American Society for Clinical Pathology (ASCP) Technologist in Cytogenetics certification (ASCP CG) Practice Test

What do the dark bands produced by routine GTG banding represent?

1. Gene-rich regions of the chromosome

2. GC-rich regions of the chromosome

3. AT-rich regions, gene-poor chromatin

4. Early replicating euchromatin

The correct answer is: AT-rich regions, gene-poor chromatin

The dark bands produced by routine GTG banding represent AT-rich regions, which are typically associated with gene-poor chromatin. GTG banding is a common cytogenetic technique used to visualize chromosomes, where the treatment of chromosomes with trypsin followed by Giemsa staining results in a distinct pattern of bands. The dark bands indicate areas of the chromosome that are more densely packed with DNA and are often less transcriptionally active, reflecting a lower density of coding genes. In contrast, the lighter bands, which are GC-rich, are usually more gene-rich regions where active transcription occurs, corresponding to euchromatin. The AT-rich dark bands, being less gene-dense, often contain repetitive sequences or heterochromatic regions, which contribute less to the overall gene repertoire. Understanding the significance of these banding patterns is critical for identifying chromosomal abnormalities and conducting genetic analyses.