(no subject)
Jennifer Martin
1st Period
Dr. D
Meiosis
Meiosis is needed for sexual reproduction and yields four haploid daughter cells, which are cells with one member of each homologous chromosome pair. Meiosis involves two nuclear and cytoplasmic divisions, and undergoes two consecutive divisions called meiosis I and meiosis II.
The two divisions of meiosis (meiosis I and meiosis II) follow only one duplication of the chromosomes. Because of this, each of the four produced daughter cells resulting from meiosis has only half as many chromosomes as the mother cell-a single haploid set of chromosomes.
Meiosis is preceded by an interphase. It is during the interphase that chromosomal duplication takes place, and at the end of the interphase, each chromosome consists of two genetically attached identical sister chromatids and the cell’s centrosomes is duplicated. The chromosomes are seen under the microscope as a mass of chromatin. There are four different stages of meiosis I: prophase I, metaphase I, anaphase I, telophase I. All the events that are unique to meiosis happen during meiosis I. In prophase I, duplicated homologous (nonsister) chromosomes pair together to form tetrads by synapsis, and crossing over occurs between the chromosomes because of an exchanging of segments. This rearranges genetic information, making important contribution to the genetic variability from sexual reproduction. Because the chromatin coils early in this phase, the chromosomes are easily seen through the use of a microscope. Prophase is the most complex phase, occupying over ninety percent of the time required for meiotic cell division. The chromosomes condense further as the nucleoli disappear as prophase progresses, and the centrosomes move away from each other. A spindle will then begin to form between them, and then nuclear envelope will break into fragments. The chromosome tetrads will be moved toward the center of the cell. During metaphase I, tetrads line up at the center of the cell. The chromosomes are condensed and thick, with the sister chromatids attached at their centromere. Spindle microtubules are attached to kinetochores. The spindle microtubules attached to one of the nonsister chromosomes come from one pole of the cell (for each tetrad) and the microtubules attached to the other homologous chromosome come from the opposite side. The homologous chromosomes of each tetrad are then prepared to move toward opposite poles of the cell. In anaphase I, sister chromatids of each chromosome stay united and move to the same pole of the cell while nonsister pairs of chromosomes separate. The sister chromatids making up each duplicated chromosome remain attached at their centromeres. Only the tetrads split up. In telophase I, the chromosomes enter at the poles of the cell. Each chromosome still consists of two sister chromatids. Cytokinesis occurs with telophase I, forming two haploid daughter cells. In some organisms, the chromosomes uncoil and the nuclear envelope redevelops, and there is an interphase before meiosis II begins. In other species, daughter cells immediately prepare for meiosis II. No chromosome duplication, however, ever occurs between telophase I and meiosis II. When meiosis I is complete, there are products of two haploid cells, but each chromosome has two sister chromatids, and meiosis II occurs. In meiosis II, there is no further chromosomal replication, but most importantly, meiosis II begins with a haploid cell where meiosis I does not. The nuclear envelope breaks down during prophase II, and a spindle forms and transports the chromosomes to the center of the cell. In metaphase II, the chromosomes are aligned in the middle of the cell. Sister chromatids separate during anaphase II and move toward opposite poles of the cell. In telophase II, nuclei forms at the cell poles, and cytokinesis occurs simultaneously. There are two daughter cells as products in meiosis II for each daughter cell of meiosis I.
An accident in gametes by meiosis is called a nondisjunction, in which the members of a chromosome pair do not separate. Sometimes a pair of homologous chromosomes does not separate in meiosis I, and all the resulting gametes end up with abnormal numbers of chromosomes. Nondisjunction can lead to an abnormal chromosome number of sex chromosomes X and Y.
Meiosis II is almost identical to mitosis and separates sister chromatids. However, mitosis and meiosis II differ because of the fact that each daughter cell produced by meiosis II has only a haploid set of chromosomes.
1st Period
Dr. D
Meiosis
Meiosis is needed for sexual reproduction and yields four haploid daughter cells, which are cells with one member of each homologous chromosome pair. Meiosis involves two nuclear and cytoplasmic divisions, and undergoes two consecutive divisions called meiosis I and meiosis II.
The two divisions of meiosis (meiosis I and meiosis II) follow only one duplication of the chromosomes. Because of this, each of the four produced daughter cells resulting from meiosis has only half as many chromosomes as the mother cell-a single haploid set of chromosomes.
Meiosis is preceded by an interphase. It is during the interphase that chromosomal duplication takes place, and at the end of the interphase, each chromosome consists of two genetically attached identical sister chromatids and the cell’s centrosomes is duplicated. The chromosomes are seen under the microscope as a mass of chromatin. There are four different stages of meiosis I: prophase I, metaphase I, anaphase I, telophase I. All the events that are unique to meiosis happen during meiosis I. In prophase I, duplicated homologous (nonsister) chromosomes pair together to form tetrads by synapsis, and crossing over occurs between the chromosomes because of an exchanging of segments. This rearranges genetic information, making important contribution to the genetic variability from sexual reproduction. Because the chromatin coils early in this phase, the chromosomes are easily seen through the use of a microscope. Prophase is the most complex phase, occupying over ninety percent of the time required for meiotic cell division. The chromosomes condense further as the nucleoli disappear as prophase progresses, and the centrosomes move away from each other. A spindle will then begin to form between them, and then nuclear envelope will break into fragments. The chromosome tetrads will be moved toward the center of the cell. During metaphase I, tetrads line up at the center of the cell. The chromosomes are condensed and thick, with the sister chromatids attached at their centromere. Spindle microtubules are attached to kinetochores. The spindle microtubules attached to one of the nonsister chromosomes come from one pole of the cell (for each tetrad) and the microtubules attached to the other homologous chromosome come from the opposite side. The homologous chromosomes of each tetrad are then prepared to move toward opposite poles of the cell. In anaphase I, sister chromatids of each chromosome stay united and move to the same pole of the cell while nonsister pairs of chromosomes separate. The sister chromatids making up each duplicated chromosome remain attached at their centromeres. Only the tetrads split up. In telophase I, the chromosomes enter at the poles of the cell. Each chromosome still consists of two sister chromatids. Cytokinesis occurs with telophase I, forming two haploid daughter cells. In some organisms, the chromosomes uncoil and the nuclear envelope redevelops, and there is an interphase before meiosis II begins. In other species, daughter cells immediately prepare for meiosis II. No chromosome duplication, however, ever occurs between telophase I and meiosis II. When meiosis I is complete, there are products of two haploid cells, but each chromosome has two sister chromatids, and meiosis II occurs. In meiosis II, there is no further chromosomal replication, but most importantly, meiosis II begins with a haploid cell where meiosis I does not. The nuclear envelope breaks down during prophase II, and a spindle forms and transports the chromosomes to the center of the cell. In metaphase II, the chromosomes are aligned in the middle of the cell. Sister chromatids separate during anaphase II and move toward opposite poles of the cell. In telophase II, nuclei forms at the cell poles, and cytokinesis occurs simultaneously. There are two daughter cells as products in meiosis II for each daughter cell of meiosis I.
An accident in gametes by meiosis is called a nondisjunction, in which the members of a chromosome pair do not separate. Sometimes a pair of homologous chromosomes does not separate in meiosis I, and all the resulting gametes end up with abnormal numbers of chromosomes. Nondisjunction can lead to an abnormal chromosome number of sex chromosomes X and Y.
Meiosis II is almost identical to mitosis and separates sister chromatids. However, mitosis and meiosis II differ because of the fact that each daughter cell produced by meiosis II has only a haploid set of chromosomes.