end show slide 1 of 35 11-4 meiosis. end show slide 2 of 35 where are genes located? on chromosomes...
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11-4 Meiosis11-4 Meiosis
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Where are genes located?
• On chromosomes in the cell nucleus
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Chromosome Number
Chromosome Number
All organisms have different numbers of chromosomes.
A body cell in an adult fruit fly (Drosophila) has 8 chromosomes: 4 from the fruit fly's male parent, and 4 from its female parent.
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Chromosome Number
These sets of chromosomes are homologous.
Which means each of the 4 chromosomes that came from the male parent has a corresponding chromosome from the female parent.
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Chromosome Number
A cell that contains both sets of homologous chromosomes is said to be diploid = 2 sets.
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.
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Haploid
• In contrast, gametes (sex cells) of sexually reproducing organisms, including fruit flies contain only a single set of chromosomes and a single set of genes.
• Haploid = 1 set (or half the number of chromosomes)
• Can be written as N = 4
• N represents the word Haploid
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Phases of Meiosis
Phases of 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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11-4 Meiosis
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Meiosis involves two divisions, meiosis I and meiosis II.
By the end of meiosis II, the diploid cell that entered meiosis has become 4 haploid cells.
Turn to pg. 276 & 277 in the book!
Phases of Meiosis
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11-4 Meiosis
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Phases of Meiosis
Prior to Meiosis I, each chromosome is replicated in Interphase I.
Meiosis I
Prophase I Metaphase I Anaphase I Telophase I and
Cytokinesis
Interphase I
Meiosis I
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Phases of Meiosis
Cells undergo a round of DNA replication, forming duplicate chromosomes.
Interphase I
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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 I
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11-4 Meiosis
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Phases of Meiosis
As homologous chromosomes pair up & form tetrads in meiosis I, they exchange portions of their chromatids in a process called crossing over.
Crossing-over produces new combinations of alleles (the different forms of a gene).
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• Metaphase, Anaphase, Telophase, and Cytokinesis all resemble the patterns of mitosis.
• Metaphase- Middle
• Anaphase- Away
• Telophase & Cytokinesis- Two
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11-4 Meiosis
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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 Interphase again.
Each of the cell’s chromosomes now have 2 chromatids instead of 4.
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Phases of Meiosis
Meiosis II
Telophase II and
CytokinesisProphase II
Metaphase II Anaphase IITelophase I and Cytokinesis I
Meiosis II
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Phases of Meiosis
Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original cell.
MEIOSIS IIProphase II
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Phases of Meiosis
The chromosomes line up in the center of cell.
MEIOSIS II Metaphase II
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Phases of Meiosis
The sister chromatids separate and move toward opposite ends of the cell.
MEIOSIS II Anaphase II
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11-4 Meiosis
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Phases of Meiosis
Meiosis II results in four haploid (N) daughter cells.
MEIOSIS II Telophase II and Cytokinesis
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Gamete Formation
Gamete Formation
In male animals, 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.
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Comparing Mitosis and Meiosis
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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Comparing Mitosis and Meiosis
Mitosis
1.A diploid cell that divides by mitosis gives rise to 2 diploid (2N) daughter cells.
2.The daughter cells have: chromosomes and alleles identical to one another and to the original cell.
3.Allows an organism’s body to grow and replace cells.
4.In ASEXUAL REPRODUCTION, a new organism is produced by mitosis of the cell or cells of the parent organism.
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Comparing Mitosis and Meiosis
Meiosis
• Begins with a diploid cell but produces 4 haploid (N) cells.
• The new cells are genetically different from the diploid cell and from one another.
• This is how SEXUALLY reproducing organisms produce gametes
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SO…..
• Did Mrs. Dayton and Mr. Dayton’s cells go through mitosis or meiosis?
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11-4
If the body cells of humans contain 46 chromosomes, a single sperm cell should have
a. 46 chromosomes.
b. 23 chromosomes.
c. 92 chromosomes.
d. between 23 and 46 chromosomes.
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11-4
During meiosis, the number of chromosomes per cell is cut in half through the separation of
a. daughter cells.
b. homologous chromosomes.
c. gametes.
d. chromatids.
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11-4
The formation of a tetrad occurs during
a. anaphase I.
b. metaphase II.
c. prophase I.
d. prophase II.
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11-4
In many female animals, meiosis results in the production of
a. only 1 egg.
b. 1 egg and 3 polar bodies.
c. 4 eggs.
d. 1 egg and 2 polar bodies.
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