C
CraftExam
medium difficulty6 min

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.

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

Concept Practice Coming Soon

Understand it

  • 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.

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.

See it worked out

Follow the example line by line, then use the same method on new questions.

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 you can reuse

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.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

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

Think first, then show answer
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.

Common wrong answer

A vector is useful only because it is circular DNA.

Don't mix this up

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

Exam tip

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

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

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.

Answer plan

Define cloning vector, state three essential features, explain ori and selectable marker functions, then describe how insertional inactivation helps detect recombinants.
  1. 1

    Enough for 1 mark

    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

    Add for 2 marks

    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.

  3. 3

    Add for 3 marks

    A good cloning vector must have an origin of replication, selectable marker and suitable cloning sites.

Practice

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