Q.81 Which of the following is/are TRUE about the Escherichia coli chromosome?
(A) It is typically bound by histones
(B) It is circular in nature
(C) It is found in the nucleoid
(D) It contains multiple origins of replication

Escherichia coli chromosome features include its circular structure and nucleoid location, but not histone binding or multiple replication origins. This analysis evaluates each option for the multiple-choice question on E. coli chromosome properties, confirming the correct choices based on established bacterial genetics.​

Option Analysis

Option (A): It is typically bound by histones
Eukaryotic chromosomes use histones to form nucleosomes for DNA compaction, but the E. coli chromosome relies on histone-like proteins such as HU, H-NS, and IHF for organization within the nucleoid. These proteins mimic some histone functions but differ structurally and are not true histones. Studies confirm no typical histone binding occurs naturally in wild-type E. coli. Thus, (A) is false.​

Option (B): It is circular in nature
The E. coli chromosome measures about 4.6 Mb and forms a single, circular DNA molecule, enabling bidirectional replication from oriC to the terminus. This circular topology supports efficient compaction and segregation during rapid cell division. Electron microscopy and sequencing confirm its circular structure. Thus, (B) is true.​

Option (C): It is found in the nucleoid
Bacterial DNA localizes to the nucleoid, a compact, irregularly shaped region occupying 15-20% of the cell volume in E. coli. The chromosome compacts ~1000-fold through supercoiling and nucleoid-associated proteins (NAPs), positioning it centrally or membrane-proximally during growth. Live-cell imaging verifies nucleoid association. Thus, (C) is true.​

Option (D): It contains multiple origins of replication
Wild-type E. coli has one origin (oriC) where DnaA initiates bidirectional replication, dividing the chromosome into two replichores meeting at ter. Experimental strains with ectopic origins exist for research, but the natural chromosome features a single oriC. Flow cytometry and genetic mapping confirm this. Thus, (D) is false.​

Correct Answer

Options (B) and (C) are true.

The Escherichia coli chromosome serves as a model for bacterial genomics, featuring a circular DNA molecule housed in the nucleoid—essential knowledge for CSIR NET Life Sciences aspirants studying molecular biology.​

Core Structural Features

The E. coli chromosome spans 4.6 million base pairs in a single circular form, compacted via supercoiling and NAPs like HU into the nucleoid, a dynamic region enabling transcription-replication coordination. Absent are eukaryotic-style histones; instead, proteins like H-NS provide architecture without nucleosome formation.​

Replication and Organization

Replication initiates solely at oriC, with forks proceeding bidirectionally—no multiple origins in wild-type cells, preventing conflicts during fast growth (20-30 min cycles). The nucleoid anchors this process near midcell, influenced by transertion and entropy.​

Exam Relevance

For competitive exams, recognize: circular (B) ✓, nucleoid (C) ✓; reject histones (A) ✗ and multiple origins (D) ✗. This distinguishes prokaryotic from eukaryotic chromosomes.