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.

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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