Question

22

Which of the following does NOT occur when a donor nucleus is transferred into an enucleate egg? Barr bodies are decondensed imprints are restored telomeres are restored chromosomal packaging is restored Question 22 (1.5 points) The Cdk/cyclin complex can trigger cell division by Ofixing damaged DNA O removing Rb from E2F O inhibiting E2F breaking down p16

Answer 1

21. The correct answer is telomeres are restored. The low expression of telomerase in most somatic cells compared to the high level in embryo and embryo stem cells revelas that normal nuclear transplant embryos would contain an increased content of telomeres on their chromosomes compared to the somatic cells from which they are derived. Thus, we can conclude that telomere replacement probably occurs after nuclear transfer and telomere length is restored in cloned cells. During nuclear transfer from donor to the receipient (enucleated egg), only telomeres are restored and no other structures are restored occur.

2. The correct answer is removing Rb from E2F. The retinoblastoma tumor suppressor protein (Rb) binds to the E2F1 transcription factor preventing it from interacting with the cell's transcription machinery. In the absence of pRb, E2F1 mediates the trans-activation of E2F1 target genes that facilitate the G1/S transition and S-phase. E2F targets genes that encode proteins involved in DNA replication and chromosomal replication. When cells are not proliferating, E2F DNA binding sites contribute to transcriptional repression. Altogether, the function of RB is to restrict cell proliferation by suppressing gene expression through direct inhibition of the E2F family of transcription factors. If Rb get phophorylated, it is removed from the E2F and succesful transcription of genes can occur leading to active cell division. This modification in the Rb-E2F complex is triggered by Cdk/cyclin complex, which here is Cdk/cyclin D causing phophorylation of Rb protein which disturbs the binding of Rb with E2F.