Transcript DNA Structure and Replication

DNA Structure and Replication
Genetics
DNA
• Can replicate
• Is the hereditary
material
• Controls cellular
activities by coding
for and controlling
protein synthesis
Long Road to Discovery
• Griffith observed transformation
• DNA passed from a harmful bacteria into a
harmless one, and the new bacteria killed
mice in the experiment
Transformation
DNA
• Hershey and Chase
confirmed that DNA,
and not protein, was
the hereditary material
• Labeled viruses and
watched what part
entered the cell
• The DNA went in, the
protein coat stayed out
Deciphering the Structure
• Watson and Crick proposed the final model
• They used the research of many others.
• Levene identified the three parts: a sugar, a
nitrogen containing base, and a
phosphorous containing part. And that they
were always present in equal numbers.
Four different bases were recognized.
Deciphering the Structure
• Chargaff showed that equal numbers of the
bases adenine and thymine were always
present; and that guanine and cytosine were
always present in equal numbers.
• Wilkins and Franklin X rayed DNA and
revealed a pattern of repeating building
blocks.
Deciphering the Structure
• From all of that
research, Watson
and Crick put
together the
double helix
model.
Deciphering the Structure
• Each repeating unit is
a nucleotide.
• The sugar is
deoxyribose, and
together with
phosphate, makes up
the sides of the DNA
ladder
• The bases make up the
rungs
Nucleotide
Deciphering the Structure
• Hydrogen bonds hold
the bases together
• Two strands of DNA
are joined in the
middle by the bases,
and then the whole
structure twists to
become a helix.
Structure of DNA
• Antiparallel: the sides of
the ladder run in
opposing directions, as if
one strand was upside
down.
• Each carbon on the
deoxyribose sugar is
numbered; one strand
runs in the 5’ to 3’
direction, the other in the
3’ to 5’ direction. Page
221
The DNA wraps
around the
histones; the
combination of
DNA and a
histone is a
nucleosome
DNA codes for Proteins
• The sequence of the bases: adenine,
thymine, cytosine, and guanine determines
the order of amino acids in a protein chain.
DNA Replication
• DNA unzips using an
enzyme called helicase
• Each strand makes a
new side
• This is
semiconservative;
each new molecule of
DNA has one old
strand and one new.
Semiconservative
Old strand
New strand
Replication
• Can be done in the lab to just certain sections or
fragments of DNA
• Requires a PRIMER, a short segment of nucleotides
recognized as a START tag by DNA polymerases
• Primers are on either end of the segment you want to
copy
Replication
• Nucleotides can only be
added to the 3’ end of the
sugar
• Nucleotides can only be
added in the 5’ to 3’
direction; the 5’ to 3’
direction refers to the
NEW strand being added
• This is easy on one side,
but the other strand is 3’ to
5’ (antiparallel)
Antiparallel structure
Replication
• The 5’ to 3’ side must be
replicated in short
fragments.
• It unzips, and then
fragments are added by
going up toward the fork
and working back down.
• Discontinuous; has a
leading strand and a
lagging strand
Replication
Replication
• Enzymes are involved
• DNA polymerase
attaches short stretches
of nucleotides to the
template
• DNA ligases connect
fragments to make a
continuous strand
DNA Repair
• Replication is very
accurate—only about
one in a million base
pairs has a mistake
• Some genes produce
repair enzymes
• Discovered when
fungi exposed to UV
radiation was repaired
by being in light
Types of damage and the fix
• Ultraviolet radiation
damages DNA
• Causes the formation
of an extra bond
between two bases on
the same strand
• Most frequently
happens to thymines
Types of damage and the fix
• This thymine dimer
causes a kink in the
strand
• Enzymes called
photolyases use light
to detect and break the
extra bond
• This type of repair is
called
photoreactivation
Types of damage and the fix
• Another way to fix
ultraviolet radiation
damage is by excision
repair
• The dimer is cut out
by a nuclease enzyme
and is replaced with
completely new
nucleotides
Types of damage and the fix
• A third type of repair
is mismatch repair
• Enzymes proofread
• Look for areas where
the bases are not
aligned properly, as if
the strand slipped
• Most common in
repeating sequences
Types of damage and the fix
• Disorders can occur
when mutations occur
in the repair genes or
there are problems in
the repair enzymes
• Conditions such as AT,
Bloom syndrome, and
Hereditary
nonpolyposis colon
cancer