleoside, and a phosphate attached to the 5 prime carbon forms a nucleotide. Two nucleotides are joined by a 3 prime to 5 prime phosphodiester bond, and the chain of sugar and phosphate units forms the backbone, with the bases projecting from it. One end of the chain carries a free 5 prime phosphate and the other a free 3 prime hydroxyl, which gives the chain its direction.

Hinglish: Nucleotide ke teen parts hote hain: base, pentose sugar aur phosphate. Base 1 prime carbon par N-glycosidic bond se judta hai, aur phosphate 5 prime carbon par; do nucleotides 3 prime se 5 prime phosphodiester bond se jurte hain. Sugar-phosphate chain backbone banati hai aur bases bahar nikalte hain. Ek end par 5 prime phosphate aur doosre par 3 prime hydroxyl hota hai, isliye chain ki direction hoti hai.

Q2 Explain the Watson-Crick model of DNA.

Answer (English): DNA is a double helix of two antiparallel polynucleotide strands. The sugar-phosphate backbones lie outside and the base pairs lie inside, perpendicular to the axis. Adenine pairs with thymine by two hydrogen bonds and guanine pairs with cytosine by three, so the two strands are complementary. The pitch of the helix is 3.4 nm, with ten base pairs per turn and a rise of 0.34 nm per base pair, and the diameter is 2 nm. The model explains stability through hydrogen bonds and base stacking.

Hinglish: Watson-Crick model mein do antiparallel strands hoti hain, backbones bahar aur base pairs andar. A-T do hydrogen bonds aur G-C teen hydrogen bonds banate hain, isliye strands complementary hoti hain. Pitch 3.4 nm, ek turn mein 10 base pairs, rise 0.34 nm, diameter 2 nm. Model se stability (hydrogen bonds aur stacking) aur replication-transcription (complementary pairing) dono samajh aate hain.

Q3 What does Chargaff's rule state?

Answer (English): In a double-stranded DNA the amount of adenine equals the amount of thymine and the amount of guanine equals the amount of cytosine. The ratio of purines to pyrimidines is therefore one. The base composition differs between species, but the equalities hold within a species for double-stranded DNA. The rule follows from the pairing between the two strands.

Hinglish: Chargaff's rule kehta hai ki double-stranded DNA mein A barabar T aur G barabar C hota hai, isliye purine aur pyrimidine ka ratio 1 hota hai. Base composition species ke hisaab se badalta hai, par yeh equalities double-stranded DNA mein hamesha rehti hain; yeh base pairing ka nateeja hai.

Q4 How is DNA packed inside a eukaryotic nucleus?

Answer (English): DNA is negatively charged and binds the positively charged histones, which are rich in lysine and arginine. Eight histones form an octamer, and about 200 base pairs of DNA wrap around it to form a nucleosome. Nucleosomes give the beaded appearance of chromatin and coil further into chromatin fibres. Loosely packed euchromatin is transcriptionally active, while densely packed heterochromatin is not. Prokaryotes lack histones and keep their DNA in the nucleoid.

Hinglish: DNA negative hota hai aur histones positive proteins (lysine, arginine rich) se bind karta hai. Aath histones ka octamer banata hai aur lagbhag 200 bp DNA uske chaaron taraf lipat kar nucleosome banata hai; inhi se beads-on-string appearance aur chromatin fibres banti hain. Euchromatin loose aur active, heterochromatin dense aur inactive hota hai. Prokaryotes mein histones nahi hote, DNA nucleoid mein rehta hai.

Q5 How did Griffith, Avery and Hershey-Chase establish that DNA is the genetic material?

Answer (English): Griffith found that heat-killed smooth pneumococci could transform live rough pneumococci into smooth ones, and called the agent the transforming principle. Avery, MacLeod and McCarty destroyed proteins, RNA and DNA in turn and found that only DNase stopped transformation, proving the principle was DNA. Hershey and Chase labelled phage DNA with phosphorus-32 and phage protein with sulphur-35, and found that only the DNA label entered the bacteria. Together the three experiments showed that DNA carries genetic information.

Hinglish: Griffith ne dekha ki heat-killed smooth bacteria live rough cells ko smooth bana dete hain, aur us agent ko transforming principle kaha. Avery, MacLeod aur McCarty ne proteins, RNA aur DNA ko alag-alag destroy kiya; sirf DNase se transformation ruka, isliye principle DNA tha. Hershey-Chase ne phage DNA ko 32P aur protein ko 35S se label kiya; bacteria mein sirf DNA label gaya. Teeno experiments ne dikhaya ki DNA genetic material hai.

Q6 Describe DNA replication and explain why it is semiconservative.

Answer (English): Replication begins at the origin of replication. Helicase unwinds the helix, topoisomerase relieves strain, and an RNA primer is laid down. DNA polymerase adds deoxyribonucleotides in the 5 prime to 3 prime direction. Because the templates are antiparallel, one new strand, the leading strand, is made continuously towards the fork, while the other, the lagging strand, is made in short Okazaki fragments that DNA ligase joins. Each daughter molecule keeps one parental strand and one new strand, which is why replication is semiconservative, and Meselson and Stahl proved it with the 15N to 14N density shift experiment.

Hinglish: Replication origin par shuru hoti hai; helicase helix kholta hai, RNA primer banta hai, aur DNA polymerase 5 prime se 3 prime direction mein nucleotides jodta hai. Templates antiparallel hain, isliye leading strand continuous aur lagging strand Okazaki fragments mein banti hai; DNA ligase fragments jodta hai. Har nayi molecule mein ek purani aur ek nayi strand hoti hai, isliye semiconservative; Meselson-Stahl ne 15N se 14N shift wale density experiment se ise prove kiya.

Q7 Explain the process of transcription in bacteria.

Answer (English): The transcription unit has a promoter, a structural gene and a terminator. RNA polymerase binds the promoter with the help of the sigma factor, the two DNA strands separate, and the template strand is read in the 3 prime to 5 prime direction. Ribonucleotides are added to the growing RNA in the 5 prime to 3 prime direction during elongation. At the terminator, the rho factor or a hairpin formed by the RNA ends transcription and the polymerase dissociates. Bacteria have one RNA polymerase and no processing of mRNA.

Hinglish: Transcription unit mein promoter, structural gene aur terminator hote hain. RNA polymerase sigma factor ki madad se promoter par bind karta hai, strands alag hoti hain, aur template strand 3 prime se 5 prime padhi jaati hai; RNA 5 prime se 3 prime banti hai. Terminator par rho factor ya RNA ka hairpin transcription khatam karta hai. Bacteria mein ek hi RNA polymerase hota hai aur mRNA ki processing nahi hoti.

Q8 What are the properties of the genetic code?

Answer (English): The code is a triplet code, because three bases make one codon. There are 64 codons, of which 61 specify amino acids and three, UAA, UAG and UGA, are stop codons. AUG is the start codon and also codes methionine. The code is degenerate because most amino acids have more than one codon, unambiguous because one codon always means one amino acid, non-overlapping and comma-less, and nearly universal across organisms.

Hinglish: Genetic code triplet hai: teen bases ek codon. 64 codons mein 61 amino acids ke liye aur 3 (UAA, UAG, UGA) stop codons hain; AUG start codon hai aur methionine bhi code karta hai. Code degenerate hai kyunki ek amino acid ke kai codons hote hain, unambiguous hai, non-overlapping aur comma-less hai, aur lagbhag universal hai.

Q9 Describe translation and the role of tRNA and ribosome.

Answer (English): Aminoacyl-tRNA synthetase attaches each amino acid to its tRNA in a reaction called charging. The small ribosomal subunit binds the mRNA and the initiator tRNA pairs with AUG, then the large subunit joins. During elongation a charged tRNA pairs with each codon, the ribosome forms a peptide bond using its peptidyl transferase activity, and it moves one codon along. At a stop codon, release factors free the polypeptide and the ribosome dissociates.

Hinglish: Aminoacyl-tRNA synthetase amino acid ko tRNA par charge karta hai. Small subunit mRNA par bind karta hai, initiator tRNA AUG se pair karta hai, phir large subunit judta hai. Elongation mein har codon par charged tRNA aata hai, ribosome peptide bond banata hai aur ek codon aage badhta hai; stop codon par release factors polypeptide chhodte hain. Ribosome mRNA aur tRNAs ko paas rakhta hai aur peptide bond catalyse karta hai.

Q10 How is the lac operon regulated, and what is DNA fingerprinting?

Answer (English): The lac operon has a promoter, an operator and the structural genes z, y and a. In the absence of lactose the repressor made by the i gene binds the operator and blocks transcription. When lactose is present, allolactose binds the repressor and removes it from the operator, so RNA polymerase transcribes the operon and the three enzymes are made. This is negative regulation, and the operon is inducible. DNA fingerprinting compares the number of repeats of short sequences at particular loci, which differs between individuals, to produce a band pattern used in forensic and paternity testing.

Hinglish: lac operon mein promoter, operator aur z, y, a genes hote hain. Lactose na ho to i gene ka repressor operator par baithta hai aur transcription rok deta hai; lactose hone par allolactose repressor ko hata deta hai, aur RNA polymerase z, y, a genes padhkar enzymes banata hai. Yeh negative regulation hai aur operon inducible hai. Regulation ka matlab yeh hai ki cell apne genes ko zarurat ke hisaab se on aur off karti hai, aur lac operon iska sabse aasan example hai. DNA fingerprinting chhote repeated sequences ke copies ki sankhya compare karke individual-specific band pattern deta hai, jo forensic aur paternity testing mein kaam aata hai.

That completes the ten questions. The five MCQ quiz below checks the facts that board papers test most often: the base-pairing rule, the transforming principle, semiconservative replication, the code and the lac operon.