After reading the sentences, I still don't get why 360,000 mRNA molecules are equal to 12,000 transcripts with 2kb. Could anyone please help?
"The majority of RNA molecules are tRNAs and rRNAs. mRNA accounts for only 1–5% of the total cellular RNA although the actual amount depends on the cell type and physiological state. Approximately 360,000 mRNA molecules are present in a single mammalian cell, made up of approximately 12,000 different transcripts with a typical length of around 2 kb. Some mRNAs comprise 3% of the mRNA pool whereas others account for less than 0.1%. These rare or low-abundance mRNAs may have a copy number of only 5–15 molecules per cell." - Qiagen
http://www.qiagen.com/knowledge-and-support/faq/?ID=06a192c2-e72d-42e8-9b40-3171e1eb4cb8
It is because some transcripts may have hundreds of copies in a cell and some may have none. Therefore the population of mRNA molecules in a cell is not a one-to-one relationship with the number of transcripts.
BTW that quote is wrong: there are far more than 12,000 transcripts in a typical mammal transcriptome.
@Christian Cole: Is one mRNA molecule equal to one nucleotide base (A/U/C/G) or one mRNA transcript? Because of the 12,000, I am confused. Also, it's being calculated by recent papers that a single mammal cell contains about 100,000 to 1,000,000 mRNA molecule. But, again, I am not sure whether one mRNA molecule is equal to one mRNA transcript. Could you please enlighten me? Thanks!
One mRNA molecule is one single chemical made up of a string of nucleotide bases (ACGU). A transcript is the name given to each unique chemical string; each can multiple copies/molecules.
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