Which one is the key step in the cloning of a DNA molecule? Cleaning the DNA from RNA and proteins Ligation of the DNA molecule into a plasmid or cloning vector Digesting the proteins bound to the DNA molecule Extension of the 3'end of the DNA molecules Cleaving the DNA molecule in half

The key step in the cloning of a DNA molecule is the ligation of the DNA molecule into a plasmid or cloning vector.

Here's a step-by-step breakdown of the process:

1. Isolation of the DNA of interest: This involves extracting the DNA from the cells and then purifying it.

2. Insertion of the DNA into the cloning vector: This is the key step. The cloning vector (often a plasmid) is a DNA molecule that will replicate within a host cell. The DNA of interest is inserted into the vector. This process is known as ligation.

3. Introduction of the vector into host cells: The vector containing the DNA of interest is introduced into host cells (often bacteria) in a process known as transformation.

4. Selection of host cells that have taken up the vector: Not all host cells will take up the vector, so it's necessary to select for those that have. This is often done by including a marker gene in the vector that confers resistance to an antibiotic. Only the cells that have taken up the vector will survive in the presence of the antibiotic.

5. Replication of the vector within the host cells: The host cells replicate, producing many copies of the vector and, therefore, the DNA of interest.

6. Extraction and purification of the cloned DNA from the host cells: The host cells are broken open and the DNA is purified.

So, while all these steps are important, the ligation of the DNA molecule into a plasmid or cloning vector is the key step that allows the DNA of interest to be replicated within the host cells.