DNA Structure & Replication
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DNA Structure
& Replication
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)
DNA Structure
• Sugar –
Phosphate
Backbone held
together by strong
Phosphodiester
Bonds (covalent)
• The Nitrogen
Bases are held
together by
Hydrogen Bonds
6
DNA Nucleotides
O=P-OO
PhosphateGroup
NNitrogenous base(A, G, C, or T)
CH2
O
C1C4
C3 C2
5
Sugar(deoxyribose)
O
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7
Nitrogenous Bases• Double ring PURINES
Adenine (A)Guanine (G)
• Single ring PYRIMIDINESThymine (T)Cytosine (C)
T or C
A or G
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Table 1. Nitrogen Base Make-Up of Different Organisms’ DNA
(in Percentages)
Organism A T G C
Mycobacterium
tuberculosis
15.1 14.6 34.9 35.4
Yeast 31.3 32.9 18.7 17.1
Wheat 27.3 27.1 22.7 22.8
Sea Urchin 32.8 32.1 17.7 17.3
Marine crab 47.3 47.3 2.7 2.7
Turtle 29.7 27.9 22.0 21.3
Rat 28.6 28.4 21.4 21.5
Human 30.9 29.4 19.9 19.8
9
Base-Pairings
• Purines ONLY pair with Pyrimidines
• Three hydrogen bonds required to bond Guanine & Cytosine
CG
3 H-bonds
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10
T A
•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.
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2 H-bonds
11
DNA
P
P
P
O
O
O
1
23
4
5
5
3
3
5
P
P
PO
O
O
1
2 3
4
5
5
3
5
3
G C
T A
copyright cmassengale
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)
13
Deoxyribose Sugar• Carbons are numbered clockwise 1’ to 5’
CH2
O
C1C4
C3 C2
5
Sugar(deoxyribose)
copyright cmassengale
Hydrogen
bonds
Nucleotide
Sugar-phosphate
backbone
Key
Adenine (A)
Thymine (T)
Cytosine (C)
Guanine (G)
Chromosome
Supercoils
Coils
Nucleosome
Histones
DNA
double
helix
16
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
DNA replication PURPOSE
DNA replication is SEMI-CONSERVATIVE.
Each daughter molecule of DNA is made of ONE
ORIGINAL strand and ONE NEW strand.
DNA Replication
• During DNA replication:
– The DNA molecule separates into 2 strands.
– 2 new complementary strands are produced following the rules of base pairing.
• Each strand of the double helix of DNA serves as a template for the new strand.
How DNA Is Replicated
• DNA replication is carried out by a
series of enzymes.
• The enzymes “unzip” (separate the
strands) of a molecule of DNA.
• The unzipping occurs when the
hydrogen bonds between the base
pairs are broken.
How DNA Is Replicated
• Each strand serves as a template for
the attachment of complimentary bases.
– Example: GTAAGC produces
CATTCG (complimentary)
• Each DNA molecule resulting from
replication has one original strand and
one new strand.
Enzymes in DNA Replication
• The principle enzyme involved in DNA
replication is called DNA polymerase.
• It attaches (or polymerizes) individual
nucleotides to produce DNA.
• DNA polymerase also “proof-reads” each
new DNA strand, helping to maximize the
odds that each molecule is a perfect copy
of the original DNA.
Summary of DNA Replication
1. Enzymes break the Hydrogen bonds between the nitrogen bases and separate the original strands of DNA.
2. Another enzyme (DNA polymerase) bonds complimentary bases to each original strand.
3. DNA polymerase then proofreads the DNA for mistakes (mutations).
• This produces 2 molecules of DNA.
• EACH molecule has 1 ORIGINAL strand and 1 NEW strand of DNA!!!
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!
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