DNA Microarray and the study of complement RNA

[noha]

Hello, I am translating a very short introduction to microbiology by  Nicholas P. Money and I don't understand how complement of rna is studied using this technique :

This is the context:

Millions of copies of each DNA sequence are dotted onto the chip in a specific two-dimensional pattern and the investigator can study which of the genes are being transcribed under a particular set of conditions by adding a labelled solution of the mRNA from a population of cells.
The mRNA is labelled with a fluorescent dye, and after this sample has been allowed to react with the microarray, mRNAs bound to the segments are identified from the positions of their complementary genes on the chip.
This snapshot of transcription is one of the sources of information used in the study of the entire complement of RNA.

[NTuft]

Double-stranded DNA is split into single strands by helicase enzyme to allow for transcription. Transcription is the process of copying a section of DNA nucleotides composed of guanine(G), adenine(A), thymine(T), and cytosine(C) into an RNA transcript composed of complementary nucleotides Cytosine(C), uracil(U), adenine(A) and guanine(G) by a polymerase enzyme. If it were going to a ribsome for translation (construction into a protein) it'd be called messenger RNA, mRNA. Note that in the RNA transcript uracil(U) is used in place of thymine(T) found in DNA.

Complementarity
DNA  -  RNA

G       -       C
A       -       U

C       -       G

C       -       G

T       -       A

in context given, it seems gene transcription was occuring on the microarray, which means DNA had been opened by helicase. This exposes a section of DNA for gene transcription. To make this "snapshot of transcription" "in the study of the entire complement of RNA" is to add a fluorescent mRNA strand that will glow when it binds to the open strand of DNA, from which a similar strand of mRNA could have been transcribed, because it is complementary.

Quote

Complementarity (molecular biology)
In molecular biology, complementarity describes a relationship between two structures each following the lock-and-key principle. In nature complementarity is the base principle of DNA replication and transcription as it is a property shared between two DNA or RNA sequences, such that when they are aligned antiparallel to each other, the nucleotide bases at each position in the sequences will be complementary, much like looking in the mirror and seeing the reverse of things.

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