Info

II 1 1 R'

step 3 warm to 72°C thermostable

DNA polymerase extends primers step 1 heat to 90°C to separate strands step 2 cool to 60°C primers anneal to their complementary sequences i—r i i i—r i i

DNA polymerase extends primers

DNA polymerase extends primers i—r i i step 1

heat to 90°C to separate strands step 2

cool to 60°C primers anneal to their complementary sequences i i

DNA polymerase extends primers

DNA polymerase extends primers

Figure 7.1 5. Amplification of a DNA sequence using the polymerase chain reaction.

Medical Microarrays and Cancer Classification Relevance Microarrays or gene chips are tiny glass wafers to which cloned DNAs are attached.

7.1 The principle of microarray technology is to isolate mRNA from a particular cell and to hybridize this to the DNAs on the chip. Because the mRNAs are tagged with a fluorescent dye, the DNAs to which they hybridize on the chip can be detected. Excess mRNA is removed, and fluorescent areas are viewed using a special scanner and microscope. Computer algorithms have been written to analyze the hybridization patterns seen for a particular microarray. The number and type of DNAs used to make a microarray is dependent on the question to be answered.

One of the spin-offs of the Human Genome Project is to identify sets of genes involved in disease. Microarrays are being used to type the mRNAs produced by different cancers in the hope that this will lead to better diagnosis and therefore better treatment. Leukemia, a cancer affecting the blood, is not a single type of disease. Microarray analysis is helping to classify different types of leukemia more precisely by cataloging the mRNAs expressed in different patients. Preliminary studies are encouraging. A microarray of 6817 cDNAs was used to compare the blood mRNAs from patients with acute lymphoblastic leukemias or acute myeloid leukemias. The mRNAs of each patient was passed over the microarray and complementary sequences hybridized. Fifty cDNA/mRNA hybrids were found that could be used to refine the classification of lymphoblastic and myeloid leukemias. In addition, the patterns allowed the classification of lymphoblastic leukemias into T-cell or B-cell classes. This separation of lymphoblastic leukemias into two classes is an important distinction when it comes to deciding on the best treatment for a patient. As more patients with leukemia are investigated using microarrays, it should be possible to design smaller chips, with fewer cDNAs, but with greater prognosis value.

A recent study has shown that the fate of women with breast cancer can be predicted by microarray analysis of their cancer tissue. Several genes were shown to be important for this prediction. If the women had a certain gene expression pattern, then the disease was likely to recur within a 5-year period. However, if the gene pattern was of a second type, the cancer almost never returned. Women that fall into this second class also show no additional benefit from chemotherapy or radiotherapy. Microarray analysis not only offers a survival prognosis for women with breast cancer but has also shown that unpleasant treatment for the disease is unnecessary for those possessing the second type of gene pattern.

micro array chip of 12 DNA samples

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