Chromosomal Landscapes
Refer to Figure 1-7 from Introduction to Genetic Analysis, Griffiths et al., 2012.
Human Chromosomal Landscapes
Refer to Figure 1-8 from Introduction to Genetic Analysis, Griffiths et al., 2012.
Molecular Basis for
Relationship between Genotype and Phenotype
genotype
DNA
transcription
DNA sequence
replication
RNA
translation
protein
function
phenotype
organism
amino acid
sequence
Replication
Replicationof
ofDNA
DNAisis
semiconservative.
semiconservative.
Each
Eachstrand
strandserves
serves
as
asaatemplate.
template.
The
Thetwo
twostrands
strands
separate
separatefrom
fromeach
each
other
otherwhen
when
hydrogen
hydrogenbonds
bonds
are
arebroken.
broken.
Refer to Figure 7-11 from Introduction to Genetic Analysis, Griffiths et al., 2012.
New
Newstrands
strandsare
are
synthesized
synthesizedby
bythe
the
addition
additionof
ofnucleotides
nucleotides
with
withbases
bases
complementary
complementaryto
to
those
thoseof
ofthe
thetemplate.
template.
DNA
DNAreplication
replicationisis
discontinuous.
discontinuous.
Two
Twoidentical
identicaldouble
double
helices
helicesresult.
result.
DNA
polymerization
requires DNA
polymerase.
Refer to Figure 7-15 from Introduction to Genetic Analysis, Griffiths et al., 2012.
DNA Polymerases
At least 5 DNA polymerases are known in E. coli .
DNA polymerase I (pol I):
•
•
•
adds nucleotides in 5’ to 3’ direction
removes mismatched bases in 3’ to 5’ direction
degrades double-stranded DNA in 5’ to 3’ direction
DNA polymerase II (pol II):
•
repairs interstrand cross-links
DNA polymerase III (pol III):
•
catalyzes DNA synthesis at replication fork in
5’ to 3’ direction and only adds nucleotides at 3’ end
of growing strand
Overview of DNA Synthesis
DNA polymerases synthesize
new strands in 5’ to 3’
direction.
Primase makes RNA primer.
Lagging strand DNA consists of
Okazaki fragments.
In E. coli, pol I fills in gaps in
the lagging strand and removes
RNA primer.
Fragments are joined by DNA
ligase.
DNA Replication at Growing Fork
DNA polymerases add nucleotides
in 5’ to 3’ direction.
Because of antiparallel nature,
synthesis of DNA is continuous for
one strand and discontinuous for
the other strand.
DNA Replication:
Synthesis of Lagging
Strand
Several components
and steps are involved
in the discontinuous
synthesis of the lagging
strand.
Note that DNA
polymerases move in 3’
to 5’ direction on the
template DNA
sequence.
DNA Replication:
Synthesis of Lagging
Strand
DNA extended from
primers are called
Okazaki fragments.
In E. coli, pol I removes
RNA primers and fills
in the gaps left in
lagging strands.
DNA ligase joins these
pieces.
Replisome and Accessory Proteins
pol III holoenzyme is a
complex of many
different proteins.
Refer to Figure 7-20 from Introduction to Genetic Analysis, Griffiths et al., 2012.
Looping of
template DNA
for the lagging
strand allows
the two new
strands to be
synthesized by
one dimer.
Priming DNA Synthesis
Primase enzyme
makes short RNA
primer sequence
complementary to
template DNA.
DNA polymerases can
extend (but cannot start)
a chain.
Primosome is a
set of proteins
that are involved
in the synthesis
of RNA primers.
Refer to Figure 7-20 from Introduction to Genetic Analysis, Griffiths et al., 2012.
DNA polymerase
extends RNA
primer with DNA.
Supercoiling results from separation of template strands
during DNA replication.
Helicases and
Topoisomerases
Helicase enzymes disrupt
hydrogen bonding between
complementary bases.
Single-stranded binding
protein stabilizes unwound
DNA.
Unwound condition
increases twisting and
coiling, which can be
relaxed by topoisomerases
(such as DNA gyrase).
Topoisomerases can either
create or relax supercoiling.
They can also induce or
remove knots.
Chromatin assembly factor I (CAF-I) and
histones are delivered to the replication fork.
CAF-I and histones bind to proliferating cell
nuclear antigen (PCNA), the eukaryotic version
of clamp protein.
Nucleosome assembly follows thereafter.
Refer to Figure 7-23 from Introduction to Genetic Analysis, Griffiths et al., 2012.
Overview of DNA Synthesis
DNA polymerases synthesize
new strands in 5’ to 3’
direction.
Primase makes RNA primer.
Lagging strand DNA consists of
Okazaki fragments.
In E. coli, pol I fills in gaps in
the lagging strand and removes
RNA primer.
Fragments are joined by DNA
ligase.