# DBT JRF 2020 — Question 41

Exam: DBT JRF 2020 · Section: B · Topic: Plant Biology · Difficulty: Hard

## Question

Which class of phytochromes is highly abundant in etiolated seedlings and is also light labile?

## Options

- **A.** Phytochrome A
- **B.** Phytochrome D
- **C.** Phytochrome B
- **D.** Phytochrome C

## Answer

Option A

## Explanation

Glycine (Gly, G) is the smallest amino acid with only a hydrogen atom as its R group. This minimal side chain creates NO steric clashes in any backbone conformation. In a Ramachandran plot ( $\phi$ vs $\psi$ angles), glycine can adopt conformations in all four quadrants — including the normally forbidden regions — because both sides of its Cα are hydrogen atoms. It therefore has the LEAST preference for any specific quadrant. Alanine is the next least sterically hindered. Val and Ile (bulky branched side chains) are highly restricted to certain Ramachandran regions. Glycine is often found at turns, tight bends, and in collagen (Gly-X-Y repeats) where it fits inside the cramped triple helix.

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