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Lesson Overview DNA Replication

Bell Ringer: • Complete the complimentary strand of

DNA to this original template strand sequence of DNA:

A-T-T-A-C-G-G-T-G-C-C-A-T-A-G

T-A-A-T-G-C-C-A-C-G-G-T-A-T-C

Lesson Overview

12.3 DNA Replication

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THINK ABOUT IT

• Before a cell divides, its DNA must

first be copied.

• How might the double-helix

structure of DNA make that

possible?


Copying the Code

• What role does DNA

polymerase play in

copying DNA?

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Copying the Code

What role does DNA polymerase play in copying DNA?

DNA polymerase is an

enzyme that joins individual

nucleotides to produce a

new strand of DNA.


Copying the Code

• Base pairing in the double helix explained how DNA could

be copied, or replicated, because each base on one strand

pairs with only one base on the opposite strand.

• 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.

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The Replication Process

• Before a cell divides, it duplicates its

DNA in a copying process called

replication.

• This process ensures that each

resulting cell has the same complete

set of DNA molecules.



• 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.

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• The two strands of the double helix separate, or

“unzip,” allowing two replication forks to form.

• The enzyme responsible for “unzipping” the two

strands is DNA helicase.


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• 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.


• The result of replication is two DNA molecules

identical to each other and to the original

molecule.

• Each DNA molecule resulting from replication has

one original strand and one new strand.


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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.

Each strand then serves as a template for the

attachment of complementary bases.

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The Role of Enzymes

• The principal enzyme involved in DNA replication

is called DNA polymerase.

• DNA polymerase is an enzyme that joins individual

nucleotides to produce a new strand of DNA.

• DNA polymerase also “proofreads” each new DNA

strand, ensuring that each molecule is a perfect

copy of the original.


Telomeres

• The tips of chromosomes are known as

telomeres.

• The ends of DNA molecules, located at the

telomeres, are particularly difficult to copy.

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Telomeres

• Over time, DNA may actually be lost from

telomeres each time a chromosome is replicated.

• An enzyme called telomerase compensates for

this problem by adding short, repeated DNA

sequences to telomeres, lengthening the

chromosomes slightly and making it less likely that

important gene sequences will be lost from the

telomeres during replication.


Telomeres

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Replication in Living Cells

How does DNA replication differ

in prokaryotic cells and

eukaryotic cells?

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How does DNA replication differ in prokaryotic cells and eukaryotic cells?

Replication in most prokaryotic cells

starts from a single point and

proceeds in two directions until the

entire chromosome is copied.



How does DNA replication differ in prokaryotic cells and eukaryotic cells?

In eukaryotic cells, replication may

begin at dozens or even hundreds of

places on the DNA molecule,

proceeding in both directions until

each chromosome is completely

copied.

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• The cells of most prokaryotes have a single,

circular DNA molecule in the cytoplasm,

containing nearly all the cell’s genetic

information.

• Eukaryotic cells, on the other hand, can

have up to 1000 times more DNA. Nearly all

of the DNA of eukaryotic cells is found in the

nucleus.


Prokaryotic DNA Replication

• In most prokaryotes, DNA

replication does not start

until regulatory proteins

bind to a single starting

point on the chromosome.

This triggers the

beginning of DNA

replication.

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• Replication in most

prokaryotic cells starts

from a single point and

proceeds in two

directions until the

entire chromosome is

copied.



Often, the two

chromosomes

produced by replication

are attached to different

points inside the cell

membrane and are

separated when the cell

splits to form two new

cells.

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• Eukaryotic chromosomes are

generally much bigger than

those of prokaryotes.

• In eukaryotic cells, replication

may begin at dozens or even

hundreds of places on the

DNA molecule, proceeding in

both directions until each

chromosome is completely

copied.

Eukaryotic DNA Replication


Eukaryotic DNA Replication

• The two copies of DNA produced by replication in each

chromosome remain closely associated until the cell enters

prophase of mitosis.

• At that point, the chromosomes condense, and the two

chromatids in each chromosome become clearly visible.

• They separate from each other in anaphase of mitosis,

producing two cells, each with a complete set of genes

coded in DNA.

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