The human genome codes for 20,000 to 25,000 proteins, but any given cell might produce only 5,000 different proteins. How is this possible?
A) Not every cell has the machinery for transcription and translation.
B) Genes are regulated so that not all genes are transcribed in all cells.
C) Every cell contains a different fraction of the genome.
D) All possible proteins are made in all cells, but those that are not needed are degraded.
E) Some cells use introns to produce proteins, and some cells use exons.
Answer: B
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