During the cell cycle, most mRNA transcription and protein translation occurs du

Question

During the cell cycle, most mRNA transcription and protein translation occurs during the so-called gaps, G1 and G2. The major exception to this is the synthesis of the histones. These proteins are only needed in large amounts during S-phase as the DNA is being replicated. If you investigate rates of histone protein synthesis during the cell cycle, you find that they go rapidly up at the beginning of S-phase, and stay high until the end of S, at which point they drop precipitously (Figure 1 , open boxes). it was proposed that this rapid rise ?and fall was due entirely to transcriptional regulation, that is, histone mRNA transcription occurs at high levels throughout S-phase, and ceases at the end. In an effort to test this hypothesis, cells in S-phase were treated with alpha-amanitin, a potent inhibitor of mRNA transcription. The rates of histone protein synthesis fell as shown in Figure 1 (closed boxes). Are these data consistent with transcriptional control being the main regulatory switch that shuts down histone synthesis at the end of S? Why or why not?

Explanation / Answer

Histones are fundamental components of chromatin that maintain and regulate appropriate chromatin conformation to support all genomic DNA-dependent processes, including transcription, replication, repair, and mitosis).Histone protein synthesis is activated as cells enter S phase to allow packaging of the newly replicated DNA into chromatin.mechanism for silencing histone expression at the end of S phase in S. pombe. Failure to shut off histone expression disrupts centromeric chromatin structure.In all eukaryotes, the levels of histone mRNAs are cell cycle regulated with their accumulation activated as cells approach entry into S phase and rapidly decreasing when DNA replication is completed. Recent findings clearly demonstrate that it is as important to shut off histone synthesis at the end of S phase as it is to initiate it at the start of S phase. The mRNAs that encode the bulk of the histone proteins, termed replication-dependent histone mRNAs, are tightly cell cycle regulated. In all organisms, there are additional histone genes that encode variant histones, such as centromeric histone H3, that are expressed outside of S phase. It is likely that one problem that arises when replication-dependent histones are overexpressed or expressed outside of S phase is that they disrupt the proper incorporation of the variant histones into chromatin.

Yes this data is consistent.

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