Question

Some prokaryotes have multiple chromosomes while other have single. Which of the following is/are true?

• A. Agrobacterium tumefaciens has 3 circular and 1 linear chromosome.
• B. Mycoplasma genitalium has one linear chromosome
• C. Sinorhizobium meliloti has 3 linear chromosomes.
• D. Escherichia coli K-12 has 2 circular chromosomes.

Hint:

Agrobacterium tumefaciens is a classic textbook example of a bacterium with a multipartite genome that contains both circular and linear chromosomes.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Historically, prokaryotic genomes were thought to consist of a single, circular double-stranded DNA chromosome.
However, genomic sequencing has revealed that many bacteria have more complex genome structures, including multiple chromosomes, linear chromosomes, or large megaplasmids.

Step 2: Detailed Explanation:
Let us analyze the genome structures of the organisms listed in the options:
- Agrobacterium tumefaciens (specifically the well-studied strain C58) has a complex genome consisting of four replicons:
A circular chromosome ($84\text{ Mb}$).
A linear chromosome ($08\text{ Mb}$).
A large circular megaplasmid called pAtC58 ($816\text{ kb}$).
The circular tumor-inducing plasmid pTiC58 ($214\text{ kb}$).
Because megaplasmids carry essential housekeeping genes, they are often classified as accessory chromosomes.
This configuration gives A. tumefaciens a genome with three circular replicons and one linear replicon, making statement A correct.
Let us evaluate the other options:
- Option B is incorrect: Mycoplasma genitalium has a small, single circular chromosome of approximately $580\text{ kb}$.
- Option C is incorrect: Sinorhizobium meliloti has a genome with three circular replicons: one circular chromosome ($65\text{ Mb}$) and two circular megaplasmids, pSymA ($35\text{ Mb}$) and pSymB ($68\text{ Mb}$).
- Option D is incorrect: Escherichia coli K-12 has a single, circular chromosome of approximately $64\text{ Mb}$.

Step 3: Final Answer:
The correct statement is that Agrobacterium tumefaciens has a genome consisting of 3 circular and 1 linear replicon.