Biology Set-1

Q1. A linear eukaryotic chromosome of length $3.0\times10^{6}$ bp is to be replicated during S‑phase lasting 30 minutes. Replication forks move at $100\ \text{nucleotides s}^{-1}$ and each origin produces two replication forks. What is the minimum number of origins required to complete replication within the S‑phase?

Q2. Consider an E. coli merodiploid constructed by introducing a plasmid into a chromosomal strain. The chromosomal genotype is $lacI^{-}\; lacO^{+}\; lacZ^{+}$ and the plasmid carries $lacI^{+}\; lacO^{c}\; lacZ^{-}$. In the absence of inducer, what is the expected level of $\beta$‑galactosidase synthesis from the chromosomal $lacZ$ gene?

Q3. A bacterial DNA polymerase with 3'→5' proofreading has an error rate of $1\times10^{-8}$ errors per base per replication. A mutant lacking proofreading shows an error rate of $1\times10^{-6}$ errors per base. For a genome size of $4.6\times10^{6}$ bp, by approximately what fold does the expected number of base‑substitution errors per genome replication increase when proofreading is lost?

Q4. Statement (S): DNA methylation of cytosine residues in CpG islands of gene promoters is commonly associated with transcriptional silencing in eukaryotes.
Reason (R): Methylation causes silencing primarily by directly blocking binding of general transcription factors (such as TFIID/TBP) to promoter DNA.

Q5. In a three‑point test cross for genes arranged A–B–C, the observed recombination frequencies are $r_{AB}=10\%$ and $r_{BC}=12\%$. Among $10{,}000$ progeny, $80$ double crossovers were observed. Interference is defined as $I = 1 - \dfrac{\text{observed double crossovers}}{\text{expected double crossovers}}$. What is the interference $I$ (nearest value)?

Q6. A circular bacterial chromosome of length $3\times10^{6}$ base pairs begins replication from a single origin and proceeds bidirectionally. If each replication fork progresses at $1000\ \text{nucleotides s}^{-1}$, the time required to complete replication (in minutes) is closest to:

Q7. In a three-point cross the recombination frequencies (from two-point tests) between gene pairs A–B and B–C are $10\%$ and $20\%$ respectively. In a three-point testcross with $1000$ progeny the observed number of double crossover (DCO) progeny is $8$. The interference value ($I$) is:

Q8. DNA polymerase has an error rate of $10^{-9}$ errors per base per replication. For a bacterial population of $10^{9}$ cells, consider a single gene of length $1500\ \text{bp}$. After one round of replication, the expected number of cells carrying at least one point mutation in that gene can be estimated approximately by $N\times L\times\mu$. The expected number is:

Q9. Assertion (A): Proofreading by DNA polymerases reduces the mutation rate during replication by removing incorrectly incorporated nucleotides.
Reason (R): Proofreading requires $3'\rightarrow5'$ exonuclease activity present in many DNA polymerases which removes mismatched nucleotides.

Q10. A linear DNA molecule of length $10\,000\ \text{bp}$ was analyzed. A single digest with enzyme A yields fragments of $4000\ \text{bp}$ and $6000\ \text{bp}$. A single digest with enzyme B yields fragments of $2000\ \text{bp}$ and $8000\ \text{bp}$. A double digest with A and B yields fragments of $2000\ \text{bp}$, $2000\ \text{bp}$ and $6000\ \text{bp}$. Which of the following maps of restriction sites on the linear molecule is consistent with these results?