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1
DNA and Replication
2
Rosalind Franklin
3
Chargaff’s Rule• Adenine must pair with
Thymine• Guanine must pair with
Cytosine• The bases form weak
hydrogen bonds
G CT A
4
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
5
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)
6
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.
11
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
14
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?
18
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’
20
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”.
25
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.