The amount of a protein that is synthesized depends on the amount of the corresponding mRNA that is available for translation. The amount of available mRNA, in turn, depends on both the rate of mRNA synthesis and the rate of mRNA degradation. Eukaryotic mRNAs are generally more stable than bacterial mRNAs, which typically last only a few minutes before being degraded. Nonetheless, there is great variation in the stability of eukaryotic mRNAs: some persist for only a few minutes, whereas others last for hours, days, or even months. These variations can produce large differences in the amount of protein that is synthesized. Cellular RNA is degraded by ribonucleases, enzymes that specifically break down RNA. Most eukaryotic cells contain 10 or more types of ribonucleases, and there are several different pathways of mRNA degradation. In one pathway, the 5′ cap is first removed, followed by 5′→3′ removal of nucleotides. A second pathway begins at the 3′ end of the mRNA and removes nucleotides in the 3′→5′ direction. In a third pathway, the mRNA is cleaved at internal sites.
Comments
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Opening sentence? I feel like you jump straight into the paragraph without introducing the topic of RNA degradation.
Suggestion
I would avoid using language such as "first," "second," and "third." This language makes your paragraph sound like you're just listing off pathways rather than explaining them in the context of the rest of your paragraph.
In all I think your paragraph
In all I think your paragraph is cohesive and well-structured. I wish you discussed a bit more about why RNA degradation in the cell happens and why different eukaryotic RNA molecules degrade at different rates.