15 lecture 10 (dna replication)update

8/7/2019 15 Lecture 10 (DNA replication)update

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Lecture 10 DNA Replication

New art: Molecular enetics


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Hereditary material must fulfill threerequ remen s

precisely, so that copies can be transmittedfrom arents to offs rin .

Information : It must contain information touide the develo ment and function of the

organism. Mutation : It must be able to change, on rare

occasions, to explain the existence of variantalleles and the evolution of new forms.


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Deoxyribonucleic acid (DNA) fulfillsall three requirements

ep ca on : rec se sem conserva vereplication of the DNA double helix.

polypeptides by the nucleotide sequence of

Mutation : Rare changes in the nucleotidese uence of DNA that chan e either thepolypeptide it encodes or the time and placethat the encoded protein is produced.


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DNA is a polymer : a molecule

subunits of DNA are nucleotides

Each nucleotide has three parts

Phosphate Nitrogenousbase

Deoxyribose sugar


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DNA is made of four different nucleotides

Two have purine basesTwo have pyrimidine bases

Guanine (G)Cytosine ( C)


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Nucleotides link together to form a chain

a sugar-phosphatebackbone with

5

nitrogenous basessticking out the side.

e c a n s rec ona :runs from a phosphategroup at the 5 end to ahydroxyl group at the 3 end.

3


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Each DNA molecule consists of two strands

that are com lementar and anti arallel

35

53


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The DNA strands are held together by hydrogen bondsthat link base pairs in opposite strands.

Hydrogen bondsas e nes n

negatively chargedmolecules O, Nin one base topositively charged

base on theopposite strand.


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Hydrogen bonds join bases in complementary pairs

A thymine in one strand is always boundto an adenine in the other strand.

A cytosine in one strand is always bound toa guanine in the other strand.


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1) When the base composition of all the DNA

was determined, 18% of the bases were found to. ,

percentage of the bases are cytosine?

95%

.B. 32%C. 36%

A. B. C. D. E.

1% 1%1%3%

.

E. 72%


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Rules of DNA base composition

The total amount of purines in a DNAdouble helix must exactl e ual the totalamount of pyrimidines.

equal the total amount of thymine (T).

equal the total amount of guanine (G).


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The double-stranded DNA molecule is in the

Major groove :t s s t e s teof mostinteractionsbetween DNA

and proteins

Each turn ofthe helix is 3.4

Minor groovelong andcontains 10


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DNA is ver stable Very strong covalent bonds connect the

nuc eo es a ong eac s ran . Large numbers of hydrogen bonds join

e wo s ran s.

The base pairs are hydrophobic, andere ore poor y so u e n wa er. edouble helix structure keeps them away

interior. This makes the helix more


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The mechanism of DNA replication


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The mechanism of DNA replication is

helix serves as a template to make a new strand

Identical daughter double helices

Parent double helix


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In E. coli , replication starts at a unique

oriC .single circular chromosome

e ou e nerepresents the twocomplementarystrands of DNA


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Viewed in detail, oriC consists of five identical 13-

base air re ions called DnaA boxesDnaA boxesAT-rich DNA

DnaA roteins bind

to the boxes andnearby DNA,-

region to unwind.

Helicase enzymesthen bind to theopen replicationbubble and

the DNA at eachend.


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Wh is the ori in of re lication AT-rich?Adenine and thymine Cytosine and guanine

hydrogen bonds.

hydrogen bonds.

Therefore, AT-rich regions are bound less strongly thanCG-rich regions. This makes them easier to unwind.


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Once the replication bubble is opened,

re lication roceeds in two directionsOrigin ofreplication

Two replication forks travel in opposite directions

from the origin. New DNA is synthesized at each fork.


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2 One strand of a DNA double helix is 5-TCGGTCA-3. What is the sequence of

95%

- - B. 3-TGACCGA-5

. - -D. 3-AGCCAGT-5

A. B. C. D. E.

1% 3% 1%0%E. 3-TCGGTCA-5


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At each replication fork, theDNA is unwound. Each

strand serves as a tem late

to make a complementary


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DNA polymerase III adds nucleotides

DNAprimer strand

DNAprimer strand


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key limitations

It can only add nucleotides to the 3en o a grow ng c a n.

It can onl extend a chain thatalready exists. It cannot start one

.


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At each replication fork, there are two growing DNAstrands: the leading strand and the lagging strand

DNA polymerase can only .


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Overview of DNA replication

The lagging strand must be synthesized

that are then joined together.

e ea ng stran s ma e y cont nuous y

extending its 3 end.


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The first step is synthesis of a short RNA.

This is necessary, because DNA polymerase can only extend a chain, not start one from scratch.


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Next, DNA polymerase III extends the primer, 5 to, .

This creates a short (1000-2000 base pair) DNA chaincalled an Okazaki fragment .


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Next, DNA polymerase I removes the.

At the same time, it extends the Okazaki.


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Finall , Li ase connects the Okazaki

fragment to the adjacent fragment.


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. . .


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Thi r win h w r li i n l in E. coli . What does the arrow point to?

A. The 5 end of an Okazaki fragmentB. The 3 end of an Okazaki fragmentC. The 5 end of the leading strandD. The 3end of the leading strand

74%

. e en o a emp a e s ran

A. B. C. D. E.

24%

1%1%1%


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4 Thi r win h w r li i n l in E. coli . What does the arrow point to?

A. The 5 end of an Okazaki fragmentB. The 3 end of an Okazaki fragmentC. The 5 end of the leading strandD. The 3end of the leading strand

82%

. e en o a emp a e s ran

A. B. C. D. E.

2%0%

3%

14%


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Replication is carried out by a complexmolecular machine called the replisome

Helicase yrase Primase po ymerase o oenzyme DNA polymerase I

Ligase Single-strand binding proteins


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Helicase catalyzes the unwindingof the DNA double helix

A DNA h li i d h d bl h li i


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As DNA helicase unwinds the double helix, itintroduces supercoils elsewhere in the DNA

molecule

Supercoils are loops in the DNA introduced when

it is unwound or

overwound.

G ( ki d f i ) l h


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Gyrase (a kind of topoisomerase) relaxes thesesupercoiled regions by cutting the DNA (allowing

it to rotate) and then rejoining the strands


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Primase makes the RNA primer necessary to start

A primer is about .

Each primer is

the DNA template strand.


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The DNA polymerase III holoenzyme

,leading strand and one for the lagging strand

The holoenzyme alsocontains manyaccessory prote ns,including

A donut-sha ed betaclamp that fastens thecatalytic core to the

A bridge that ties thetwo cores together.

Primase and DNA polymerase III start


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Primase and DNA polymerase III startand extend each Okazaki fragment

Previous Okazakifra ment

Tem late strandPrimase builds a new RNA primer

po ymerase ex en s a newOkazaki fragment from the primer

DNA polymerase I removes the RNA


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DNA polymerase I removes the RNArimer and fills in the a with DNA

Li j i h Ok ki f i h


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Ligase joins the Okazaki fragments with

Single strand binding (SSB) proteins stabilize


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Single-strand binding (SSB) proteins stabilize

the ex osed sin le-stranded DNASingle stranded

hydrogen bond to itself and form hairpin structures

SSB proteins

exposed bases.

DNA pol III removes the SSB proteins as it builds the DNA strand.

Overview of the


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Overview of the(Gyrase)

.

15 lecture 10 (dna replication)update

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