January 5, 2014
1) Staple winter break hw (DNA model and
questions) together and turn in bin.
2) Have homework packet assessments out and
start working on Bellringer that you picked
up. (I will be walking around stamping hw
packet)
Protein Synthesis
• 3 major processes:
– Replication → DNA copied to form 2 new DNA
molecules
• Nucleus
– Transcription → DNA info copied to RNA
• Nucleus
– Translation → building a protein according to
RNA instructions
• Cytoplasm
• Why is DNA replication (copying) important for
cells?
• Where would the 2 strands of DNA open?
– Between the bases
or break the backbone?
DNA Replication
DNA Replication
• DNA Replication = DNA
 DNA
– Parent DNA makes 2
exact copies of DNA
– Occurs in nucleus
– Why??
• Occurs in Cell Cycle
before MITOSIS so
each new cell can
have its own FULL
copy of DNA
How does DNA replication happen?
• DNA stores the genetic information,
but proteins and enzymes do the work
•DNA helicase (enzyme) separates
(unzips) the two strands of DNA
•Opening are called “origins of
replication”
•This occur at several places along
segment of DNA
How much DNA is from the
original DNA strand at the end
of replication?
•DNA replicates in a semi-conservative
model.
•Each parent strand is now a template
(pattern) that determines the order of the
new bases
•Forms a “complementary” strand to
original strand
•The newly synthesized double helix is a
combination of one old (or original) and
one new DNA strand
Models of DNA Replication
http://www.sumanasinc.com/webcontent/animations/content/meselson.html
Segments of single-stranded DNA are called
template strands.
Copied strand is called the complement strand
(think “c” for copy)
BEGINNING OF DNA REPLICATION
(INITIATION)
• DNA helicase (think “helix”)
– binds to the DNA at the replication fork (origin of
replication)
– DNA strand separates into TWO
– untwist (“unzips”) DNA using energy from ATP
by breaking hydrogen bonds between base pairs at
several places along the segments of DNA 
called “origins of replication”
• Single-stranded DNA-binding proteins
(SSBP)
– stabilize the single-stranded template DNA during
the process so they don’t bond back together.
DNA Polymerase
• DNA Polymerase- adds nucleotides to the DNA
strands makes POLYNUCLEOTIDES (1st
function)
Helicase unzips the DNA
molecule
DNA Polymerase adds
nucleotides to create two
NEW identical daughter
molecules (A to T) and (G
to C)
• Complementary bases match up
– Two new strands are formed
– A with T
– C with G
Elongation
Antiparallel nature:
• Sugar (3’end)/phosphate (5’
end) backbone runs in
opposite directions
– one strand runs 5’  3’,
– other runs 3’  5’
• DNA polymerase only adds
nucleotides at the free 3’
end of NEW STRAND
forming new DNA strands in
the
5’  3’ direction
only!!!
DNA Replication (Elongation)
After SSBP’s bind to each template…
•
Primase
– primase is required for DNA synthesis
– Like a “key” for a car ignition
– makes a short RNA primers
•
Short pieces of RNA needed for DNA synthesis
DNA polymerase
– adds nucleotides to RNA primer  makes
POLYNUCLEOTIDES (1st function)
– After all nucleotides are added to compliment
strand…
• RNA primer is removed and replaced with
DNA by DNA polymerase (2nd function)
– Proofreads the strand before the backbone is
finished (3rd function)
• DNA ligase
– “seals” the gaps in DNA
•
–
Connects DNA pieces by making phosphodiester bonds
Elongation (con’t)
• Leading (daughter) strand
– NEW strand made toward the
replication fork (only in 5’  3’
direction from the 3’  5’ template
strand)
– Needs ONE RNA primer made
by Primase
– This new leading strand is made
CONTINOUSLY
Elongation (con’t)
Lagging (daughter) strand
• NEW strand synthesis away from
replication fork
• Replicate DISCONTINUOUSLY
– Creates Okazaki fragments
• Short pieces of DNA
– Okazaki fragments joined by DNA ligase
• “Stitches” fragments together
– Needs MANY RNA primer made by
Primase
Supercoiled DNA relaxed by gyrase & unwound by
helicase + proteins:
5’
SSB Proteins
DNA Polymerase
1
Okazaki
Fragments
ATP
2
Lagging strand
3
Helicase
3’
primase
base pairs
5’
DNA Polymerase
RNA primer replaced by
DNA Polymerase & gap is
sealed by DNA ligase
Leading strand
RNA Primer
3’
Why Replication again?
DNA replication is necessary to create
identical copies of DNA so it can be passed
onto a new cell (cell division & reproduction)
Summary
Accuracy of Replication
• Very low mistake rate (1/billion!) because
cells have enzymes (like DNA Polymerase)
that proofread, recognize,
and fix mistakes!
• HOWEVER, mistakes
can happen 
MUTATIONS (cancer)
Mistakes Made during DNA
Replication
• Mutation
– Change in DNA (genetic material)
• Frameshift(s)
–extra or missing base(s).
• Substitutions
–when the wrong nucleotide is
incorporated (mismatch mutation).
• Deletions
–Nucleotides are deleted shortening the
DNA
Review: What is DNA
replication?
1. A new sugar-phosphate backbone is made for each
new strand
2. Base pairs are added
3. Two strands are created in place of the original
strand
http://sites.fas.harvard.edu/~biotext/a
nimations/replication1.swf
http://highered.mheducation.com/sites/0072
943696/student_view0/chapter3/animation_
_dna_replication__quiz_1_.html