Q.16 Small circular DNAs such as plasmids and small viral DNAs when have no breaks in
either strand are referred as
(1)Nucleosome
(2)Close circular DNA
(3)Nicked DNA
(4)Chromatid
Small circular DNAs like plasmids and viral DNAs with no breaks in either strand are referred to as closed circular DNA. These covalently closed circular (ccc) molecules maintain superhelical tension due to unbroken phosphodiester backbones in both strands, distinguishing them from nicked or relaxed forms.
Option Analysis
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(1) Nucleosome: Eukaryotic DNA-histone complex forming chromatin beads; unrelated to circular bacterial/viral DNA topology.
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(2) Closed circular DNA: Correct—Plasmids (e.g., pBR322) and SV40 viral DNA exist as intact, supercoiled cccDNA where both strands form continuous circles without nicks, enabling topological constraints like linking number (Lk).
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(3) Nicked DNA: Open circular (oc) form with single-strand breaks (nicks) relieving supercoils; slower sedimenting than cccDNA in CsCl gradients.
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(4) Chromatid: One of two identical chromosome arms post-DNA replication; linear eukaryotic structure, not applicable to small circular DNAs.
Answer: (2) Closed circular DNA.
Introduction to Closed Circular DNA
Small circular DNAs such as plasmids and small viral DNAs when having no breaks in either strand are referred to as closed circular DNA (cccDNA). These supercoiled molecules maintain fixed topology, critical for bacterial replication and viral persistence (e.g., HBV cccDNA).
Structural Characteristics
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Both polynucleotide chains form continuous, unbroken circles.
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Superhelical density (σ = -0.06 typical) creates writhe and twist.
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Distinguished from nicked (oc) or linear forms by gel electrophoresis mobility and buoyant density.
Biological Significance
Plasmids use ccc form for stable maintenance; viral cccDNAs (SV40, polyoma) resist exonucleases; topology affects transcription, replication origin usage.
GATE Molecular Biology Focus
Tests DNA topology: ccc vs. oc vs. linear. Key for plasmid biology, viral persistence, supercoiling enzyme questions (topoisomerases).
