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DNA and Replication

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Rosalind Franklin

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Chargaff’s Rule• Adenine must pair with

Thymine• Guanine must pair with

Cytosine• The bases form weak

hydrogen bonds

G CT A

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DNA• Two strands coiled

called a double helix• Sides made of a

pentose sugar Deoxyribose bonded to phosphate (PO4) groups

• Center made of nitrogen bases bonded together by weak hydrogen bonds

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Three Parts of the DNA Nucleotide

OO=P-O O

Phosphate Group

NNitrogenous base (A, G, C, or T)

CH2

O

CC

C C

Sugar(deoxyribose)

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DNA

P

P

P

O

O

O

P

P

PO

O

O

G C

T A

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Question

• If there is 30% Adenine, how much Cytosine is present?

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Answer

• There would be 20% Cytosine

• Adenine (30%) = Thymine (30%)

• Guanine (20%) = Cytosine (20%)

• Therefore, 60% A-T and 40% C-G

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DNA Replicati

on

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Replication Facts

• DNA is copied in a cell before a cell divides by mitosis, meiosis, or binary fission.

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DNA Replication

• Begins at Origins of Replication• Two strands open forming

Replication Forks (Y-shaped region)

• New strands grow at the forks

ReplicationFork

Parental DNA Molecule

3’

5’

3’

5’

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DNA as a Zipper

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DNA Replication• Enzyme Helicase

unwinds and separates the 2 DNA strands by breaking the weak hydrogen bonds

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Question

• How does the structure of DNA make it easy to copy?

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DNA Replication• DNA polymerase (enzyme)

adds the new nucleotides• Covalent bonds form between

deoxyribose sugar and phosphate group.

• Hydrogen bonds between nitrogenous bases.

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Zipper Movement

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Question

• Are the nucleotides of the new sequences of the new strand identical to the original?

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Synthesis of the New DNA Strands

• The Leading Strand is synthesized as a single strand from the point of origin toward the opening replication fork

RNAPrimerDNA PolymeraseNucleotides

3’5’

5’

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Synthesis of the New DNA Strands

• The Lagging Strand is made in MANY short segments It is replicated from the replication fork toward the origin

• Leaves gaps in new strand. Gaps filled in by DNA ligase

RNA Primer

Leading Strand

DNA Polymerase

5’

5’

3’

3’

Lagging Strand

5’

5’

3’

3’

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Replication of Strands

Replication Fork

Point of Origin

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Semiconservative Model of Replication

• New DNA consists of 1 PARENTAL (template) and 1 NEW (complementary) strand of DNA

Parental DNA

DNA Template

New DNA

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Question

• Why is the new strand complementary to the original strand?

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Question

• After a cell’s DNA is replicated, the cell may divide in two. Each new cell receives one copy of the original cell’s DNA. How are the new strands and the original strands divided between the two new cells?

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• Explain the statement, “DNA replication depends on specific base pairs”.

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Activity:• Is photography a document

similar to DNA replication? Think of the original materials, the copying process, and the final products. Explain how the two processes are alike. Identify major differences.