1. DNA Organization and Replication
Genetics Unit I-Part B
DNA Organization and Replication

2. An Introduction
“A genetic material must carry out two jobs: duplicate itself and control the development of the rest of the cell in a specific way.”
-Francis Crick
"It has not escaped our notice that the specific (base) pairing we have postulated immediately suggests a possible copying mechanism for the genetic material.“
-Watson and Crick April 25, 1953

3. the fun’s just getting underway
Eukaryotic DNA Organization
DNA is very organized.
The amount of DNA packed into a nucleus would equal 5 miles of string packed into a film cannister
The double helix is wrapped around protein particles called nucleosomes. These nucleosomes are made up of 8 proteins called histones.
the DNA then resembles “beads on a string”
This allows the DNA to be very tightly packed and organized
These nucleosomes must be unwound in order for the DNA to replicate

4. Nucleosome diagram

5. Replication of the Genetic Material
Replication is defined as the process by which the genetic material is reproduced, leaving 2 exact copies to be passed on to the daughter cells during mitosis.
For many years scientists debated how replication took place. Three models were generally discussed:

6. Models for DNA replication
1) Semiconservative model:
Daughter DNA molecules contain one parental strand and one newly-replicated strand
2) Conservative model:
Parent strands transfer information to an
intermediate, then the intermediate gets copied. The parent helix is conserved, the daughter helix is completely new
3) Dispersive model:
Parent helix is broken into fragments, dispersed, copied then assembled into two new helices.
New and old DNA are completely dispersed

7. MODELS OF DNA REPLICATION
Semi-conservative replication
Conservative replication
Dispersive replication

8. Which model is correct? Proven by Meselson-Stahl Experiment
Materials used
-Ultracentrifuge, bacteria, two types of radioactive nitrogen, 15N (heavy), 14N (light), Cultured bacteria
-Method used:
Grew bacteria in medium containing heavy nitrogen, until all DNA contained this isotope
Next, grew the bacteria in light nitrogen for 1 cell division, the DNA was then isolated and centrifuged
The DNA bands in the centrifuge were examined, results analyzed

9. Which model is correct?
This experiment was continued over further generations, the results showed replication took place in a semi-conservative manner.

10. Features of DNA Replication
DNA replication is semi-conservative
Each strand of template DNA is being copied.
DNA replication is bidirectional
Bidirectional replication involves two replication forks, which move in opposite directions
DNA replication is semi-discontinuous
The leading strand copies continuously
The lagging strand copies in segments (Okazaki fragments)
which must be joined
DNA replication involves many enzymes and factors

11. DNA Replication is bi-directional

12. Discontinuous synthesis
DNA replication is semi-discontinuous
Discontinuous synthesis

13. Notes to diagram
Okazaki showed that the 2nd strand is replicated discontinuously, in short pieces.
1st, RNA primers are synthesized, then DNA is added. The RNA primers are then degraded + DNA is filled in.
The pieces are called Okazaki fragments.

14. DNA replication involves many enzymes and factors
The method of DNA replication
Many enzymes + factors are involved in the process.
The main class of enzymes are the DNA polymerases (DNA pol’s)

15. The DNA Polymerase Family
A total of 5 different DNAPs have been reported:
DNAP I: functions in repair and replication
DNAP II: functions in DNA repair (proven in 1999)
DNAP III: principal DNA replication enzyme
DNAP IV: functions in DNA repair (discovered in 1999)
DNAP V: functions in DNA repair (discovered in 1999)

16. DNA replication involves many enzymes and factors
Other Enzymes and Factors Needed
1.) helicase – enzyme which unwinds the DNA double helix
2.) RNA primase – enzyme which makes the RNA primers
3.) DNA ligase – enzyme which binds Okazaki fragments together
4.) single-stranded binding proteins (SSB’s) – keep single-stranded DNA stable when it’s unwound
5.) gyrase – prevents DNA from tangling as it unwinds (aka the “Conditioner” enzyme