#circular DNA

Mitochondria Plastids Ribosomes and Other Organelles

Organelles X and Y both contain circular DNA and 70S ribosomes. X has cristae, while Y contains thylakoids surrounded by

Cristae identify X as a mitochondrion, while thylakoids and stroma identify Y as a chloroplast. Mitochondria perform aerobic respiration and generate ATP; chloroplasts trap light energy. Their circular DNA and 70S ribosomes support some protein synthesis within both organelles.

Ref: NCERT Class 11 Biology Chapter 8: Cell: The Unit of Life Mitochondria Plastids Ribosomes and Other Organelles

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Mitochondria Plastids Ribosomes and Other Organelles

An organelle is double-membrane-bound and contains circular DNA and 70S ribosomes. Its internal membranous sacs bear pig

Both mitochondria and chloroplasts contain circular DNA and 70S ribosomes, so those features alone do not distinguish them. Pigment-bearing thylakoids and enzymes for carbohydrate synthesis in the stroma identify the organelle as a chloroplast.

Ref: NCERT Class 11 Biology Chapter 8: Cell: The Unit of Life Mitochondria Plastids Ribosomes and Other Organelles

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Replication Basics

Theta (θ) replication occurs in

Theta replication describes intermediate structures resembling Greek letter θ arising when circular double-stranded DNA undergoes bidirectional replication from single origin. As forks proceed, parental circle remains topologically linked to two newly synthesized circles forming bubble that expands until termination, producing intertwined catenated products separated by topoisomerase IV. This mode is characteristic of circular bacterial chromosomes, many plasmids, bacteriophage lambda and mitochondrial DNA. Linear chromosomes replicate via Y-shaped forks rather than theta, and rolling circle produces sigma structure with single-stranded tail. Visualization by electron microscopy confirmed Cairns theta model.

Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 5: Theta Replication Occurs in Circular DNA

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Replication Basics

Bacterial chromosome is usually

Bacterial chromosomes possess single origin of replication oriC where DnaA-ATP binds DnaA boxes, unwinds DUE and recruits DnaB helicase with DnaC loader. Replication proceeds bidirectionally toward terminus region defined by Tus-Ter complexes. Because only one origin fires per chromosome per cell cycle, bacterial chromosome is monorepliconic. Some bacteria with secondary chromosomes or megaplasmids may appear multirepliconic but canonical E. coli remains model monoreplicon system. Regulation ensures initiation once per cycle via SeqA sequestration and DnaA inactivation, preventing over-replication and maintaining copy number control.

Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 5: Bacterial Chromosome is Usually Monorepliconic

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Restriction Digestion & Mapping

A circular DNA digested with 2 asymmetric sites gives:

Circular DNA cleaved at two distinct positions yields as many fragments as cuts. Asymmetric sites divide circle into two arcs of different contour lengths, producing two double-stranded fragments of unequal size. Complete digestion separates circle into those two linear pieces. Agarose gel resolves them as two distinct bands with different migration. Symmetric sites would give one band due to co-migration. One band would indicate single site, three bands would need three sites. Therefore two asymmetric sites logically generate two bands.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Restriction Digestion & Mapping

How many fragments are expected from circular DNA with one restriction site?

Circular plasmids are continuous duplexes. Introducing a single restriction break interrupts both strands at the recognition sequence, converting the circle into a linear duplex. Physical counting of phosphodiester backbones shows two single-stranded fragments now with free termini, though double-stranded gel analysis displays one length band. Many introductory keys teach that one cut in a circle generates two ends, interpreted as two fragments in terms of molecular ends produced. NCERT-based problems often equate circular one-site digestion with two fragments representing two single strands separating after cleavage, distinguishing from linear one-site outcome.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Restriction Digestion & Mapping

What does symmetric cutting at 2 sites in circular DNA result in?

In circular DNA, two restriction sites placed diametrically opposite divide the circle into two equal halves. After complete digestion, two fragments of identical molecular weight are produced. During agarose gel electrophoresis, migration rate depends on size, not sequence content. Fragments of same length co-migrate, overlapping as a single intensely stained band despite being two distinct physical pieces. Asymmetric placement would give two bands of different sizes. Recognizing symmetry explains why band number may appear less than fragment number and intensity doubles.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Restriction Digestion & Mapping

How many fragments are generated by digesting circular DNA with 3 restriction sites?

Circular DNA lacks free ends, forming a continuous loop. A single restriction cut converts the circle into a linear molecule of full length. Each additional cut divides one existing fragment into two. Consequently, in complete digestion the number of fragments generated equals exactly the number of recognition sites cleaved. With three distinct sites, three separate fragments appear whose sizes sum to the plasmid length. This contrasts with linear DNA where fragments are n+1. Understanding circular topology helps interpret plasmid mapping gels correctly during cloning validation experiments.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Restriction Digestion & Mapping

Which technique allows mapping of restriction sites in circular DNA?

Restriction mapping relies on systematic cleavage with single enzymes and combinations, followed by agarose gel electrophoresis to measure fragment sizes accurately. By overlapping patterns from single and double digests, distances between recognition sequences can be ordered around a circle, producing a circular map with relative positions. Ethidium bromide staining only visualizes DNA without mapping, Southern blot detects specific sequences after digestion, and PCR amplifies regions but does not locate sites. Only deliberate restriction digestion provides positional information about cleavage sites essential for cloning design, subcloning, and RFLP analysis in molecular biology.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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Restriction Digestion & Mapping

How many fragments result from double digestion of circular DNA with 2 unique sites?

Circular DNA remains topologically closed, so a single unique restriction cut converts it into one linear molecule of unchanged length. A second unique cut at a different position divides that linear molecule into two distinct fragments. Counting bands on agarose gel therefore directly reveals number of cut sites when enzymes are unique. For circular plasmids, number of fragments equals number of sites cut. Two enzymes each cutting once generate two fragments whose sizes add up to total plasmid size, confirming double digestion and enabling circular map construction for vector analysis.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

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