Mitosis Flashcards
What is the cell cycle
- ordered sequence of events in cell division
- interphase (G1, S, G2) → mitotic phase/mitosis = NUCLEAR DIVISION (prophase, metaphase, anaphase, telophase, cytokinesis)
Late interphase → cell will be large/ same size as cells undergoing mitosis due to cell growth
First stage of interphase
G1 phase (2n/46, 2x)
- cell synthesises organelles such as mitochondria and ribosomes
- builds up large store of energy
- manufactures proteins such as histones, ribosomal proteins and tubulin (subunit of spindle fibres)
Second stage of interphase
S phase (2n/46, 4x)
- DNA replication / synthesis
- microtubule-organising structure called the centrosome is duplicated
Third stage of interphase
G2 phase (2n/46, 4x)
- cell continues to store energy and manufacture proteins and organelles
- only cells with the right conditions can proceed to the M phase to ensure that damaged or incomplete DNA is not passed on to the daughter cells
What comes after interphase
M phase
- mitosis (nuclear division) -> does not occur in bacteria/prokaryotes as they do not have a nucleus
- cytokinesis
What are centrosomes
- nonmembranous organelles found only in animal cells
- function as the microtubule organising center (MTOC)
- in animal cells: each centrosome is composed of 2 centrioles positioned perpendicular to each other
What are centrioles
- microtubule-based cylinders of defined length (500nm) and diameter (200nm)
- composed of 9 triplets of microtubules
- found in animal cells, missing in most plant cells
- 2 centrioles perpendicular to each other = 1 centrosome
- at the beginning of nuclear division, long protein fibres (microtubules) extend from centrioles to form spindle fibres
- movement of centrioles to poles of cell during prophase determines polarity of cell
What are spindle fibres
- formed by microtubules that extend from centrioles
- long hollow tubes, 24-25nm in diameter
- made up of protein subunits called tubulin
- some attach to the kinetochores of the chromosomes = kinetochore microtubules
- shortening of kinetochore microtubules by removal of tubulin subunits separates the chromatids and pulls them to opposite poles (anaphase)
- spindle fibres that do not attach to kinetochores = nonkinetochore microtubules (interact with other nonkinetochore microtubules from opp pole of cell, responsible for elongating the whole cell during anaphase)
- Colchicine (chemical) used to prevent formation of spindle fibre -> sister chromatic remain attached in metaphase plate, allows for observation of number and structure of chromosomes, as well as karyotyping
First stage of mitosis
Prophase (2n/46, 4x)
- condensation of chromatin into chromosome (becomes visible under light microscope)
- 1 chromosome = 2 identical sister chromatids joined at the centromere
- nucleolus disappears
- centrosomes move to opposite ends of cell
- short microtubules develop from each pair of centrioles in the centrosomes, star-shaped structure called an aster is formed
- phosphorylation of various proteins on the inner surface of the nuclear envelope cause the nuclear envelope to disintegrate into small membrane vesicles
- microtubules extend from the centrioles to form spindle fibres which extend from each pole towards cell equator
- 2 ends of spindle = poles
- kinetochore (specialised protein structure) assembles at the centromere of the chromosomes
- kinetochore microtubules extend to attach specifically to the kinetochores
- nonkinetochore/polar molecules are spindle fibres that extend from one pole to the opposite pole
Second stage of mitosis
Metaphase (2n/46, 4x)
- longest stage (20mins)
- centrosomes are at opposite ends of cell
- chromosomes align on metaphase plate/equator (imaginary plane that is equidistant between the spindle’s 2 poles)
- chromosome’s centromeres lie on metaphase plate
- kinetochore microtubules are attached to kinetochores at the centromere of each chromosome
Third stage of mitosis
Anaphase (4n/92, 4x)
- shortest stage (few mins)
- centromeres of each chromosome divides and 2 sister chromatids of each chromosome separate
- each chromatid becomes a full-fledged chromosome
- 2 daughter chromatids move, centromere first, to opposite poles of the spindle due to the shortening of their spindle fibres/kinetochore microtubules via the removal of tubulin subunits
- cell elongates as nonkinetochore microtubules lengthen
Fourth stage of mitosis
Telophase (4n/92, 4x)
- chromosomes reach their respective poles to become the genetic material of daughter nuclei
- chromosomes uncoil and return to chromatin form
- spindle fibres disintegrate
- nuclear envelope reforms around the chromosomes at each pole
- nucleolus reappears in each new nucleus
What comes after mitosis
Cytokinesis (2n/46, 2x)
- Animal cells: cleavage furrow develops in cell membrane. Cell membranes in the furrows eventually join up and separate the 2 daughter nuclei
- Plant cells: series of Golgi Vesicles line up in the middle of the parent cell. Fuse to form the cell plate, which extends outwards across the equator of the parent cell
: content of Golgi Vesicles contributes to the cell wall of the daughter cell while their membranes form the cell membranes of daughter cells
: cell plate eventually fuses with parent cell wall and cell membrane, separating the 2 daughter cells
Significance of mitosis
- 2 daughter nuclei formed contains genetically identical sets of chromosomes as parent (same no/type of chromosomes, genetic makeup = clones)
: semi-conservative replication of DNA that occurs before mitosis (ensure each chromosome of parent nucleus is duplicated to form 2 identical sister chromatids)
: proper arrangement of chromosomes along equator = chromosomes shared equally between 2 daughter nuclei, as each sister chromatid of every chromosome is pulled towards opposite poles
: proper formation and subsequent shortening of mitotic spindle (ensures proper separation of chromosomes during mitosis) - maintains genetic stability of organism, important during : growth, repair of worn-out parts of body, asexual reproduction
What are cell-cycle checkpoints
- critical control point where the stop & go-ahead signals can regulate the cycle
- ensure the cell is only allowed to proceed to the next phase of the cell cycle if it is properly completed in the previous phase
- monitor for DNA replication, DNA damage and chromosome-to-spindle attachments
- 3 major checkpoints during G1, G2, and M phases
