Download DNA Replication - Aurora City Schools

Discovery of DNA
• 1850’s Gregor Mendel discovered
hereditary “factors”, but did not know
that DNA was what genes were made
of.
• 1928…Frederick Griffith showed
genes were responsible for heredity
• 1944…Oswald Avery showed DNA was
responsible for heredity.
• 1952…Alfred Hershey and Martha
Chase backed up Avery’s
experiments
Discovery of DNA
• 1949 Edwin Chargaff noticed that
A&T and G&C appear almost
equally in DNA, no matter what the
organism!
• Early 1950’s…Rosalind Franklin took
first Xray of DNA molecule
• 1953 James Watson and
Francis Crick announced
double helix shape
(based on RF’s Xray).
DNA Structure
• Deoxyribonucleic Acid
• Makes up chromosomes
• Chromosomes have small areas called
genes
• Genes are a section of DNA that is a
code for a protein that does some
function in your body (more about
this in next unit)
DNA Structure
DNA Structure
• Double helix like a twisted ladder made of
subunits called nucleotide. Has three
parts:
– deoxyribose (sugar)
– phosphate group
– nitrogen base
• 4 different bases:
adenine
guanine
cytosine
thymine
• Bases match each other (complementary), A
matches with T, G matches with C
• Order of bases called the base sequence
• (DNA overview)
DNA Replication
• DNA Copies itself during the S phase of the
cell cycle, so daughter cells have complete
copy of all genes.
• DNA Helicase (an enzyme) “unzips” the DNA
molecule by breaking the hydrogen (weak)
bonds between nitrogen bases.
• Area where “unzips” is called replication fork.
• DNA polymerase (another enzyme) matches
base pairs with their complement (A with
T, G with C…Chagraff’s Rule) on both
strands at once. The base pairs are
already in the nucleus.
DNA Replication
DNA Replication
• Because one strand is upside down
(antiparallel), one strand is copied as
one whole piece (leading strand) and
one is copied in chunks (Okazaki
fragments) and pieced together
(lagging strand)
• DNA polymerase continues until whole
strand is copied. Since two new
strands have 1 original and 1 new,
it’s called semi-conservative
replication.
• (DNA Replication)
Prokaryotic DNA
• In Prokaryotes (bacteria),
DNA is a ring and
replication starts at one
point and goes around
both ways until it’s done.
• Thus there will be only two
replication forks.
• In eukaryotes, replication will start in many
places, so there are multiple replication
forks.
• (Replication Forks)
Mistakes
• Mistakes in copying DNA can be (and
often are) made, but are usually
caught by DNA polymerase and
fixed.
• If not fixed, called mutation, and the
base sequence changes.
• Sometimes this is good and leads to a
new adaptation (evolution)
• Sometimes its bad (cancer, other genetic
disorders)
Mistakes
• (Mutation)
• There are also mutations to chromosomes, but
we’ll talk about all of this in the next unit.