1. Providing Genetic Variation
Meiosis
Providing Genetic
Variation

2. Genes, Chromosomes, and Numbers
Chromosomes occur in pairs.
Body cells have two of each kind of chromosome.
Diploid = 2n
Organisms have two factors – alleles – for each trait
One allele is located on each paired chromosome.

3. The Need for Meiosis
In order to reproduce, organisms must pass on their DNA (chromosomes) to their offspring.
What would happen if organisms gave a complete set of chromosomes to their offspring?
Ex. Organism has 10 chromosomes.
How many chromosomes would its offspring have?
How many would its grandchildren have?
Great-grandchildren?

4. Diploid and Haploid Cells
Gametes contain one of each kind of chromosome.
Haploid=1N or N
(n = number of chromosomes)
Parent organisms give one allele, for each trait to each of their offspring.

5. Human Karyotype

6. Chromosomes
Each species of organisms contains a characteristic number of chromosomes.
Large range of numbers among species
Not related to complexity of organism

7. Homologous chromosomes
Paired chromosomes = homologous chromosomes
Have genes for the SAME trait arranged in the SAME order
However, they are NOT identical
Ex. Even though both have hair color alleles, one allele could be for dark hair, and other other could be for light hair.

8. Meiosis Two separate divisions – Meiosis I and II
Meiosis I starts with one diploid cell (2n)
By the end of Meiosis II there are four haploid (n) cells
In animals and most plants these are called gametes or sex cells.

9. SEX CELLS! Male gametes = sperm Female gametes = eggs
A fertilized egg is called a zygote.
This pattern of reproduction that involves production and fusion of haploid sex cells is called sexual reproduction.

10. Human Spermatogenesis and Oogenesis

11. The Phases of Meiosis Consist of 2 stages (Meiosis I and Meiosis II)

12. Interphase - Review Cell carries out its usual metabolic activities
Replicates its chromosomes

13. Prophase I Chromosomes come together to form a tetrad
Consists of 2 pairs of homologous chromosomes (4 total)
Nuclear envelope disappears

14. Crossing Over Homologous chromosomes pair up & form tetrads
May exchange portions of their chromatids
Results in an exchange of alleles to produce new combinations

15. Metaphase I Spindle fibers attach to the chromosomes
Chromosome pairs (tetrads) are arranged in the middle or equator of the spindle
Homologous chromosomes are lined up together in pairs

16. Anaphase I
Spindle fibers separate homologous pairs and pull them to opposite sides of the cell
Centromeres do not split as they do in mitosis
This step ensures that each new cell will receive only one chromosome from each homologous pair

17. Telophase I Spindle fibers break down Chromosome uncoil
The cytoplasm divides to yield two new cells
Nucleus and nuclear envelope reform
Each cell has only half of the genetic information of the original cell

18. Cytokinesis
Cytoplasm splits to form 2 haploid cells with sister chromatid

19. Prophase II
Spindle reforms
Chromosomes condense
2 haploid cells

20. Metaphase II
Chromosomes line up on the equator of the cell attached to spindle fibers

21. Anaphase II Centromere splits
Sister chromatids separate and move to opposite sides of the cell

22. Telophase II Cytoplasm divides
Results in four haploid (n) daughter cells

23. Genetic Recombination
Peas have 7 pairs of chromosomes
Peas can form up to 128 different kinds of sperm
Humans have 23 pairs of chromosomes
They can form up to 8 million different kinds of sperm or eggs

24. Why Meiosis? Genetic Diversity
Cells formed by mitosis are identical to each other and the parent cell
Meiosis shuffles chromosomes and genetic information

25. Unique Individuals
In addition to recombination, crossing over helps provide an endless number of possible chromosomes.
This is called genetic recombination.

26. Mitosis
Meiosis
Number of Divisions 1 2
Number of Cells Made
2 Diploid Cells
4 Haploid Cells
Cells Made Are
Identical to Original Cell
Different from Original Cell
Purpose
Growth and Replacement of Body Cells
Asexual Reproduction in Some Organisms
Sexual Reproduction