Cell Division and differentiation Flashcards
Why do cells grow and divide?
- to generate tissues e.g., during development
- to make those tissues grow
- to make up for wear and tear on those tissues
What does halting cell division cause?
- causes death
What are the ways that cells can grown and divide?
- cell division = making a new cell (hyperplasia)
- hypertrophy = a cell gets physically bigger
What is the definition of cell division?
- the process by which cells double their content, then divide to produce two daughter cells. These daughter cells have half of the cell contents each
The cell cycle is repeating sequences of events - there are 3 concurrent cycles that are interdependent what are these?
- the chromosome cycle
- cytoplasmic cycle
- the centrosome cycle
What is the chromosome cycle?
- DNA replication and segregation (mitosis)
What is the cytoplasmic cycle?
- organelle replication and physical division (cytokinesis)
What is the centrosome cycle?
- replication of the mitotic spindle (in S-phase)
Light microscopy looking at the cells suggests that there are two phases of the cell cycle - what are these?
- resting
- dividing
What is the G1 phase?
- this is the start point of the cycle
- the phase of normal cell activity e.g., biosynthesis - like resting
- longest phase
What does the G stand for in the G1 phase?
- G = gap
What is the S phase?
- phase of synthesis, where cell contents doubles (DNA and cytoplasm)
- the chromosome cycle occurs here
- duration varies, dependent on DNA content
What does the S stand for in the S phase?
- synthesis
What do the chromosomes do in the S phase?
- chromosomes duplicate to form sister chromatids
When is the S phase long?
- in species that have a lot of DNA
What is the G2 phase?
- the second “gap” phase
What happens in the second gap phase?
- normal biosynthesis occurs during this phase
- S-G2 occurs only when all DNA has replicated
What does the M stand for in the M phase?
- mitosis
What happens during the M phase?
- very short phase
- involves both mitosis (nuclear division) and cytokinesis (cell division)
- has 6 distinct stages
What is the G0 phase?
- a potential phase for all cells, not all do this though
- where cells exit normal cell cycle and go into a phase where they are neither dividing or preparing to divide - this is the true resting phase
What happens to cells once they have entered the G0 phase?
- some cells stay here permeant such as neuronal cells once they have reached final differentiation, and skeletal muscle
- for other cells its transient and they sit and wait for signal to re-enter the cell cycle - such as osteocytes
During the cell cycle there are a specific number of check points that the cells must undergo - what does this ensure?
- this ensures each stage is completed in the correct manner and the correct way
- if a cell has a flaw it wont progess any further and it will die
What can evasion of cell check points lead to?
- leads to issues such as cancer (uncontrolled cell growth)
What does the M-phase and mechanics of cell division involve and require?
- separation and segregation of chromosomes
- requires cytoskeleton to form structure
What are the 2 different cytoskeletal machines?
- mitotic spindle = microtubules
- contractile ring = actin and myosin - will physically separate cells
The mitotic spindle is formed by what?
- by the centrosome
What is the centrosome?
- its an organelle that contains a pair of cylindrical centrioles
What are the stages of mitosis in order?
- prophase
- prometaphase
- metaphase
- anaphase
- telophase
How many stages of cytokinesis are there?
- cytokinesis
What happens in prophase?
- chromosomes begin to condense
- outside the nucleus, mitotic spindle begins to assemble
What happens in prometaphase?
- nuclear membrane breaks down
- spindle and chromosomes now meet
- attachment is via kinetochores
- spindle begins to move to the poles of the cell
What happens during metaphase?
- sister chromatids align on the cell equator
- forms the metaphase plate
What happens during anaphase?
- sister chromatids separated by separase (enzyme)
Describe how chromosomes are physically separated by:
- spindle poles moving outward
- microtubules shorten
- chromosomes slide along microtubules
What happens during telophase?
- chromosomes arrive at the poles
- chromosomes decondense
- nuclear envelop forms
- cells split into 2
Where does cytokinesis start and end?
- starts in anaphase
- ends in telophase
What happens during cytokinesis?
- actinomyosin ring assembles under the cell membrane
- contraction of the ring pinches the cell into 2
- each cell has 1 full set of chromosomes, a centrosome a and about half the cytoplasm
What does cytokinesis require?
- requires membrane manufacture
What are embryonic stem cells like before cell differentiation?
- embryonic stem cells are all alike and have unlimited capacity to proliferate
What is the process called that makes stem cells turn into specific cell types?
- a process called cell differentiation
What are differentiated cells like?
- they have reached their final specialised form
- they differ from each other both structurally and functionally
Differentiated cells are often post mitotic - what does this mean?
- they cannot undergo further cell division and are in G0
- some differentiated cells can be slowly dividing however
What do differentiated cells express specific sets of?
- express specific sets of the genes that are characteristics of that particular type of cell
- this means that will have a specific set of proteins coded for by those genes
What direct a cell to begin differentiation?
- generally, signals from other cells, the extracellular matrix and hormones do this
What are the 2 steps in differentiation?
- specification
- determination
What is specification?
- capable of autonomous differentiation in isolation (i.e. in test tubes or dish) can be reversed
- Cells in this stage are called “NOT fate committed”.
What is determination?
- when the cell differentiated into a specific cell type, even when it’s placed amongst cells if a different type.
- This is irreversible and “fate committed” (or is it)
In many cell types what must cease for differentiation to occur?
- division
What is it called when division must cease for differentation to occur?
- this is called terminal differentiation
- examples are nerves and skeletal muscle
Some other cell types significantly slow division to differentiate - when these cells divide what can they only form?
- can only form the parent cell type
- they do not regain ability to make multiple cell types
Can a cell change its differentiated state?
- sometimes!
What is an example of a cell changing its differentiated state?
- During regeneration – some animals can regenerate body parts after injury
- Cells at the injured site de-differentiate come out of final differentiated stage back into a semi stage .
- An example is the newt limb. Injury prompts muscle cells to de-differentiate and then re-differentiate into cartilage
An example of de-differentiation to make multiple cell types would be dolly the sheep - what was she derived from?
- from a cell derived from an adult udder (a slow dividing, differentiated cell)
How are viable cloned mice produced?
- have been produced from terminally differentiated olfactory neurons cells - but this is controversial.
