Download DNA history and structure KS

DNA
History, Structure
and Function
What is DNA?
• Deoxyribonucleic Acid
• It is a long chain molecule that
carries the code for the
characteristics of living things.
• Sections of this chain are called
genes.
DNA Code
• The code is made of molecules
called nitrogen bases.
– Adenine (A)
– Thymine (T)
– Guanine (G)
– Cytosine (C)
• The code is represented in strings
of letters that stand for the nitrogen
bases
Ex: ATTAAGGCATA
TAATTCCGTAT
HISTORY
Oswald Avery
1944
• Discovered that DNA
stores and passes on
genetic information
from one generation
to the next
Before 1950
Chargaff’s Rules
In any DNA, the number of A’s was the same as the number of
T’s, and the number of C’s was the same as the number of G’s
In other words:
A must pair with T and
G must pair with C
A=T
G=C
X-ray Crystallography
Rosalind Franklin • Used concentrated x-ray
beams to take pictures of
early 1950’s
DNA molecules
• Could only get partial
pictures
• She didn’t realize what the
picture meant for the
structure of DNA.
A chemist with extensive
knowledge on X-ray
crystallography
James Watson
• An American chemist from Chicago
• Entered the University of Chicago at 15
• He met Francis Crick at Cambridge
University.
Francis Crick
• Born in England
• A Physicist that
studied proteins using
x-ray diffraction
• Knowing the form
(structure) gives clues
to the function of
molecules.
Watson and Crick
Had been trying to solve the
puzzle of the structure of DNA
1953: Watson viewed a
copy of Franklin’s
x-rays.
One week later: they solved
the puzzle together
DNA Vocabulary
GENOME
• The nucleus of a human cell
contains 20,000 to 30,000 genes
in the form of DNA called the
GENOME.
CHROMOSOMES
• CHROMOSOMES are coiled DNA and
proteins.
• Humans have 23 pairs of
chromosomes in the NUCLEUS of any
one of your cells
GENE
• Segment of DNA that codes for
an organism’s trait.
Structure of DNA
• shape of a double helix
– a twisted ladder.
• made up of 4 nitrogen
bases that bond together,
with hydrogen bonds.
– the "rungs" of the ladder.
• Nitrogen Bases are
attached to a phosphatesugar backbone
– the “rails” of the ladder.
More on Structure
• Right handed double helix
• Made of units called nucleotides:
Thymine
– One 5 carbon sugar ring (deoxyribose)
– One phosphate group
– One nitrogen base
One Nucleotide
P
S
Phosphate
Sugar
Nitrogen Base
Nitrogen Bases
• Purines (longer): adenine and guanine
• Pyrimidines (short): thymine and cytosine
• These pair one short with one long
– Complimentary bases
Thymine
Cytosine
Adenine
Guanine
Purines and Pyrimidines
• A and T are bonded by
2 hydrogen bonds.
• C and G are bonded by
3 hydrogen bonds.
• Pyrimidines (T and C) have a single ring
structure.
• Purines (A and G) have a double ring
structure.
Sides
• Alternating sugar and phosphate
molecules
• Nitrogen bases
are connected
to the sugar
molecules
One nucleotide
What is a DNA Sequence?
• The particular order of the
nitrogen bases is called the
DNA SEQUENCE. The first
letter of the base is used:
• Ex. A T G C C G A T C
DNA Replication
• Replicate = to copy
• Enzyme = a
chemical
that makes a
reaction
go faster
• DNA replicates
with the help of
an enzymes
One nucleotide
Enzymes in DNA replication
• HELICASE unwinds parental
double helix
• DNA POLYMERASE binds
new nucleotides to form
new strands
• LIGASE joins Okazaki fragments and seals
other breaks in sugar-phosphate backbone
• http://www.youtube.com/watch?v=zdDkiRw1PdU
Replication
Step 1:
HELICASE unzips DNA down the middle by
breaking Hydrogen bonds.
Step 2:
• DNA POLYMERASE
moves along the 2 open
parent strands (in 5’ to 3’
direction) and adds NEW
complimentary
nucleotides to each
original parent strand.
• New DNA is proofread
as it is made.
Step 3:
Replication moves in opposite directions on
each side -- only in the 5’ to 3’ direction. So,
small DNA segments called OKAZAKI
FRAGMENTS are made on the other strand.
Step 4:
• DNA LIGASE seals up all of the gaps in
the DNA molecule, like the Okazaki
fragments.
Semi-Conservative Model
• Each open side of the parental DNA acts
as a template for a new complementary
strand
• New DNA consists of 1 PARENTAL
(original) and 1 NEW strand of DNA