Download Chapter 16

The Molecular Basis of
Inheritance
Chapter 16 – AP Biology
The Search for the Genetic Material
• From Previous Chapters:
– Sutton said the genes were on the
chromosomes
– Morgan proved this with his work with fruit
flys
• Still, it was unknown what PART of the
chromosomes carried the genes…
Chromosome Structure
• Chromosomes are composed of two
main chemicals
– Nucleic Acid (DNA)
– Protein
Which part of the Chromosomes
carries the Genetic Material?
• While it was widely believed (thanks to
Sutton and Morgan) that the genes
were located on the chromosomes, it
was unknown which chemical
component (nucleic acid or protein)
actually carried them.
• Up until the middle part of the 20th
century, it was thought that the protein
parts of the chromosomes were most
likely the genetic material.
– This was because proteins were being found at
Experiments Leading to DNA as the
Genetic Material
• (Bacteria were found to be the
organisms best suited for determining
the nature of the genetic material)
Griffith and Avery
• Griffith killed virulent (disease causing bacteria
• He mixed the remains of the killed bacteria with
living, harmless bacteria
• Upon exposure to the dead virulent bacteria, the
harmless bacteria were transformed into virulent
bacteria
• Griffith concluded that some component of the dead
cells was causing an inheritable change in the living,
harmless cells.
• This was called transformation.
• The question remained:
– What was causing the transformation?
Griffith and Avery
• Avery took Griffith’s work further.
• He isolated and purified the different
chemicals in the killed bacteria
• He exposed the harmless bacteria to
each different chemical that he
obtained from the killed virulent
bacteria
• The only chemical that caused the
harmless bacteria to transform was the
DNA.
• Still, the results were not widely
Diagram: Griffith and Avery’s
Experiment
Fred Griffith (with “Bobby”) and
Oswald Avery
Griffith and Avery
• They never met, though they did
respect each other’s work
• Griffith was English and was killed in
the Blitz in London in 1941.
• Avery was American and lived from
1877 to 1955.
• For more click HERE.
Hershey and Chase
• Worked with
Bacteriophages
– Viruses that infect
bacteria
– Viruses are
composed of
• Protein
• Nucleic Acid
Alfred Hershey and Martha Chase
Hershey and Chase
• Designed and experiment to determine,
without question, which component of
a bacteriophage infected a bacterium –
the protein or the nucleic acid
Hershey and Chase
• Tagged the protein coat of the virus
with one type of radioactive marker
that they could see and follow
• Tagged the DNA of the virus with a
different radioactive marker that they
could see and follow
• Allowed bacteria to be infected by the
tagged viruses
• Observed which markers actually ended
up inside the bacteria
Diagram: Hershey and Chase’s
Experiment
Discovering the Structure of DNA
• Once it was determined that DNA was
the genetic material, the next step was
to determine the structure of the
molecule
• It was hoped once the structure was
determined, clues would be evident
about how the molecule worked.
Scientists Working on the DNA
Molecule in the 1950s
• Linus Pauling
– Very well known scientist
– Made many important
discoveries in molecular
biology
• Especially important
discoveries about
proteins
• Discovered the alpha
helix shape of some
protein molecules
– Still, he ultimately “lost”
the race in determining
the structure of DNA
Scientists Working on the DNA
Molecule in the 1950s
• Rosalind Franklin
– Working the the lab of
Maurice Wilkins at King’s
College (in England)
– Made excellent X-ray
crystallography images
of the DNA molecule that
were ultimately used to
discover DNA’s structure
Photo 51
• The x-ray
diffraction image
made by Franklin
and shown to
Watson by
Maurice Wilkins
Scientists Working on the DNA
Molecule in the 1950s
• Maurice Wilkins
– It was Wilkins’ lab in
which Rosalind
Franklin came to
work at King’s
College
– Was, along with
Watson and Crick,
awarded the Nobel
Prize.
Scientists Working on the DNA
Molecule in the 1950s
• James Watson
– Very young American
– Came to study at the
Cavendish in England
– Interested in DNA
– Using Rosalind Franklin’s
DNA images, he and
Francis Crick determined
DNA’s structure and were
awarded the Nobel Prize.
Scientists Working on the DNA
Molecule in the 1950s
• Francis Crick
– British
– Ph.D student at at the
Cavendish Laboratory in
England.
– Shared a common
interest with James
Watson in DNA
– Using Rosalind Franklin’s
DNA images, he and
Watson determined
DNA’s structure and were
awarded the Nobel Prize
Puzzle of DNA Structure
• Initially, Watson tried to make the
bases fit together “like with like”
• Purine + purine was too wide and
pyrimidine + pyrimidine was too narrow
– the structure simply could not fit
together.
• Eventually he realized that purine and
pyrimidine would fit together were like
with like would not.
Puzzle of DNA Structure
• Figuring out the
A-T and C-G
arrangement of
DNA also
explained
“Chargaff’s
Rules”.
– That A’s and T’s
occurred in a 1:1
ratio as did C’s and
G’s.
DNA Structure
• Though the rungs of the DNA ladder
must always be A-T and C-G, the
SEQUENCE of these bases along the
length of the strand is NOT restricted.
• Thus the sequence can be varied in
many ways, allowing for nearly
unlimited variety among organisms.
Announcement of the Structure of
DNA
• Watson and Crick
announced their
findings in the April
1953 issue of the
journal “Nature”.
• Their article was only
one page long.
• The beauty of the
model that Watson and
Crick described was
that it immediately
suggested a mechanism
by which DNA could
replicate itself
DNA Structure
• DNA is a NUCLEIC ACID
• Nucleic Acids are POLYMERS
• The MONOMERS that make up Nucleic
Acid polymers are NUCLEOTIDES.
Nucleotide Structure
• Each nucleotide is
composed of 3 parts
– Sugar
– Nitrogenous base
– Phosphate group
• Variation only occurs
only in nitrogenous
base
The Bases - Two Families
– Purines - LARGER (TWO
Rings)
• Adenine
• Guanine
– Pyrimidines -(smaller and
one ring)
• Cytosine
• Thymine - (DNA)
• Uracil - (RNA)
The Sugar
• 5 carbon sugar (also
known as a pentose
sugar)
– DNA - deoxyribose
– RNA - ribose
• carbons are numbered
to indicate position
The Phosphate Group
• The phosphate
group is always
attached to the 5’
carbon
Linking Nucleotides together to make
DNA
• Covalent bonds link one
nucleotide to the next
– a phosphodiester
bond
• Formed between sugar
of one nucleotide and
phosphate group of the
next nucleotide
Double Helix
• The DNA molecule is
composed of two
“polynucleotide”
molecules that spiral
around an imaginary
axis.
• This is known as a
“double helix”.
• Click here for another
DNA model site.
Double Helix Structure
• Sugar/phosphate (backbones) are on the outside of the helix
• Nitrogenous bases are paired in the inside of the helix
• Hydrogen bonds between bases hold the two strands of DNA
together
Double Helix Structure
• Only certain bases
are compatible with
each other
– Purine + Pyrimidine
• Cytosine + Guanine
• Adenine + Thymine
• (RNA = Adenine matches
with Uracil)
Double Helix Structure
• Complementarity
•
– The two strands of
the double helix are
said to be
complementary.
– This means that
each is a predictable
counterpart of the
other.
Antiparallel means that sugar/phosphate
backbones run in opposite directions.
DNA Replication
• During replication, the pairing of the
bases enables existing DNA strands to
serve as templates for new,
complementary strands.
DNA Replication
• The first step in replication is the
separation of the strands
DNA Replication
• The second step in replication
– Each “old” strand serves as a template that the
determines the order of nucleotides along the new
complementary strands.
DNA Replication
• The third step in replication
– The nucleotides are connected to for the
sugar/phosphate backbones of the new strands.
Each DNA molecule now consists of one old
strand and one new strand
DNA Replication
• Each DNA molecule completed in
replication is identical to the parent
molecule
• Term for the two daughter DNA
molecules that are composed of one
OLD strand and one NEW strand =
SEMICONSERVATIVE
Semiconservative Model of
Replication (diagram b)
How replication of DNA is carried out
• A large team of Enzymes and other
proteins carries out the process of DNA
replication
Keep in mind…
• There are 46 DNA molecules (that is,
chromosomes) in each of your cells
• That’s 6 billion base pairs
• It would take about 900 AP Biology
books to print it all out (A’s, T’s, C’s and
G’s)
• It takes a cell just a few hours to copy
all of that information
• And the cells are VERY good at it – only
1 error per BILLION nucleotides, on
Replication – the Process – Page 301
• Origins of Replication
– Sites on the chromosome where replication
begins
– A bacterial chromosome has only one
origin of replication
– Eukaryotic chromosomes have multiple
origin sites.
• This makes replication faster
Origin of Replication
• stretch of DNA that has a specific
sequence of nucleotides
• Proteins that initiate DNA replication
recognize this sequence and attach to
the DNA at these points.
• This separates the DNA into two
strands and opens up a replication
bubble.
Origins of Replication - Diagram
What happens at a replication
bubble?
• Replication occurs in both directions
from the origin
• Eukaryotic cells - multiple (hundreds of
thousands) replication bubbles fuse.
– Multiple bubbles speeds up replication
What is a replication fork?
• Y-shaped region at either end of a replication bubble
• New strands of DNA are being elongated at these
points.
Elongating the DNA Strand
• DNA Polymerase – the enzyme that
catalyzes the elongation of new DNA at
a replication fork.
• elongates a DNA strand at rates of
– 500 nucleotides/sec in bacteria
– 50 nucleotides/sec in human cells
Elongating the DNA Strand - Energy
• a nucleotide is a triphosphate.
• Cleavage of phosphate groups from the
nucleotides themselves provide energy
for their attachment to the strand.
• Remember ATP!
Explanation of Antiparallel Strands
• two
sugar/phosphat
e backbones
that make up a
DNA double
helix are
arranged
“upside down”
or antiparallel to
each other.
Why Do We Care??
• Important in the mechanism of
replication
• DNA Polymerase can ONLY ADD
NUCLEOTIDES TO THE 3’ END OF AN
ELONGATING DNA MOLECULE
– new DNA strand can elongate ONLY IN
THE 3’ DIRECTION
– NEVER elongate in the 5’ DIRECTION
First Look at Replication
• VIEW ANIMATION
54
How DNA Polymerase works…
• Along ONE of the
template
(old/parent)
strands of DNA,
DNA Polymerase
can create a
CONTINUOUS
complementary
strand – elongating the new DNA in the 5’ to
3’ direction
– CALLED the LEADING strand.
How DNA Polymerase works…
• On the OTHER side of the replication
fork, the process is DIFFERENT.
– This is because DNA Polymerase can ONLY add to
the 3’ end of the elongating strand.
• DNA Polymerase must work in the
Tet
direction leading AWAY
from the
• The
strand fork.
replication
made in this
direction is
called the
LAGGING
strand.
How the Lagging Strand is Made…
• Replication bubble opens
• DNA polymerase molecule can work away
from the fork and make a short segment
of DNA.
• As the bubble opens up a bit more,
polymerase can leap frog back up the
fork and slide back out of the fork again
until it bumps into the strand it just made
• Thus, the lagging strand is made in a
series of short segments
– Okazaki fragments
– 100-200 nucleotides long in eukaryotes
More Problems for DNA Polymerase…
• Additionally, DNA polymerase CANNOT
initiate the synthesis of a strand of DNA
• DNA Polymerase can ONLY add
nucleotides to a pre-existing
polynucleotide
Solution…
• Another enzyme, this one called
primase, places a short stretch of RNA
that is complementary to the template
strand.
• Provides a “hook” upon which DNA
polymerase can “hang” the nucleotides
needed to elongate the new DNA
strand
• RNA “hook” called the “Primer”
More on Primers and Primase
• Only 1 primer is required for the
synthesis of the LEADING DNA strand.
• However, for the LAGGING strand, each
new segment created must have its
own primer
Other Enzymes and Proteins in
Replication – Page 304
• Helicase – enzyme that untwists the
DNA molecule at the site of the
replication fork and separates the two
“old” strands.
• Single strand binding proteins –
enzymes that hold the open strands of
the template DNA apart while new
strands are being made.
• DNA Ligase – joins the 3’ end of DNA to
rest of the strand
Repairing Enzymes – page 305
• Nuclease – cuts out damaged strands
of DNA