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11-4 Meiosis
11-4 Meiosis
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You have somatic cells and gametes.
Body cells are also called somatic cells and make up most of the body tissues and organs.
Not passed on to offspring.
Germ cells develop into gametes.
Germ cells are located in the ovaries and testes. Gametes are sex cells: egg and sperm (sex cells!)
Gametes have DNA that can be passed to offspring.
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3. In humans, there are 23 pairs of chromosomes (or 46 chromosomes).
Chromosome Number
In humans, there are 23 pairs of chromosomes (or 46 chromosomes).
Your cells have autosomes and sex chromosomes.
Autosomes are chromosomes that contain genes not directly related to the sex of an organism.

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Chromosome Number
Homologous chromosomes are pairs of chromosomes that have the same structure.
For each homologous pair, one chromosome comes from each parent.
Chromosome pairs 1-22 are autosomes.
Sex chromosomes, X and Y, determine gender in mammals. XX=female; XY=male.
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Chromosome Number
A cell that contains both sets of homologous chromosomes is said to be diploid. (di=two)
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. For humans, the diploid number is 46, which can be written as 2N=46.
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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. For humans, the haploid number is 23, which can be written as N=23.
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Phases of Meiosis
Meiosis
Meiosis is a process of reduction division in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell.
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8. Meiosis differs from mitosis in significant ways.
Meiosis has two cell divisions while mitosis has one.
In mitosis, homologous chromosomes never pair up.
Meiosis results in haploid cells; mitosis results in diploid cells.

9. Cells go through two rounds of division in meiosis.
Meiosis reduces chromosome number and creates genetic diversity.

10. Pairs of homologous chromosomes separate in meiosis I.
Meisois involves 2 divisions. Meiosis I and meiosis II each have four phases, similar to those in mitosis.
Pairs of homologous chromosomes separate in meiosis I.
Homologous chromosomes are similar but not identical.
Sister chromatids divide in meiosis II.
Sister chromatids are copies of the same chromosome.
homologous chromosomes
sister
chromatids
sister
chromatids

11. Meiosis I occurs after DNA has been replicated.
Meiosis I divides homologous chromosomes in four phases.

12. Meiosis II divides sister chromatids in four phases.
DNA is not replicated between meiosis I and meiosis II.

13. 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
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Phases of Meiosis
Cells undergo a round of DNA replication, forming duplicate chromosomes.
Meiosis I occurs after DNA has been replicated.
Interphase I
Interphase I - Cells undergo a round of DNA replication, forming duplicate chromosomes.
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Phases of Meiosis
Chromosomes condense
Homologous chromosomes pair up
The nuclear envelope breaks down and spindle fibers form.
MEIOSIS I
Prophase I
MEIOSIS I Prophase I - Each chromosome pairs with its corresponding homologous chromosome to form a tetrad.
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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.
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Phases of Meiosis
Spindle fibers attach to the homologous chromosomes and align along the equator.
MEIOSIS I
Metaphase I
MEIOSIS I Metaphase I - Spindle fibers attach to the chromosomes.
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Phases of Meiosis
MEIOSIS I
Anaphase I
The fibers pull the homologous chromosomes toward opposite ends of the cell
the sister chromatids remain attached together.
MEIOSIS I Anaphase I - The fibers pull the homologous chromosomes toward opposite ends of the cell.
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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.
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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.
Each of the cell’s chromosomes has 2 chromatids.
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21. 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
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Phases of Meiosis
MEIOSIS II
Prophase II
Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original cell.
The nuclear envelope breaks and spindle fibers form.
MEIOSIS II Prophase II - Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original cell.
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Phases of Meiosis
MEIOSIS II
Metaphase II
Spindle fibers align and 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.
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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.
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Phases of Meiosis
MEIOSIS II
Telophase II and Cytokinesis
Nuclear membranes form around chromosomes, chromosomes begin to uncoil, spindle fibers fall apart and the cell undergoes 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.
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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.
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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.
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28. Comparing Mitosis and Meiosis
Mitosis results in the production of two genetically identical diploid cells. Meiosis produces four genetically different haploid cells.
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29. Comparing Mitosis and Meiosis
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30. Independent assortment and crossing over during meiosis result in genetic diversity.
Sexual reproduction creates unique combination of genes.
independent assortment of chromosomes in meiosis
random fertilization of gametes
Unique phenotypes may give a reproductive advantage to some organisms.

31. Fertilization is in itself a random process and ensures a different combination of genes each time, depending on which sperm or pollen combines with the egg.
In humans, the chance of getting any combination of chromosomes from one set of parents is 223 x 223 or one in 64 trillion.
When homologous chromosomes, one from each parent, pair up along the equator during metaphase 1 of meiosis, the particular arrangement is determined by chance.
In humans, about 223 or 8 million combinations of chromosomes could result.

32. Crossing over during meiosis increases genetic diversity.
Crossing over is the exchange of chromosome segments between homologous chromosomes.
occurs during prophase I of meiosis I
results in new combinations of genes

33. Chromosomes contain many genes. Genetic Linkage:
The farther apart two genes are located on a chromosome, the more likely they are to be separated by crossing over.
Genes located close together on a chromosome tend to be inherited together, which is called genetic linkage.
Not found by Mendel because he studied traits on separate chromosomes or traits located far apart on the same chromosome which means not all genes follow the law of independent assortment.

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11-4
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If the body cells of humans contain 46 chromosomes, a single sperm cell should have
46 chromosomes.
23 chromosomes.
92 chromosomes.
between 23 and 46 chromosomes.
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During meiosis, the number of chromosomes per cell is cut in half through the separation of
daughter cells.
homologous chromosomes.
gametes.
chromatids.
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11-4
The formation of a tetrad occurs during
anaphase I.
metaphase II.
prophase I.
prophase II.
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In many female animals, meiosis results in the production of
only 1 egg.
1 egg and 3 polar bodies.
4 eggs.
1 egg and 2 polar bodies.
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Compared to egg cells formed during meiosis, daughter cells formed during mitosis are
genetically different, while eggs are genetically identical.
genetically different, just as egg cells are.
genetically identical, just as egg cells are.
genetically identical, while egg cells are genetically different.
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