DNA: History, Structure and Replication
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Transcript of DNA: History, Structure and Replication
DNA: History, Structure and Replication
What does DNA stand for?
Deoxyribonucleic Acid
ALL LIVING THINGS ON EARTH, SHARE THE GENETIC CODE FOUND IN DNA
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DNA History
How did scientists know that DNA carries the information for life?
• Many thought that proteins instead of DNA were inherited from parent to offspring.
• Scientists (Griffith, Hershey, and Chase) showed that DNA actually carries the directions on how to make proteins.
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Who discovered that DNA is the blueprint for life?
• In 1952 Alfred Hershey and Martha Chase performed an experiment using radioactively labeled viruses that infect bacteria.
• Because viruses are protein and DNA only, they figured out that viral DNA (not viral protein) could force the bacteria to make new viruses.
• This was evidence that DNA can determine cell activity.
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Who discovered the double helix structure?
• In 1953, Watson and Crick proposed that DNA is made of two chains of nucleotides held together by nitrogenous bases and twisted together.
• They called it a double helix.
• They used Rosalind Franklin’s X-ray crystallography work to figure this out.
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DNA Structure
DNA is a Polymer
• A polymer is a chemical structure made of repeating units called NUCLEOTIDE
• DNA nucleotides always have:– phosphate group – deoxyribose sugar (5-Carbon Sugar)– nitrogenous base.
DNA nucleotide
Four DNA Nitrogenous Bases
• A nitrogenous bases in DNA are referred as the GENETIC CODE.
• ALL traits such as hitch hiker thumb’s, widow’s peak, attached earlobes, etc are part of the genetic code.
Adenine (A)
Guanine (G)
Thymine (T)Cytosine (C)
• In DNA, there are four possible nitrogenous bases:– Adenine (A) – Guanine (G)– Cytosine (C)– Thymine (T)
Double Helix Structure
DNA Nucleotide – Draw This!
OO=P-O O
Phosphate Group
N
Nitrogenous base (A, G, C, or T)
CH2
O
C1C4
C3 C2
5
Sugar(deoxyribose)
DNA Double Helix – Draw This!
NitrogenousBase (A,T,G or C)
“Rungs of ladder”
“Legs of ladder”
Phosphate &Sugar Backbone
DNA Double Helix-DRAW THIS
P
P
P
O
O
P
P
PO
O
O
1
2 3
4
5
5
3
5
3
G C
T A
Hydrogen BondCovalent Bond
Nucleotide
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Chargaff’s Rule
• Adenine must pair with Thymine
• Guanine must pair with Cytosine
• Their amounts in a given DNA molecule will be about the same.
G CT A
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BASE-PAIRINGS
CG
H-bonds
T A
Complementary DNA pairing
C-A-C-T-G-G-T
G-T-G-A-C-C-A
Now you…
A-C-A-G-T-T-G
Before Replication Begins…
• Chromosomes must be “unwound” back to chromatin through the removing of histone proteins.
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Enzymes for DNA Replication
• Helicase (“Unzipper”)• DNA polymerase• Ligase (“Glue”)
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Helicase (“Unzipper”)
• Breaks hydrogen bonds between nitrogen bases of nucleotides
• Opens double helix starting at origin of replication
Helicase
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DNA polymerase
5’ – A A A T T C G T
3’ – T T T A A G C A
AT
CG A T T A C A - 3’
T A A T G T - 5’
A
TT
A C
GG
C T A A T G T - 5’
Leading strand
Lagging strand
• Enzyme that makes the new DNA strand.
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DNA replication in 5’ to 3’ direction
Growth
Growth
Replication fork
DNA polymerase
New strand
Original strand DNA
polymerase
Replication fork
Original strand
New strand
DNA replication in 5’ to 3’ direction
Leading vs. Lagging strands
• Leading strand: made continuously• Lagging strand: made in fragments (called
Okazaki fragments)
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Ligase (“Glue”)
• Reforms HYDROGEN bonds between parts of the nucleotides and between 2 nucleotides.
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DNA Synthesis Overview
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IMAGE IN YOUR NOTEBOOK IN THE NEXT RIGHT BLANK PAGE AND TITLE IT:“CELL CYCLE”