Question

A typical plasmid-free bacterial cell having a single chromosome consists of $\sim 55%$ protein, $\sim 3%$ DNA and $\sim 21%$ RNA (percent of dry weight). Assuming there are 2500 different types of protein molecules (with at least one copy number) being expressed in a bacterial cell at any given time, which of the following statements is/are NOT correct with respect to the number of intracellular DNA, RNA and protein molecules?

• A. DNA = RNA = Protein $\ge 2500$
• B. DNA = RNA = 1; Protein $\ge 2500$
• C. DNA = 1; RNA = Protein $\ge 2500$
• D. DNA = 1; RNA>Protein $\ge 2500$

Hint:

In bacteria: DNA molecules = 1; RNA molecules $\gg$ protein molecules; protein molecules are thousands in number due to gene expression.

Answer 1

The Correct Option is A

A plasmid-free bacterial cell contains a single circular chromosome, which means the number of DNA molecules is exactly:
DNA = 1.
Protein molecules are the most abundant macromolecules in any bacterial cell.
Given that there are 2500 different protein types, each with at least one copy, the total protein count must satisfy:
Protein $\ge 2500$.
RNA constitutes 21% of bacterial cell dry weight and includes:
• mRNA (many copies of each gene transcript),
• tRNA (high copy number),
• rRNA (thousands of copies).
Therefore, the total RNA molecule count is much larger than 2500.
Thus, RNA $\gg$ Protein $\ge 2500$.
Now examine each option:
(A) DNA = RNA = Protein $\ge 2500$
This is incorrect because DNA = 1, while RNA and proteins are far more numerous. Hence (A) is NOT correct.
(B) DNA = RNA = 1; Protein $\ge 2500$
This is incorrect because RNA molecule count is never equal to 1; it is extremely large. Thus (B) is NOT correct.
(C) DNA = 1; RNA = Protein $\ge 2500$
RNA count is far greater than protein count, so this is also incorrect. Thus (C) is NOT correct.
(D) DNA = 1; RNA>Protein $\ge 2500$
This is the only correct statement. RNA is indeed larger in number than proteins, and both exceed 2500 molecules.
Therefore, the statements that are NOT correct are (A), (B), and (C).
Final Answer: (A), (B), (C)