A blood sample holds 8 copies of the DNA of a virus…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A blood sample holds copies of the DNA of a virus. A test needs at least copies for a signal. In each cycle of PCR, the number of copies doubles, with no loss.
- How many cycles are needed?
- How many copies are there after that many cycles?
- A second sample holds only copy. How many cycles does it need?
Need a reminder? The key ideaShow
Gene therapy delivers a normal gene to take over the work of a non-functional one. The first clinical gene therapy, in 1990, was for a four-year-old girl with ADA deficiency, which is caused by the deletion of the gene for adenosine deaminase. Lymphocytes are grown outside the body, given a functional ADA cDNA by a retroviral vector, and returned. They are not immortal, so the infusions are repeated. Very low amounts of a germ can be found early by PCR, which copies the nucleic acid, and by ELISA, which uses antigen and antibody interaction.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Given. Start: copies. Needed: copies. Copies double in each cycle.
Plan. Find the smallest whole with .
Step 1. The condition. . Now is too small, and is enough. So .
Step 2. Copies. copies. After cycles there are , which is too few.
Step 3. One copy. . is too small, and is enough. So cycles.
Answer: cycles; copies; cycles.
Check. , so . The two samples reach the same number, and the first one starts three cycles ahead.
Watch out. Real PCR does not double perfectly in every cycle. The number is an ideal one.
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