Which of the following statements are correct about poly-A-tail addition to a pre-mRNA:
A. Polyadenylation signals in mammalian cells consist of the hexanucleotide AAUAAA in addition to upstream and downstream (U- or GU-rich) elements.
B. Polyadenylation signals consist of the hexanucleotide GGUAAA.
C. Poly-A polymerase adds a poly-A tail consisting of about 50 Adenines to the 3' end.
D. Poly-A polymerase adds a poly-A tail consisting of about 200 Adenines to the 3' end.