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
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
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)
• 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