1. Meiosis
How
Reproductive
Cells Divide &
Pass on DNA

2. Why do Cells Divide?
• To replace old/worn
out or damaged cells
• skin, red blood cells,
intestines
• To fight disease
• white blood cells
identical copies
• To reproduce
• gametes (sex cells)
• male sperm cell
• female ova cell
variation

3. DNA: Secret Code of the Cell
 Traits are characteristics inherited from parents
• i.e.: leaf shape, eye/hair color, tree height, etc.
 Instructions that determine these traits are found in
sections of DNA called genes
• most cells have two genes that control each trait
• one gene comes from the mother (maternal)
• one gene comes from the father (paternal)
 This gene might give
instructions for nose
width in a human.

4. Mama Genes & Papa Genes
 Homologous chromosomes
• every body (somatic) cell has a set of two similar chromosomes
• one paternal, one maternal
• genes of homologous chromosomes code for the same traits
• genes for similar traits are usually found at the same location on
the two different homologous chromosomes

5. Homologues
 Same length
 Same centromere position
 Carry genes that control the
same inherited traits
• i.e.: both carry the gene for hair
color - one is for brown hair and
one is for red hair
 One is from mother, one is
from father = a matching set
 NOT identical

6. Haploid vs. Diploid Cells
 Cells with only one set of chromosomes
(one gene for each trait) are haploid
• symbolized by N
• all gametes (sex cells) are haploid
• haploid cells are the result of meiosis
 Cells having homologous chromosomes
(two genes for each trait) are diploid
• symbolized by 2N
• all somatic cells (body cells) are diploid
• diploid cells are created through mitosis

7. Sexual Reproduction
 The nuclei of haploid gametes combine in fertilization,
resulting in a diploid zygote
haploid gametes
diploid
zygote

8. Does Chromosome # determine complexity?
Comparative Number of Chromosomes
Organism Body Cell (2n) Gamete (n)
Fruit fly 8 4
Garden pea 14 7
Flatworm 16 8
Corn & Green algae 20 10
Frog 26 13
Apple 34 17
Cat 38 19
Monkey 42 21
Human 46 23
Potato & Chimpanzee 48 24
Dog 78 39
Adder's tongue fern 1260 630

9. Mitosis vs. Meiosis
Meiosis I
Meiosis II

10. Mitosis vs Meiosis

11. Meiosis
 Interphase
• Chromosomes replicate
• Chromatin condenses
 Prophase I Interphase
• Replicated homologous
chromosomes pair up (synapsis)
• Chromosomes each consist of two
identical sister chromatids
• Nuclear membrane breaks down
• Spindles form
• Crossing over begins
Prophase I

12. Meiosis
 Crossing over
• chromosomal segments are
exchanged between pairs of
homologous chromosomes
• produces variation in genetic
information passed to offspring Prophase I

13. Meiosis
 Metaphase I
• Chromosome kinetochores
attach to spindle fibers.
• Homologous chromosome
pairs line up at the Metaphase I
equatorial plate.

14. Meiosis
 Anaphase I
• Homologues separate
and move to opposite
poles of the cell
• Sister chromatids Anaphase I
remain attached!

15. Meiosis
 Telophase I
• Spindles break down
• Chromosomes uncoil
• Two nuclei form Telophase I
• Cytoplasm divides
(cytokinesis)
 Cells have only half as many chromosomes
(they are now haploid)
• homologous pairs have separated into two cells
• chromosomes have already replicated

16. Meiosis I

17. Meiosis
 Prophase II
• A second set of phases
begins in both haploid
daughter cells
• Spindle apparatus forms
Prophase II
• Chromosomes
condense
• Nuclear membrane
breaks down
 No DNA replication occurs between Meiosis I and
Meiosis II (interphase is skipped)

18. Meiosis
 Metaphase II
• A haploid number of
chromosomes line up at
the equatorial plate
• Spindle fibers attach to
Metaphase II
kinetochores

19. Meiosis
 Anaphase II
• Sister chromatids are
pulled apart at the
centromere
• Spindle fibers pull Anaphase II
chromatids toward the
opposite poles of the cell
 Telophase II
• chromosomes reach the
poles
• nuclear membranes and
nuclei reform
Telophase II

20. Meiosis
 Cytokinesis
• Plasma membranes and
cytoplasm of cells divide
• Four gametes are the
result, each with n
number of
chromosomes Cytokinesis

21. Meiosis II (same as mitosis)

22. Meiosis Provides for Variation

23. Meiosis - summary
 Germs cells (reproductive cells produced in the
testes & ovaries in humans) divide by meiosis
 In meiosis, a cell divides twice to produce four
daughter cells
 Cells arising from meiosis are haploid, which
means they half the number of chromosomes
 Provides genetic variability, because daughter
cells are not identical (as in mitosis)

24. Week 13 Lab
Cell Division Project Presentations