# DBT JRF 2020 — Question 64

Exam: DBT JRF 2020 · Section: B · Topic: Microbiology · Difficulty: Medium

## Question

A dilution of a microbial culture was prepared by adding 1 mL of the culture to 9 mL of sterile blank. Further, 200 μL from the diluted culture was spread on an agar plate; and 150 colonies were observed after the incubation period. Calculate the CFU/mL of the original sample.

## Options

- **A.** 75
- **B.** 7500
- **C.** 75000
- **D.** 750

## Answer

Option B

## Explanation

$\gamma$ - $^{32}$ P-ATP contains the radioactive $^{32}$ P label on the TERMINAL (gamma) phosphate group. T4 Polynucleotide Kinase (PNK) transfers the $\gamma$ -phosphate of ATP to the 5'-OH end of DNA or RNA, creating a 5'- $^{32}$ P-labelled end (end-labelling). This is used to label oligonucleotides or restriction fragments for: gel shift assays (EMSA), footprinting, sequencing by Maxam-Gilbert method, and probe preparation. In contrast: Nick translation uses $\alpha$ - $^{32}$ P-dNTPs (the label goes into the phosphodiester backbone of the new strand). Reverse transcription uses $\alpha$ - $^{32}$ P-dNTPs. The $\gamma$ -phosphate is unique in that it becomes the TERMINAL phosphate after the kinase reaction — therefore $\gamma$ - $^{32}$ P-ATP is ONLY useful for end-labelling (not nick translation or reverse transcription).

Source: https://mypyqbuddy.com/questions/a-dilution-of-a-microbial-culture-was-prepared-6a05a8a02adafec470293258