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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.
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
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
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
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
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.