Lesson Overview - Mrs. Hazel's Class Websitelmhsbio.weebly.com/uploads/1/0/6/0/10601628/12.3_notes.pdf · Lesson Overview DNA Replication THINK ABOUT IT Before a cell divides, its

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?


Review Question!

• At what stage of the cell cycle do cells duplicate their DNA?

Mitosis-prophase-metaphase-anaphase-telophase

G1 G2

Sphase

interphase

DNA replication takesplace in the S phase.


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.


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.


Copying the Code

What role does DNA polymerase play in

copying DNA?


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.


The Replication Process

Before a cell divides, it duplicates its DNA in a

copying process called replication.

Why?

This process ensures that each resulting cell

has the same complete set of DNA molecules.



During replication

• DNA molecule separates into

two strands

• Then produces two new

complementary strands

following base pairing rules



• Each strand of the double helix of DNA serves

as a template, or model, for the new strand.

• DNA Replication Animation

http://www.youtube.com/watch?v=uxeLRvsNytg


Let’s Model this with our paper!

• Fold your paper in half hamburger style.

• On each side of the paper, fold back the flaps to meet at the fold (this “hides” the middle section of your paper)

• On the top portion of your paper, record the following DNA sequence:

ACT GGG ATT TGC CCC ATG GTA AAA CCC TTT

• On the bottom portion of your paper, write the complementary strand. They should line up when the bottom of your paper is “hidden”.


• Now, mimic DNA replication:

Divide the two strands and match up the complementary base pairs. Use a different color to do this

• Identify the parent strands

• Identify the complementary strands

• Are the two resulting copies of DNA identical?


The two strands of the double helix separate, or

“unzip,” allowing two replication forks to form.

The Replication Process (Skip)


The Replication Process (Skip)

As each new strand forms, new bases are

added following the rules of base pairing.

Review:

What would pair with Adenine? Guanine?


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.


DNA Template

New DNA

Parental DNA


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.


The Role of Enzymes

The principal enzyme involved in DNA replication is

called DNA polymerase.

• Joins individual nucleotides to produce a new

strand of DNA.

• “Proofreads” each new DNA strand, ensuring that

each molecule is a perfect copy of the original.

Animation with Helicase

& DNA Polymerase

http://www.youtube.com/watch?v=zdDkiRw1PdU&feature=related


Telomeres

The tips of chromosomes are known as telomeres.

The ends of DNA molecules, located at the telomeres,

are particularly difficult to copy.

Over time, DNA may actually be lost

from telomeres each time a chromosome

is replicated.


TelomeresAn enzyme called telomerase compensates for this problem by

adding short, repeated DNA sequences to telomeres

• Lengthening the chromosomes slightly

• Makes it less likely that important gene sequences will be lost

from the telomeres during replication.


Replication in Living Cells

How does DNA replication differ in prokaryotic cells and

eukaryotic cells?



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.

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.



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.

Replication in most prokaryotic cells

starts from a single point and

proceeds in two directions until the

entire chromosome is copied.


Prokaryotic DNA Replication

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.


Eukaryotic chromosomes are generally much bigger

than those of prokaryotes.

Eukaryotic DNA Replication


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 ReplicationThe 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.


What do BOTH prokaryotes & eukaryotes have in common with DNA Replication?

• Hydrogen bonds are broken, strands unwind & separate

• Each add complementary base pairs using parent strand as a template

• Both result in 2 identical copies of DNA, each with one parent strand and one complementary strand

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Lesson Overview - Mrs. Hazel's Class Websitelmhsbio.weebly.com/uploads/1/0/6/0/10601628/12.3_notes.pdf · Lesson Overview DNA Replication THINK ABOUT IT Before a cell divides, its