19-11 Microarray / DNA Chip - Part 2 (Cambridge AS A Level Biology, 9700)

Microarray Function: Determining Gene Expression
📌 The second primary function of a microarray is to determine gene expression, which means checking if a gene is undergoing transcription to produce mRNA.
🔬 Transcription involves DNA separating and RNA nucleotides forming complementary base pairs to create mRNA (represented as blue molecules in the example).
🧪 To detect expression, scientists extract the mRNA from the cytoplasm and treat it with reverse transcriptase to convert it back into complementary DNA (cDNA).
🔴 This resulting cDNA is then labeled with a red fluorescent tag before being added to the microarray for hybridization with DNA probes.

Microarray Procedure for Gene Expression Detection
🔗 If the cDNA hybridizes (sticks) to the DNA probes on the microarray, it indicates that the corresponding gene was expressed, showing a glow under UV light after washing away unbound material.
🚫 If a gene is present but not expressed (no transcription), no mRNA is produced, thus no cDNA is made, resulting in no glow under UV light.
📊 The intensity of the red glow observed at specific probe positions directly correlates with the amount of cDNA present, indicating the rate of gene expression (higher glow = higher rate of transcription/more mRNA).

Distinction Between Gene Presence and Gene Expression Testing
🧬 To detect the presence of a gene, DNA is extracted, amplified (PCR), denatured, tagged, hybridized, and viewed, with a glow confirming the gene's existence in the organism.
🔬 To detect gene expression, mRNA is extracted, converted to cDNA, tagged, hybridized, and viewed; the glow confirms the gene was active (transcribed).

Key Points & Insights
➡️ Gene expression is confirmed when labeled cDNA (derived from mRNA) binds to DNA probes on the microarray, producing a visible red fluorescent signal under UV light.
➡️ The microarray technique differentiates between merely possessing a gene (by testing DNA) and the gene being actively transcribed (by testing mRNA).
📈 The intensity of the light signal provides quantitative data, showing that stronger glowing areas mean the gene had a higher rate of expression (more mRNA produced).

📸 Video summarized with SummaryTube.com on Feb 23, 2026, 08:58 UTC