Teacher Notes

This PPT was revised June 25, 2007. This PPT is a companion PPT to Mitosis &

Meiosis. There is a handout for this PPT.

Reproduction &DNA Replication

How does DNA copy itself?

Mitosis – Why Divide?

to increase surface area

to repair damaged cells

to grow

Cell Reproduction

Cells reproduce either sexually or asexually.

Bacteria

ant2 chromosomes

Chromosome Number

Every species has a specific number of chromosomes that every body cell is supposed to have.

crayfish200 chromosomes human

46 chromosomes

dog78 chromosomes

Chromosome Number

Since you inherit half of your chromosomes from each parent, you carry two complete sets of chromosomes.

mother crayfish100

chromosomes

father crayfish100

chromosomes

1 set of 1001 set of 1002 sets = 200

crayfish200 chromosomes

Chromosome Number

The single set of one chromosome of each type inherited from one parent is called haploid.

Haploid:1 set of chromosomes (n)found in gametes (egg or sperm)

23 Chromosomes

Chromosome Number

A cell (such as a fertilized egg) with two sets of homologous chromosomes is called diploid.

Diploid:2 sets of chromosomes (2n)found in body cells (somatic)

46 chromosomes

Chromosome Number

Recall that every body cell in an organism must have the specific 2n chromosome number for that species.

If it doesn’t, the cell will not be able to function properly.

humans must have46 chromosomes

How does this happen?

When we grow by our cells dividing to make two new cells, all of those new cells must have 46 chromosomes.

How do we get 92 chromosomes (46 for each new cell) from a cell with only 46 chromosomes?

46

46

46

Chromosome Number

The doubling of the chromosomes is actually DNA making an exact copy of itself. This is called DNA replication.

46

46

Then each new cell can receive the correct number of chromosomes (DNA).

46 9292

A Cell’s Life Cycle

When does a cell divide? Most of a cell’s

life cycle is spent in Interphase when it performs normal life functions

M

G1

S

G2

C

G1

A Cell’s Life Cycle

When does a cell divide?

M

G1

S

G2

C

Only a small part of the cell cycle is spent in division.

The RED section marked letter M

A Cell’s Life Cycle

When does DNA replication occur?

During Interphase, just before division, the DNA replicates (makes a copy of itself).

M

G1

S

G2

C

G1

DNA Replication

Before a cell divides the DNA must replicate (make an exact copy of itself) so the new cells have the right 2n # of chromosomes.

DNA Replication

Each new cell must have the same chromosome number (and same DNA) as the parent cell to function properly.

DNA Replication

Replication occurs in 3 easy steps…

DNA Replication

A. DNA UnzipsAn enzyme called DNA

Helicase unzips the molecule by breaking the hydrogen bonds that hold the nitrogen bases together.

DNA Replication: 1. DNA Unzips

3’End

5’End

A

T

A

G

G

C

3’End

5’EndA

T

T

C

C

G

Left 5’ Strand of Original DNA Right 3’ Strand of Original DNA

5” End

The point where the original DNA strand separates is called the replication fork

B. Bases Pair

An enzyme called DNA Polymerase adds complementary bases to bond with each original base in the open helix

What is the complementary base pairing rule?

A = TG C

DNA Base Pairing – Step 2

3’End

5’End

A

T

A

G

G

C

3’End

A

T

T

C

C

G

C. Two Identical DNA molecules

Two new DNA molecules each have the same DNA nitrogen base sequence.

The new DNA molecules are exactly like the original.

T

C

C

A

G

G

T

C

C

A

G

G

T

C

C

A

G

G

DNA Replication Review

Explain what is happening at each step in DNA Replication.

Step 1 Step 2 Step 3

What molecule is at I? Original DNA

DNA Replication Review

What process is occurring? DNA replication

What is happening at II (the arrow)? Replication forkWhat are the

structures at III (circle)? Free nucleotides

I

II

V VIII

IV

What are the 2 molecules at V? identical DNA

DNA Replication Review

What is happening at IV?? Complementary base-pairing How many

strands are formed?

I

II

V VIII

IV

What are the new strands like compared to the original? identical

What are the new strands like compared to each other? identical

MITOSIS

Now that DNA has replicated, Mitosis (cell division) can begin.

The cell now has two identical sets of DNA/chromosomes.

46 92

T

C

C

A

G

G

T

C

C

A

G

G

What is the purpose of DNA replication?

make identical copies of DNA

TRY THIS!

On your paper create a 3 Column Chart for Cell Reproduction and DNA Replication

What I Know…

What I Don’t Know…

What I Wish I Knew…

Works Cited

There is a nice animation of DNA Replication at http://www.ncc.gmu.edu/dna/repanim.htm

DNA Replication diagram, artist Darryl Leja, National Human Genome Research Institute (NHGRI) http://www.genome.gov/page.cfm?pageID=10000552

Background DNA Replicaiton image, DOE Human Genome project, Department of Energy, (http://www.ornl.gov/hgmis)

DNA Replication diagram, artist Darryl Leja, National Human Genome Research Institute (NHGRI) http://www.genome.gov/page.cfm?pageID=10000552

“Fertilization - sperm fertilizing egg”, (No Date). Wikipedia.com, Retrieved June 20, 2006 from the World Wide Web: http://en.wikipedia.org/wiki/Image:Fertilisation.jpg, Website Copyright Permission: This image is copyrighted. However, the copyright holder has irrevocably released all rights to it, allowing it to be freely reproduced, distributed, transmitted, used, modified, built upon, or otherwise exploited in any way by anyone for any purpose, commercial or non-commercial, with or without attribution of the author, as if in the public domain.