1.5 Flashcards
What is the cytoskeleton?
a network of proteins extending through the cytosol, it is anchored to proteins in the plasma membrane and is dynamic in nature, constantly breaking down and reforming.
What are the functions of the cytoskeleton?
- gives mechanical support and shape to cells
- anchorage for many organelles and some enzymes
- enables whole cell to move
- enables organelles to move
What are microtubules?
hollow straight cylinders composed of globular proteins called tubulins
What are microtubules made up of?
alternative dimers of beta tubulin
What is the role of microtubules?
they control the movement of membrane-bound organelles and chromosomes
Where do microtubules originate?
from the microtubule organising centre (MTOC), contains centrioles
Where is the MTOC in the cell?
found near the nucleus
What does formation and breakdown of microtubules involve?
involves polymerisation and depolymerisation of tubulin
What are spindle fibres?
microtubules that form during cell division
What is the result of an uncontrolled reduction in the cell cycle rate?
May result in a degenerative disease, alteration in normal cell cycle control thought to lead to certain proteins being expressed in association with Alzheimer’s disease, resulting in neuronal cell death.
What is the result of an uncontrolled increase in the cell cycle rate?
May result in tumour formation, proto-oncogene is a normal gene that can mutate to form tumour-promoting oncogene.
What are the two stages of the cell cycle?
Interphase and the mitotic phase
Describe interphase…
it lasts much longer than mitotic phase; cannot see interphase through microscope; active period of growth; protein synthesis takes place, cytoplasmic organelles synthesised and cell grows/ replicates it chromosomes
What are the three sub-phases of interphase?
G1, S phase and G2
Describe G1…
a growth period where proteins and organelles are synthesised
Describe S phase…
cell continues to grow and DNA is replicated in preparation for mitosis
Describe G2…
second growth period during which proteins and organelles are synthesised
What are the two stages of the mitotic phase?
Mitosis - chromosomal material separated by spindle microtubules and nucleus separates contents to divide
cytokinesis - separation of cytoplasm into daughter cells
Describe the first stage of mitosis…
prophase - chromatin condense into discrete chromosomes; each chromosome is 2 identical chromatids joined at centromere; nuclear membrane breaks down; spindle microtubules extend from MTOC and attach to kinetochores in centromere region
Describe the second stage of mitosis…
metaphase - microtubule bundles line up chromosomes on metaphase plate in euqator and cells then enter anaphase
Describe the third stage of mitosis…
anaphase - spindle microtubules shorten, sister chromatids are separated to opposite poles
Describe the fourth stage of mitosis…
the chromosomes decondense and nuclear membrane forms around them
Describe cytokinesis…
cytosol separates into identical daughter cells with identical genetic material
What are checkpoints?
mechanisms in the cell that assess the condition of cell during the cell cycle and halt progression until certain requirements are met
What is required for a cell to commit to the next stage in cycle?
Sufficient nutrients and growth factors must be present
What is the G1 checkpoint?
Appears the most important in mammalian cells; if cell passes this checkpoint then is usually completes cycle; if not it exits the cycle and goes into a non-dividing state (G0); most somatic cell are in G0 phase as cell is not dividing or is preparing to
What are cyclins?
proteins that accumulate during cell growth involved in regulation of cell cycle
What do cyclins combine with?
They combine with and activate cyclin-dependent kinases (CDK)
What does the active cyclin-CDK complex do?
They phosphorylate proteins that regulate progression through the cell cycle.
If sufficient phosphorylation is reached then the cell progresses.
What is retinoblastoma (Rb)?
a cell cycle regulatory protein, it’s non-phosphorylated form restricts progression from G1 to S phase, tumour suppressor protein
How is Rb a tumour suppressor?
It inhibits the transcription of genes that code for proteins needed for DNA replication; It acts by binding to a transcription factor, therefore preventing transcription of certain genes required for S phase
How Rb released?
when the cell enters S phase the G1 CDKs phosphorylate the Rb plasma; this inhibits Rb activity so no longer binds to transcription factors; this brings about the transcription of genes needed to initiate DNA replication, allowing S phase
What do checkpoints also check for?
They asses DNA damage and prevent it from continuing the cell cycle
What does DNA damage trigger?
It triggers the activation of several proteins that can stimulate DNA repair, arrest the cell cycle or cause apoptosis
Name a protein activated by DNA damage…
p53
What are majority of human cancers linked to?
They are linked to mutations in the p53 gene
What is the G2 checkpoint?
It assesses success of DNA replication and DNA damage
What is the G2 checkpoint?
It assesses success of DNA replication and DNA damage
Describe the responses of p53…
-activate DNA repair proteins to repair damaged cell
- arrest cell cycle at G1 checkpoint so cell cycle halts to allow repair proteins time to recognise DNA damage so cell can restart cell cycle
- initiate apoptosis if DNA damage is severe
What is the metaphase checkpoint?
during metaphase, it controls anaphase entry by checking chromosome alignment and microtubule attachment. ensures each daughter cell receives the correct number of chromosomes
What is apoptosis?
The controlled destruction of cells in a multicellular organism, important throughout growth and development of an organism.
What signals does apoptosis trigger?
It triggers both external or internal cell death signals
Give an example of an external death signal…
production of cell death signal molecules from lymphocytes
Give an example of an internal death signal…
DNA damage is an internal death signal
What is the importance of apoptosis?
It is essential during development of an organism to remove cells no longer required as development progresses or during metamorphosis.
Give an example of apoptosis and organ sculpting…
during embryonic development of an organism to remove cells between fingers/toes allows individual digits
Give an example of apoptosis and organ sculpting…
during embryonic development of an organism to remove cells between fingers/toes allows individual digits
How do external death signals work?
They bind to a surface receptor protein and trigger a protein cascade within the cytoplasm
What do internal death signals do?
Results from DNA damage causing p53 activation
What do both external and internal cell death signals result in?
both result in caspase activation (types of protease enzyme) which cause cell destruction
What else may initiate apoptosis?
in the absence of cell growth factors, cells may also initiate apoptosis
