CELL DIVISION
I. Mitosis
- somatic cells
- diploid = 46 chromosomes
Homologous pair = diploid cell having 2 chromosomes of each kind
- result in 2 daughter cells
Requisites before the onset of division:
DNA replication
twice the normal diploid complement (92)
Storage of energy
Interphase = period between successive episodes
Only nucleus & condensed chromatin are seen
Chromosomes are in extended state
1. Prophase
Chromosomes become visible
Chromosomes continue to condense, become shorter & thicker
Chromosomes consist of 2 parallel strands / chromatids
Chromatids joined to one another at the Centromere
Centromere = constricted segment common to both strands
Kinetochore = trilaminar disc at the centromere
Centrioles replicate & migrate to opposite poles of the cell
Breakdown of nuclear envelope
2. Metaphase
Alignment of chromosomes in the same plane in the middle of the cell forming the equatorial plate
Equatorial plate - associated with the development of mitotic spindle
Mitotic spindle = fusiform array of microtubules which extend from centrioles to chromosomes or extend from pole to pole
3. Anaphase
Separation of single kinetochore of each pair of chromatids into 2 so each chromatid has its own
Chromatids move to opposite poles as separate chromosomes
Kinetochore of each chromosome attaches to 1 or more spindle microtubules
Microtubules are shorten by depolymerization at one end
DYNEIN
4. Telophase
Chromosomes are clustered at the spindle poles
Segments of nuclear envelope are formed around
Chromosomes uncoil except those destined to remain condensed
Karyokinesis is complete
▪ Nucleoli are reformed
▪ Nuclear envelope coalesce
Cleavage furrow deepens until it encounters the spindle microtubules
= constriction of the cytoplasm midway between the nuclei
For a short time, daughter cells remain connected by a slender cytoplasm bridge occupied by residual spindle microtubules
bound together at their midpoint by dense amorphous material (midbody)
Depolymerization of microtubules completes cytokinesis
In female cells, 1 of the x-chromosomes remains condensed during interphase.
II. Meiosis
- germ cells in ovary & testis
- haploid = 23 chromosomes, results in 4 daughter cells
A. 1st Divison / Reduction
- members of each homologous pair separates & go to opposite poles thus reducing the # of chromosomes in daughter cells by ½
1. Prophase
a. Leptotene
- chromosomes become visible in nucleus
b. Zygotene
- homologous chromosomes begin to come together in close lateral opposition
Synapsis = pairing
- tetrad formation
c. Pachytene
- chromosomes coil becoming shorter & thicker
- homologous chromosomes may give erroneous impression they are single chromosome due to very close apposition
I. Mitosis
- somatic cells
- diploid = 46 chromosomes
Homologous pair = diploid cell having 2 chromosomes of each kind
- result in 2 daughter cells
Requisites before the onset of division:
DNA replication
twice the normal diploid complement (92)
Storage of energy
Interphase = period between successive episodes
Only nucleus & condensed chromatin are seen
Chromosomes are in extended state
1. Prophase
Chromosomes become visible
Chromosomes continue to condense, become shorter & thicker
Chromosomes consist of 2 parallel strands / chromatids
Chromatids joined to one another at the Centromere
Centromere = constricted segment common to both strands
Kinetochore = trilaminar disc at the centromere
Centrioles replicate & migrate to opposite poles of the cell
Breakdown of nuclear envelope
2. Metaphase
Alignment of chromosomes in the same plane in the middle of the cell forming the equatorial plate
Equatorial plate - associated with the development of mitotic spindle
Mitotic spindle = fusiform array of microtubules which extend from centrioles to chromosomes or extend from pole to pole
3. Anaphase
Separation of single kinetochore of each pair of chromatids into 2 so each chromatid has its own
Chromatids move to opposite poles as separate chromosomes
Kinetochore of each chromosome attaches to 1 or more spindle microtubules
Microtubules are shorten by depolymerization at one end
DYNEIN
4. Telophase
Chromosomes are clustered at the spindle poles
Segments of nuclear envelope are formed around
Chromosomes uncoil except those destined to remain condensed
Karyokinesis is complete
▪ Nucleoli are reformed
▪ Nuclear envelope coalesce
Cleavage furrow deepens until it encounters the spindle microtubules
= constriction of the cytoplasm midway between the nuclei
For a short time, daughter cells remain connected by a slender cytoplasm bridge occupied by residual spindle microtubules
bound together at their midpoint by dense amorphous material (midbody)
Depolymerization of microtubules completes cytokinesis
In female cells, 1 of the x-chromosomes remains condensed during interphase.
II. Meiosis
- germ cells in ovary & testis
- haploid = 23 chromosomes, results in 4 daughter cells
A. 1st Divison / Reduction
- members of each homologous pair separates & go to opposite poles thus reducing the # of chromosomes in daughter cells by ½
1. Prophase
a. Leptotene
- chromosomes become visible in nucleus
b. Zygotene
- homologous chromosomes begin to come together in close lateral opposition
Synapsis = pairing
- tetrad formation
c. Pachytene
- chromosomes coil becoming shorter & thicker
- homologous chromosomes may give erroneous impression they are single chromosome due to very close apposition
