The Molecular Basis of
Inheritance
Chapter 16
AP Biology
Search For The Genetic Material
• Once chromosomes were known to carry
genes, the question became which of two
organic compounds make up chromosomes.
• Was DNA or protein the genetic material?
• In 1952, Alfred Hershey and Martha Chase
showed that DNA must be the genetic
material.
• Phage - virus
that attacks
bacteria and
reprograms host
to produce more
viruses.
Hershey & Chase Experiment
Viruses Infecting Bacteria
Questions About DNA Structure
• James Watson and Francis Crick were the first
to solve the puzzle of DNA structure.
• Maurice Wilkins and Rosalind Franklin
contributed to their success.
• X-ray crystallography produced a rough
blueprint of the molecule.
Rosalind Franklin’s Photo of DNA
Purines – Pure As Gold (2 gold
wedding rings)
Watson and Crick’s Model
• Determined three major features of DNA
– DNA is a double helix (with Franklin’s image).
– The nitrogenous bases are A (adenine), T
(thymine), C (cytosine), and G (guanine). In DNA,
A only pairs with T and C only pairs with G.
– The strands are antiparallel. They run in opposite,
upside-down directions.
How does DNA Replicate?
How does DNA Replicate?
•
•
•
•
Meselson – Stahl, late 1950’s
Grew bacteria on two isotopes of N.
Started on 15N, switched to 14N.
Looked at weight of DNA after one, then
2 rounds of replication.
DNA Replication
• DNA replication is semiconservative.
– This means at the end of replication, each of the
daughter molecules has one old strand, derived
from the parent strand of DNA, and one strand
that is newly synthesized.
Replication - Preview
• DNA splits by breaking the H-bonds between
the backbones.
• Then DNA builds the missing backbone using
the bases on the old backbone as a template.
How does DNA replicate?
Priming
• DNA pol III cannot initiate DNA synthesis.
• Nucleotides can be added only to an existing
chain called a Primer.
Primer
•
•
•
•
Make of RNA.
10 nucleotides long.
Added to DNA by an enzyme called Primase.
DNA is then added to the RNA primer.
Priming
• A primer is needed for each DNA elongation
site.
Anti-Parallel Alignment
Problem of Antiparallel DNA
• The two DNA strands run antiparallel to each
other.
• DNA can only elongate in the 5’--> 3’
direction.
Leading Strand
• Continuous replication toward the replication
fork in the 5’--> 3’ direction.
Lagging Strand
• Discontinuous synthesis away from the
replication fork.
• Replicated in short segments as more
template becomes opened up.
Okazaki Fragments
• Short segments (100-200 bases) that are
made on the lagging strand.
• All Okazaki fragments must be primed.
• RNA primer is removed after DNA is added.
DNA Replication – Major Points
• The replication of DNA begins at sites called the
origins of replication.
• Initiation proteins bind to the origin of replication
and separate the two strands, forming a replication
bubble. DNA replication proceeds in both directions
along the DNA strand.
• A group of enzymes called DNA polymerases catalyze
the elongation of new DNA.
• DNA polymerase adds nucleotides working in the 5’
to 3’ direction.
DNA Replication
• The strand running in the 5’ to 3’ direction is called
the leading strand. Replication occurs continuously
along this strand.
• The other strand running in the 3’ to 5’ direction is
called the lagging strand. It is synthesized in separate
fragments called Okazaki fragments, which are then
sealed together by DNA ligase, forming a continuous
DNA strand.
DNA replication animation
• http://highered.mheducation.com/sites/0072
943696/student_view0/chapter3/animation__
dna_replication__quiz_1_.html
Summary of DNA Replication
Overview
Origin of replication
Lagging strand
Leading strand
Leading strand
Lagging strand
Overall directions
of replication
Single-strand
binding protein
Helicase
5
Leading strand
3
DNA pol III
3
Parental DNA
Primer
5
Primase
3
DNA pol III
Lagging strand
5
4
DNA pol I
3 5
3
2
DNA ligase
1
3
5
Enzymes
• DNA pol I - replaces RNA
primers with DNA nucleotides.
• DNA pol III – adds nucleotides
in the 5’ -> 3’ direction
• DNA Ligase - joins all DNA
fragments together.
Other Proteins in Replication
• Topoisomerase – relieves
strain ahead of replication
forks.
• Helicase - unwinds the DNA
double helix.
• Single-Strand Binding Proteins
- help hold the DNA strands
apart.
DNA animation DNAi
• https://www.youtube.com/watch
?v=I9ArIJWYZHI
Energy for Replication
• From the triphosphate monomers.
• Loses two phosphates as each monomer is
added.
DNA Replication Error Rate
• 1 in 1 billion base pairs.
• About 3 mistakes in our DNA each time it’s
replicated.
Proofreading and Repairing DNA
• During DNA replication, DNA polymerases
proofread each nucleotide against its template
as soon as it is added to the growing chain.
• An incorrectly paired nucleotide is removed
and the correct one inserted.
• This action is similar to hitting the “delete” key
and then entering the correct letter.
Mismatch Repair
• Mismatched nucleotides sometimes evade
proofreading by DNA polymerases.
• Special repair enzymes fix incorrectly paired
nucleotides.
• If one of these enzymes was structured
incorrectly (from a mutation), what would be
the result?
Nucleotide Repair
Damage in skin
cells due to
ultraviolet
radiation.
Repair prevents
the onset of skin
cancer.
Nucleotide Excision Repair
• Incorrectly paired or altered nucleotides can
also arise after replication.
• DNA bases can undergo spontaneous chemical
changes under normal conditions, but certain
environmental conditions can also contribute
to change.
• Nucleases which are DNA cutting enzymes, cut
out the damaged part and replace it with the
correct nucleotides.
Telomeres
• Eukaryotic chromosomes have special
nucleotide sequences at their ends.
• These are multiple repetitions (100-1000) of
one short nucleotide sequence (TTAGGG in
humans).
• These sequences are called telomeres.
• They do not prevent shortening of the DNA
molecule in replication, they just postpone the
erosion of genes near the ends of the molecule.
Shortening of the
Ends of Linear DNA
Molecules
With each round of
replication the DNA
molecule becomes
shorter and shorter.
This may limit the
number of times a cell
can divide.
Telomerase
• What prevents the loss of genes is gametes?
• Germ cells (which give rise to sex cells)
produce an enzyme called telomerase.
• Telomerase catalyzes the lengthening of
telomeres in the germ cells.
• Zygote receives chromosomes with the
maximum length of telomeres.
• Telomerase is not active in body cells.
Telomerase in Cancer Cells
• Cancer cells produce the enzyme telomerase.
• This prevents the erosion of the ends of
chromosomes and allows for unlimited cell
division.
Nucleosomes
Chromatin Packing
Effects of DNA Packing
• As DNA becomes more highly packed, it
becomes less accessible for transcription.
• This is one way gene expression is controlled
in cells.
– DNA methylation
– DNA acetylation
• Evidence of effect: Barr bodies (inactivated X
chromosome in females) is highly packed.