DNA and DNA Replication

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DNAandDNA Replication
Used and Modified with permission from Robert
Goodman, Biology Department,Citrus Community
College
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DNA see link for more on discovery

• Discovery of the DNA double helix
• A. 1950s
• B. Rosalind Franklin - X-ray photo of DNA.

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DNA

• Discovery of the DNA double helix
• A. 1950s
• B. Rosalind Franklin - X-ray photo of DNA.
• C. Watson and Crick - described the DNA
  molecule from Franklins X-ray.

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Question

• What is DNA?

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Deoxyribonucleic Acid (DNA)

• Made up of nucleotides (DNA molecule) in a DNA
  double helix.
• Nucleotide
• 1. Phosphate group

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DNA Nucleotide

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Deoxyribonucleic Acid (DNA)

• Made up of nucleotides (DNA molecule) in a DNA
  double helix.
• Nucleotide
• 1. Phosphate group
• 2. 5-carbon sugar

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DNA Nucleotide

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Deoxyribonucleic Acid (DNA)

• Made up of nucleotides (DNA molecule) in a DNA
  double helix.
• Nucleotide
• 1. Phosphate group
• 2. 5-carbon sugar
• 3. Nitrogenous base
• 2 nm wide

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DNA Nucleotide

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Nitrogenous Bases

• PURINES
• 1. Adenine (A)
• 2. Guanine (G)

• PYRIMIDINES
• 3. Thymine (T)
• 4. Cytosine (C)

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BASE-PAIRINGS

• Base of
• Purines Pyrimidines Pairs H-Bonds
• Adenine (A) Thymine (T) A T 2

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BASE-PAIRINGS

• Base of
• Purines Pyrimidines Pairs H-Bonds
• Adenine (A) Thymine (T) A T 2
• Guanine (G) Cytosine (C) C G 3

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Chargaffs Rule

• Adenine must pair with Thymine
• Guanine must pair with Cytosine
• Their amounts in a given DNA molecule will be
  about the same.

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• Question
• If there is 30 Adenine, how much Cytosine is
  present?
• Answer
• Adenine (30) Thymine (30)
• Guanine (20) Cytosine (20) (50)
  (50)
• There would be 20 Cytosine.

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Antiparallel Double Helix

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DNA Double Helix

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Information
Information is contained in the linear sequence
of the bases in the polynucleotide chain. DNA
from all organisms has the same structure, but
different sequence different information
AT or GC are different for different species.
(Chargaffs rules)
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View DNA Movie
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Question

• When and where does DNA Replication take place?

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Synthesis Phase (S phase)

• S phase in interphase of the cell cycle.
• Nucleus of eukaryotes

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DNA Replication Basic Concept
Each daughter molecule has one parental strand
and one new strand.
After separation, each parental strand becomes a
template
Complementary pairing
Replication is semiconservative
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Synthesis of new DNA

• New DNA is synthesized by adding one nucleotide
  at a time
• The nucleotide comes in as a triphosphate
• Depending on which one is needed, it could be
  dATP, dTTP, dGTP or dCTP

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Bases are added using DNA polymerase to the 3
end of the growing strand
DNA polymerase catalyzes this reaction
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DNA Synthesis in both Strands

• Enzyme only works from 5 ? 3
• Antiparallel
• (one goes 5? 3, the other goes 3 ? 5).
• New synthesis has to go in opposite directions.

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3
3
5
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Origins of Replication

• Replication Forks
• Hundreds of Y-shaped regions of replicating DNA
  molecules where new strands are growing.

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Copying by Two Mechanisms
Continuous Discontinuous
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DNA replication starts from a specific origin
Bubbles form!
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Origin of Replication

• A particular sequence of nucleotides on the DNA
  that serves as the signal for the starting point
  of DNA replication.

• The number varies depending on the size of the
  DNA.

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We can see these bubbles!
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Okazaki Fragments
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DNA Replication

• Semiconservative Model
• 1. Watson and Crick showed the two strands of
  the parental molecule separate, and each
  functions as a template for synthesis of a new
  complementary strand.

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DNA Repair

• Excision repair
• 1. Damaged segment is excised by a repair
  enzyme (there are over 50 repair enzymes).
• 2. DNA polymerase and DNA ligase replace and
  bond the new nucleotides together.

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Remember!!!!

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Remember!!!!!

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Question

• What would be the complementary DNA strand for
  the following DNA sequence?
• DNA 5-GCGTATG-3