Mitosis Flashcards
What are the phases in the cell cycle?
Interphase : G1 phase, S phase, G2 phase
M phase : mitosis and cytokinesis
What happens in G1 phase ?
- Cell synthesises organelles such as mitochondria and ribosomes
- builds up large store of energy
- manufacture proteins such as histones, ribosomal proteins and tubulin (a subunit of spindle fibres)
What happens in S phase?
- DNA replication / synthesis occurs
- a microtubule-organising structure (centrosome) is duplicated
What happens in G2 phase?
- cells continues to store energy and manufacture proteins and organelles
- only cells with right conditions can proceed to the M phase to ensure that damaged or incomplete DNA is not passed on to daughter cells
What are centrosomes?
Non-membranous organelles found in both plant and animal cells
- in animal cells : each centrosome is composed of 2 centrioles positioned perpendicular to each other
They function as the microtubule organising center (MTOC) to organise the cell’s microtubule throughout the cell cycle
What are centrioles ?
Microtubule-based cylinders of defined length and diameter, found in animal cells but absent in plant cells
Each centriole is composed of 9 triplets of microtubules
What are spindle fibres?
Long hollow tubes about 24-25nm in diameter
Made up of tubulin (protein subunit)
Formed from microtubules extending from the centrioles at the beginning of nuclear division
What are kinetochore microtubules?
Spindle fibres which attach to the kinetochore of the chromosome
Shortening of these fibres by removal of tubulin subunits separates the chromatids and pull them to opposite poles
What are non-kinetochore microtubules ?
Spindle fibres not attached to kinetochores
They interact with non kinetochore microtubules from the opposite pole of the cell
- in a dividing animal cell : these are responsible for elongating the whole cell during anaphase
What is colchicine?
A chemical commonly used to prevent formation of spindle fibre in actively diving cells
- sister chromatids remain attached in the metaphase plate allowing observation of the number and structure of chromosomes as well as karyotyping
What are the four phases of mitosis?
(PMAT)
Prophase
Metaphase
Anaphase
Telophase
(Mitosis)
What happens in prophase?
- nucleolus disappears
- in animal cells centrosomes move to opposite poles of the cell, short microtubules develop form each pair of centrioles in the centrosomes forming an aster
- phosphorylation of various proteins on the inner surface of the nuclear envelope causes the nuclear envelope to disintegrate into small membrane vesicles
- microtubules extend from centrioles to form spindle fibre
- kinetochore assembles at each centromere
- both non kinetochore and kinetochore spindle fibres extend
What happens in metaphase?
(Longest stage of mitosis ~20 mins)
- centrosomes are now at opposite ends of the cell
- chromosome align on the metaphase plate/equator
- centromere lies on the plate - spindle fibres are now attached to kinetochores located at the centromere of each chromosome
What is the metaphase plate/equator?
An imaginary plane that is equidistant between the spindle’s two poles
(Mitosis)
What happens during anaphase?
(Shortest stage ~ a few mins)
- centromere of each chromosome divides and two sister chromatids of each chromosome separate
- each chromatid becomes a chromosome
- the two daughter chromosomes move, centromere first to the opposite poles of the spindle due to shortening of their spindle fibres : distinct V-shape of the chromosomes
- the cell elongates as the nonkinetochore microtubules lengthen
What happens in telophase ?
- the chromosome reach their respective poles to become the genetic material of daughter nuclei
- chromosomes uncoil and return to their chromatin form
- spindle fibres disintegrate
- nuclear envelope reforms around chromosomes at each pole
- nucleolus reappears in each new nucleus
What happens during cytokinesis in animal cells?
A cleavage furrow develops in the cell membrane (top and bottom) which joins up eventually and completely separates the two daughter nuclei
What happens during cytokinesis in plant cells?
- a series of golgi vesicles line up in the middle of the parent cell, fuse to form the cell plate (vertical) which extends outward across the equator (horizontal) of the parent cell
- the content of the golgi vesicles contributes to the cell wall of the daughter cell while their membranes form the cell membrane of the daughter cell
- the cell plate eventually fuses with the parent cell wall and cell membrane separating the two daughter cells
What is the significance of mitosis?
Mitosis ensures that the two daughter nuclei formed contained genetically indentical sets of chromosomes as the parent nuclei : same number of chromosomes, same genetic make-up
Mitosis maintains the gentil stability of an organism and is important in the following processes
1. Growth of a multicellular organism
2. Repair of worn-out parts of the body
3. Asexual reproduction - production of offspring without the fusion of gametes
What are the two features of mitosis that ensure daughter nuclei receive exactly the same number and type of chromosome as parent nucleus?
- Semi-conservative replication of DNA must occur before mitosis to ensure that each chromosome of the parent nucleus is duplicated to form two identical sister chromatids
- Arrangement of the chromosomes along the equator ensures that the chromosomes are shared equally between the two daughter nuclei as each sister chromatid of every chromosome is pilled towards opposite poles
What are cell cycle checkpoints?
A critical control point where the stop and go-ahead signals can regulate the cycle
Important as they ensure the cell is only allows to proceed to the next phase of the cell cycle if it has properly completed the previous phase
Basically monitor for DNA replication, DNA damage and chromosome-to-spindle attachments
Where are the 3 major checkpoints in the cell cycle found ?
G1 phase - restriction/start checkpoint
G2 phase - G2/M checkpoint
M phase - spindle/metaphase checkpoint
What factors are checked in G1? (4)
- size of cell : ensures that the cell is large enough to divide
- nutrients availability : ensures enough nutrients are available to support daughter cells
- DNA integrity : ensures that genome is intact, damage to DNA can halt the cycle at this point
- molecular signals : ensures that cell has received an appropriate growth signal
What does G1 checkpoint do ?
Primary point where the cell decides whether to divide or not
If cell does not receive the growth signal, it will exit the cell cycle and switch to a non-diving state (G0)
- most cells in the human body are in G0
What are factors G2 checkpoint checks for?
DNA integrity : ensures that DNA is undamaged
DNA replication : ensures that DNA replication was successfully completed
How does G2 checkpoint ensure that damaged DNA is not passed on to daughter cells?
- if DNA damage are detected or DNA is not accurately replicated, the cell can stall the cycle at G2 checkpoint to allow for DNA repairs
- if DNA damage is extensive / beyond repair, p35 triggers the cell to undergo programmed cell death (apoptosis)
What factors are checked at M checkpoint?
- all chromosomes are attached to spindle fibres to repare for anaphase
- proper tension is placed on the paired kinetochores
What is the significant characteristic of a cell in metaphase under a microscope?
From the polar view (view from one pole or from the top) the chromosomes form a nice circle
