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Cloning Vectors and Selectable Markers

A cloning vector is a DNA molecule that can carry foreign DNA into a host cell and replicate there; selectable markers help identify cells that have received the vector.

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Student-friendly explanation

A good cloning vector must have an origin of replication, selectable marker and suitable cloning sites. The origin of replication controls copy number and allows the vector to multiply inside the host. Selectable markers, such as antibiotic resistance genes, help distinguish transformed cells from non-transformed cells. Multiple cloning sites provide recognition sequences for restriction enzymes. In insertional inactivation, insertion of foreign DNA disrupts a marker gene, so recombinant colonies can be identified by a changed phenotype. The exam logic is structure-function: every part of the vector has a reason, and marker-based selection prevents wasting time on cells that did not take up the vector.

How to write this in exams

  1. 1

    Start with the exact idea

    A cloning vector is a DNA molecule that can carry foreign DNA into a host cell and replicate there; selectable markers help identify cells that have received the vector.

  2. 2

    Then show how to use it

    Check whether the vector has ori; identify selectable markers; locate suitable restriction sites; insert the desired DNA at the cloning site; use marker response to identify transformed or recombinant cells.

  3. 3

    Add one concrete example

    In pBR322, antibiotic resistance genes can serve as selectable markers, and insertion of foreign DNA into a marker region may inactivate that marker, helping identify recombinant cells.

  4. 4

    Avoid this incomplete answer

    A vector is useful only because it is circular DNA.

Definition

A cloning vector is a DNA molecule that can carry foreign DNA into a host cell and replicate there; selectable markers help identify cells that have received the vector.

Example

In pBR322, antibiotic resistance genes can serve as selectable markers, and insertion of foreign DNA into a marker region may inactivate that marker, helping identify recombinant cells.

Rule to remember

Vector rule: ori permits replication, selectable marker permits identification, and cloning sites permit insertion of foreign DNA.

Memory hook

Remember ORI-MARK-SITE: ori copies the plasmid, marker finds the transformed cell, and cloning site receives the foreign DNA insert.

Examples and method

Worked example

If a plasmid has an antibiotic resistance gene interrupted by an inserted DNA fragment, colonies losing that resistance can indicate recombinants under the appropriate screening method.

Method to apply

Check whether the vector has ori; identify selectable markers; locate suitable restriction sites; insert the desired DNA at the cloning site; use marker response to identify transformed or recombinant cells.

Diagram support

A plasmid vector diagram should show ori, selectable marker genes, restriction sites and the site where foreign DNA is inserted; students should connect labels to functions.

How CBSE asks it

Asked as short answers on features of a cloning vector, assertion-reason on selectable markers, and diagram-based questions on pBR322 labels.

Avoid common mistakes

Common confusion

Students often write that selectable markers select the desired gene directly; actually, they help identify transformed or recombinant host cells.

Common wrong answer

A vector is useful only because it is circular DNA.

Exam tip

For vector questions, always mention ori, selectable marker and cloning site; for insertional inactivation, mention disruption of marker expression.

Quick check

Why is an origin of replication necessary in a cloning vector?

The origin of replication allows the vector to replicate inside the host cell. Without ori, the inserted gene would not be copied and maintained properly in the host population.

Answer writing and exam use

1-mark answer

Define cloning vector, state three essential features, explain ori and selectable marker functions, then describe how insertional inactivation helps detect recombinants.

2-mark answer

Define cloning vector, state three essential features, explain ori and selectable marker functions, then describe how insertional inactivation helps detect recombinants.

3-mark answer

Define cloning vector, state three essential features, explain ori and selectable marker functions, then describe how insertional inactivation helps detect recombinants.
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