Recombinant DNA and Molecular Cloning

How restriction enzymes, ligases, and vectors are used to cut and join DNA from different sources and propagate it in host cells — the founding technology of genetic engineering.

Definition

Recombinant DNA and molecular cloning is the set of methods for joining DNA fragments from different sources into a vector and propagating the resulting molecule in host cells, so that a chosen DNA sequence can be amplified, maintained, and studied.

Scope

This topic covers the construction of recombinant DNA and its cloning: restriction enzymes and the cutting of DNA at specific sites, joining fragments with DNA ligase, the use of plasmid and other vectors, introduction into host cells, and selection of recombinants. It treats the principles of cloning; downstream amplification, sequencing, and editing are covered in companion topics.

Core questions

• How do restriction enzymes cut DNA at specific sequences?
• How are DNA fragments joined into a vector?
• What are vectors and how do they carry foreign DNA into host cells?
• How are cells carrying the desired recombinant molecule selected?

Key theories

Sequence-specific cutting and joining

Restriction enzymes recognise and cut defined DNA sequences, often leaving complementary ends, which DNA ligase can join, allowing fragments from different sources to be recombined predictably.

Vector-borne propagation

Inserting a fragment into a replicating vector and introducing it into host cells lets the recombinant DNA be copied along with the host, yielding many identical clones of the chosen sequence.

Mechanisms

A restriction enzyme cuts both the target DNA and a vector at specific recognition sites, frequently generating matching single-stranded overhangs. The fragments are mixed and joined by DNA ligase to form a recombinant molecule, which is introduced into host cells by transformation or related methods. Because the vector carries an origin of replication and selectable markers, host cells that take up and maintain the recombinant DNA can be identified and grown, producing large amounts of the cloned sequence for further use.

Clinical relevance

Molecular cloning underpins the production of recombinant proteins such as insulin and antibodies and the construction of vectors for gene therapy and vaccines; offered as significance, not clinical guidance.

History

The characterisation of restriction enzymes by Arber, Smith, and Nathans, and the construction of the first recombinant DNA molecules by Berg, Cohen, and Boyer in the early 1970s, established molecular cloning and earned Nobel recognition, making genetic engineering routine.

Key figures

• Hamilton Smith
• Daniel Nathans
• Werner Arber
• Paul Berg
• Herbert Boyer
• Stanley Cohen

Seminal works

• smith1970
• watson2013

Frequently asked questions

What does a restriction enzyme do?

It recognises a specific short DNA sequence and cuts the DNA there, often leaving ends that can be joined to other fragments cut by the same enzyme.

Why are vectors needed in cloning?

A vector carries the inserted DNA into host cells and replicates there, so the cloned sequence is copied and can be selected and propagated.