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NOTES: 12.2 – 12.3DNA Structure & Replication
12.2: DNA Structure
● it was known that DNA was made up of
nucleotides joined into long strands by
covalent bonds…but HOW were they
connected…what was the structure?
DNA Structure
● Made up of nucleotides (monomer)
● Each nucleotide composed of 3 parts:
– 5-carbon sugar (DEOXYRIBOSE)
– Phosphate group
– Nitrogen-containing base
● Adenine (A)
● Thymine (T)
● Guanine (G)
● Cytosine (C)
Chargaff’s Rules
What do these data suggest to you?
Percentage of Bases in Four Organisms
Source of DNA A T G C
Streptococcus 29.8 31.6 20.5 18.0
Yeast 31.3 32.9 18.7 17.1
Herring 27.8 27.5 22.2 22.6
Human 30.9 29.4 19.9 19.8
● A = T (A pairs with T)
● C = G (C pairs with G)
What DNA Looks Like:
● Rosalind Franklin: used X-ray
diffraction patterns to discover that DNA
strands twisted around each other like a HELIX
● Watson & Crick: while
trying to build a 3-D
model of DNA, Watson
saw Franklin’s
photograph and
measurements and
within weeks, he and
Crick figured out the
structure of DNA
● Backbone
– deoxyribose &
phosphate group of
each nucleotide
●“Steps of Ladder”
– bases (A, G, C, T)
● Any base sequence is
possible!
DNA Structure = DOUBLE HELIX
(“twisted ladder”)
What’s holding the strands together?
● HYDROGEN BONDS!!!
● Hydrogen Bonds form between A and T base pairs as well as betweenC and G base pairs
12.3 - DNA Replication
Vocabulary:
• Chromatin
• Replication
• DNA polymerase
Key Concept:
•What happens during DNA Replication?
The Review
● Prokaryotic cells do not have a nucleus
– DNA molecules are located in the cytoplasm
– Usually a circular DNA molecule and it is referred
to as the cell’s chromosome
● Eukaryotic cells have a nucleus
– Can have 1000x more DNA than prokaryotic cells
– DNA is located in the form of a number of
chromosomes
– # of chromosomes varies widely from species to species
DNA molecules are long…how does DNA fit in the nucleus?
● It forms chromosomes!
● DNA coils around proteinscalled histones and then this chromatin supercoils around itself until chromosomes form
DNA Replication
● before a cell divides, it must copy its DNA so that all cells have a copy of the genetic instructions
● this process involves A LOT of ENZYMES!
● each half of the DNA double helix can serve as a "template" for the replication of another DNA double helix molecule
– The strands are said to be
complementary
DNA REPLICATION:
THE PROCESS
1) An enzyme unwinds
& "unzips" DNA
(separates the 2
DNA strands) by
breaking the
hydrogen bonds
between base pairs
Hydrogen Bonds Breaking!
2) a complementary base is inserted to each
side of the DNA strand with the help of DNA
POLYMERASE
Why does the
“blue” always pair
with the “green?”
3) The sugar-phosphate
groups are covalently
bonded to the
growing DNA chain (new "backbone")
1 nucleotide
COVALENT BOND
4) an enzyme (DNA
polymerase) will
"proof-read" the
order of bases &
make corrections
A SIMPLIFIED VERSION…
A Little More Complicated…