Do Now
1. What are the three parts of a nucleotide?
- Sugar (deoxyribose), phosphate, nitrogen
base
2. Adenine (A) bonds with ____________
- Thymine (T)
3. Guanine (G) bonds with ____________
- Cytosine (C)
Objective: Describe DNA replication. Explain the importance of DNA
replication. Model DNA replication.
DNA Replication
Making New DNA Molecules
Objective: Describe DNA replication. Explain the importance of DNA
replication. Model DNA replication.
Overview of DNA Replication
• DNA replication is the process by which the cell
copies its DNA.
– Happens in the S phase (synthesis) of cell cycle.
• The double helix separates, and each strand
serves as a template on which a new strand is
built.
• Each new piece of DNA has one old strand and
one new strand.
Step 1: Separation of the Strands
• In order for the DNA to replicate, the
two strands need to be separated.
– This happens easily because the hydrogen
bonds holding them together are weak.
• An enzyme called helicase breaks the
hydrogen bonds and separates the
strands.
Step 2: Building New DNA
• A second enzyme called DNA polymerase attaches to
the DNA strands to attach complementary nucleotides.
• The DNA polymerase joins the nucleotides by
covalent bonds in the backbone and hydrogen bonds
between the 2 strands.
Animation
animation 2
Objective: Describe DNA replication. Explain the importance of DNA
replication. Model DNA replication.
•http://207.207.4.198/pub/flash/24/24.html
Objective: Describe DNA replication. Explain the importance of DNA
replication. Model DNA replication.
Step 3: Proofreading
• DNA replication is very accurate, with only
about one error in 10,000 paired nucleotides.
• Mutations are mistakes or changes to the
DNA.
– Some of them can be fixed, and some cannot.
– Some can have serious effects, and some do not
affect the cell at all.
• Enzymes proofread and repair many of the
mutations that can occur.
– This reduces the error rate to about 1 in every 1
billion base pairs.
End Result of Replication
• Replication results in the creation
of two identical molecules of DNA.
• The process is described as being
semi-conservative.
– “semi” = half
– “conserve” = to save
– Replication is semiconservative, because each
new molecule is made of one
old strand of DNA and one
newly created strand.
Activity time!
• Now you are going to take your DNA model from
yesterday and simulate replication.
• You need your model, scissors, glue and
colored pencils, using same colors as yesterday!
Sugars – Red
Adenine - Purple
Phosphates – Black
Thymine - Pink
Guanine – Green
Cytosine – Blue
• Listen to the instructions as they are given!!