2_3 Flashcards
cell cycle
- Multicellular eukaryotes depend on cell division for Development from a fertilized cell, Growth, Repair
- Cell division is an integral part of the cell cycle, the life of a cell from formation to its own division.
- Most cell divisions result in daughter cells with identical genetic information.
- The exception is meiosis
genome
- All the DNA in a cell constitutes the cell’s genome.
- A genome can consist of a single DNA molecule (common in prokaryotic cells) or a number of DNA molecules (common in eukaryotic cells).
- DNA molecules in a cell are packaged into chromosomes.
binary fission
- Cell Division in a prokaryote
1. DNA contained in a single circular chromosome with an origin of replication. First step is DNA replication.
2. Cell gets larger as chromosomes move to opposite ends of the cell.
3. After replication, cell pinches in two, resulting in two identical daughter cells.
Eukaryotic chromosomes
- Eukaryotic chromosomes consist of chromatin, a complex of DNA and protein that condenses during cell division.
- Every eukaryotic species has a characteristic number of chromosomes in each cell nucleus.
Cell division in eukaryotic cells more complicated because
because they have…
More than one chromosome
Cell organelles
Cytoskeleton
cell cycle phases
interphase
- G1 (40 percent) - separates M phase from S phase. Cell grows and accumulates substrates needed for DNA replication. Centrioles in centrosome separate.
- S (39 percent) - DNA synthesis
- G2 (19 percent) - separates S from M. Cell produces molecules needed for mitosis.
M phase
(2 percent); Mitosis (nuclear division) and cytokinesis (division of the cytoplasm)
G0
(variable) The non-dividing (resting) phase in which most cells exist
cell cycle differences
- The frequency of cell division varies with the type of cell
- These cell cycle differences result from regulation at the molecular level
cell cycle experiments
- The cell cycle appears to be driven by specific chemical signals present in the cytoplasm
- Some evidence for this hypothesis comes from experiments in which cultured mammalian cells at different phases of the cell cycle were fused to form a single cell with two nuclei
- one cell in S; one in G1; G1 nucleus immediately entered S phase
checkpoints
- In normal cells that divide, the cell cycle does not proceed unchecked. Cells maintain control over the stages of the cell cycle through what are called checkpoints—steps along the cycle that must be completed before the next step is started.
- Many of these signals are kinases (cyclin-dependent kinases - CDKs) that activate other proteins and allow the next phase to proceed.
G1 checkpt
regulates the entry of the cell into the S-phase of DNA replication.
One of the conditions that must be met is:
Is DNA undamaged?
G2 checkpt
regulates the entry of the cell into the M-phase of mitosis
One of the conditions that must be met is:
Has all of the DNA been replicated?
M checkpt
regulates the entry of the cell from metaphase to anaphase
One of the conditions that must be met is:
Are the chromosomes aligned at the metaphase plate?
Cdk/cyclin concentrations
- Cdks are proteins that activate other proteins and allow the next phase to proceed. Their concentrations do not vary throughout the cell cycle. However, they must be bound to cyclins to be active.
- Cyclin concentrations do vary during the cell cycle.
Cdk/cyclin compelx and example
- Cyclins and CDK bind together to form complexes that control the progression of the cell cycle.
- ex: Maturation (or M-phase)-promoting factor (MPF)
- This protein allows the cell to progress past the G2 checkpoint and enter mitosis. It is formed when a cyclin binds to a CDK.
- Activated MPF is a kinase and controls some of the processes associated with mitosis, including the breakdown of the nuclear membrane.
- MPF is only a transiently active molecule - only when it is bound to cyclin.
MPF concentration
- MPF also initiates a sequence that leads to its separation back into a cyclin and the inactive cyclin-dependent kinase. The cyclin is then broken down, and levels remain low until the correct signal occurs to increase cyclin concentrations.
- MPF activity tracks (and lags behind) changes in the cyclin concentration
internal signal at checkpt
An example of an internal signal is that kinetochores not attached to spindle microtubules send a molecular signal that delays anaphase
external signal at checkpt
- Some external signals are growth factors, proteins released by certain cells that stimulate other cells to divide
- Another example of external signals is density-dependent inhibition, in which crowded cells stop dividing
PDGF
- example of growth factor (external signal)
- platelet-derived growth factor (PDGF) stimulates the division of human fibroblast cells in culture
experiment:
1. sample of human connective tissue is cut up into small pieces w/ scalpel, pieces placed in petri dish
2. enzymes digest the ECM, resulting ins uspension of free fibrobasts
3. cells transferred to culture vessels; PDGF added to half of the vessels
4. vessels w/ PDGF: many more cells than vessels w/o
anchorage dependence
- example of external signal
- Most animal cells, in addition to density-dependent inhibition, exhibit anchorage dependence, in which they must be attached to a substratum in order to divide
- Cancer cells exhibit neither density-dependent inhibition nor anchorage dependence
density-dependent inhibition
- crowded cells stop dividing
- cells form a single layer; cells divide to fill a gap and then stop
S-Cdk
S-Cdk controls G1 checkpoint (entry into S-phase)
- if DNA is damaged, S-Cdk is inhibited so cell won’t
enter S-phase and replicate damaged DNA.
Ubiquitylation
is the targeting of molecules to be destroyed
when p53 gene is normal
- Mdm2 binds to the unphosphorylated p53 and directs it to be destroyed. (Ubiquitylation)
- In response to the presence of damaged DNA, kinases are activated that phosphorylate and thus increase the concentration of p53 (it’s no longer ubiquitylated).
- Active p53 binds to a regulatory site on the DNA just upstream from a gene that codes for a protein called p21.
- This stimulates the transcription of the p21 gene and through translation increases the production of the p21 protein.
- p21 protein is a CDK inhibitor and it binds to and inhibits the activity of the S-cyclin CDK complex.