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Mitosis

Homologous chromosomes = 1 pair of chromosomes

1 chromosome = a homologue

1. homologous chromosomes are similar in size,

shape, centromere position, staining pattern.

2. one homologue from male parent, one female

3. pair up during Prophase I of meiosis

4. 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 forcarrying 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)

internsive cellular synthesis toform new organelles

substances produced to inhibit or

stimulate onset of next phase

S

(Synthesis) DNA replication+ synthesis

(DNA content doubles)

each chromosome has now two

chromatids

G2

(2nd growthphase

continuation of intensive cellularsynthesis

centrioles replicate (not in higher

plants)

mitotic spindle begins to form

plants: make a lot of mitochondriaand chloroplasts, build up large

energy store for nuclear division,cytokinesis.)

Prophase

- chromatin fibres shorten and thicken byspiralisation 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 nowdevelops a structure at the centromere calledthe kinetochore.

- Bundles of microtubules forming spindlefibres 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]

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

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- spindle fibres disappear, just beneath theplasma membrane, the cell membrane begins

to invaginate as a band of microfilaments at

the former spindle equatorcontracts forming

a cleavage furrow. Contractions continueform 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 smallfluid-filled vesicles which first appears in

centre of cell. Guided by microtubules, the

vesicles merge to form cell plate which growsacross equatorial plane. Contents of Golgi

vesicles contribute to new cell walls of

daughter cells while membranes form newcell membranes. Spreading cell plate fuseswith 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. binaryfission 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, regenerationof missing parts to varying degrees in multi-cellular organisms

Meiosis

In sexual reproduction, there is a need to form

haploid GAMETES.

- Nuclei of male and female gametes fuse toproduce zygote with diploid number of

chromosomes. Diploid condition of cells is

restored and maintained.

Meiosis creates genetic VARIATION

- accumulate heritable variations that arefavoured by particular environment

- indiv. In population have appropriate genes tocope with environment change

Mendels 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. mendals law of independent

assortment

3. random fertilization