1. Meiosis Chapter 6 The student is expected to:
6A identify components of DNA, and describe how information for specifying the traits of an organism is carried in the DNA and
6G recognize the significance of meiosis to sexual reproduction

2. Requirements of Genetics
Each organism must inherit a single copy of each gene from both “parents”
When this organism creates its own gametes, the two sets of parental genes must separate from each other
Gamete: reproductive cell; has one copy of each chromosome (half the number of chromosomes of body cells); ex: sperm, egg
Somatic Cells: also known as body cells; have BOTH copies of each chromosome from the parents; ex: hair, muscles, nerve

3. Chromosome Number All organisms have different numbers of chromosomes.
A body cell in an adult fruit fly has 8 chromosomes:
4 from the fruit fly's male parent
4 from its female parent.

4. Chromosome Number Human body cells have 23 pairs of chromosomes.
Homologous pairs of chromosomes have the same structure.
For each homologous pair, one chromosome comes from each parent.
Chromosome pairs 1-22 are autosomes.
Sex chromosomes, X and Y, determine gender in mammals.

5. Chromosome Number
A cell containing both sets of homologous chromosomes is called diploid.
Occurred when fertilization between an egg and a sperm during sexual reproduction.
The number of chromosomes in a diploid cell is represented by the symbol 2N.

6. Chromosome Number
Gametes of sexually reproducing organisms contain a single set of chromosomes, and therefore only a single set of genes.
These cells are haploid and are represented by the symbol N.

7. Chromosome Number
Chromosome number must be maintained in animals; if not, drastic mutations and/or spontaneous abortions (miscarriages) can occur.
Many plants have more than two copies of each chromosome – referred to as polyploidy.
This can result in larger, more fruitful plants.

8. Meiosis
Process of reduction division - the number of chromosomes per cell is divided in half when homologous chromosomes are separated.
It makes haploid cells from diploid cells!
Two Divisions:
Meiosis I & Meiosis II
Meiosis also helps creates genetic diversity!

9. Meiosis

10. Meiosis I Pairs of homologous chromosomes separate in Meiosis I
Remember…homologous chromosomes are SIMILAR, but NOT identical!
Interphase I
Prophase I
Metaphase I
Anaphase I
Telophase I & Cytokinesis

11. Meiosis I
Interphase:
Cells undergo a round of DNA replication, forming duplicate chromosomes.
Meiosis I occurs after DNA has been replicated.

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

13. Meiosis I
Prophase I:
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 (versions of genes).

14. Meiosis I
Metaphase I:
Spindle fibers attach to the chromosomes.

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

16. 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 and from the
diploid cell that
entered meiosis I.

17. Meiosis I

18. Meiosis II
The two cells produced by meiosis I now enter a second meiotic division.
Neither of the 2 haploid cells go through DNA replication between Meiosis I and Meiosis II!

19. Telophase II & Cytokinesis
Meiosis II
Telophase II & Cytokinesis
Prophase II
Metaphase II
Anaphase II

20. Meiosis II
Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original cell.
The sister chromaids have not separated, but this does NOT mean that there are two copies of each chromosome.

21. Meiosis II
Prophase II
Begins with the two haploid daughter cells formed by Meiosis I
Chromosomes are condensed.
No crossing over, no tetrads

22. Meiosis II
Metaphase II
The chromosomes line up in the center of cell.

23. Meiosis II
Anaphase II
The sister chromatids separate and move toward opposite ends of the cell.

24. Meiosis II Telophase II and Cytokinesis Meiosis II results in four
haploid (N) daughter cells.

25. Gamete Formation Males
Meiosis results in four equal-sized gametes called sperm.

26. Gamete Formation Females Only one egg results from meiosis.
The other three cells, called polar bodies, are usually not involved in reproduction.

27. Genetic Variation
Sexual reproduction creates unique combinations of genes.
independent assortment of chromosomes in meiosis
random fertilization of gametes
Crossing over results in new combinations of genes

28. Genetic Variation Chromosomes contain many genes.
The farther apart two genes are located on a chromosome, the more likely they are to be separated by crossing over.
Genes located close together on a chromosome tend to be inherited together, which is called genetic linkage.

29. Comparing Mitosis & Meiosis
Produces two genetically identical diploid cells.
Cells have the same number of chromosomes and type of alleles as the parent cell.
Allows an organism to grow and replace cells.
Some organisms reproduce asexually by mitosis (ex: bacteria).

30. Comparing Mitosis & Meiosis
Produces four genetically different haploid cells.
Cells have half the number of chromosomes as the parent cell.
Cells are genetically different from the diploid parent cell and from each other.
How sexually-reproducing organisms produce gametes.