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Lesson Overview 12.3 DNA Replication
Lesson Overview DNA Replication
THINK ABOUT IT Before a cell divides, its DNA must first be
copied. How might the double-helix structure of DNA
make that possible?
Lesson Overview DNA Replication
Copying the Code
Each strand of the double helix has all the information needed to reconstruct the other half by the mechanism of base pairing.
Because each strand can be used to make the
other strand, the strands are said to be complementary.
Lesson Overview DNA Replication
The DNA Replication Process Before a cell divides, it duplicates its DNA in a copying process called
replication.
Replication: the process of copying DNA before the cell divides
This process ensures that each resulting cell has all the DNA it needs
Lesson Overview DNA Replication
The Replication Process During replication, the DNA molecule separates into two strands and then
produces two new complementary strands following the rules of base pairing.
Each strand of the double helix of DNA serves as a template, or model, for the new strand.
Lesson Overview DNA Replication
Lesson Overview DNA Replication
The two strands of the double helix separate, or “unzip,” allowing two replication forks to form.
The Replication Process
Lesson Overview DNA Replication
The Replication Process
As each new strand forms, new bases are added following the rules of base pairing.
If the base on the old strand is adenine, then thymine is added to the newly
forming strand. Likewise, guanine is always paired to cytosine.
Lesson Overview DNA Replication
The result of replication is two DNA molecules identical to each other and to the original molecule.
Both of the DNA molecules now have one original strand and one new strand.
The Replication Process
Lesson Overview DNA Replication
The Role of Enzymes DNA replication is carried out by a series of enzymes.
They first “unzip” a molecule of DNA by breaking the hydrogen bonds between base pairs and unwinding the two strands of the molecule.
.
Lesson Overview DNA Replication
The Role of Enzymes The principal enzyme involved in DNA replication is
called DNA polymerase. -It joins individual nucleotides to produce a new strand of
DNA. -It also “proofreads” each new DNA strand, ensuring that
each molecule is a perfect copy of the original.
Lesson Overview DNA Replication Fig. 16-9-3
A T
G C
T A
T A
G C
(a) Parent molecule
A T
G C
T A
T A G C
(c) “Daughter” DNA molecules, each consisting of one parental strand and one new strand
(b) Separation of strands
A T
G C
T A
T A
G C
A T
G C
T A
T A G C
Lesson Overview DNA Replication Fig. 16-10
Original DNA Strand
First replication
Second replication
Semi-conservative DNA Replication
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