Cell reproduction and division

why do cells divide

new organisms, growth, cell replacement

cell cycle stages

G1, S, G2, M

g1

cells grow, prepares to replicate dna, checks to see if environment is right for replication

s phase

cell grows & dna copied

G2 phase

intact nuclear envelope - chromosomes replicated (indistinct as loosely packed chromatin fibres) - centrosome replicated, microtubules extend - radially forming asters

prophase

chromatin fibres condense - forms discrete chromosomes

nucleoli disappear, centrosomes move away from each other, mitotic spindle begins to form

prometaphase

breakdown of the nuclear envelope, some microtubules attach to chromosomes at their kinetophores, others interact with those from opposite poles

metaphase

centrosomes at opposite poles

chromosomes align on the metaphase plate, sister kinetochores attached to microtubules coming from opposite poles

anaphase

begins when centromeres separate, sister chromatids move towards opposite poles of cell, each chromatid becomes a new chromosome

poles move further apart

telophase

elongation of cell by polar microtubules, daughter nuclei begin to form at poles of cell, nuclear envelopes form, chromatin begins to decondense

mitotic spindle consists of

tubules (astral, kinetochore, non-kinetochore), centrosome, chromatid pairs

astral tubules

anchors centrosomes to cell membranes, dynein walks along structures - microtubules depolymerise and shorten

kinetochore tubules

pulls chromosomes to poles

uses dynein - pulls of microtubules during anaphase to move them to poles

non-kinetochore tubules

extend from centrosomes and overlap with microtubules - uses kinesin - microtubules polymerise so cell elongates

centrosome function

organises microtubules and their functions

chromosomal separation - pulling

uses dynein - kinetochore motors pull chromosomes towards centrosome, astral motors pulls centrosomes towards inner face of plasma membrane - both shorten and depolymerise microtubules

chromosomal separation - pushing

non-kinetochore/polar add subunits to microtubules to drive poles of spindle apart, elongates cell for telophase

physical separation of chromosome pairs

happens in anaphase - proteins holding sister chromatids together are inactivated, chromatids separate, kinetochore dyneins walk chromosomes to nearest poles, microtubules shorten by depolymerisation

cytokinesis

microfilaments form a ring at the furrow, ring contracts due to actin and myosin interaction, furrow deepends until cell is pinched in two

mitosis in plant cells

cell plate at equatorial plane of cell, cell wall forms plate contents