1. The DNA double helix is held together by two types of bonds, covalent and
hydrogen. Covalent bonds occur within each linear strand and strongly bond the
bases, sugars, and phosphate groups (both within each component and between
components). Hydrogen bonds occur between the two strands and involve a base
from one strand with a base from the second in complementary pairing. These
hydrogen bonds are individually weak but collectively quite strong.
3. A primer is a short segment of RNA that is synthesized by primase using DNA as
a template during DNA replication. Once the primer is synthesized, DNA
polymerase then adds DNA to the 3´ end of the RNA. Primers are required
because the major DNA polymerase involved with DNA replication is unable to
initiate DNA synthesis and, rather, requires a 3´ end. (It is the 3´ -OH group that
is required to create the next phosphodiester bond.) The RNA is subsequently
removed and replaced with DNA so that no gaps exist in the final product.
4. Helicases are enzymes that disrupt the hydrogen bonds that hold the two DNA
strands together in a double helix. This breakage is required for both RNA and
DNA synthesis. Topoisomerases are enzymes that create and relax supercoiling in
the DNA double helix. The supercoiling itself is a result of the twisting of the
DNA helix that occurs when the two strands separate.
5. Because the DNA polymerase is capable of adding new nucleotides only at the 3´
end of a DNA strand, and because the two strands are antiparallel, at least two
molecules of DNA polymerase must be involved in the replication of any specific
region of DNA. When a region becomes single-stranded, the two strands have an
opposite orientation. Imagine a single-stranded region that runs from right to left.
The 5´ end is at the right, with the 3´ end pointing to the left; synthesis can initiate
and continue uninterrupted toward the right end of this strand. Remember: new
nucleotides are added in a 5´→ 3´ direction, so the template must be copied from
its 3´ end. The other strand has a 5´ end at the left with the 3´ end pointing right.
Thus, the two strands are oriented in opposite directions (antiparallel), and
synthesis (which is 5´→ 3´) must proceed in opposite directions. For the leading
strand (say, the top strand) replication is to the right, following the replication fork.
It is continuous and may be thought of as moving “downstream.” Replication on
the bottom strand cannot move in the direction of the fork (to the right) because,
for this strand, that would mean adding nucleotides to its 5´ end. Therefore, this
strand must replicate discontinuously: as the fork creates a new single-stranded
stretch of DNA, this is replicated to the left (away from the direction of fork
movement). For this lagging strand, the replication fork is always opening new
single-stranded DNA for replication upstream of the previously replicated stretch,
and a new fragment of DNA is replicated back to the previously created fragment.
Thus, one (Okazaki) fragment follows the other in the direction of the replication
fork, but each fragment is created in the opposite direction.
14. d. Replication would take twice as long.
15. a. Prior to the S phase, each chromosome has two telomeres, so in the case of
2n = 14, there are 14 chromosomes and 28 telomeres.
b. After S, each chromosome consists of two chromatids, each with two
telomeres, for a total of four telomeres per chromosome. So, for 14
chromosomes, there would be 14 × 4 = 56 telomeres.
