1. Copyright Pearson Prentice Hall
11-4 Meiosis
11-4 Meiosis
Copyright Pearson Prentice Hall

2. Copyright Pearson Prentice Hall
Each organism must inherit a single copy (or allele) of every gene from each of its “parents.”
That means that gametes must be formed by a process that separates the two alleles of each gene so that each gamete ends up with just one allele.
Copyright Pearson Prentice Hall

3. Chromosome Number
Review:
Chromosomes—strands of DNA and protein inside the cell nucleus—are the carriers of genes.
The genes are located in specific positions on chromosomes.

4. Copyright Pearson Prentice Hall
Chromosome Number
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, and 4 from its female parent.
These chromosomes are from a fruit fly. Each of the fruit fly’s body cells has 8 chromosomes.
Copyright Pearson Prentice Hall

5. Copyright Pearson Prentice Hall
Chromosome Number
The chromosomes can be arranged in pairs. Each of the 4 chromosomes that came from the male parent has a corresponding chromosome (carrying the genes for the same traits) from the female parent.
These two sets chromosomes are homologous.
Copyright Pearson Prentice Hall

6. Copyright Pearson Prentice Hall
Chromosome Number
A cell that contains both sets of homologous chromosomes is said to be diploid.
The number of chromosomes in a diploid cell is sometimes represented by the symbol 2N.
For Drosophila, the diploid number is 8, which can be written as 2N=8.
The diploid cells of most adult organisms contain two complete sets of inherited chromosomes and two complete sets of genes.
Copyright Pearson Prentice Hall

7. Copyright Pearson Prentice Hall
Chromosome Number
The gametes of sexually reproducing organisms contain only a single set of chromosomes, and therefore only a single set of genes.
These cells are haploid. Haploid cells are represented by the symbol N.
For Drosophila, the haploid number is 4, which can be written as N=4.
Copyright Pearson Prentice Hall

8. Copyright Pearson Prentice Hall
Phases of Meiosis
Phases of Meiosis
Meiosis is a process of reduction division in which the number of chromosomes per cell (2N) is cut in half through the separation of homologous chromosomes in a diploid cell.
Gametes are produced in meiosis.
Copyright Pearson Prentice Hall

9. Copyright Pearson Prentice Hall
Phases of Meiosis
Meiosis involves two divisions, meiosis I and meiosis II.
At the end of meiosis I, there are 2 haploid cells. By the end of meiosis II, the diploid cell that entered meiosis has become 4 haploid cells.
Copyright Pearson Prentice Hall

10. Telophase I and Cytokinesis
Phases of Meiosis
Meiosis I
Meiosis I
Interphase I
During meiosis, the number of chromosomes per cell is cut in half through the separation of the homologous chromosomes. The result of meiosis is 4 haploid cells that are genetically different from one another and from the original cell.
Prophase I
Metaphase I
Anaphase I
Telophase I and Cytokinesis
Copyright Pearson Prentice Hall

11. Copyright Pearson Prentice Hall
Phases of Meiosis
Cells undergo a round of DNA replication, forming duplicate chromosomes.
Interphase I
Interphase I - Cells undergo a round of DNA replication, forming duplicate chromosomes.
Copyright Pearson Prentice Hall

12. Copyright Pearson Prentice Hall
Phases of Meiosis
Each chromosome pairs with its corresponding homologous chromosome to form a tetrad.
There are 4 chromatids in a tetrad.
MEIOSIS I
Prophase I
MEIOSIS I Prophase I - Each chromosome pairs with its corresponding homologous chromosome to form a tetrad.
Copyright Pearson Prentice Hall

13. Copyright Pearson Prentice Hall
Phases of Meiosis
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.
Crossing-over occurs during meiosis. (1) Homologous chromosomes form a tetrad. (2) Chromatids cross over one another. (3) The crossed sections of the chromatids are exchanged.
Copyright Pearson Prentice Hall

14. Copyright Pearson Prentice Hall
Phases of Meiosis
Spindle fibers attach to the chromosomes.
MEIOSIS I
Metaphase I
MEIOSIS I Metaphase I - Spindle fibers attach to the chromosomes.
Copyright Pearson Prentice Hall

15. Copyright Pearson Prentice Hall
Phases of Meiosis
MEIOSIS I
Anaphase I
The fibers pull the homologous chromosomes toward opposite ends of the cell.
MEIOSIS I Anaphase I - The fibers pull the homologous chromosomes toward opposite ends of the cell.
Copyright Pearson Prentice Hall

16. Copyright Pearson Prentice Hall
Phases of Meiosis
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.
MEIOSIS I Telophase I and Cytokinesis - Nuclear membranes form. The cell separates into two cells.
Copyright Pearson Prentice Hall

17. Copyright Pearson Prentice Hall
Phases of Meiosis
Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original cell.
Because each pair of homologous chromosomes was separated, neither daughter cell has the two complete sets of chromosomes.
MEIOSIS II Prophase II - Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original cell.
Copyright Pearson Prentice Hall

18. Copyright Pearson Prentice Hall
Phases of Meiosis
Meiosis II
The two cells produced by meiosis I now enter a second meiotic division.
Unlike meiosis I, neither cell goes through chromosome replication.
Remember: Each of the cell’s chromosomes has 2 chromatids (from replication that occurred back at the beginning of meiosis.)
Copyright Pearson Prentice Hall

19. Telophase II and Cytokinesis
Phases of Meiosis
Meiosis II
During meiosis, the number of chromosomes per cell is cut in half through the separation of the homologous chromosomes. The result of meiosis is 4 haploid cells that are genetically different from one another and from the original cell.
Meiosis II
Telophase I and Cytokinesis I
Metaphase II
Anaphase II
Telophase II and Cytokinesis
Prophase II
Copyright Pearson Prentice Hall

20. Copyright Pearson Prentice Hall
Phases of Meiosis
MEIOSIS II
Prophase II
Metaphase II
A new spindle forms.
The chromosomes line up in the center of cell.
MEIOSIS II Metaphase II - The chromosomes line up in a similar way to the metaphase state of mitosis.
Copyright Pearson Prentice Hall

21. Copyright Pearson Prentice Hall
Phases of Meiosis
MEIOSIS II
Anaphase II
The sister chromatids separate and move toward opposite ends of the cell.
MEIOSIS II Anaphase II - The sister chromatids separate and move toward opposite ends of the cell.
Copyright Pearson Prentice Hall

22. Copyright Pearson Prentice Hall
Phases of Meiosis
MEIOSIS II
Telophase II and Cytokinesis
Meiosis II results in four haploid (N) daughter cells.
MEIOSIS II Telophase II and Cytokinesis - Meiosis II results in four haploid (N) daughter cells.
Copyright Pearson Prentice Hall

23. Copyright Pearson Prentice Hall
Gamete Formation
Gamete Formation
In male animals, meiosis results in four equal-sized gametes called sperm.
Meiosis produces four genetically different haploid cells. In males, meiosis results in four equal-sized gametes called sperm.
Copyright Pearson Prentice Hall

24. Copyright Pearson Prentice Hall
Gamete Formation
In many female animals, only one egg results from meiosis. The other three cells, called polar bodies, are usually not involved in reproduction.
Meiosis produces four genetically different haploid cells. In females, only one large egg cell results from meiosis. The other three cells, called polar bodies, usually are not involved in reproduction.
Copyright Pearson Prentice Hall

25. Gametes to Zygotes
Fertilization—the fusion of male and female gametes—generates new combinations of alleles in a zygote.
The zygote undergoes cell division by mitosis and eventually forms a new organism.

26. Comparing Mitosis and Meiosis
In mitosis, when the two sets of genetic material separate, each daughter cell receives one complete set of chromosomes. In meiosis, homologous chromosomes line up and then move to separate daughter cells.
Mitosis does not normally change the chromosome number of the original cell. This is not the case for meiosis, which reduces the chromosome number by half.

27. Comparing Mitosis and Meiosis
Mitosis results in the production of two genetically identical diploid cells. Meiosis produces four genetically different haploid cells.
Copyright Pearson Prentice Hall

28. Comparing Mitosis and Meiosis
Cells produced by mitosis have the same number of chromosomes and alleles as the original cell.
Mitosis is a single cell division.
Mitosis allows an organism to grow and replace cells.
Some organisms reproduce asexually by mitosis.
Copyright Pearson Prentice Hall

29. Comparing Mitosis and Meiosis
Cells produced by meiosis have half the number of chromosomes as the parent cell.
Meiosis requires two rounds of cell division.
These cells are genetically different from the diploid cell and from each other.
Meiosis is how sexually-reproducing organisms produce gametes.
Copyright Pearson Prentice Hall

30. Copyright Pearson Prentice Hall
Gene Linkage
Gene Linkage
Thomas Hunt Morgan’s research on fruit flies led him to the principle of linkage.
Morgan discovered that many of the more than 50 Drosophila genes he had identified appeared to be “linked” (inherited) together.
They seemed to violate the principle of independent assortment.
Copyright Pearson Prentice Hall

31. Gene Linkage
For example, Morgan used a fly with reddish-orange eyes and miniature wings in a series of test crosses.
His results showed that the genes for those two traits were almost always inherited together.
Only rarely did the genes separate from each other.

32. Copyright Pearson Prentice Hall
Gene Linkage
Morgan and his associates grouped all of the fly’s genes into four linkage groups.
Each group assorted independently from the other groups BUT all the genes in one group were inherited together.
As it turns out, Drosophila has four linkage groups and four pairs of chromosomes.
Copyright Pearson Prentice Hall

33. Gene Linkage Morgan’s findings led to two remarkable conclusions:
First, each chromosome is actually a group of linked genes.
Second, it is the chromosomes that assort independently, not individual genes.
Alleles of different genes tend to be inherited together when those genes are located on the same chromosome.

34. Copyright Pearson Prentice Hall
Gene Maps
Gene Mapping
Remember that:
Crossing-over during meiosis sometimes separates genes that had been on the same chromosomes onto the homologous chromosome.
Crossover events occasionally separate linked genes and produce new combinations of alleles.
Copyright Pearson Prentice Hall

35. Copyright Pearson Prentice Hall
Gene Maps
Gene Mapping
Alfred Sturtevant, a student of Morgan, reasoned that the farther apart two genes were on a chromosome, the more likely it would be that a crossover event would occur between them.
If two genes are close together, then crossovers between them should be rare. If two genes are far apart, then crossovers between them should be more common.
Copyright Pearson Prentice Hall

36. Gene Mapping
By this reasoning, he could use the frequency of crossing-over between genes to determine their distances from each other.
Sturtevant gathered lab data and presented a gene map showing the relative locations of each known gene on one of the Drosophila chromosomes.
Sturtevant’s method has been used to construct gene maps ever since this discovery.