[The ‘S’ phase of the Cell Cycle finally explained!]
1. DNA (chromosome) replication occurs in the nucleus of __________________ ("true
nucleus") cells. The DNA is replicated so that when a cell divides, each daughter cell receives
the full complement of genetic instructions.
2. The strands of DNA are "________________________". One strand runs in the 5' to 3'
direction and the other runs in the 3' to 5' direction. Thus, the nucleotides on one original
strand are upside down compared to the other original strand.
3. The enzyme ___________________ begins by breaking the ________________ bonds
between complementary bases (_____ with T, ____ with G) on opposite strands of DNA.
Often, the enzyme will begin cutting someplace in the middle of the DNA molecule (ie. not at
one end). This produces a "replication ___________________" as the strands separate.
Another enzyme ______________ helps to unwind the DNA from its helical structure.
(See Fig. 3 P. 219)
4. New ______________________ are added to each original (parental) strand by the enzyme
___________ _________________. This enzyme can only add nucleotides in the 5' to 3'
direction. This enzyme requires a ____________ of 10-60 base pairs of DNA to _________
to the template strand. Another enzyme called primase makes this primer. Thus DNA is
always synthesized more easily on the _____________ strand to form what is called a
_______________________ strand. (See Fig. 5 P. 220)
5. On the other original strand (lower strand in Fig. 8 P. 222), another __________
_______________ enzyme attaches complementary nucleotides in the 5' to 3' direction (with
respect to the new strand) through complementary base pairing in the ‘opposite’ direction
compared to the leading strand. The two DNA polymerase enzymes are thus moving in
opposite directions. (Fig. 5 P. 220)
6. The new DNA strand on the lower side in this diagram cannot be synthesized continuously as
in the first strand (step 4 above). This strand has to be built in pieces (discontinuously) in the
opposite direction to the replication fork and is called the ______________ strand. Primers
are continuously added as the replication fork forms and the enzyme _________________
attaches to this original strand in the 5’ to 3’ direction adding short DNA fragments. This means
that there are now fragments of new nucleotides (______________________ fragments).
[These primers are replaced with deoxyribonucleotides by the enzyme __________________.]
The sugar-______________________ backbones of these fragments are joined together by a
DNA _____________________ enzyme which makes the ________________________ bond
between the sugar and the phosphate. (Fig. 7 P. 221)
7. The DNA is thus completely replicated and now there are 2 genetically ___________________
DNA molecules (ie. chromosomes) for each original (parental) DNA. Depending on the type of
cell, _____________ (for somatic cells) or _____________________ (for gametes) will