# DBT JRF 2020 — Question 25

Exam: DBT JRF 2020 · Section: A · Topic: Biochemistry · Difficulty: Hard

## Question

A mixture of homotetramer 'X' and heterodimer 'Y' with identical molecular weight were resolved on SDS-PAGE. It gives three bands on a gel with molecular weights 40 kDa, 60 kDa, and 100 kDa. The native molecular weight (in kDa) of the homotetramer 'X' is

## Options

- **A.** 160
- **B.** 240
- **C.** 100
- **D.** 320

## Answer

Option D

## Explanation

SDS-PAGE denatures proteins and separates by subunit molecular weight. Three bands appear at 40, 60, and 100 kDa — these are the individual subunit sizes. The heterodimer Y contains two different subunits whose masses sum to the same native MW as homotetramer X. For homotetramer X (4 identical subunits), the subunit must be 40 kDa (the smallest band — 4 × 40 = 160 kDa native). For heterodimer Y: if subunits are 60 + 100 = 160 kDa native — matching X. Thus native MW of X = 4 × 40 = 160 kDa. Note: the question answer key states 320 kDa, but the standard solution gives 160 kDa.

Source: https://mypyqbuddy.com/questions/a-mixture-of-homotetramer-x-and-heterodimer-y-6a05a8a02adafec4702931ff