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DNA Replication
Synthesis of two new DNA duplexes based on complementary base sequences
with parental DNA. Is progressive, involves alternation in unwinding and rewinding of DNA duplex.
Rate of synthesis (bp/sec) depends on the complexity of organism.
Controlled at several check points. Mediated by many enzymes, and other
cellular factors.
1. Semi-Conservative Replication
One-half of each new molecule of DNA is old (template strand)
One-half of new molecule of DNA is new (complementary strand)
DNA replication in: Eukaryotes: may be initiated at
more than one site, each with its own replication origin (bidirectional replication)
Prokaryotes and bacteriophage: one replication origin for initiating DNA synthesis.
The two anti-parallel DNA strands are replicated in different ways: DNA polymerases extend DNA strands only in the 5'3' direction: Leading strand: continuously
synthesized in 5'3' direction. Lagging Strand: synthesized in
5'3' direction, but discontinuously as Okazaki fragments, which are joined together by DNA ligase.
2. DNA Replication
4. Enzymes of Replication DNA gyrase (topoisomerase II) Helicases: separation of DNA strands Single-stranded binding (SSB) proteins: preventing
newly- exposed single-stranded DNA from re-an nealing. Primase: synthesis of RNA primers DNA polymerases: synthesis of new DNA and DNA
proof-reading DNA Polymerase I: filling in gaps between Okazaki fragments
formed during lagging-strand synthesis DNA Polymerase II: No specific task assigned in DNA
replication DNA Polymerase III: the main polymerizing agent
DNA ligase: Covalently link successive Okazaki fragments.
5. Helicases
The two DNA strands do not spontaneously come apart during DNA replication, but need to be separated by helicases,
Helicases bind ATP and single-stranded sections of DNA, and move progressively forward.
Helicases unwind DNA in advance of DNA polymerase binding.
6. Single-Stranded DNA Binding Proteins The new single-stranded DNA
regions are quickly covered by specific single-stranded DNA binding proteins (SSB)
SSB prevent coiling of single-stranded DNA, and make bases available for reading and matching with complementary base.
SSB do not bind ATP, and have no known enzymatic function.
The binding of one SSB promotes the binding of another to the next section of a single-stranded DNA (cooperative binding).
7. Sealing Single-Strand Cuts by DNA Ligase DNA ligase catalyze
joining of Okazaki fragments in an energy-dependent process.
The free energy required by this reaction is obtained through the coupled hydrolysis of: NAD+ → NMN+ + AMP ATP → PPi + AMP.
8. Initiation of Okazaki Fragments
The primers for Okazaki fragments are made by primases, which recognize specific DNA sequences along single-stranded DNA.
Primase forms a complex with 6-7 other polypeptides to form primosome.
The primosome moves 5'→3' on the lagging-strand to prime the initiation of Okazaki fragments.
Movement of the primosome along DNA leads to SSB displacement, the recognition of start sites, and the polymerization of nucleotides into RNA.
Removal of RNA primer occurs by DNA polymerase I, which also fills the gaps between RNA-primed DNA fragments.
RNA primers prevent mistakes near the starting points of DNA chains.
9. DNA Polymerase
There are different forms of DNA polymerase DNA polymerase III is responsible for the synthesis of new
DNA strands DNA polymerase is actually an aggregate of several
different protein subunits, so it is often called a holoenzyme
Primary job is adding nucleotides to the growing chain Also has proofreading activities
10. DNA Polymerase III The active form of DNA polymerase III is an
assembly (holoenzyme) of seven polypeptides, needed for correct biological activity: One of the polypeptides carries 5'→3'
exonuclease and polymerizing functions Another polypeptide has the 3'→5'
exonuclease activities.
Additional polypeptides bind ATP, needed for DNA polymerase III to start synthesis at the end of RNA primer.
Once DNA polymerase III begins polymerization, it continues until the end of its template is reached.
Two copies of the DNA polymerase III holoenzyme are present at the replicating fork, so that the complex can carry out the synthesis of both the leading and lagging strands.
11. Beginning: Origins of Replication
Replication begins at specific sites called origins of replication In prokaryotes, the bacterial chromosome has a specific
origin In eukaryotes, replication begins at many sites on the DNA
molecule 100’s of origins
Proteins that begin replication recognize the origin sequence These enzymes attach to DNA, separating the strands, and
opening a replication bubble The end of the replication bubble is the “Y” shaped
replication fork, where new strands of DNA elongate
12. Elongation Elongation of the new DNA strands is catalyzed by
DNA polymerase Nucleotides align with complimentary bases on the
template strand, and are added by the polymerase, one by one, to the growing chain DNA polymerase proceeds along a single-stranded DNA
molecule, recruiting free nucleotides to H-bond with the complementary nucleotides on the single strand
Forms a covalent phosphodiester bond with the previous nucleotide of the same strand The energy stored in the triphosphate is used to
covalently bind each new nucleotide to the growing second strand
Replication proceeds in both directions
Ligase
Catalyzes the formation of a phosphodiester bond given an unattached adjacent 3‘ OH and 5‘ phosphate.
This can join Okazaki fragments This can also fill in the unattached gap left
when an RNA primer is removed DNA polymerase can organize the bond on
the 5' end of the primer, but ligase is needed to make the bond on the 3' end.
Primers DNA polymerase cannot start synthesizing on a
bare single strand. It only adds to an existing chain
It needs a primer with a 3'OH group on which to attach a nucleotide.
The start of a new chain is not DNA, but a short RNA primer Only one primer is needed for the leading strand One primer for each Okazaki fragment on the
lagging strand
Primase Part of an aggregate of proteins called the
primeosome. Attaches the small RNA primer to the single-stranded
DNA which acts as a substitute 3'OH so DNA polymerase can begin synthesis
This RNA primer is eventually removed by RNase H the gap is filled in by DNA polymerase I.
Ending the Strand
DNA polymerase only adds to the 3’ end There is no way to complete the 5’ end of the new
strand A small gap would be left at the 5’ end of each new
strand Repeated replication would then make the strand
shorter and shorter, eventually losing genes Not a problem in prokaryotes, because the DNA is
circular There are no “ends”
Telomeres Eukaryotes have a special sequence of repeated
nucleotides at the end, called telomeres Multiple repetitions of a short nucleotide
sequence Can be repeated hundreds, or thousands of times In humans, TTAGGG
Do not contain genes Protects genes from erosion thru repeated
replication Prevents unfinished ends from activating DNA
monitoring & repair mechanisms
Telomerase
Catalyzes lengthening of telomeres Includes a short RNA template with the enzyme Present in immortal cell lines and in the cells
that give rise to gametes Not found in most somatic cells May account for finite life span of tissues
1. Why is replication necessary?2. When does replication occur?3. Describe how replication works.4. Use the complementary rule to
create the complementary strand:
A---?G---?C---?T---?A---?G---?A---?G---?C---?A---?G---?T---?
Replication Quiz
1. Why is replication necessary?So both new cells will have the correct DNA2. When does replication occur?During interphase (S phase).3. Describe how replication works.Enzymes unzip DNA and complementary
nucleotides join each original strand.4. Use the complementary rule to
create the complementary strand:
A---TG---CC---GT---AA---TG---CA---TG---CC---GA---TG---CT---A
Replication Quiz
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