Lecture 4: Cell cycle Flashcards
(29 cards)
Single cell Vs multicellular organisms
> Unicellular organisms
-complete new organism
Multicellular organisms
-Require long and complex series of division to form functioning organisms
Events of the cell cycle
> 24 hours In typical human cell
- 1 hour for mitosis
- 23 hours for interphase
most cells are in golgi apparatus
- cells are senescent/ quiescent or differentiated
- can remain In this state for long periods e.g. neurones
Why do cells need to replicate?
> UV
Chemical damage
themal damage
Mutation
Premature ageing
Duration of cell cycle
> frog embryo ~ 30 minutes
Yeast cells ~ 1.5 hours
Human intestinal cells ~ 12 hours
Human fibroblasts ~ 20 hours
Skin cells ~ 24 hours
Human liver cells ~ 1 year
Neurones ~ never
Features of cell cycle
> Clock or timer
Continous processes
- Organelle/protein synthesis
Stepwise processes
-once per cell cycle
-correct order
- to completion
Purpose of cell cycle
> 1st three phases (interphase)
-duplicate cell content (DNA and other cellular macromolecules)
- Transcribes genes, synthesises proteins etc
Mitosis
- Divide the genetic material into 2 exact halves
Cytokinesis
- Divide the rest of the cell contents into 2 cells (except nucleus)
Checkpoints
> the cycle is controlled at key checkpoints which monitor the status of the cell
- cell size
- adequate supply on nutrients
- DNA quantity and quality
- Favourable environment
Checkpoints can be regulated by signals from
- other cells
- growth factors
- other extracellular signal molecules
Types of cell division
> Mitosis
- occurs in somatic cells (body)
- growth, development and repair
- Asexual reproduction (yields identical cells)
meiosis
- Occurs in specific reproductive cells
- Sexual reproduction (yields different cells)
Mitosis
> Production of two genetically identical daughter cells which are also genetically identical to the parent cell
5 phases
- prophase
- prometaphase
- metaphase
- anaphase
- telophase
Prophase
> genetic material condenses to prevent chromosomes tangling I.e. easier to separate
Centrosomes replicate and move to opposite sides of nucleus
- Driven by motor proteins and ATP
mitotic spindle assembles between the two centrosomes
- Used to separate the two sister chromatids
Nucleus disappears
Prometaphase
> The nuclear membrane breaks down, releasing the centrosomes (normally associated with nuclear membrane)
-Allows centrosomes to
interact with
chromosomes to build the
mitotic spindle
Chromosomes further condense
A kinetochore (point where the sister chromatids are attached) forms at the centromere
-Contains two regions:
-The inner kinetochore
tightly associates with the
centromere DNA
-The outer kinetochore
interacts with microtubules
Microtubules attach at the kinetochore
-The chromosomes attach
to the mitotic spindle
Metaphase
> Chromosomes align on an axis (the metaphase plate)
-The spindle consists of
microtubules, on attached
to each chromosome
Chromosomes act independently and align randomly
Anaphase
> Each centromere splits
-Results in two free
chromatids (called sister
chromatids)
Each chromatid moves towards a pole
Cell changes shape becoming elongated due to microtubules not associated with the kinetochore
Telophase
> Formation of nucleolus and nuclear membrane
Short and thick chromosomes begin to elongate to form long and thin chromatin
Formation of the cleavage furrow (a shallow groove in the cell near the old metaphase plate)
-Position is determined by
the spindle
Cytokinesis
> Pinching of cytoplasm due to contraction of ring of actin and myosin II filaments
-Location of ring
determined by position of
mitotic spindle
-Cleavage occurs in plane
perpendicular to the spindle
Meiosis
> Meiosis means ‘to make smaller’
-Duplicated cells have a
reduced chromosome
number
Diploid germ cells (2N) undergo DNA replication followed by two rounds of division (meiosis I and meiosis II)
-Results in haploid cells (N)
called gametes
-Essential for sexual
reproduction
-Each gamete contains a
complete set of
chromosomes
–>Half of the genetic
content of the original
cell
Genetic diversity
> Meiosis enables genetic diversity
Important as it:
-Facilitates evolution
-Lack of diversity promotes
genetic disease
-A healthy population has a
large gene pool
Genetic diversity arises by recombination
-Independent assortment
of chromosomes
-Crossing over (genetic
exchange)
Meiosis
> Meiosis I
-Prophase I
-Metaphase I
-Anaphase I
-Telophase
Meiosis II
-Prophase II
-Metaphase II
-Anaphase II
-Telophase
Meiosis I
> Each starting diploid cells contains 2 copies of each chromosome
-1 from father 1 from
mother
First step is duplication of chromosomes, which proceeds meiosis in interphase
Interphase (G1, S, G2) is the same in cells undergoing mitosis or meiosis
Prophase I
> Virtually identical to prophase in mitosis
-Development of spindle
-Breakdown of membrane
-Centrosomes move
towards poles
Chromosomes pairs form
-Paired chromosomes are
bivalent (or tetrads)
Chromosomes partially separate in late prophase
-Areas where crossing over
occurred remain attached
and re called chiasmata
Metaphase I
> Critical difference between mitosis and meiosis
The orientation is random
-There is a 50-50 chance for
the daughter cells to get
either the mothers or
fathers homologue for each
chromosome
Crossing over can also occur during this phase
Anaphase I
> Kinetochore spindle fibres contract
Pulls homologous pairs away from each other towards cell poles
Chromosomes, each with two chromatids, move to separate poles
-Each daughter cell is
haploid
-Centromeres do not
separate so each
chromosomes has two
chromatids
Telophase I
> A cleavage furrow forms followed by cytokinesis
Nuclear envelope usually is not reformed and chromosomes do not disappear
Each daughter cell has a single set of chromosomes, half the total number in the original cell
-Original cell was diploid
-Daughter cells are haploid
Meiosis II
> Meiosis II is similar to to mitosis
Chromatids of each chromosome are no longer identical because of recombination
Meiosis II separates the chromatids producing two daughter cells each with haploid chromosomes
Each chromosome only has one chromatid
