DNA Structure and Function
Transcript of DNA Structure and Function
Rosalind Franklin:
A Female Scientist Ahead of her
Time
• When we talk about Watson and Crick, Rosalind Franklin provided crucial X-ray diffraction photos that allowed Watson and Crick to develop the DNA model.
Rosalind Franklin’s Work
• Was an expert in x-ray crystallography
• Used this technique to examine DNA
fibers
• Concluded that DNA was some sort of
helix
Mystery of the
Hereditary Material
• Originally believed to be an unknown
class of proteins
• Thinking was
– Heritable traits are diverse
– Molecules encoding traits must be diverse
– Proteins are made of 20 amino acids and
are structurally diverse
Structure of the
Hereditary Material
• Experiments in the 1950s showed that DNA is the hereditary material
• Scientists raced to determine the structure of DNA
• 1953 - Watson and Crick proposed that DNA is a double helix
Structure of Nucleotides
in DNA
• Each nucleotide consists of
– Deoxyribose sugar (5-carbon sugar)
– Phosphate group
– A nitrogen-containing base
• Four bases
– Adenine, Guanine, Thymine, Cytosine
Watson-Crick Model
• DNA consists of two nucleotide strands
• Strands run in opposite directions
• Strands are held together by hydrogen bonds
between bases
• A binds with T and C with G
• Molecule is a double helix
Nucleotide Bases
sugar
(deoxyribose)
adenine
A
base w ith a
double-ring
structure
guanine
(G )
base w ith a
double-ring
structure
cytosine
(C )
base w ith a
s ingle-ring
structure
thym ine
(T )
base w ith a
s ingle-ring
structure
The DNA Code
• The information in DNA is encoded in the
sequence of bases
• A-T (double hydrogen bonded)
• G-C (triple hydrogen bonded)
A (adenine) T (thymine)
C (cytosine) G (guanine)
• The DNA sequence in a gene specifies which
amino acids go into making a particular
protein, and in what order
Base Pairing
• The bases on opposite strands pair up with
each other in very specific ways
• A large, two-ring base must always pair with a
smaller, one-ring base
• For this reason alone, A could only pair with
either T
• Hydrogen-bond compatability restricts pairing
to A with T, and C with G, always
DNA Strands are Complementary
• If one strand has an “A” at one spot, the other
strand must have a “T” at the same spot
• Likewise, “C” is matched with “G”
• This means that if we have one complete
strand, we can predict the sequence of the
other strand
Semiconservative DNA Copying
• When DNA is
copied, each old
strand functions as
a template to
construct a new,
complementary
strand
• Each resulting
DNA double-helix
is half old and half
new material
Strands Run in Opposite Directions
Direction of a strand is defined by orientation of the
5’ and 3’ carbon atoms in the sugars in its backbone
Strands Run in Opposite Directions
• The two strands of DNA run past each other
in opposite directions
• DNA polymerase always assembles strand
in a particular direction, from the 5’ carbon
toward the 3’ carbon on its nucleotides
Enzymes in Replication
• Enzymes unwind the two strands
• DNA polymerase attaches
complementary nucleotides
• DNA ligase fills in gaps
• Enzymes wind two strands together
http://www.steve.gb.com/images/science/okazaki_fragments.png
Okazaki Fragments
• The leading strand is
continuously assembled
• The lagging strand must
be assembled backward
(relative to movement of
the replication fork)
• DNA Polymerases move
backwards along
lagging strand, making
many short segments,
later attached together
Genes and DNA
• DNA gives directions for making proteins
• Differences in the DNA sequence are the basis
of different alleles for genes
• Sequences may be longer or shorter between
different alleles, or may otherwise differ
• DNA can be chemically chopped into fragments,
that are then separated by size
• Different alleles may be cut into different size
fragments, which are then sorted by size
Electrophoresis Separates DNA Fragments
• DNA found at a crime
scene, as well as DNA
from seven suspects,
is cut into fragments
with special enzymes
• Electrophoresis
separates DNA
fragments by size,
producing unique
pattern
• Who did it?
Composition of DNA
• Chargaff showed:
– Amount of adenine relative to guanine
differs among species
– Amount of adenine always equals amount
of thymine and amount of guanine always
equals amount of cytosine
A=T and G=C
DNA Structure Helps
Explain How It Duplicates
• DNA is two nucleotide strands held
together by hydrogen bonds
• Hydrogen bonds between two strands
are easily broken
• Each single strand then serves as
template for new strand
DNA
Replication
newnew old old
• Each parent
strand remains
intact
• Every DNA
molecule is half
“old” and half
“new”
DNA Repair
• Mistakes can occur during replication
• DNA polymerase can read correct
sequence from complementary strand
and, together with DNA ligase, can
repair mistakes in incorrect strand
What about Cloning?
Moral Dilemma or Technological
Revolution
• Making a genetically identical copy of
an individual
• Researchers have been creating clones
for decades
• These clones were created by embryo
splitting
• Showed that differentiated cells could
be used to create clones
• Sheep udder cell was combined with
enucleated egg cell
• Dolly is genetically identical to the
sheep that donated the udder cell
Dolly:
Cloned from an Adult Cell