Understanding Mitosis and Meiosis: Asexual and Sexual Cell Division
Mitosis (Asexual Cell Division)
Mitosis results in 2 daughter cells that are clones of the parent cell and therefore also identical to each other. This type of cell division allows multicellular organisms to grow and repair damaged tissue.
Interphase
G₁ Phase
– Chromosomes are not visible because they are uncoiled and in the form of chromatin.
– The cell has 6 chromosomes present as chromatin.
S Phase
– DNA is duplicated.
– Centrosome (centrioles x 2) is duplicated.
– The cell has 6 chromosomes present as chromatin.
G₂ Phase
– The chromatin has been duplicated during the previous “S” phase.
– Centrosomes (centrioles x 4) remain close together.
– The cell has 6 chromosomes present as chromatin.
Prophase
– Chromatin coils and condenses to chromosomes.
– Nuclear membrane disintegrates into vesicles.
– Centrosomes move to opposite poles of the cell.
– Spindle microtubules are visible.
– Nucleolus disappears from view.
– The cell has 6 chromosomes.
– Each chromosome is composed of two sister chromatids.
Metaphase
– Spindle microtubules extend and attach to kinetochores, aligning the chromosomes along the metaphase (equatorial) plate.
– The cell has 6 chromosomes.
– Each chromosome has two sister chromatids.
Anaphase
– Sister chromatids separate, as they are pulled apart by spindle microtubules.
– Anaphase usually lasts only a few minutes.
– The cell has 12 daughter chromosomes.
Telophase
– Daughter chromosomes arrive at poles.
– Envelope begins to reform around each group of chromosomes.
– Nucleolus also reappears.
– Chromosomes begin to “de-condense.”
– Each pole receives 6 daughter chromosomes.
Cytokinesis (division of cytoplasm)
– It begins in late anaphase & continues to after telophase.
– Each daughter cell receives about half cytoplasm and cell components.
– Daughter cells separate and enter interphase G₁.
– Each cell has 6 daughter chromosomes, but now extended into chromatin.
Meiosis (Sex Cell Division)
Meiosis results in 4 daughter cells that are NOT clones of the parent cell and are NOT identical to each other. This type of cell division allows multicellular organisms to reproduce offspring with maximum genetic diversity.
Meiosis I
Early Prophase I
– Chromatin coils and condenses.
– Centrosomes move to opposite poles of the cell.
– Spindle microtubules are visible.
– Nuclear membrane disintegrates into vesicles.
Prophase I
– Nucleolus disappears from view.
– Spindle microtubules attach to and move chromosomes towards the center of the cell as in mitosis.
Late Prophase I
– Homologous chromosomes align close together in synapsis, forming tetrads.
– Homologous chromosomes exchange fragments by a process called crossing over (recombination).
– By the end of prophase 1, the cell still has 6 chromosomes, grouped as tetrads. However, the sister chromatids are no longer identical because of crossing over and they are referred to as double-chromatid chromosomes.
Metaphase I
– Chromosomes become aligned along the metaphase (equatorial) plate.
– The assortment of homologous chromosomes along the metaphase plate is independent and random.
Anaphase I
– The homologous chromosomes separate, NOT individual chromatids.
– Each chromosome of the homologous pair is pulled apart by the microtubules and moved to opposite poles.
– The cell has 6 chromosomes that are separating.
– The chromatids do NOT separate.
– It is the homologues that separate from their counterparts.
– At the end of anaphase 1, the cluster of chromosomes at each pole contains one member of each pair of homologous chromosomes.
Telophase I
– The spindle microtubules are disassembled.
– Cytokinesis commonly occurs.
– The nuclear membrane reforms.
– This stage is absent in some species.
– The nucleolus re-appears.
– Chromosomes may or may not decondense.
– Each cell now has 3 chromosomes. The chromatids have NOT separated. It is the homologues that have separated.
– Each meiosis I daughter cell has a haploid number of chromosomes relative to the parent cell.
Interkinesis
– Interkinesis, which may or may not occur, is similar to interphase except there is no DNA replication. However, centrosomes are duplicated again.
Meiosis II (Separation of Chromatids – From N to N)
The events that occur during meiosis II are similar to mitosis.
Prophase II
– Chromatin coils and condenses.
– Nuclear membrane disintegrates into vesicles.
– Spindle microtubules are visible.
– Nucleolus disappears from view.
– Centrosomes move to opposite poles of the cell.
– Each cell has 3 chromosomes with each chromosome composed of double chromatids.
Metaphase II
– Chromosomes become aligned along the equatorial plate.
– Each cell has 3 chromosomes. Each chromosome is composed of double chromatids.
Anaphase II
– Double chromatids separate, pulled apart by microtubules.
– Each cell has 6 daughter chromosomes that are separating.
Telophase II
– Each cell divides in two and gives rise to 2 daughter cells for a total of 4 daughter cells.
– The chromosomes begin to decondense.
– The nucleus reforms. The nucleolus also re-appears.
– The spindle apparatus disassembles.
– Each daughter cell has 3 chromosomes, a haploid number.
Cytokinesis
– Cytokinesis (division of cytoplasm).
– It begins in late anaphase & continues to after telophase.
– Each daughter cell receives about half cytoplasm and cell components.
– Daughter cells separate.
– Each daughter cell has 3 chromosomes, a haploid number and now in the form of chromatin.