Restriction enzymes, sometimes called restriction endonucleases, recognise specific palindromic recognition sequences in DNA.

They then cut the DNA at these sites through a hydrolysis reaction.

Palindromic sequences are sections of DNA where the sequence of nucleotides reads the same in opposite directions on antiparallel strands.

Each restriction enzyme has an active site that is complementary to a specific DNA sequence, so it only cuts at that unique recognition sequence.

If recognition sequences are at either end of a desired DNA fragment, restriction enzymes can cut these sites to separate the fragment from the rest of the DNA.

This means we can obtain the desired gene from an organism's DNA.

Sticky ends are short, overhanging sequences of unpaired bases at the end of a DNA fragment cut by restriction enzymes.

These overhanging sequences are used to insert genes into vectors.

mRNA is reverse transcribed using the enzyme reverse transcriptase, to form a strand of complementary DNA (cDNA) that is the same as the original DNA coding sequence.

cDNA, free nucleotides, and the enzyme DNA polymerase can then be used to form the other strand of DNA, reforming the desired gene.

Recombinant DNA is DNA that is altered to contain nucleotides from two different organisms, creating a genetically modified or transgenic organism.

This allows the transgenic organism to express new genetic traits.

1. The gene can be made from the mRNA of a donor organism using reverse transcriptase and cDNA
2. The gene can be cut from the DNA of a donor organism using restriction enzymes
3. The gene can be synthesised from nucleotides (using a gene machine)

A vector is a carrier used to deliver a gene to an organism's cells.

Some examples of vectors in genetic engineering involve plasmids, viruses, or liposomes.

1. Choose codons for the desired amino acid sequence from a known protein structure.
2. Use a computer to direct the synthesis of short fragments of DNA (oligonucleotides) in DNA synthesiser machines, sometimes called 'gene machines'.
3. Join the fragments together to make a longer sequence of nucleotides, forming the desired gene.
4. The polymerase chain reaction (PCR) constructs a complementary DNA strand and amplifies the gene to produce multiple copies.

1. The desired gene is identified and isolated
2. Multiple copies of the gene are made using the polymerase chain reaction (PCR)
3. The gene is inserted into a vector
4. Vector delivers the gene into cells
5. Cells with the new gene are identified, such as by using marker genes
6. Cells with the new gene are cloned

Genetic engineering is the deliberate manipulation of genetic material to modify an organism's characteristics, often involving gene transfer.