Prophase is the first phase of mitosis. During prophase I the chromosomes begin to shorten and thicken and become visible under a microscope. In animal cells, small protein bodies in the cytoplasm called centrioles separate and move to opposite poles of the cell. Centrioles are involved in the formation and organization of spindle fibres, which attach to the chromosomes during cell division. The centromere helps anchor the chromosomes to the spindle fibres. Most plant and fungi cells lack centrioles but still produce a similar set of spindle fibres.
During prophase, the nuclear membrane starts to dissolve. This will eventually allow the chromosomes to separate into the two new daughter cells.
Metaphase
The second phase of mitosis is metaphase. During metaphase, the spindle fibres begin moving and aligning the chromosomes. Each chromosome, composed of sister chromatids, is pulled toward the centre of the cell, where its centromere becomes aligned across the middle, or equator, of the cell.
sister chromatid: the identical copy of a single chromosome that remains attached to the original chromosome at the centromere
Anaphase
The third phase of mitosis is anaphase. The centromeres divide, and the sister chromatids, now referred to as chromosomes, move to opposite poles of the cell. The chromosomes are being pulled by their centromeres, resulting in a distinctive pattern with the ends of the chromosomes trailing behind. If mitosis proceeds correctly, the same number and type of chromosomes will be found at each pole of the cell.
Telophase
The last phase of mitosis is telophase. Here, chromosomes reach the opposite poles of the cell and begin to unwind. As they do so, the spindle fibres dissolve and nuclear membranes form around the chromosomes. The result is two daughter nuclei.
Cytokinesis
Mitosis is followed immediately by cytokinesis, the stage of the cell cycle when the cytoplasm of a eukaryotic cell is divided to form two new cells. Although the nucleus is divided during mitosis, the division of the cell content into two new daughter cells occurs during cytokinesis. Hence, cell division occurs in two stages—mitosis and cytokinesis.
cytokinesis: the process in which a eukaryotic cell divides its cytoplasm into two new daughter cells
MEIOSIS: CREATION OF SEX CELLS IN SEXUAL REPRODUCTION
meiosis: a two-stage cell division in which the resulting daughter cells have half the number of chromosomes as the parent cell; results in the formation of gametes or spores
MEIOSIS I
As in mitosis, DNA replication in meiosis occurs during interphase (prior to the start of meiosis). At the beginning of prophase I, the chromosomes start to shorten and thicken. Each chromosome has been replicated during interphase and now consists of two sister chromatids joined at the centromere. The nuclear membrane begins to dissolve; the centrioles separate and move to opposite poles of the cell, and spindle fibres form.
As prophase continues, the chromosomes come together in homologous pairs. Each chromosome of the pair is composed of a pair of sister chromatids.
homologous chromosomes: matching pairs of chromosomes, similar in size and carrying information for the same genes
During metaphase I, the tetrads (made up of pairs of homologous chromosomes) migrate toward the centre of the cell and align their centromeres across the middle of the cell.
In anaphase I, homologous chromosomes move to opposite poles of the cell. At this point in meiosis, reduction division occurs. Only one chromosome from each homologous pair will be found in each new daughter cell. Each chromosome in the new cells consists of two sister chromatids.
In telophase I, nuclear membranes begin to form around the chromosomes at each end of the cell and the cell begins to divide. Unlike in mitosis, the chromosomes in the two nuclei are not identical with respect to their gene content. Each daughter nucleus receives only one member of each original chromosome pair. These cells are now haploid—containing only n chromosomes—and are ready to begin the second stage of meiosis.
MEIOSIS II
Meiosis II usually begins immediately after telophase I and the first cell division. Each haploid daughter cell contains one set of chromosomes. The chromosomes still consist of two sister chromatids. As a result of crossing over events, the sister chromatids are no longer identical. Note that there is no duplication of DNA between meiosis I and meiosis II.
Prophase II signals the beginning of the second division. During this stage, the nuclear membrane dissolves and the spindle fibres begin to form.
Metaphase II is identified by the arrangement of the chromosomes, each with two sister chromatids, across the middle of the cell. The sister chromatids remain attached by their centromeres.
In anaphase II, sister chromatids separate and move to opposite poles of the cell. The nuclear membrane begins to form around the chromatids, now called chromosomes.
The cells then enter telophase II, the final stage of meiosis II. During this stage, the second nuclear division is completed and the second division of cytoplasm follows.
Four haploid daughter cells (gametes) have been produced. The recombination of genetic information that occurs during the crossing over stage of meiosis produces gametes (sex cells) that are genetically different from one another.
MITOSIS: ASEXUAL REPRODUCTION
Prophase
Prophase is the first phase of mitosis. During prophase I the chromosomes begin to shorten and thicken and become visible under a microscope. In animal cells, small protein bodies in the cytoplasm called centrioles separate and move to opposite poles of the cell. Centrioles are involved in the formation and organization of spindle fibres, which attach to the chromosomes during cell division. The centromere helps anchor the chromosomes to the spindle fibres. Most plant and fungi cells lack centrioles but still produce a similar set of spindle fibres.
During prophase, the nuclear membrane starts to dissolve. This will eventually allow the chromosomes to separate into the two new daughter cells.
Metaphase
The second phase of mitosis is metaphase. During metaphase, the spindle fibres begin moving and aligning the chromosomes. Each chromosome, composed of sister chromatids, is pulled toward the centre of the cell, where its centromere becomes aligned across the middle, or equator, of the cell.
sister chromatid: the identical copy of a single chromosome that remains attached to the original chromosome at the centromere
Anaphase
The third phase of mitosis is anaphase. The centromeres divide, and the sister chromatids, now referred to as chromosomes, move to opposite poles of the cell. The chromosomes are being pulled by their centromeres, resulting in a distinctive pattern with the ends of the chromosomes trailing behind. If mitosis proceeds correctly, the same number and type of chromosomes will be found at each pole of the cell.
Telophase
The last phase of mitosis is telophase. Here, chromosomes reach the opposite poles of the cell and begin to unwind. As they do so, the spindle fibres dissolve and nuclear membranes form around the chromosomes. The result is two daughter nuclei.
Cytokinesis
Mitosis is followed immediately by cytokinesis, the stage of the cell cycle when the cytoplasm of a eukaryotic cell is divided to form two new cells. Although the nucleus is divided during mitosis, the division of the cell content into two new daughter cells occurs during cytokinesis. Hence, cell division occurs in two stages—mitosis and cytokinesis.
cytokinesis: the process in which a eukaryotic cell divides its cytoplasm into two new daughter cells
MEIOSIS: CREATION OF SEX CELLS IN SEXUAL REPRODUCTION
meiosis: a two-stage cell division in which the resulting daughter cells have half the number of chromosomes as the parent cell; results in the formation of gametes or spores
MEIOSIS I
As in mitosis, DNA replication in meiosis occurs during interphase (prior to the start of meiosis). At the beginning of prophase I, the chromosomes start to shorten and thicken. Each chromosome has been replicated during interphase and now consists of two sister chromatids joined at the centromere. The nuclear membrane begins to dissolve; the centrioles separate and move to opposite poles of the cell, and spindle fibres form.
As prophase continues, the chromosomes come together in homologous pairs. Each chromosome of the pair is composed of a pair of sister chromatids.
homologous chromosomes: matching pairs of chromosomes, similar in size and carrying information for the same genes
During metaphase I, the tetrads (made up of pairs of homologous chromosomes) migrate toward the centre of the cell and align their centromeres across the middle of the cell.
In anaphase I, homologous chromosomes move to opposite poles of the cell. At this point in meiosis, reduction division occurs. Only one chromosome from each homologous pair will be found in each new daughter cell. Each chromosome in the new cells consists of two sister chromatids.
In telophase I, nuclear membranes begin to form around the chromosomes at each end of the cell and the cell begins to divide. Unlike in mitosis, the chromosomes in the two nuclei are not identical with respect to their gene content. Each daughter nucleus receives only one member of each original chromosome pair. These cells are now haploid—containing only n chromosomes—and are ready to begin the second stage of meiosis.
MEIOSIS II
Meiosis II usually begins immediately after telophase I and the first cell division. Each haploid daughter cell contains one set of chromosomes. The chromosomes still consist of two sister chromatids. As a result of crossing over events, the sister chromatids are no longer identical. Note that there is no duplication of DNA between meiosis I and meiosis II.
Prophase II signals the beginning of the second division. During this stage, the nuclear membrane dissolves and the spindle fibres begin to form.
Metaphase II is identified by the arrangement of the chromosomes, each with two sister chromatids, across the middle of the cell. The sister chromatids remain attached by their centromeres.
In anaphase II, sister chromatids separate and move to opposite poles of the cell. The nuclear membrane begins to form around the chromatids, now called chromosomes.
The cells then enter telophase II, the final stage of meiosis II. During this stage, the second nuclear division is completed and the second division of cytoplasm follows.
Four haploid daughter cells (gametes) have been produced. The recombination of genetic information that occurs during the crossing over stage of meiosis produces gametes (sex cells) that are genetically different from one another.