Mitosis
Homologous chromosomes = 1 pair of chromosomes
1 chromosome = a homologue
1.
2.
3.
4.
homologous chromosomes are similar in size,
shape, centromere position, staining pattern.
one homologue from male parent, one female
pair up during Prophase I of meiosis
Genes for same characters at corresponding loci
(singular – locus), determine same traits, might
not be identical for what they code for.
Diploid cell (2n)
-
species with 2 sets of chromosomes are diploid
cells have 2 sets of chromosomes, one set from
each parent
Haploid cell
gametes of animal species and flowering plants
Cell Cycle
Sequence of events between the formation of a cell
and the division into daughter cells
(A) Interphase
Longest phase of cell cycle
Period of synthesis and growth
Cell produces many materials required for
carrying out its functions
Cell replicates DNA, prepare for B
(B) Nuclear Division
Mitosis or Meiosis
(C) Cytokinesis
Division of cytoplasmic contents into 2
daughter cells
Interphase
Preparation for mitosis/meiosis
Non diving phase, 90% of cell cycle
G1
(1st growth
phase)
S
(Synthesis)
G2
(2nd growth
phase
 internsive cellular synthesis to
form new organelles
 substances produced to inhibit or
stimulate onset of next phase
 DNA replication+ synthesis
(DNA content doubles)
 each chromosome has now two
chromatids
 continuation of intensive cellular
synthesis
 centrioles replicate (not in higher
plants)
 mitotic spindle begins to form
 plants: make a lot of mitochondria
and chloroplasts, build up large
energy store for nuclear division,
cytokinesis.)
Interphase (resting stage)
- G1, S, G2
- Chromosomes are duplicated but exists as chromatin
(cannot be distinguished individually)
- Nucleus and nucleolus well defined and bounded by
nuclear envelope
- Single centrosome replicated to form 2 centrosomes
Prophase
chromatin fibres shorten and thicken by
spiralisation and condensation
each chromosome appears as 2 identical sister
chromatids joined together at centromere
Nucleoli disappear
Centrioles move to opposite poles of cell
From each pair of centroles, short
microtubules develop and form a star-shaped
structure called an aster.
-
Mitotic spondle begins to form
Prometaphase
nuclear envelope fragments
each of 2 chromatids of a chromosome now
develops a structure at the centromere called
the kinetochore.
Bundles of microtubules forming spindle
fibres extend from each pole toward cell
equator and attach to the kinetochore at the
centromere.
Chromosomes start moving to centre
Metaphase
chromosomes line up at metaphase plate
attached by their centromeres to spindle fibres
Anaphase
rapid stage, centromere of each chromosome
divides into two, sister chromatids separate
kinetochore microtubules shorten and pull
chromatids to opposite poles
sister chromatids move to opposite ends of the
spindle
non kinetochore microtubules lengthen
elongate the cell
Telophase
chromatids reach poles of cell
chromatids uncoil back to chromatin
spindle fibres disintegrate
nuclear envelope reforms
nucleoli reappear
Cytokinesis
[animals]
spindle fibres disappear, just beneath the
plasma membrane, the cell membrane begins
to invaginate as a band of microfilaments at
the former spindle equator contracts forming
a cleavage furrow. Contractions continue
form 2 daughter cells
[plants]  centrioles absent, asters not formed,
cytokinesis occurs by forming cell plate, occurs in
meristemic tissue
-
Golgi apparatus produces a number of small
fluid-filled vesicles which first appears in
centre of cell. Guided by microtubules, the
vesicles merge to form cell plate which grows
across equatorial plane. Contents of Golgi
vesicles contribute to new cell walls of
daughter cells while membranes form new
cell membranes. Spreading cell plate fuses
with parents cell wall, separates 2 daughter
cells.
SIGNIFICANCE OF MITOSIS
(1) Maintains genetic stability
Mitotic division produces 2 daughter cells,
each with the same number of chromosomes
and hereditary information as parent cell.
Daughter cells genetically identical to parent
cell and genetic stability is maintained within
populations of cells
(2) Asexual reproduction
mitotic division is the means for some plant
and animal to produce asexually. (e.g. binary
fission in Amoeba, development of new
potato from tuber)
(3) Growth
mitosis takes place during growth of organism
number of existing cells within organism
increases and new cells are identical to
existing
(4) Regeneration and Cell replacement
mitotic division ensures that damage cells in
an organism are replaced by new cells that are
identical to original cells
mitosis helps in cell replacement, regeneration
of missing parts to varying degrees in multicellular organisms
Meiosis
In sexual reproduction, there is a need to form
haploid GAMETES.
-
Nuclei of male and female gametes fuse to
produce zygote with diploid number of
chromosomes. Diploid condition of cells is
restored and maintained.
Meiosis creates genetic VARIATION
-
accumulate heritable variations that are
favoured by particular environment
indiv. In population have appropriate genes to
cope with environment change
Mendel’s law of independent assortment
During
, homologous chromosomes, each
consisting of one maternal and one paternal
chromosome are randomly arranged at equator.
Arrangement of one pair of homologous
chromosome at the equator is independent of the
other pairs of homologous chromosomes.
During
, chromosomes of one
homologous pair will separate independently of the
other pairs to form daughter cells. How one
homologous pair/each chromosome
Factors leading to genetic variation
1. crossing over Prophase I
2. mendal’s law of independent
assortment
3. random fertilization