1. Meiosis

2. Bellwork
Why do your siblings not look exactly like you or your Mom or Dad?

3. Objectives
Explain the difference between asexual and sexual reproduction
Explain how meiosis creates haploid cells
Describe how sexual reproduction creates genetic variation in the offspring

4. Key Terminology Asexual reproduction Sexual reproduction Meiosis
Homologous chromosome
Sister chromatid
Crossing over
Independent assortment of chromosomes

5. Asexual Reproduction Involves mitosis – duplication of cells
Offspring formed from a single parent without uniting egg and sperm
Typically, a piece of parent is broken off and grows into a new organism by means of mitotic division
Ex. prickly pear, African violets, some microorganisms
Organisms produced by asexual reproduction are genetically identical
Considered genetic clones

6. PROPERTY OF PIMA COUNTY JTED, 2010
Think-Pair-Share
Speculate why you might not want a whole greenhouse of plants that are genetic clones.
3. Think-pair-share
teacher presents a question
teacher gives wait time for student to form answer
teacher instructs students to share their answer with a partner
teacher calls on non-volunteers to share with the class
PROPERTY OF PIMA COUNTY JTED, 2010 3

7. Individual Variation
Why are you not a clone of your biological parents?
What gives rise to that individual variation?

8. Sexual Reproduction Asexual reproduction Sexual Reproduction –
Via mitosis only produces clones- genetically identical offspring
Sexual Reproduction –
enables the shuffling of DNA among individuals
produces genetically unique offspring

9. PROPERTY OF PIMA COUNTY JTED, 2010
Think-Pair-Share
Speculate why sexual reproduction is important to the survival of a species- including Humans?
3. Think-pair-share
teacher presents a question
teacher gives wait time for student to form answer
teacher instructs students to share their answer with a partner
teacher calls on non-volunteers to share with the class
PROPERTY OF PIMA COUNTY JTED, 2010 3

10. Sexual Reproduction Organisms that reproduce by sexual reproduction
alternate between diploid (2 of each chromosomes) and haploid number (1 of each chromosome)
Human somatic (body) cells
contain 46 chromosomes
23 homologous pairs; 2n =46; diploid
Human gametes(egg or sperm)
contain 23 chromosomes
n=23; haploid

11. Sexual Reproduction (cont.)

12. Meiosis
Involves the production of haploid cells with unpaired chromosomes
Meiosis comes from the Greek word to diminish

13. PROPERTY OF PIMA COUNTY JTED, 2010
Think-Pair-Share
Speculate why you would not want the egg and sperm to be diploid (have a pair of each chromosome).
3. Think-pair-share
teacher presents a question
teacher gives wait time for student to form answer
teacher instructs students to share their answer with a partner
teacher calls on non-volunteers to share with the class
PROPERTY OF PIMA COUNTY JTED, 2010 3

14. Homologous Chromosomes
Two chromosomes that make up a matched pair
One member of the pair donated from the father
One donated from the mother

15. Sister Chromatids Sister chromatids-
an identical duplicated copy of the chromosome donated by the father or by the mother

16. Meiosis –divided into two major phases
Meiosis I – separate homologous chromosomes
Meiosis II – separate sister chromatids
Each phase is divided into stages similar to mitosis – prophase, metaphase, anaphase, and telophase

17. Meiosis I Interphase Enter into Meiosis I Prophase I
duplication of chromosomes
Enter into Meiosis I
Prophase I
Homologous chromosomes line up side by side
Chromosomes condense
Crossing over event occurs
exchange segments between maternal and paternal chromosomes
Disappearance of nuclear envelope

18. Meiosis I (cont) Metaphase I
Homologous chromosomes line up at equator of cell
Homologous chromosomes face opposite poles
What pole the chromosomes face is random
known as independent assortment of chromosomes

19. Meiosis I (cont) Anaphase I Homologous chromosomes separate
At end of anaphase I, each pole has only one member of homologous chromosome (donated by either father or mother)

20. Meiosis I (cont) Telophase I and cytokinesis
Nuclear envelope reappears
Cytokinesis occurs

21. Meiosis II
Little or no intervening interphase between meiosis I and II
Sister chromatids separate
identical to mitosis

22. Meiosis

23. Sexual Reproduction Promotes Genetic Variation
Independent assortment of chromosomes = 2n
n = number of chromosomes
humans 223 = 8 million possible chromosome combinations
Crossing over during prophase I
Random Fertilization
fusion of egg and sperm to form the zygote
223 x 223 = 70 trillion combinations

24. PROPERTY OF PIMA COUNTY JTED, 2010
3 Minute Paper
You have three minutes to write down 2 key points of today’s lesson.
Be prepared to share.
29. Minute paper or Quick Write
teacher provides a prompt from the lesson
students have one minute to write down a summary of the key points
students compare with a partner
teacher uses a strategy to check all
PROPERTY OF PIMA COUNTY JTED, 2010 24