1. DNA Replication

2. Replication Facts
Remember, your body cells can divide by mitosis so your body can grow and repair wounds.
Remember, parents must create sex cells by meiosis to pass their DNA (half of it!) to their children.
DNA must be copied before mitosis and before the first division of meiosis.

3. Replication Facts
DNA replication takes place in the S phase during interphase of the cell cycle.
DNA is replicated in the nucleus of cells.
Mitosis
-prophase
-metaphase
-anaphase
-telophase G1 G2 S
phase
interphase
DNA replication takes
place in the S phase.

4. Thymine (T)
Adenine (A)
Cytosine (C)
Guanine (G)
Phosphate
deoxyribose sugar
DNA nucleotide
Deoxyribose sugar
phosphate
Nitrogenous base
(guanine)
Sugar / phosphate “sides”
Nitrogenous base “step”
Hydrogen bond
(H-bonds)
Take a look at this key. Now, lets use it to learn the how DNA replicates!!
DNA Replication

5. DNA Replication Step 1: Hydrogen bonds between base pairs break.
The DNA “unzips”.

6. DNA Replication
Step 2: DNA strands pull apart from each other.

7. DNA Replication Step 3: DNA nucleotides in the cell
match up with each side of the
“unzipped” DNA.
Each “unzipped” old strand forms a
template (a model) for a new strand.

8. DNA Replication
Step 4: Each “old’ strand forms a template for a “new” strand.
Two identical DNA
molecules form, one strand is “new”, one strand is “old”. This is referred to as semi-conservative replication.
“new” strand, identical
sequence to the original
“old” (original) strand

9. DNA Helicase
DNA helicase – is a special protein called an enzyme which unwinds or “unzips” the DNA by breaking the hydrogen bonds between the bases.
Unzipping the DNA forms a region called a replication fork.
Replication Fork: “Y-shaped region” in DNA molecule where helicase separates the two strands.
Replication
Fork
DNA Molecule

10. DNA Polymerase
DNA polymerase – is an enzyme responsible for adding new nucleotides to the growing strand.
DNA Polymerase
Nucleotide 5’ 3’

12. DNA Replication

13. DNA Polymerase
DNA polymerase initially makes about 1 in 10,000 base pairing errors.
DNA polymerase proofreads and corrects many of these mistakes.
For example, if an adenine pairs with a cytosine instead of a thymine, DNA polymerase can repair the error by removing the mispaired cytosine and replacing it with a thymine.
The new error rate for DNA that has been proofread is 1 in 1 billion base pairing errors.

17. Mutations Some errors escape repair.
When mistakes in DNA replication do occur, the base sequence of the newly formed DNA differs from the base sequence of the original DNA.
A change in the nucleotide sequence of a DNA molecule is called a mutation.
Mutations can be harmful, helpful, or neutral.
You will learn more about the effect mutations can have on an organism later in the chapter.