Chapter 16 Study Guide
Vocabulary:
Word Roots:
helic- = a spiral
liga- = bound or tied
semi- = half
telos- = an end
Key Terms:
DNA ligase: A linking enzyme essential for DNA replication
DNA polymerase: An enzyme that catalyzes the elongation of new DNA at a replication fork by the
addition of nucleotides to the existing chain.
Double helix: The form of DNA, referring to its two adjacent polynucleotide strands wound into a
spiral shape.
Helicase: An enzyme that untwists the double helix of DNA at the replication forks.
Lagging strand: A discontinuously synthesized DNA strand that elongates in a direction away from
the replication fork.
Leading strand: The new continuous complementary DNA strand synthesized along the template
strand in the mandatory 5’ to 3’ direction.
Mismatch repair: The cellular process that uses special enzymes to fix incorrectly paired nucleotides.
Nuclease: An enzyme that can break DNA and RNA into their component nucleotides.
Nucleotide excision repair: The process of removing and then correctly replacing a damaged
segment of DNA using the undamaged strand as a guide.
Okazaki fragment: A short segment of DNA synthesized on a template strand during DNA replication.
Many Okazaki fragments make up the lagging strand of newly synthesized DNA.
Origin of replication: Site where the replication of a DNA molecule begins.
Primase : An enzyme that joins RNA nucleotides to make the primer.
Primer: An RNA polynucleotide with a free 3´ end, bound by complementary base pairing to the
template strand
Replication fork: A Y-shaped region on a replicating DNA molecule where new strands are growing.
Semiconservative model: Type of DNA replication in which the replicated double helix consists of
one old strand, derived from the old molecule, and one newly made strand.
Single-strand binding protein: Molecules that line up along the unpaired DNA strands, holding them
apart while the DNA strands serve as templates for the synthesis of complementary strands of DNA.
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Telomerase: An enzyme that catalyzes the lengthening of telomeres.
Telomere: The protective structure at each end of a eukaryotic chromosome.
Topoisomerase: A protein that functions in DNA replication, helping to relieve strain in the double
helix ahead of the replication fork.
Chapter 16 Important Points:
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DNA stands for deoxyribonucleic acid and is the molecule that encodes our heredity
information by providing the instructions for the manufacture of proteins
o This information is passed from parent to offspring and from cell to cell during cell
division
DNA is a double stranded helical molecule with each strand a polynucleotide comprised of
nucleotides
o Each nucleotide is made up of a phosphate group covalently bound to deoxyribose
(a pentose sugar) and the sugar is covalently bound to one of 4 nitrogenous bases
 Adenine (A)
 Thymine (T)
 Guanine (G)
 Cytosine (C)
o The order of bases contains the genetic code for making proteins
o The 2 strands of DNA are held together by hydrogen bonds between the base pairs
and are antiparallel
 Chargaff’s Rule states that in any species:
 Adenine amounts match with Thymine amounts
 Cytosine amounts matches with Guanine amounts
 These base pair rules allows one strand of DNA to serve as a
template or pattern so that DNA can easily make identical copies of
itself
DNA is replicated or copied during the S phase of the cell cycle in a semi-conservative
manner
o Of the 2 resulting daughter DNA molecules, each is a mixture of one parental strand
and one daughter strand
Eukaryotic chromosomes are linear and DNA replication begins at multiple origins of
replication and can proceed in both directions away from the origins
o When the parental DNA strands are separated, a replication bubble forms to allow
the new daughter nucleotides access to the template strands of DNA
Synthesizing a new DNA strand or repair of an existing strand requires several enzymes
o DNA polymerases is a family of enzymes responsible for adding new nucleotides to
the growing strand, replacing damaged nucleotides or replacing RNA primers
o Helicase untwists the double helix and separates the parental DNA strands
o Single strand binding proteins keeps the 2 parental strands from reforming their
original base pairs during replication
o Topoisomerase prevents overwinding of the DNA strands
o Primase assembles RNA nucleotides into primers
o DNA ligase seals fragments of DNA together
o Nuclease can cut out damaged parts of DNA
DNA polymerase requires the binding of an RNA primer to initiate replication
o Once the RNA primer is in place (base pairing with the DNA parental strands), DNA
polymerase can add DNA nucleotides to the 3’ end of the primer so DNA replication
proceeds in a 5’ to 3’ direction on the new daughter strand
o From each origin of replication, replication can proceed in both directions
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The new daughter strand that has a free 3’ end is called the leading strand
and only requires one primer
 The new daughter strand that does not have a free 3’ end is called the
lagging strand and must be made in a series of small pieces called Okasaki
fragments, each fragment requiring its own primer
After both strands are replicated, the RNA primers are removed and DNA polymerase
replaces the RNA nucleotides with DNA nucleotides and DNA ligase seals the fragments
If an incorrect base pairing occurs (mismatch), DNA polymerase can often immediately
correct the mistake
Later damage to the DNA molecule can sometimes be fixed by a nucleotide excision repair in
which the damaged section is removed by nuclease, DNA polymerase adds back correct
nucleotides and DNA ligase seals the fragment
Mutations are changes in the DNA sequence and may or may not pass on to future
generations
The end replication problem is caused because at the end of linear chromosomes, the last
Okasaki fragment’s primer is removed but can not be replaced by DNA polymerase because
there is no free 3’ end for the enzyme to work with
o This causes chromosomes to become shorter with each round of replication
o Telomere regions at the ends of eukaryotic chromosomes do not contain coding
regions and therefore are protective “caps” that prevent coding genes from being
degraded for a time
 When real coding regions are affected, the cell typically ceases to function
o Telomerase is an enzyme present in germ line cells which allows chromosomes to
be lengthened for gamete formation
 Some cancer cells also have active telomerase which may explain some of
their “immortal” characteristics
DNA is packaged in the nucleus along with proteins called histones that wind the DNA into
bead like structures called nucleosomes
o Euchromatin is more loosely packaged and will allow gene transcription for protein
synthesis during interphase
o Heterochomatin, which is present at the telomere and centromere regions, is more
tightly packed and prevents access to the DNA located there so these regions do not
code for any proteins
o DNA is relatively diffuse during most of the cell cycle but condenses and loops
around the histones during cell divisions to allow easier separation of the
chromosomes
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