In most prokaryotes, the chromosome number is:
- (a)4
- (b)3
- (c)2
- (d)1
Correct — D, 1. A prokaryote such as a bacterium has no true nucleus; its genetic material lies free in the cytoplasm in a region called the nucleoid, and it is typically a single, circular, double-stranded DNA molecule. That one circular DNA molecule is its chromosome, so most prokaryotes have a chromosome number of 1. Any extra small circular DNA rings a cell carries are plasmids, which replicate independently and are not counted as chromosomes.
- (a)4 — A count of four chromosomes describes some eukaryotes, not prokaryotes; a typical bacterium carries just one circular chromosome.
- (b)3 — Prokaryotes are not organised into several chromosomes like eukaryotes; the standard number is a single circular chromosome, not three.
- (c)2 — Two might be mistaken by counting a plasmid alongside the chromosome, but plasmids are not chromosomes; most prokaryotes have one chromosome.
Prokaryotes — bacteria and archaea — are cells without a membrane-bound nucleus. Their DNA is usually a single circular chromosome that sits in an irregular region of the cytoplasm called the nucleoid. This 'naked' DNA is not wrapped around histone proteins the way eukaryotic chromosomes are, and it replicates before the cell divides by binary fission.
The point to remember is that prokaryotes keep their genome as one circular chromosome, unlike eukaryotes, which have several linear chromosomes inside a nucleus. Do not count plasmids as chromosomes — they are optional, small, extrachromosomal DNA rings that carry accessory genes such as antibiotic resistance.
- Most prokaryotes have a single, circular, double-stranded DNA chromosome.
- The chromosome lies in the nucleoid, an area of the cytoplasm not enclosed by a membrane.
- Plasmids are small extra circular DNA molecules that replicate independently and are not counted as chromosomes.
- Prokaryotic DNA is called 'naked' because it is not packaged with histone proteins as eukaryotic DNA is.
- Counting plasmids as chromosomes — plasmids are extra, small circular DNA and are not the chromosome.
- Assuming prokaryotes have several chromosomes like eukaryotes — most carry just one.
A direct factual question on prokaryotic genome organisation — most prokaryotes carry a single circular chromosome, so the number is 1.
No directly related past PYQ was found.
- practice — not a real PYQ
In a bacterial cell, the single circular chromosome is located in a region called the
- (a)nucleus
- (b)nucleoid
- (c)nucleolus
- (d)vacuole
Answer(b) nucleoid — the membrane-less region of the cytoplasm that holds the prokaryotic chromosome.
- practice — not a real PYQ
Small circular DNA molecules in bacteria that replicate independently of the main chromosome are called
- (a)plasmids
- (b)ribosomes
- (c)lysosomes
- (d)centromeres
Answer(a) plasmids — extrachromosomal DNA rings that often carry accessory genes such as antibiotic resistance.