BIOLOGI-MEIOSIS

The Biology Project > Cell Biology > Meiosis > Meiosis I & II Meiosis Tutorial Meiosis I & II What is meiosis I? In meiosis I, chromosomes in a diploid cell resegregate, producing four haploid daughter cells. It is this step in meiosis that generates genetic diversity. The phases of meiosis I & II Prophase I DNA replication precedes the start of meiosis I. During prophase I, homologous chromosomes pair and form synapses, a step unique to meiosis. The paired chromosomes are called bivalents, and the formation of chiasmata caused by genetic recombination becomes apparent. Chromosomal condensation allows these to be viewed in the microscope. Note that the bivalent has two chromosomes and four chromatids, with one chromosome coming from each parent. Prometaphase I The nuclear membrane disappears. One kinetochore forms per chromosome rather than one per chromatid, and the chromosomes attached to spindle fibers begin to move. Metaphase I Bivalents, each composed of two chromosomes (four chromatids) align at the metaphase plate. The orientation is random, with either parental homologue on a side. This means that there is a 50-50 chance for the daughter cells to get either the mother's or father's homologue for each chromosome. Anaphase I Chiasmata separate. Chromosomes, each with two chromatids, move to separate poles. Each of the daughter cells is now haploid (23 chromosomes), but each chromosome has two chromatids. Telophase I Nuclear envelopes may reform, or the cell may quickly start meiosis II. Cytokinesis Analogous to mitosis where two complete daughter cells form. Meiosis I Animation (360 kb) Meiosis II Animation (360 kb) Meiosis II is similar to mitosis. However, there is no "S" phase. The chromatids of each chromosome are no longer identical because of recombination. Meiosis II separates the chromatids producing two daughter cells each with 23 chromosomes (haploid), and each chromosome has only one chromatid. Previous | Next Vocabulary The Biology Project > Cell Biology > Meiosis > Meiosis 1 & 2 The Biology Project Department of Biochemistry and Molecular Biophysics University of Arizona April 1997 Revised: August 2004 Contact the Development Team http://www.biology.arizona.edu All contents copyright © 1997 - 2004. All rights reserved.

Meiosis Tutorial

A review of meiosis

Comparing Meiosis and Mitosis

• Chromosome behavior
1. Mitosis: Homologous chromosomes independent
2. Meiosis: Homologous chromosomes pair forming bivalents until anaphase I
• Chromosome number- reduction in meiosis
1. Mitosis- identical daughter cells
2. Meiosis- daughter cells haploid
• Genetic identity of progeny:
1. Mitosis: identical daughter cells
2. Meiosis: daughter cells have new assortment of parental chromosomes
3. Meiosis: chromatids not identical, crossing over

Meiotic errors

• Nondisjunction- homologues don't separate in meiosis 1
1. Results in aneuploidy
2. Usually embryo lethal
3. Trisomy 21, exception leading to Downs syndrome
4. Sex chromosomes
1. Turner syndrome: monosomy X
2. Klinefelter syndrome: XXY
• Translocation and deletion: transfer of a piece of one chromosome to another or loss of fragment of a chromosome.

Mitosis, Meiosis, and Ploidy

• Mitosis can proceed independent of ploidy of cell, homologous chromosomes behave independently
• Meiosis can only proceed if the nucleus contains an even number of chromosomes (diploid, tetraploid).
• Trisomy 21 does not prevent meiosis

The Biology Project > Cell Biology > Meiosis > Meiosis I & II Meiosis Tutorial Meiosis I & II What is meiosis I? In meiosis I, chromosomes in a diploid cell resegregate, producing four haploid daughter cells. It is this step in meiosis that generates genetic diversity. The phases of meiosis I & II Prophase I DNA replication precedes the start of meiosis I. During prophase I, homologous chromosomes pair and form synapses, a step unique to meiosis. The paired chromosomes are called bivalents, and the formation of chiasmata caused by genetic recombination becomes apparent. Chromosomal condensation allows these to be viewed in the microscope. Note that the bivalent has two chromosomes and four chromatids, with one chromosome coming from each parent. Prometaphase I The nuclear membrane disappears. One kinetochore forms per chromosome rather than one per chromatid, and the chromosomes attached to spindle fibers begin to move. Metaphase I Bivalents, each composed of two chromosomes (four chromatids) align at the metaphase plate. The orientation is random, with either parental homologue on a side. This means that there is a 50-50 chance for the daughter cells to get either the mother's or father's homologue for each chromosome. Anaphase I Chiasmata separate. Chromosomes, each with two chromatids, move to separate poles. Each of the daughter cells is now haploid (23 chromosomes), but each chromosome has two chromatids. Telophase I Nuclear envelopes may reform, or the cell may quickly start meiosis II. Cytokinesis Analogous to mitosis where two complete daughter cells form. Meiosis I Animation (360 kb) Meiosis II Animation (360 kb) Meiosis II is similar to mitosis. However, there is no "S" phase. The chromatids of each chromosome are no longer identical because of recombination. Meiosis II separates the chromatids producing two daughter cells each with 23 chromosomes (haploid), and each chromosome has only one chromatid. Previous | Next Vocabulary The Biology Project > Cell Biology > Meiosis > Meiosis 1 & 2 The Biology Project Department of Biochemistry and Molecular Biophysics University of Arizona April 1997 Revised: August 2004 Contact the Development Team http://www.biology.arizona.edu All contents copyright © 1997 - 2004. All rights reserved.

Meiosis Tutorial

A review of meiosis

Comparing Meiosis and Mitosis

• Chromosome behavior
1. Mitosis: Homologous chromosomes independent
2. Meiosis: Homologous chromosomes pair forming bivalents until anaphase I
• Chromosome number- reduction in meiosis
1. Mitosis- identical daughter cells
2. Meiosis- daughter cells haploid
• Genetic identity of progeny:
1. Mitosis: identical daughter cells
2. Meiosis: daughter cells have new assortment of parental chromosomes
3. Meiosis: chromatids not identical, crossing over

Meiotic errors

• Nondisjunction- homologues don't separate in meiosis 1
1. Results in aneuploidy
2. Usually embryo lethal
3. Trisomy 21, exception leading to Downs syndrome
4. Sex chromosomes
1. Turner syndrome: monosomy X
2. Klinefelter syndrome: XXY
• Translocation and deletion: transfer of a piece of one chromosome to another or loss of fragment of a chromosome.

Mitosis, Meiosis, and Ploidy

• Mitosis can proceed independent of ploidy of cell, homologous chromosomes behave independently
• Meiosis can only proceed if the nucleus contains an even number of chromosomes (diploid, tetraploid).
• Trisomy 21 does not prevent meiosis

The Biology Project > Cell Biology > Meiosis > Meiosis I & II Meiosis Tutorial Meiosis I & II What is meiosis I? In meiosis I, chromosomes in a diploid cell resegregate, producing four haploid daughter cells. It is this step in meiosis that generates genetic diversity. The phases of meiosis I & II Prophase I DNA replication precedes the start of meiosis I. During prophase I, homologous chromosomes pair and form synapses, a step unique to meiosis. The paired chromosomes are called bivalents, and the formation of chiasmata caused by genetic recombination becomes apparent. Chromosomal condensation allows these to be viewed in the microscope. Note that the bivalent has two chromosomes and four chromatids, with one chromosome coming from each parent. Prometaphase I The nuclear membrane disappears. One kinetochore forms per chromosome rather than one per chromatid, and the chromosomes attached to spindle fibers begin to move. Metaphase I Bivalents, each composed of two chromosomes (four chromatids) align at the metaphase plate. The orientation is random, with either parental homologue on a side. This means that there is a 50-50 chance for the daughter cells to get either the mother's or father's homologue for each chromosome. Anaphase I Chiasmata separate. Chromosomes, each with two chromatids, move to separate poles. Each of the daughter cells is now haploid (23 chromosomes), but each chromosome has two chromatids. Telophase I Nuclear envelopes may reform, or the cell may quickly start meiosis II. Cytokinesis Analogous to mitosis where two complete daughter cells form. Meiosis I Animation (360 kb) Meiosis II Animation (360 kb) Meiosis II is similar to mitosis. However, there is no "S" phase. The chromatids of each chromosome are no longer identical because of recombination. Meiosis II separates the chromatids producing two daughter cells each with 23 chromosomes (haploid), and each chromosome has only one chromatid. Previous | Next Vocabulary The Biology Project > Cell Biology > Meiosis > Meiosis 1 & 2 The Biology Project Department of Biochemistry and Molecular Biophysics University of Arizona April 1997 Revised: August 2004 Contact the Development Team http://www.biology.arizona.edu All contents copyright © 1997 - 2004. All rights reserved.

Meiosis Tutorial

A review of meiosis

Comparing Meiosis and Mitosis

• Chromosome behavior
1. Mitosis: Homologous chromosomes independent
2. Meiosis: Homologous chromosomes pair forming bivalents until anaphase I
• Chromosome number- reduction in meiosis
1. Mitosis- identical daughter cells
2. Meiosis- daughter cells haploid
• Genetic identity of progeny:
1. Mitosis: identical daughter cells
2. Meiosis: daughter cells have new assortment of parental chromosomes
3. Meiosis: chromatids not identical, crossing over

Meiotic errors

• Nondisjunction- homologues don't separate in meiosis 1
1. Results in aneuploidy
2. Usually embryo lethal
3. Trisomy 21, exception leading to Downs syndrome
4. Sex chromosomes
1. Turner syndrome: monosomy X
2. Klinefelter syndrome: XXY
• Translocation and deletion: transfer of a piece of one chromosome to another or loss of fragment of a chromosome.

Mitosis, Meiosis, and Ploidy

• Mitosis can proceed independent of ploidy of cell, homologous chromosomes behave independently
• Meiosis can only proceed if the nucleus contains an even number of chromosomes (diploid, tetraploid).
• Trisomy 21 does not prevent meiosis

The Biology Project > Cell Biology > Meiosis > Meiosis I & II Meiosis Tutorial Meiosis I & II What is meiosis I? In meiosis I, chromosomes in a diploid cell resegregate, producing four haploid daughter cells. It is this step in meiosis that generates genetic diversity. The phases of meiosis I & II Prophase I DNA replication precedes the start of meiosis I. During prophase I, homologous chromosomes pair and form synapses, a step unique to meiosis. The paired chromosomes are called bivalents, and the formation of chiasmata caused by genetic recombination becomes apparent. Chromosomal condensation allows these to be viewed in the microscope. Note that the bivalent has two chromosomes and four chromatids, with one chromosome coming from each parent. Prometaphase I The nuclear membrane disappears. One kinetochore forms per chromosome rather than one per chromatid, and the chromosomes attached to spindle fibers begin to move. Metaphase I Bivalents, each composed of two chromosomes (four chromatids) align at the metaphase plate. The orientation is random, with either parental homologue on a side. This means that there is a 50-50 chance for the daughter cells to get either the mother's or father's homologue for each chromosome. Anaphase I Chiasmata separate. Chromosomes, each with two chromatids, move to separate poles. Each of the daughter cells is now haploid (23 chromosomes), but each chromosome has two chromatids. Telophase I Nuclear envelopes may reform, or the cell may quickly start meiosis II. Cytokinesis Analogous to mitosis where two complete daughter cells form. Meiosis I Animation (360 kb) Meiosis II Animation (360 kb) Meiosis II is similar to mitosis. However, there is no "S" phase. The chromatids of each chromosome are no longer identical because of recombination. Meiosis II separates the chromatids producing two daughter cells each with 23 chromosomes (haploid), and each chromosome has only one chromatid. Previous | Next Vocabulary The Biology Project > Cell Biology > Meiosis > Meiosis 1 & 2 The Biology Project Department of Biochemistry and Molecular Biophysics University of Arizona April 1997 Revised: August 2004 Contact the Development Team http://www.biology.arizona.edu All contents copyright © 1997 - 2004. All rights reserved.

Meiosis Tutorial

A review of meiosis

Comparing Meiosis and Mitosis

• Chromosome behavior
1. Mitosis: Homologous chromosomes independent
2. Meiosis: Homologous chromosomes pair forming bivalents until anaphase I
• Chromosome number- reduction in meiosis
1. Mitosis- identical daughter cells
2. Meiosis- daughter cells haploid
• Genetic identity of progeny:
1. Mitosis: identical daughter cells
2. Meiosis: daughter cells have new assortment of parental chromosomes
3. Meiosis: chromatids not identical, crossing over

Meiotic errors

• Nondisjunction- homologues don't separate in meiosis 1
1. Results in aneuploidy
2. Usually embryo lethal
3. Trisomy 21, exception leading to Downs syndrome
4. Sex chromosomes
1. Turner syndrome: monosomy X
2. Klinefelter syndrome: XXY
• Translocation and deletion: transfer of a piece of one chromosome to another or loss of fragment of a chromosome.

Mitosis, Meiosis, and Ploidy

• Mitosis can proceed independent of ploidy of cell, homologous chromosomes behave independently
• Meiosis can only proceed if the nucleus contains an even number of chromosomes (diploid, tetraploid).
• Trisomy 21 does not prevent meiosis