1. The Little Things That Make You

2. Trace a line connecting the picture of the phase of mitosis to its name and to the major event that occurs during that phase.

3. What is sexual reproduction?
Sexual reproduction is the fusion of two gametes to produce a single zygote.
Gametes are haploid
Zygote is diploid
Offspring produced sexually are genetically different from either parent.
Introduces greater genetic variation, allows genetic recombination.

4. Meiosis
CELLULAR DIVISION

5. Meiosis

7. Vocabulary
Homologous Chromosomes: chromosome pairs (one from each parent) that are similar in length, gene position, and centromere location. The position of the genes on are the same, but the genes may contain different alleles.

8. Tetrad Diversity Crossing Over: Exchange of genes between
homologous chromosomes, which lead to different characteristics
in offspring( diversity)
Diversity

9. Independent assortment: allele pairs separate independently during the formation of gametes therefore traits are transmitted to offspring independently of one another.

10. Why is Meiosis necessary?
The reduction of the chromosome number during meiosis is most important for —maintaining the chromosome number during sexual reproduction .

11. What is meiosis?
Meiosis is a process of nuclear division that reduces the number of chromosomes in new cells to half the number in the original cell.
The purpose of meiosis is to produce haploid eggs and sperm.
Meiosis consists of two cell divisions.
Meiosis I separates pairs of homologous chromosomes producing haploid cells.
Meiosis II, like mitosis, separates sister chromatids.
Characteristic of eukaryotes only: not in prokaryotes.
Normal cells are diploid: 2 copies of every gene. Diploidy is useful because 2 copies of every gene means that there a backup copy if one gets mutated. Mutations are very frequent in the cells of large organisms. We wouldn’t survive with just one copy of each gene.
Gametes are haploid: 1 copy of every gene. Need to choose 1 copy of each gene randomly.
Why have sexual reproduction? Shuffling of alleles between parents and offspring leads to new combinations. Bad combinations die without reproducing; good combinations survive and reproduce more offspring.

14. Diploid
Haploid

15. How do eggs and sperm form?
Meiosis is the primary event in the formation of gametes.
Occurs in the ovaries and testes.
In males, meiosis results in 4 sperm cells.
Process is called spermatogenesis
In females, meiosis results in 1 egg cell and 3 polar bodies.
Process is called oogenesis.

17. What happens during Prophase I?
DNA coils tightly into chromosomes.
Spindle fibers appear.
Nuclear membrane break downs.
Each chromosome pairs with its corresponding homologous chromosome to form a tetrad.
There are 4 chromosomes in a tetrad.
Crossing-over occurs here
In each tetrad, chromatids of the homologous chromosomes are aligned length-wise so that the genes on one chromosome are adjacent to the corresponding genes on the other chromosome.
Crossing-over allows the exchange of genetic material between maternal and paternal chromosomes. It increases genetic diversity, because a new mixture of genetic material is created.

18. How does crossing-over work?
How does crossing-over ensure genetic variation?
How does crossing-over ensure genetic variation? Crossing-over permits the exchange of genetic material between maternal and paternal chromosomes, producing a new combination of genes.

19. What happens during Metaphase I?
Homologous chromosomes, or tetrads, line up randomly along the middle of the cell.
This random orientation of homologous chromosomes produces new genetic combinations.
Spindle fibers attach to the centromere of each homologous chromosome.
Children can never look exactly like their parents because of the random orientation taken by chromosomes at metaphase I and because of crossing-over at prophase I.

20. What happens during Anaphase I?
Homologous chromosomes are pulled to opposite sides of the cell.
This random separation of the homologous chromosomes is called independent assortment.
Results in genetic variation.
Why is genetic variation so important for living things?
How does anaphase I in meiosis differ from anaphase in mitosis?
Why is genetic diversity important? Genetic diversity plays a very important role in survival and adaptability of a species because when a species’ environment changes, slight gene variations are necessary to produce changes in the organisms that enable it to adapt and survive. A species that has a large degree of genetic diversity among its population will have more variations from which to choose. Species that have very little genetic variation are at a great risk.
How does anaphase I in meiosis differ from anaphase in mitosis? In anaphase during mitosis, sister chromatids separate. In anaphase I of meiosis, homologous pairs separate, but sister chromatids do not.

21. What happens during Telophase I?
Chromosomes reach opposite sides of the cell.
Nuclear membrane forms.
Cytokinesis begins.
The new cells now contain a haploid number of chromosomes.
Chromosomes are now “sister chromatids”

22. What happens during Prophase II?
Spindle fibers form.
Nuclear membrane break down.
Chromatin condenses back into chromosomes.

23. What happens during Metaphase II?
“Sister chromatids” move to the middle of dividing cells.
Each chromatid faces opposite poles of the cell.
Spindle fibers attach to each sister chromatid.

24. What happens during Anaphase II?
“Sister chromatids” separate and move toward opposite poles of the cell.

25. What happens during Telophase II?
Four cells are produced.
Each cell contains half of the original cell’s numbers.
Nuclear membrane forms around the chromosomes.

26. Independent assortment PROPHASE 1

27. Summary of Meiosis Consists of 2 cell divisions.
Starts with 2 copies of each chromosome, each with 2 chromatids (a tetrad).
In meiosis I, crossing over in prophase mixes alleles between the homologous chromosomes.
In metaphase I, homologous chromosomes pair up randomly and independently.
Meiosis I produces two haploid cells.
Meiosis II is just like mitosis in that it splits the “sister chromatids”.
The centromeres divide in anaphase II, giving rise to a total of 4 cells, each with 1 copy of each chromosome, and each chromosome with only 1 chromatid.

28. Questions? What is the significance of meiosis to sexual reproduction?
If an organism’s body cells contain 48 chromosomes, how many chromosomes will its gamete have?
How many chromosomes do fertilized eggs contain?
Are fertilized eggs haploid cells or diploid cells?
During which stage of meiosis does crossing over take place?
How does meiosis lead to increase genetic variation
What is the significance of meiosis to sexual reproduction? Meiosis reduces the chromosome number in gametes. When sexual reproduction occurs, the zygote recombines the chromosome and maintains the 2n condition.
If human somatic cells have 46 chromosomes, how many chromosomes do human gametes have? Human gametes (sex cells) are haploid cells, meaning that they have only one complete set of chromosomes. (23)
How many chromosomes do fertilized eggs contain? Are fertilized eggs haploid cells or diploid cells? When fertilization occurs, the haploid sperm cell and haploid egg cell join, producing a fertilized zygote. This"restores" the diploid chromosome number.

29. Questions?
6. What is the result of the first meiotic division? 7. What is the result of the second meiotic division? 8. During which stage of meiosis is the diploid number of chromosomes reduced to the haploid number of chromosomes?

30. Mitosis (vs) Meiosis

31. DO NOW

32. Mitosis vs. Meiosis Mitosis Meiosis
DNA is replicated during interphase
Cytokinesis
Results in
2 Diploid Cells (2N)
4 Haploid Cells (N)
Cells are
Genetically Identical
Genetically Different
Occurs in
Somatic Cells
Sex Cells
Crossing-over
Does not occur
Occurs during prophase

33. Diploid
Haploid

35. Questions?
How are the cells at the end of meiosis different from the cells at the beginning of meiosis? Use the terms chromosomes number, haploid, and diploid in your answer.

36. Questions?
How are meiosis and mitosis different and similar?

37. Compare and Contrast Mitosis Meiosis Cell Division Cytokinesis PMAT
Growth and Repair
Diploid (2n)
Somatic (body) Cells
1 division
Identical Cells
Reproduction
Haploid (n)
Gametes (Sex Cells)
2 divisions
Genetic Diversity