The genome possesses:
1. Exonal segments of DNA whose sequences encode the pre-mRNA, and ultimately polypeptide and protein sequences.
2. Intronal segments that are excised by pre-mRNA splicing before transport of mature mRNA through nuclear pores to the cytoplasm where ribosomal translation into ribosomal polypeptides and proteins occurs.
3. A start site for transcription, called the initiator gene.
4. Promoters, both a basal or a core promoter located within about 40 bp of the start site, and an upstream promoter, which may extend over as many as 200 bp farther upstream.
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