Chapter 14: DNAand the Gene: Synthesis and Repair
-Genes are made of DNA. When DNAis copied, each strand of a DNAdouble helix serves as the template
for the synthesis of a complementary strand.
-DNAis synthesized in the 5’ 3’direction. When a DNAmolecule is being copied, a large suite of
specialized enzymes is involved in unwinding the double helix, continuously synthesizing the leading strand
is the 5’ 3’direction and synthesizing the lagging strand as a series of fragments that are then linked
-Specialized enzymes repair mistakes in DNAsynthesis and damaged DNA. If these repair enzymes are
defective, the mutation rate increases. Mutations in many types of genes can lead to cancer.
14.1 DNAas the Hereditary Material
Transformation: experiments carried out by Frederick Griffith reported mysterious phenomenon
involving hereditary traits. Experiment with mice and bacteria strains.
Strains: Population of genetically identical individuals.
Virulence: ability to cause disease and death.
Medium: liquid or solid that is suitable for growing cells.
Culture: Collection of cells that grows under controlled conditions.
Question: When different strains of a bacterium infect the same host, how do the strains interact?
Hypothesis: None; experiment was exploratory in nature. Experimental setup:
It had been known since the later 1800s that chromosomes are a complex of DNAand proteins.
Because the chromosome theory had been confirmed around 1920, it was clear that Griffith’s
transforming factor had to consist of either protein or DNA. OswaldAvery, Colin M. Macleod, Maclyn McCarty’s experiment:
Question: Does the transforming factor consist of DNA, RNA, or protein?
Hypothesis 1: Transforming factor is protein.
Hypothesis 2: Transforming factor is RNA.
Hypothesis 3: transforming factor is DNA.
Prediction: No explicit prediction.
Conclusion: DNAis the transforming factor.
Do viral genes consist of DNAor protein?
1) Proteins present in T2 contain sulfur but not phosphorous.
2) DNAcontains phosphorous but not sulfur. Question: Do viral genes consist of DNAor protein?
DNAhypothesis: Viral genes consist of DNA.
Protein hypothesis: Viral genes consist of protein.
Prediction of DNAhypothesis: Radioactive DNAwill be located within pellet.
Prediction of protein hypothesis: radioactive protein will be located within pellet.
Conclusion: Viral genes consist of DNA. Viral coats consist of protein. 14.2 Testing Early Hypothesis about DNASynthesis
-DNAlink together into a polymer when a phosphodiester bond forms between a hydroxyl group
on the 3’carbon of deoxyribose and the phosphate group attached to the 5’carbon.
-Primary structure of a DNAmolecule:
1) A“backbone” made up of the sugar and phosphate groups of deoxyribonucleotides.
2) Aseries of nitrogen-containing bases that project from the backbone. -Astrand of DNAhas a directionality or polarity: One end has an exposed hydroxyl group on the
3’carbon of deoxyribose, while the other end has an exposed phosphate group on a 5’carbon.
tructure of a deoxyribonucleotide:
Structure of a deoxyribonucleotide:
Primary structure of DNA:
y base pairing A-T G-C
- Double helix.
- Watson and Crick suggested existing strands of DNAserve as a template strand for DNA
to replicate. Bases are added to existing strand. (S-phase of mitosis and meiosis when
Alternate hypothesis: 1. Semi-conservative replication: If the old strand of DNAseparated, each could then be
used as a template for the synthesis of a new strand. Each new DNAwill contain one old
and one new strand.
2. Conservative replication: If bases temporarily turned outward so that complementary
strands no longer face each other, they could serve as a template for the synthesis of a
new double helix. Intact parallel strands and daughter DNAmolecule consisting of
entirely new strands.
3. Dispersive replication: If the parent helix was cut and unwound in short sections before
being copied and put back together, the new and old strands would intermingle.
Genome: Entire complement of DNA.
*Draw Figure 14.8 The Meselson-Stahl Experiment
14.3 AComprehensive Model for DNASynthesis
DNAPolymerase: Polymerizes deoxyribonucleotides to DNA. Catalyzes DNAsynthesis.
-DNApolymerases can add deoxyribonucleotides to the 3’end of a strand, therefore DNA
synthesis always proceeds in the 5’3’direction. -Reaction is exergonic. Monomers added to the strand are deoxyribonucleoside triphosphates
(dNTPs). N=A,T,C,G. High potential energy. High enough to make the formation of
phosphodiester bonds in a growing DNAstrand exergonic.
How Replication Gets Started:
Replication bubble forms at a specific sequence of bases called origin of replication. Synthesis
is bidirectional (two strands; always 5 to 3) Eukaryotes have multiple replication bubbles.
Replication fork: Y-shaped region where the parent DNAdouble helix is split into two single
strands which are then copied.
How is the helix opened and stabilized?
-Enzyme helicase catalyzes the breaking of hydrogen bonds between deoxyribonucleotides.
Causes strands to separate.
- Proteins called single-strand DNA-binding proteins (SSBPs) attach to separated strands and
prevent them from snapping back into a double helix.
-Topoisomerase: cuts and rejoins the DNAdownstream of the replication fork. (unzipping helix
causes tension further down, this fixes it) Cutting and pasting relieves twists and knots.
How is the leading/continuous strand synthesized?
-Enzyme primase (type of RNApolymerase) has to synthesize a short stretch of RNA that acts as
a primer for DNApolymerase. Simple match ribonucleotides directly by complementary base
baring on single-stranded DNA.
-DNApolymerase III only works in the 5 to 3 direction. To start a s