Take the toxin gene from a stretch of Bt DNA and combine it with a vector.

You've added a gene from the DNA of Bt to a vector. The vector is a short piece of DNA capable of replicating on its own when inside a bacterial cell. The DNA of the gene and the DNA of the vector now form a continuous loop of DNA.

Another gene has also been added to the vector. This gene will make plants resistant to a specific herbicide.

Add the vector to the bacterium.

The vector, which incorporates the Bt gene, is now inside an Agrobacterium cell.

Agrobacterium, a bacterium that causes disease in plants, has the ability to transfer a portion of its DNA into plant cells. Once inside a plant cell, its DNA will be incorporated into the DNA of one of the plant's chromosomes. The vector that carries the Bt gene and the herbicide resistance gene is designed to be transferred to plant cells by agrobacterium.

Move the bacteria to the growth medium.

The Agrobacterium cell grows in number by dividing. Each time one of its cell divides, so do the vectors within it.

Soon, there are a million or more copies of the bacterium and of the vector that contains the Bt gene.

Add pieces of the tomato plant's leaf to the bacteria.

You have added small pieces of a tomato plant's leaf to the Agrobacteria. The DNA of the bacteria enter the cells of the leaf and the cells' nuclei.

The vector's DNA (which includes the Bt and herbicide-resistant genes) becomes integrated with the plant cells' DNA.

Move the plant cells to the growth medium for plants.

This growth medium promotes the regeneration of plants. As the plant cells multiply, they form stems, roots, and new leaves.

Spray herbicide on the plant cuttings.

You have exposed the plant cells to a herbicide. Only the cells that include the herbicide-resistant gene survive. Since the Bt gene is only present in those cells that include the herbicide-resistant gene, only those plant cells with the Bt gene are capable of growing.

Note: In Step 5, the plant cells are transferred to a growth medium that already contains the herbicide. We have added a separate "spray herbicide" step to better illustrate what happens.

Transfer the plant to the growth chamber.

The regenerated cutting is planted in soil (or a mixture that resembles soil) and allowed to mature.

Examine the plant to determine if it has the desired trait.

Congratulations!

To test if this tomato plant is resistant to a targeted pest (in this case a caterpillar), the pest was allowed to eat the plant's leaves. Since only plants that have successfully incorporated the Bt gene will kill the pest, the death of this and other caterpillars indicates that you were successful.