Download - Watson and Crick-1953
1
Division Ave. High School Ms. Foglia AP Biology
AP Biology 2007-2008
DNA Replication
AP Biology
Watson and Crick-1953
2
Division Ave. High School Ms. Foglia AP Biology
AP Biology
Double helix structure of DNA
“It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material.” Watson & Crick
AP Biology
Directionality of DNA § You need to
number the carbons! u The 5’end of
the DNA strand will be near the phosphate group.
OH
CH2 O
4ʹ
5ʹ
3ʹ 2ʹ
1ʹ
PO4
N base
ribose
Nucleotide
3
Division Ave. High School Ms. Foglia AP Biology
AP Biology
The DNA Backbone § Putting the DNA
backbone together u refer to the 3ʹ and 5ʹ
ends of the DNA § New nucleotides are
added to the 3’ carbon of the sugar.
§ This will form a special covalent bond called a phosphodiester bond
OH
O
3ʹ
PO4
base
CH2 O
base
O P O
C
O –O
CH2
1ʹ
2ʹ
4ʹ
5ʹ
1ʹ
2ʹ
3ʹ
3ʹ
4ʹ
5ʹ
5ʹ
AP Biology
Anti-Parallel Strands § Nucleotides in DNA
backbone are bonded from phosphate to sugar between 3ʹ & 5ʹ carbons u DNA molecule has “direction”
u complementary strand runs in opposite direction
3ʹ
5ʹ
5ʹ
3ʹ
4
Division Ave. High School Ms. Foglia AP Biology
AP Biology
Bonding in DNA
3ʹ
5ʹ 3ʹ
5ʹ
covalent phosphodiester
bonds
hydrogen bonds
AP Biology
Base pairing in DNA § Purines (2 RINGS)
u adenine (A) u guanine (G)
§ Pyrimidines (1 RING) u thymine (T) u cytosine (C)
§ Pairing u A : T
§ 2 bonds u C : G
§ 3 bonds
5
Division Ave. High School Ms. Foglia AP Biology
AP Biology
Copying DNA § Replication of DNA
u base pairing allows each strand to serve as a template for a new strand
u new strand is 1/2 parent template & 1/2 new DNA
u SEMI CONSERVATIVE
AP Biology
DNA Replication § Large team of enzymes coordinates replication using
multiple origins.
6
Division Ave. High School Ms. Foglia AP Biology
AP Biology
DNA Replication in Prokaryotes § One circular chromosome will form two
circular chromosomes.
AP Biology
Replication: 1st step § Unwind DNA using topoisomerase
u Helicase enzyme § Separates the DNA helix § stabilized by single-stranded binding proteins
single-stranded binding proteins replication fork
helicase
7
Division Ave. High School Ms. Foglia AP Biology
AP Biology
Replication: 2nd step § Build daughter DNA strand
u add new complementary bases
u DNA polymerase III u Energy comes from
hydrolyzing ATP
AP Biology
§ Adding bases u Can only add nucleotides to
3ʹ end of a growing DNA strand § One strand is called the LEADING STRAND, and
this process will happen continually as fast as the two strands separate.
u strand only grows 5ʹ→3ʹ
Replication
8
Division Ave. High School Ms. Foglia AP Biology
AP Biology
Limits of DNA polymerase III u can only build onto 3ʹ end of
an existing DNA strand
Leading & Lagging Strands
5ʹ
5ʹ
5ʹ
5ʹ
3ʹ
3ʹ
3ʹ
5ʹ 3ʹ
5ʹ 3ʹ 3ʹ
Leading strand
Lagging strand
Okazaki fragments
ligase
Okazaki
Leading strand u continuous synthesis
Lagging strand u Okazaki fragments u joined by ligase
§ “spot welder” enzyme
DNA polymerase III
ü
û
3ʹ
5ʹ
growing replication fork
AP Biology
DNA polymerase III
RNA primase u Lays down an RNA primer u serves as starter sequence
for DNA polymerase III
Limits of DNA polymerase III u can only build onto 3ʹ end of
an existing DNA strand
STEP 1-RNA Primase
5ʹ
5ʹ
5ʹ
3ʹ
3ʹ
3ʹ
5ʹ
3ʹ 5ʹ 3ʹ 5ʹ 3ʹ
growing replication fork primase
RNA
9
Division Ave. High School Ms. Foglia AP Biology
AP Biology
DNA polymerase III
Occurs in both directions- The lagging strand will create pieces.
DNA polymerase III u Copies DNA adding to the 3’
end
STEP 2-DNA Polymerase III Copies
5ʹ
5ʹ
5ʹ
3ʹ
3ʹ
3ʹ
5ʹ
3ʹ 5ʹ 3ʹ 5ʹ 3ʹ
growing replication fork primase
RNA
AP BiologyDNA Polymerase 1 swaps RNA primer for DNA
STEP 3-Replacing RNA primers with DNA
5ʹ
5ʹ
5ʹ
5ʹ
3ʹ
3ʹ
3ʹ
3ʹ
growing replication fork
DNA polymerase I
RNA
10
Division Ave. High School Ms. Foglia AP Biology
AP Biology
DNA Ligase will fuse the Okazaki Fragments to form a continuous strand.
STEP 4-Fuse the Okazaki Fragments
5ʹ
5ʹ
5ʹ
5ʹ
3ʹ
3ʹ
3ʹ
3ʹ
growing replication fork
RNA
DNA ligase
AP Biology
Loss of bases at 5ʹ ends in every replication u chromosomes get shorter with each replication u limit to number of cell divisions?
DNA polymerase III
All DNA polymerases can only add to 3ʹ end of an existing DNA strand
Chromosome Erosion
5ʹ
5ʹ
5ʹ
5ʹ
3ʹ
3ʹ
3ʹ
3ʹ
growing replication fork
DNA polymerase I
RNA
11
Division Ave. High School Ms. Foglia AP Biology
AP Biology
Repeating, non-coding sequences at the end of chromosomes = protective cap u limit to ~50 cell divisions
Telomerase u enzyme extends telomeres u can add DNA bases at 5ʹ end u different level of activity in different cells
§ high in stem cells & cancers -- Why?
telomerase
Telomeres
5ʹ
5ʹ
5ʹ
5ʹ
3ʹ
3ʹ
3ʹ
3ʹ
growing replication fork
TTAAGGG TTAAGGG TTAAGGG
AP Biology
Replication fork
3’
5’ 3’
5’
5’
3’ 3’ 5’
helicase
direction of replication SSB = single-stranded binding proteins
primase
DNA polymerase III
DNA polymerase III
DNA polymerase I
ligase
Okazaki fragments
leading strand
lagging strand
SSB
12
Division Ave. High School Ms. Foglia AP Biology
AP Biology
DNA Polymerases § DNA polymerase III
u 1000 bases/second! u main DNA builder
§ DNA polymerase I u 20 bases/second u editing, repair & primer removal
DNA polymerase III enzyme
Arthur Kornberg 1959
Roger Kornberg 2006
AP Biology
Editing & Proofreading DNA § 1000 bases/second =
lots of typos!
§ DNA polymerase I u proofreads & corrects
typos u repairs mismatched bases u removes abnormal bases
§ repairs damage throughout life
u reduces error rate from 1 in 10,000 to 1 in 100 million bases
13
Division Ave. High School Ms. Foglia AP Biology
AP Biology
Fast & accurate! § It takes E. coli <1 hour to copy
5 million base pairs in its single chromosome u divide to form 2 identical daughter cells
§ Human cell copies its 6 billion bases & divide into daughter cells in only few hours u remarkably accurate u only ~1 error per 100 million bases u ~30 errors per cell cycle