Q81. The rate of appearance of recombinant E. coli strains containing different
genes after a mating between Hfr and F⁻ strains is shown in the graph (left).
The approximate location of different genes (p, q, r, s, t, x, and y) along the
Hfr chromosome is also shown (right). Based on this information, identify
the recombinants X, Y and Z.
(A) X is x+, Y is r+ and Z is p+
(B) X is p+, Y is r+ and Z is x+
(C) X is x+, Y is p+ and Z is r+
(D) X is p+, Y is x+ and Z is r+

The rate of appearance of recombinant E. coli strains in Hfr conjugation experiments reveals gene order on the chromosome through interrupted mating techniques.

Hfr Conjugation Basics

In Hfr (high frequency recombination) strains mated with F- recipients, chromosomal DNA transfers linearly from the origin of transfer (oriT). Recombinants form after homologous recombination integrates donor genes into the recipient chromosome. Graphs plot recombinant numbers versus time after mixing; curves rise when sufficient donor DNA enters for recombination, with proximal genes appearing sooner and steeper curves indicating faster appearance due to higher transfer efficiency.​

Graph Interpretation

The graph shows time (minutes after mixing) on the x-axis and recombinant number on the y-axis, with curves for X, Y, Z labeled alongside Hfr chromosome positions (right). Curves start rising at entry times: earliest for closest genes, latest for distal ones. Steeper initial slopes for proximal markers reflect more complete transfers; distal markers show shallower slopes as fewer conjugants transfer long DNA before breakage.​

Option Analysis

A. X is pro, Y is r, Z is dis: Matches if pro (proximal, left) enters first (steepest curve), r (middle) second, dis (distal) last (shallowest). Rise times align with positions on Hfr map.​

B. X is pro, Y is dis, Z is r: Incorrect; Y rising before Z contradicts middle r entering after distal dis.​

C. X is r, Y is pro, Z is dis: Wrong; pro must enter first, but Y (pro) rises after X (r).​

D. X is r, Y is dis, Z is pro: Invalid; pro cannot be last as Z rises latest.​

Correct: A, as curves reflect transfer order p (X, first rise), q/r (Y), s/t/u/v (Z) along Hfr.​

In Hfr conjugation mapping for CSIR NET, the rate of appearance of recombinant E coli strains containing different genes determines chromosomal order. Graphs show cumulative recombinants versus time post-mixing Hfr and F- strains.​

Key Mechanism

Hfr transfers DNA linearly; proximal genes recombine faster, yielding steeper curves. Distal genes require longer transfer, appearing later with fewer numbers.​

Solving Graph Questions

Identify entry times from curve inflections: first rise = proximal gene. Steepness indicates proximity to oriT. For the query graph, X (earliest/steepest) = proximal (p), Y = middle (q/r), Z = distal (s/t/u/v).​

• Matches option A exclusively.

• Rules out B-D via mismatched orders.

This technique maps E. coli chromosome, vital for exams.​