1. DNA Structure & Replication

3. DNA Structure Chromosomes are made of DNA.
The DNA is wound around proteins called Histones.
When chromosomes are unraveled, the DNA strands are called Chromatin.
DNA is made of Nucleotides.
Deoxyribose (5 carbon sugar)
Phosphate group
Nitrogen Base (A, T, G, C)

4. DNA Structure
Sugar – Phosphate Backbone held together by strong Phosphodiester Bonds (covalent)
The Nitrogen Bases are held together by Hydrogen Bonds

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DNA Nucleotides
O=P-O O
Phosphate
Group N
Nitrogenous base
(A, G, C, or T)
CH2 C1 C4 C3 C2 5
Sugar
(deoxyribose)
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Nitrogenous Bases
Double ring PURINES
Adenine (A)
Guanine (G)
Single ring PYRIMIDINES
Thymine (T)
Cytosine (C)
A or G
T or C
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Base-Pairings
Purines ONLY pair with Pyrimidines
Three hydrogen bonds required to bond Guanine & Cytosine C G
3 H-bonds
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Two hydrogen bonds are required to bond Adenine & Thymine The hydrogen bonds between A=T and G=C create the twist in the double helix. Each twist of DNA is about 10 bases long. T A
2 H-bonds
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DNA P O 1 2 3 4 5 P O 1 2 3 4 5 G C T A
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11. DNA
One strand begins with the 5’ end of the sugar (5’ to 3’ = Lagging Strand)
The other strand begins with the 3’ end of the sugar (3’ to 5’ = Leading Strand)
Strands are ANTIPARALLEL (opposite)

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Deoxyribose Sugar
Carbons are numbered clockwise 1’ to 5’
CH2 O C1 C4 C3 C2 5
Sugar
(deoxyribose)
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13. Sugar-phosphate backbone
Hydrogen bonds
Nucleotide
Sugar-phosphate backbone
Key
Adenine (A)
Thymine (T)
Cytosine (C)
Guanine (G)

14. Chromosome
Supercoils
Coils
Nucleosome
Histones
DNA
double
helix

15. DNA replication PURPOSE
Every cell in your body has the SAME DNA
DNA has to be copied before a cell divides
DNA is copied during interphase
New cells will need identical DNA strands

16. DNA replication PURPOSE
DNA replication is SEMI-CONSERVATIVE. Each daughter molecule of DNA is made of ONE ORIGINAL strand and ONE NEW strand.

18. Replication Bubbles
Replication starts in MANY places at the same time in Eukaryotes. Replication Bubbles form.
In Prokaryotes, only ONE Replication Bubble forms.
The beginning of the Replication Bubble is called the Point of Origin. Replication begins at the Point of Origin.

19. Replication Bubbles
The Replication Bubble opens in BOTH directions (Replication Forks). Replication moves towards the Replication Forks.
As the strands separate the Replication Bubbles grow. Eventually they will connect. This speeds up the process.

20. Step 1: Strand Separation
Helicase unwinds and separates the 2 DNA strands.
Single-Strand Binding proteins attach to the DNA strands to keep the strands from twisting.
Topoisomerase (another protein) attaches to the strands to relieve stress and keep the strands from breaking during replication.

21. Step 1: Strand Separation

22. Step 2: Adding RNA Primer
DNA nucleotides cannot bond to “empty” space, so an RNA Primer is used.
Primase is the enzyme responsible for adding the RNA Primer.

23. Step 2: Adding RNA Primer
Primase attaches a short sequence of complementary RNA nucleotides to the original strand.
Primase adds one RNA primer at the Point of Origin on the Leading Strand (3’ – 5’).
Primase has to add multiple RNA Primers on the Lagging Strand (5’ – 3’).

24. Step 3: Forming New DNA Strand
DNA polymerase III is the main enzyme responsible for replication.
DNA polymerase III adds DNA nucleotides to the RNA Primer.

25. Step 3: Forming New DNA Strand
The DNA nucleotides are complementary to the original DNA strand G = C & A = T
DNA polymerase III can only move 3’ to 5’ on the original strand.
This means that the NEW strand is built in the opposite direction (5’ to 3’).

26. Step 4: Joining Fragments
The Leading Strand (3’ – 5’) is built continuously. DNA polymerase makes one long strand of complementary DNA.

27. Step 4: Joining Fragments
Because DNA polymerase can only move 3’ to 5’, the Lagging Strand is built DISCONTINUOUSLY.
Many RNA Primers are laid and DNA polymerase III works from primer to primer.
Forms short segments called Okazaki Fragments

29. Step 4: Joining Fragments
DNA polymerase I removes the RNA primer and replaces it with DNA nucleotides
The enzyme DNA Ligase seals the gap between these fragments by adding a phosphodiester bond.

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Step 5: Proofreading
DNA polymerase initially makes about 1 in 10,000 base pairing errors
DNA polymerase II proofreads and corrects these mistakes
The new error rate for DNA that has been proofread is 1 in 1 billion base pairing errors
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33. DNA replication BIG IDEA
Base Pairing ensures that when
DNA is replicated, you get
2 identical copies of DNA
each with ONE NEW STRAND and
ONE ORIGINAL STRAND!