Describe the process of ‘reading’ a microarray in order to compare the expression of genes in two different tissues--
one that is normal and one that represents a disease such as cancer. What will be an ideal response?
ANSWER: For an expression study, mRNA is isolated from normal cells and cancer cells derived from
the same cell type as the normal cells. The isolated mRNAs are copied into single-stranded
DNA molecules called cDNA (complementary DNA), using an enzyme called reverse
transcriptase. cDNA molecules from the two cell types are labeled with different fluorescent
dyes (green for normal, red for cancer). The labeled cDNA molecules are mixed together and
applied to a microarray containing DNA fields representing thousands of genes. Unbound
cDNA molecules are washed off, a laser scans the chip, and the image is transferred to a
computer for storage and analysis. The pattern on the microarray appears as a series of dots;
each dot represents one field. A field is the location to which single-stranded DNA fragments
are attached. If a gene is expressed only in normal cells, the dot will be green. If the field is
red, it means that the gene is expressed only in cancer cells. If the dot is yellow, it means that the gene is expressed equally in both the normal cells and the cancer cells (mixing red and
green produces yellow). Intermediate colors mean that there are different amounts of
complementary single-stranded DNA in the normal and cancer cells and different levels of
expression of these genes in the two cell types
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