1. Sexually Reproducing Organisms:
Chromosome Structure
Sexually Reproducing Organisms:
Chromosomes occurs in pairs (One from the mother and 1 from the father)
These pairs are called homologous chromosomes
Carry directions/alleles for traits but they do not always carry same version for the same trait.

2. Organism – Kingdom
Body/Somatic Cell (2n)
Sex Cell/Gamete/Eggs & Sperm (n)
All Bacteria 1
Yeast – Fungi 32 16
Bread mold – Fungi 8 4
Potato – Plant 48 24
Tobacco – Plant
Corn – Plant 20 10
Adder’s Tongue Fern – Plant
1,262
631
Chicken 78 39
Dog
Human 46 23
Chimpanzee

3. Cell Types and their Chromosomes Number
Cell type w/examples
Cell type alternate names
Sets of chromosomes per cell
Naming sets of chromosomes
Body Cells
kidney, bone, liver, muscle or
Sex Cells
eggs, sperm *

4. Chromosome Number
A cell that contains both sets of homologous chromosomes are Diploid
“Two Sets”
Diploid=2N
Cells containing One set of chromosome are Haploid
“One Set”’
Haploid=1N
Sex Cells ONLY!

5. All living things can reproduce.
Sex Cell Reproduction
All living things can reproduce.
Reproduction occurs in one of two ways:
Sexually
The Union of two cells, each from a different parent and containing one set of chromosomes
To produce the first cell of a new organism, having TWO full sets of chromosomes
Asexually
Process by which a single parent reproduces by itself (Ex. Budding)

6. Meiosis Takes place in the Gametes of an organism
People have a Chromosome count of 46
When an egg joins a sperm the count must stay at 46 to remain human
So, the egg can only have 23 chromosomes, and the sperm can only have 23 chromosomes
But, the integrity of the organism must be maintained.
How does this happen?

7. During Meiosis gamete (sex) cells undergo a “double division”, maintaining the DNA, but reducing the chromosomal count to 23 + =
Sperm (23) Egg (23) = Fertilized Cell (46)

8. Original Gamete Metaphase Anaphase Telophase Cytokinesis
2 Daughter Cells Metaphase 2 Anaphase 2 Telophase 2 Cytokinesis – 4 Gametes

9. Meiosis involves two divisions, meiosis 1 and meiosis II
Phases of Meiosis
Meiosis involves two divisions, meiosis 1 and meiosis II
By the end of meiosis II, the diploid cell that entered meiosis has become 4 haploid cells

10. Phases of Meiosis I

11. Phases of Meiosis Interphase 1
Cells undergo a round of DNA replication, forming duplicate chromosomes.
Interphase 1
Interphase I - Cells undergo a round of DNA replication, forming duplicate chromosomes.

12. Phases of Meiosis
Each chromosome pairs with its corresponding homologous chromosome to form a tetrad.
There are 4 chromatids in a tetrad.
MEIOSIS I
Prophase I
MEIOSIS I Prophase I - Each chromosome pairs with its corresponding homologous chromosome to form a tetrad.

13. Phases of Meiosis
When homologous chromosomes form tetrads in meiosis I, they exchange portions of their chromatids in a process called crossing over.
Crossing-over produces new combinations of alleles.
Crossing-over occurs during meiosis. (1) Homologous chromosomes form a tetrad. (2) Chromatids cross over one another. (3) The crossed sections of the chromatids are exchanged.

14. Spindle fibers attach to the chromosomes.
Phases of Meiosis
Spindle fibers attach to the chromosomes.
MEIOSIS I
Metaphase I
MEIOSIS I Metaphase I - Spindle fibers attach to the chromosomes.

15. Phases of Meiosis
MEIOSIS I
Anaphase I
The fibers pull the homologous chromosomes toward opposite ends of the cell.
MEIOSIS I Anaphase I - The fibers pull the homologous chromosomes toward opposite ends of the cell.

16. Phases of Meiosis Telophase I and Cytokinesis Nuclear membranes form.
MEIOSIS I
Telophase I and
Cytokinesis
Nuclear membranes form.
The cell separates into two cells.
The two cells produced by meiosis I have chromosomes and alleles that are different from each other.
MEIOSIS I Telophase I and Cytokinesis - Nuclear membranes form. The cell separates into two cells.

17. Meiosis II Phases of Meiosis
The two cells produced by meiosis I now enter a second meiotic division.
Unlike meiosis I, neither cell goes through chromosome replication.
Each of the cell’s chromosomes has 2 chromatids.

18. MEIOSIS II
Prophase II
Phases of Meiosis
Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original cell.
MEIOSIS II Prophase II - Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original cell.

19. The chromosomes line up in the center of cell.
Phases of Meiosis
MEIOSIS II
Metaphase II
The chromosomes line up in the center of cell.
MEIOSIS II Metaphase II - The chromosomes line up in a similar way to the metaphase state of mitosis.

20. MEIOSIS II
Anaphase II
Phases of Meiosis
The sister chromatids separate and move toward opposite ends of the cell.
MEIOSIS II Anaphase II - The sister chromatids separate and move toward opposite ends of the cell.

21. MEIOSIS II
Telophase II and Cytokinesis
Phases of Meiosis
Meiosis II results in four haploid (N) daughter cells.
Cytokinesis: results in formation of:
4 sperm cells in males
1 Egg and 3 polar bodies in females
MEIOSIS II Telophase II and Cytokinesis - Meiosis II results in four haploid (N) daughter cells.

25. How is Meiosis different from Mitosis?

26. Comparing Mitosis and Meiosis
Mitosis results in the production of two genetically identical diploid cells.
Meiosis produces 4 genetically different haploid cells.

27. Comparing Mitosis and Meiosis
Cells produced by mitosis have the same number of chromosomes and alleles as the original cell.
Mitosis allows an organism to grow and replace cells.
Some organisms reproduce asexually by mitosis.

28. Comparing Mitosis and Meiosis
Cells produced by meiosis have half the number of chromosomes as the parent cell.
These cells are genetically different from the diploid cell and from each other.
Meiosis is how sexually-reproducing organisms produce gametes.

29. Mitosis Meiosis What is the number of divisions of the nucleus?
Mitosis
Meiosis
What is the number of divisions of the nucleus?
What is the number of daughter cells produced?
Are the daughter cells identical or not identical to one another?
Are the daughter cells identical or not identical to the parent cell?
Do the homologous chromosomes pair up to form a tetrad?
Is the parent cell haploid or diploid?
Are the daughter cells haploid or diploid?
Is this the nuclear division for somatic or sex cells?