17. When errors in nucleotide sequencing occur,

Question

a. DNA polymerases replaces the incorrect nucleotide with the correct nucleotide
b. enzymes dissolve the incorrect nucleotide so DNA polymerase can add the correct one
c. purines replace pyrimidines in the DNA molecule
d. DNA polymerase proofreads for errors while another enzyme corrects it
A?

18. Which of the following is NOT a step in transcription?
a. RNA polymerase unwinds the double helix of the transfer RNA
b. RNA polymerase attaches to a gene's promoter region on a strand of DNA
c. RNA polymerase adds complementary RNA nucleotides to the growing strand of RNA
d. RNA polymerase unwinds and separates the DNA strands
D?

19. During translation, the amino acid detaches from the transfer RNA molecule and attaches to the end of a growing protein chain when
a. the ribosomal RNA anticodon binds to the messenger RNA codon
b. the transfer RNA anticodon binds to the messenger RNA codon
c. a "stop" codon is encountered
d. the protein chain sends a signal through the nerve cells to the brain
Not sure...

21. Which of the following procedures is NOT a usual step in a genetic engineering experiment?
a. inducing a mutation in a source of chromosome
b. cleaving DNA with a restriction enzyme
c. recombining pieces of DNA from different species
d. cloning and screening target cells
I'm not sure between A and C, but I think it's C...

Thanks
-MC