Download DNA

DNA
Deoxyribonucleic acid
History of DNA
• Early scientists thought protein
was the cell’s hereditary material
because it was more complex than
DNA
• Proteins were composed of 20
different amino acids in long
polypeptide chains
2
History of DNA
• Chromosomes are made of both
DNA and protein
• Experiments on bacteriophage
viruses by Hershey & Chase
proved that DNA was the cell’s
genetic material
History of DNA
• 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. Their model of
DNA was a double helix.
video
Video 2
DNA Structure
• Double Helix - Helix is a shape
that twists like the threads of a
screw.
• The two sides of the ladder are
made up of molecules of sugar
called deoxyribose, alternating
with molecules known as
phosphates.
video
DNA Structure
• A long molecule made up of units
called nucleotides.
• Each nucleotide is made up of
three basic parts.
– 5 carbon sugar
– Phosphate group
– Nitrogen base
video
DNA Structure
• Each rung of the DNA ladder is
made up of a pair of molecules
called nitrogen bases.
• There are four kinds of Nitrogen
Bases:
1.Adenine (A)
2.Thymine (T)
3.Guanine (G)
4.Cytosine (C)
Chargaff’s Rule
• Adenine must pair with
Thymine
• Guanine must pair with
Cytosine
• The bases form weak
hydrogen bonds
T
A
G
C
8
Chromosomes and DNA
Replication
Chromosome Structure
• Eukaryotic chromosomes contain
DNA and protein, tightly packed
together to form chromatin.
• Chromatin consists of DNA tightly
coiled around proteins called
histones.
Video
Video 2
Chromosomes & Genes
Duplicating DNA
• Before a cell divides, it duplicates
its DNA in a process called
replication.
• Replication ensures that each
resulting cell will have a complete
set of DNA.
What phase of cell division is
this happening?
video
DNA Replication
• How does it occur?
– Step 1: Enzymes “unzip” a molecule of
DNA. This happens when the hydrogen
bonds between base pairs is broken
and the two strands of the molecule
unwind.
Video
DNA Replication
• During DNA replication, the DNA
molecule separates into two
strands, then produces two new
complementary strands following
the rules of base pairing. Each
strand of the double helix of DNA
serves as a template for the new
strand.
New Strand
Original strand
Nitrogen Bases
Growth
Growth
Replication Fork
Replication Fork
DNA Polymerase
DNA Replication
How Replication Occurs
DNA replication is carried out by enzymes that
“unzip” a molecule of DNA.
Hydrogen bonds between base pairs are broken
and the two strands of DNA unwind.
Copyright Pearson Prentice Hall
DNA Replication
•The principal enzyme involved in
DNA replication is DNA
polymerase.
•DNA polymerase joins individual
nucleotides to produce a DNA
molecule and then “proofreads”
each new DNA strand.