Cell division and cell cycles Flashcards
Interphase
- consists of several phases
- G1 -Gap; growing phase
- S - Synthesis of DNA
- G2 - Gap; growing phase
- cell spends most of its time here, carrying out its normal functions
Steps of Mitosis
- prophase
- prometaphase
- metaphase
- anaphase
- telophase
Prophase
- Key factor: Intact nuclear envelope
- condensed chromosomes with two sister chromatids
- start of a mitotic spindle
Prometaphase
- nuclear envelope in fragments
- chromosome in active motion before lining up across center of cell
Metaphase
-Kinetochores of chromosomes aligned in plane midway between the tow spindle poles
Anaphase
- Daughter chromosomes are split
- microtubules of mitotic spindles shorten
- spindle poles start moving outward
Telophase
- set of daughter chromosomes at each spindle pole
- nuclear envelopes start to develop
- contractile ring appears
Cytokinesis
- completed nuclear envelope surrounds decondensing chromosomes
- reformation of interphase array of microtubules nucleated by centrosome
- contractile ring creating cleavage furrow
Permanent cells
“non-dividing cells”
- Have left the cell cycle and cannot reenter
- If damaged, they are not replaced
- ex - nerve cells, cardiac myocytes, skeletal muscle
Stable cells
“quiescent cells”
Cells that have left the cell cycle and rest in G0, but can be persuaded to reenter if necessary
-ex - parenchymal cells of liver, kidney, pancreas, vascular endothelial cells, smooth muscle cells
Labile
“continuously diving cells”
- proceed continuously through cell cycle
- ex -epithelia of skin, oral cavity, reproductive organs, salivary glands, exocrine pancreas, gastrointestinal epithelium, hematopoietic cells of bone marrow
Growth signals
- Mammalian cells grow only when told to do so
- Mostly more than one growth signal is necessary to initiate growth
- Growth signals are transmitted by:
- growth factors
- steroid hormones
- other hormones such as norepinephrine
G1/S Checkpoint
- growth signal?
- cell size?
- environment?
- DNA ok?
Why is it important to check the size of the cell before proceeding into the next step?
Cell would become smaller if no size check and just divided at a given interval.
While ensuring the right size may delay when division occurs, the size of the cell will remain constant throughout divisions.
G2/M checkpoint
- DNA duplication ok?
- cell size?
M checkpoint
-chromosomes correctly distributed?
What 3 questions must be answered “yes” for cyclin-dependent kinase (CDK) activity to turn on?
- Has the phosphate been removed?
- Has this phosphate been added?
- Is cyclin present?
Mechanism at G1/S checkpoint
- pRB holds transcription factors inactive and thus prevents transcription until a growth signal arrives
- Growth signals cause transcription of 3 cyclins: D, A, and B
- CDK forms a complex with a cyclin.
(In the case of cyclin D/CDK complex, pRB is phosphorylated - Phosphorylation of pRB releases transcription factors
- Transcription factors induce transcription of proteins, which are needed for the duplication of chromosomes in the S-phase
Injury to DNA at Mechanism at G1/S checkpoint
- Cell senses injury
- cell transcribes p53
- p53 halts cell cycle and induces transcription of repair enzymes until DNA can be corrected
