Recall basic facts about RNA

RNA stands for ribonucleic acid. It plays important roles in the cell, especially in protein synthesis. Common forms include mRNA, tRNA, and rRNA.

Check each statement

A: It controls the synthesis of protein

• RNA is directly involved in protein synthesis:
  • mRNA carries the genetic message,
  • tRNA brings amino acids,
  • rRNA forms part of ribosomes.
• So this statement is true.

B: It is present in the nucleus of the cell

• RNA is synthesized in the nucleus in eukaryotic cells.
• Therefore, RNA is indeed present in the nucleus.
• So this statement is true.

C: It usually does not replicate

• In normal cells, RNA is transcribed from DNA and generally does not self-replicate.
• Replication is mainly a property of DNA, though some RNA viruses are exceptions.
• Since the statement says usually, it is true.

D: It has always double stranded $\alpha$-helix structure

• RNA is generally single-stranded, not always double-stranded.
• Also, the typical double-helical structure is associated with DNA, not RNA in general.
• Hence this statement is not true.

Conclusion

The statement which is not true about RNA is: $\boxed{\text{D}}$

Comparison with stored correct answer

• Stored correct answer: D
• Derived answer: D
• They match.