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AP Biology 2007-2008 DNA Replication By- Saurav K. Rawat Rawat DA Greatt

Dna replication

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DNA replication..how does it occur actually.. very easy way.must watch.

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Page 1: Dna replication

AP Biology 2007-2008

DNA ReplicationBy- Saurav K. Rawat

Rawat DA Greatt

Page 2: Dna replication

AP Biology

Watson and Crick1953 article in Nature

Page 3: Dna replication

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

Page 4: Dna replication

AP Biology

Directionality of DNA You need to

number the carbons! it matters!

OH

CH2

O

4

5

3 2

1

PO4

N base

ribose

nucleotide

This will beIMPORTANT!!

Page 5: Dna replication

AP Biology

The DNA backbone Putting the DNA

backbone together refer to the 3 and 5

ends of the DNA the last trailing carbon

OH

O

3

PO4

base

CH2

O

base

OPO

C

O–O

CH2

1

2

4

5

1

2

3

3

4

5

5

Sounds trivial, but…this will beIMPORTANT!!

Page 6: Dna replication

AP Biology

Anti-parallel strands Nucleotides in DNA

backbone are bonded from phosphate to sugar between 3 & 5 carbons DNA molecule has

“direction” complementary strand runs

in opposite direction

3

5

5

3

Page 7: Dna replication

AP Biology

Bonding in DNA

….strong or weak bonds?How do the bonds fit the mechanism for copying DNA?

3

5 3

5

covalentphosphodiester

bonds

hydrogenbonds

Page 8: Dna replication

AP Biology

Base pairing in DNA Purines

adenine (A) guanine (G)

Pyrimidines thymine (T) cytosine (C)

Pairing A : T

2 bonds C : G

3 bonds

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AP Biology

Copying DNA Replication of DNA

base pairing allows each strand to serve as a template for a new strand

new strand is 1/2 parent template & 1/2 new DNA

Page 10: Dna replication

AP Biology

DNA Replication Large team of enzymes coordinates replication

Let’s meetthe team…

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AP Biology

Replication: 1st step Unwind DNA

helicase enzyme unwinds part of DNA helix stabilized by single-stranded binding proteins

single-stranded binding proteins replication fork

helicase

Page 12: Dna replication

AP Biology

DNAPolymerase III

Replication: 2nd step

But…We’re missing something!

What?

Where’s theENERGY

for the bonding!

Build daughter DNA strand add new

complementary bases DNA polymerase III

Page 13: Dna replication

AP Biology

energy

ATPGTPTTPCTP

Energy of ReplicationWhere does energy for bonding usually come from?

ADPAMPGMPTMPCMPmodified nucleotide

energy

We comewith our ownenergy!

And weleave behind anucleotide!

YourememberATP!

Are there other ways

to get energyout of it?

Are thereother energynucleotides?You bet!

Page 14: Dna replication

AP Biology

Energy of Replication The nucleotides arrive as nucleosides

DNA bases with P–P–P P-P-P = energy for bonding

DNA bases arrive with their own energy source for bonding

bonded by enzyme: DNA polymerase III

ATP GTP TTP CTP

Page 15: Dna replication

AP Biology

Adding bases can only add

nucleotides to 3 end of a growing DNA strand need a “starter”

nucleotide to bond to

strand only grows 53

DNAPolymerase III

DNAPolymerase III

DNAPolymerase III

DNAPolymerase III

energy

energy

energy

Replication energy

3

3

5B.Y.O. ENERGY!The energy rules

the process

5

Page 16: Dna replication

AP Biology

energy

35

5

5

3

need “primer” bases to add on to

energy

energy

energy

3

no energy to bond

energy

energy

energy

ligase

3 5

Page 17: Dna replication

AP Biology

Limits of DNA polymerase III can only build onto 3 end of

an existing DNA strand

Leading & Lagging strands

5

5

5

5

3

3

3

53

53 3

Leading strand

Lagging strand

Okazaki fragments

ligase

Okazaki

Leading strand continuous synthesis

Lagging strand Okazaki fragments joined by ligase

“spot welder” enzyme

DNA polymerase III

3

5

growing replication fork

Page 18: Dna replication

AP Biology

DNA polymerase III

Replication fork / Replication bubble

5

3 5

3

leading strand

lagging strand

leading strand

lagging strandleading strand

5

3

3

5

5

3

5

3

5

3 5

3

growing replication fork

growing replication fork

5

5

5

5

53

3

5

5lagging strand

5 3

Page 19: Dna replication

AP Biology

DNA polymerase III

RNA primer built by primase serves as starter sequence

for DNA polymerase III

Limits of DNA polymerase III can only build onto 3 end of

an existing DNA strand

Starting DNA synthesis: RNA primers

5

5

5

3

3

3

5

3 53 5 3

growing replication fork

primase

RNA

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AP Biology

DNA polymerase I removes sections of RNA

primer and replaces with DNA nucleotides

But DNA polymerase I still can only build onto 3 end of an existing DNA strand

Replacing RNA primers with DNA

5

5

5

5

3

3

3

3

growing replication fork

DNA polymerase I

RNA

ligase

Page 21: Dna replication

AP Biology

Loss of bases at 5 ends in every replication chromosomes get shorter with each replication limit to number of cell divisions?

DNA polymerase III

All DNA polymerases can only add to 3 end of an existing DNA strand

Chromosome erosion

5

5

5

5

3

3

3

3

growing replication fork

DNA polymerase I

RNA

Houston, we have a problem!

Page 22: Dna replication

AP Biology

Repeating, non-coding sequences at the end of chromosomes = protective cap limit to ~50 cell divisions

Telomerase enzyme extends telomeres can add DNA bases at 5 end different level of activity in different cells

high in stem cells & cancers -- Why?

telomerase

Telomeres

5

5

5

5

3

3

3

3

growing replication fork

TTAAGGGTTAAGGGTTAAGGG

Page 23: Dna replication

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

Page 24: Dna replication

AP Biology

DNA polymerases DNA polymerase III

1000 bases/second! main DNA builder

DNA polymerase I 20 bases/second editing, repair & primer removal

DNA polymerase III enzyme

Arthur Kornberg1959

Roger Kornberg2006

Page 25: Dna replication

AP Biology

Editing & proofreading DNA 1000 bases/second =

lots of typos!

DNA polymerase I proofreads & corrects

typos repairs mismatched bases removes abnormal bases

repairs damage throughout life

reduces error rate from 1 in 10,000 to 1 in 100 million bases

Page 26: Dna replication

AP Biology

Fast & accurate! It takes E. coli <1 hour to copy

5 million base pairs in its single chromosome divide to form 2 identical daughter cells

Human cell copies its 6 billion bases & divide into daughter cells in only few hours remarkably accurate only ~1 error per 100 million bases ~30 errors per cell cycle

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AP Biology

1

2

3

4

What does it really look like?

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AP Biology 2007-2008

Any Questions??

Page 29: Dna replication

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