dna and replication - biowilson.weebly.com · dna replication • before new dna strands can form,...
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DNA and Replication
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DNA REPLICATION
• Remember in the S phase or Synthesis phase of Cell cycle chromosomes double or duplicate.
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Remember?
• Chromosomes are made of DNA and histone proteins.
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History of DNA discovery
• Experiments on bacteriophage viruses by Hershey & Chase proved that DNA was the cell’s genetic material
Radioactive P32 was injected into E. coli bacteria!
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DNA (P32)
was detected
in bacteria
cell not
Sulfur (S35)…
DNA was the cell’s genetic
material
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Discovery of DNA Structure
• Erwin Chargaff showed the amounts of the four bases (A,T,C,G) on DNA
• In a body or somatic cell:
Adenine (A) = 30.3%
Thymine (T) = 30.3%
Guanine (G)= 19.5%
Cytosine (C)= 19.5%
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Chargaff’s Rule • Adenine must pair with Thymine
• Guanine must pair with Cytosine
• The bases form weak hydrogen bonds
G C T A
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DNA Structure • Rosalind Franklin took diffraction x-ray photographs of DNA crystals
• In the 1950’s, Watson & Crick built the first model of DNA using Franklin’s x-rays
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DNA
• Two strands coiled called a double helix
• Sides made of a 5C sugar deoxyribose bonded to phosphate (PO4) groups
• Center made of nitrogen bases bonded together by weak hydrogen bonds
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DNA Double Helix
Nitrogenous Base (A,T,G or C)
“Rungs of ladder”
“Legs of ladder”
Phosphate & Sugar Backbone
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Helix
• Most DNA has a right-hand twist with 10 base pairs in a complete turn
• Left twisted DNA is called Z-DNA or southpaw DNA
• Hot spots occur where right and left twisted DNA meet… producing mutations
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DNA • Stands for Deoxyribonucleic acid
• Made up of subunits called nucleotides
• Nucleotide made of: 1.Phosphate group 2.5-carbon sugar
3.Nitrogenous base
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DNA Nucleotide
O=P-O O
Phosphate Group
N Nitrogenous base (A, G, C, or T)
CH2
O
C1 C4
C3 C2
5
Sugar (deoxyribose)
O
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Nitrogenous Bases
• Double ring PURINES Adenine (A) Guanine (G) • Single ring PYRIMIDINES Thymine (T) Cytosine (C) T or C
A or G
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• The pentose sugars (sides) of the double helix DNA molecule are anti-parallel to each other – Meaning the two linear strands are wound around each other and each going opposite directions.
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• One side has carbon 5 of the pentose sugar pointing up…5’
• One side has carbon 3 of the pentose sugar pointing up…3’
CH2
O
C1 C4
C3 C2
5
Sugar (deoxyribose)
CH2 O
C1 C4
C3 C2
5
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DNA Replication • Enzyme helicase unwinds and separates the 2 DNA strands by breaking the weak hydrogen bonds
• Single-Strand Binding Proteins (SSB) attach and keep the 2 DNA strands separated and untwisted
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DNA Replication (DNA copying)
• After the two strands untwist and separate, they form a Replication Fork (Y-shaped region)
• Then new strands will begin to form at the fork.
Replication Fork
Parental DNA Molecule
3’
5’
3’
5’
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DNA Replication • Before new DNA strands can
form, there must be RNA primers present to start the addition of new nucleotides
• Primase is the enzyme that synthesizes the RNA Primer…it prepares the DNA molecule to receive the new nucleotides.
• DNA polymerase can then add the new complimentary base pairs (nucleotides).
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Synthesis of the New DNA Strands
• One nucleotide at a time…The Leading Strand receives the nucleotides from the point of origin toward the opening replication fork
RNA Primer DNA Polymerase Nucleotides
3’ 5’
5’
Point of origin
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• The Lagging Strand receives nucleotides from the opposite direction of replication and is done in MANY short segments (Okazaki fragments)
• It is replicated from the replication fork toward the origin
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PROOFREADING NEW DNA • DNA polymerase initially makes about 1 in 10,000 base pairing errors
• Enzymes proofread and correct these mistakes
• The new error rate for DNA that has been proofread is 1 in 1 billion base pairing errors
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SEMI CONSERVATIVE MODEL FOR DNA REPLICATION
• The two strands of the parental molecule separate, and each acts as a template for a new complementary strand
• New DNA consists of 1 PARENTAL (original) and 1 NEW strand of DNA
Parental DNA New DNA
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DNA DAMAGE & REPAIR • Chemicals & ultraviolet radiation damage the DNA in our body cells
• Cells must continuously repair DAMAGED DNA
• Excision repair occurs when repair enzymes remove damaged parts of DNA
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DNA polymerase and DNA ligase replace and bond the new
nucleotides together