Download DNA Structure and Replication Notes

The Story of Our DNA.
Chapter 11, Section 1 Notes
Important Discoveries: DNA’s
Function
• Frederick Griffith
(1928) – Transformation
• -Turned non-lethal
bacteria into lethal
• -Hypothesized that a
gene was responsible
(He did NOT know what
biomolecule genes were
made of)
Avery (1944)
• DNA is the molecule
responsible for heredity
• a. Treated an extract
from the heat-killed
bacteria with enzymes
to destroy everything
except the nucleic acid.
• Resulted in an identical
outcome to Griffith’s
experiment
Hershey-Chase (1952)
-Studied
bacteriophages
-Determined that the
DNA not the
protein coat was
the source of
genetic
information
Now that scientists agree that
DNA carries the information…
How is DNA organized?
Chargraff (1950)
Determined that nucleotides (the building
blocks of DNA) occur in specific ratios
- Guanine = cytosine
- Adenine = thymine
Did not know why this was true, only that
every organism followed this rule
Franklin (1950’s)
- Used x-ray diffraction
- Three important pieces of
information
1) Each strand is coiled (helix)
2) Two strands of in the structure
3) The nitrogen bases are near the
center
Watson and Crick (1953)
- Used Franklin’s experiment to determine
the 3D structure of DNA
-The double helix explained both how
DNA could carry information and how
that information could be copied
- Their model explained why A=T and C=G
What we know now about the
structure of DNA
•
•
•
•
Double helix
Made of a phosphate backbone
“Rungs” are the nucleotide bases
Bases attach in certain patterns
–CG
– AT
• One side is called the 3’ side the other is called 5’
• On the ends of the chromosomes in eukaryotes
are structures called telomeres that are repeating
bases that protect the DNA
DNA Replication
The process of duplicating DNA
Commonalities in all organisms
• One side of the double helix gives the
instructions for the other
• DNA replication occurs in both
directions
• The site where DNA separation and
duplication occurs is called a fork
Prokaryotes
• More simple
– DNA is not in nucleus
– Plasmid – single, circular DNA molecule
– Much less DNA than a eukaryote
• DNA replication begins at a single point
Eukaryotes
• DNA is more complex
– DNA is in a membrane-bound nucleus
– DNA is wrapped tightly around proteins to form
chromosomes
– More DNA than prokaryotes
• DNA replication occurs at hundreds of points
How Replication Occurs
• Step 1: DNA is opened
– DNA Helicase – reduces the strain caused by
the unwinding of the DNA
Step 2: New DNA is formed
• DNA Polymerase – Helps the daughter strand
form by matching base pairs
– Also proofreads the DNA and corrects mistakes
• Complimentary base pairs are attached to each
side of the now single-sided DNA
– Each strand acts as a template
– New strands of DNA are ½ new and ½ original
– DNA has a “north” and “south” – they are called 5’
and 3’
– Base pairs are copied following the 3’ to the 5’
direction of the parent strand
•Leading strand – the side of the DNA that
is replicated faster – follows the 3’  5’ of
the parent strand
•Lagging strand – It is replicated more
slowly
•Okazaki fragment – the pieces that are
formed in the lagging strand, because
replication can only occur in the 3’ to 5’
direction as the DNA is unwound.
Step 3: The pieces of DNA are
connected
• DNA ligase
attaches the
Okazaki fragments
together