Lecture 13- The cell cycle Flashcards
What do unicellular organisms primarily use cell division for?
Reproduction
What 4 events must occur for a cell to divide?
- Reproductive signal
- Replication of DNA (and other cell components)
- Segregation
- Cytokinesis
What is segregation?
When the cell distributes replicated DNA to each of the two new cells
How do prokaryotes divide?
Binary fission
What do reproductive rate in prokaryotes respond to?
conditions in the environment
What bacterium is commonly used in genetic studies?
Escherichia coli
How long does it typically take for E.coli to divide at 37 degrees Celsius?
40 minutes
How can the rate of division of E.coli be sped up?
Abundant carbohydrates and mineral nutrients
What bacterium stops dividing when food supplies are low and continues dividing when conditions improve?
Bacillus subtilis
What is the structure of the E.coli DNA?
A continuous molecule called a circular chromosome
How does DNA fit into a cell?
It is compacted
How is DNA compacted in prokaryotes?
It folds in on its self- + proteins bind to negative DNA to contribute to this folding
Where are circular chromosomes found?
Prokaryotes,
some viruses,
chloroplasts and mitochondria of eukaryotic cells
What are the two regions of the prokaryotic chromosome that play a functional role in reproduction?
ori
ter
What is the ori?
The site where replication of the circle starts
What is the ter?
The site where replication ends
What is the DNA threaded through as chromosome replication takes place?
A replication complex of proteins including DNA polymerase
What happens when the DNA is being replicated in prokaryotic cells?
The ori regions move towards opposite ends of the cell
What does DNA adjacent to the ori do?
Binds proteins essential for this segregation
What type of process is segregation in prokaryotes?
Active process- binding proteins hydrolyze ATP
What else is involved in DNA segregation?
The prokaryotic cytoskeleton
How does the prokaryotic cytoskeleton help in DNA segregation?
Either actively moving DNA along or passively acting as a ‘railroad’ track along which DNA moves
What is cytokinesis?
Cell separation
When does cytokinesis begin in prokaryotes?
After chromosome replication is finished
What is the first event in prokaryotic cytokinesis?
Pinching in of the plasma membrane to form a ring structure similar to a purse.
What happens in prokaryotes after the plasma mebrane has been pinched in?
The membrane pinches in, new cell wall materials are synthesised which separates the cell.
What is the major component of the ring that is made when the pinching in of plasma membranes form a ring in prokaryotes?
Fibers composed of proteins similar to eukaryotic tubulin are major components of this ring.
By what two processes do eukaryotic cells divide?
Mitosis, meiosis
What is development?
The formation of a multicellular organism from a fertilized egg
What does development involve?
Cell reproduction
Cell specialization
How does the rate of division in eukaryotic and prokaryotic cells differ?
Eukaryotic cells do not constantly divide whenever environmental conditions are adequete
How do eukaryotes and prokaryotes differ in regards to the number of chromosomes?
Eukaryotes usually have many
Prokayotes usually have a single main chromosome
How many chromosomes do humans have?
46
What does having more chromosomes in eukaryotes do?
Makes segregation more intricate
What are newly replicated chromosomes in eukaryotes known as?
Sister chromatids
What mechanism is used to segregate chromosomes in eukaryotes?
Mitosis
What does the presence of a nucleus in eukaryotes do?
Makes cytokinesis distinct from segregation (because the nucleus divides first)
Why does cytokinesis proceed differently in plant cells/
Plant cells have a cell wall
Animal cells do not
What happens to some cells such as red blood cells as they mature?
They lose the ability to divide
What type of cells divide rarely?
Cortical cells in plant stems
What cells are specialized for rapid division?
Cells in a developing embryo
What is the cell cycle?
The events that occur to produce two eukaryotic cells from one
What is the phase between divisions called?
Interphase
What are the three sub phases of interphase?
G1, S, G2
What happens during S (synthesis) phase?
The cells DNA replicates
What is G1?
Gap 1
Period between the end of mitosis and the onset of S phase
What is G2?
Gap 2
Separates the end of S phase and the beginning of mitosis
What is mitosis and cytokinesis reffered to as in the cell cycle?
M phase
What happens during G1 phase?
Preparing for S phase
What is the structure of DNA during G1?
Single, unreplicated structure
How long is G1?
Variable in different cell types
What happens when a cell in G1 enters a resting phase?
It enters G0
What is needed to enter leave G0 phase?
Special internal and external cells prompt a cell to leave G0 and reenter G1
What is the transition between G1 to S called?
The commitment to cell division
How many chromosomes are there at the end of S phase?
Double
Two sister chromatids jointed together, awaiting segregation
What happens during G2 phase?
Cell makes preparations for mitosis:
-synthesizing microtubule components to move chromatids to opposite ends of dividing cell
How was the hypothesis that a cell in S phase contains an activator of DNA replication tested experimentally?
Fuse cells- 1 in S phase and one in G1.
Both enter S phase
What is the conclusion to the experiment that tested that a cell in S phase contains an activator of DNA replication?
The S phase cell produces a substance that diffuses to the G1 nucleus and activates DNA replication
How are cells induced to fuse to certain substances?
In sugar alcohol polytheylene glycol, plasma membranes are disaggregated
What is the fused cells in the experiment called?
Binucleate cell
What does the transition from G1 to S phase depend on?
An activator protein called cyclin-dependent kinase (Cdk)
How does phosphorylation effect an enzyme?
Phosphorylation changes the shape and function of a protein by changing its charges
What experiment was taking place when Cdk was discovered?
James Maller
University of Colorado
Studying immature sea urchin eggs
Finding out how they are stimulated to divide
What protein was found by James Maller at the University of Colorado?
Maturation promoting factor
Prodded immature eggs into division
What experiment was also taking place when maturation promoting factor was discovered?
Leland Hartwell
University of Washington
Studying cell cycle in yeast
What did Leland Hartwell find when studying cell cycle in yeast?
A strain of yeast stalled in G1-S boundary that lacked Cdk
What was discovered about Leland Hartwell’s strain?
yeast Cdk very similar to sea urchins maturating promoting factor
How are Cdk’s activated?
Binding to cyclin (a protein)
What is the binding of Cdk to cyclin an example of?
Allosteric regulation
How does cyclin activate Cdk?
Changes its shape, exposes active site
What does the cyclin-Cdk complex act as?
A protein kinase
What happens after cyclin-Cdk complex triggers transition from G1 to S phase?
Cyclin breaks down, Cdk is inactivated
What is the restriction point?
A key decision point beyond which the rest of the cell cycle is normally inevitable
What activator acts in the middle of G1 to move the cell past the restriction point?
Cyclin-D-Cdk4
What other activator acts in the middle of G1?
Cyclin-E-Cdk2
What does cyclin-E-Cdk2 do?
Works in concert with Cyclin-D-Cdk4 to move the cell past the restriction point
What act at the G2-M boundary?
Cyclin-B-Cdk1- initiates the transition into meiosis
What does Cyclin-A-Cdk2 do?
Acts during S phase to stimulate DNA replication
What does retinoblastoma do?
Normally inhibits the cell cycle
What happens when retinoblastoma is phosphorylated by a protein kinase?
It becomes inactive
Doesn’t block restriction point
Cell progresses past G1 into S
What do cyclin-Cdk complexes act as?
Checkpoints- points which a cell cycle’s progress is monitored to determine whether the next step can be taken
Give an example of a cyclin-Cdk complex acting as a checkpoint
When DNA is damaged by radiation during G1, p21 binds to G1’s Cdk’s to prevent their activation by cyclins
What is p21?
A tumour suppressor
What does the 21 stand for in p21?
Its molecular weight
21,000 daltons
What happens to p21 after cell repairs have been made?
It breaks down
What checkpoint is there at the end of S phase?
A checkpoint for complete DNA replication- if not complete, the cell cycle stops.
What is disrupted in cancer cells?
Cyclin-Cdk controls
What do some fast-growing cancers have?
Too much cyclin D overstimulates Cdk4 and thus cell division
What does p53 do?
Prevents normal cells dividing by stimulating synthesis of p21
What do more than half of human cancers have?
Faulty p53- absence of cell cycle controls
What is RB, p52 and p21 examples of?
Tumor suppressors
How do cells that no longer divide enter the cell cycle?
Stimulated by external chemical signals called growth factors
Give an example of a growth factor
Platelet derived growth factor
produced by platelets after a cut
What does platelet derived growth factor do?
Diffuses to adjacent cells in the skin and stimulates them to divide and heal wounds
What type of growth factor does white blood cells produce?
Interleukins
What do interleukins do?
Promote cell division in other cells essential for the bodies immune system
What growth factor does the kidney produce?
Erythropoietin
What does erythropoietin do?
Stimulates the division of bone marrow cells and production of red blood cells
Additionally, what else can act as a growth factor?
Hormones which promote division in many cells types
How do growth factors initiate the cell cycle?
Bind to target cells via specialized receptors on target cells surface
Binding triggers events within the target cell to initiate the cell cycle
How are growth factors and cancer related?
Cancer cells divide inappropriately because they make their own growth factors of no longer require growth factors to start cycling
What can mutations in retinoblastoma gene do?
render it inactive
What happens when mutations render retinoblastoma inactive?
The cell cycle progresses through the cell cycle
What happens in retinoblastoma tumors in retinal cells?
Retinal cells divide uncontrollably
This leads to blindness
What is a risk factor for retinoblastoma tumors in the retina?
Hereditary/ inherited
What are the two types of cell death?
Necrosis
Apoptosis
When does necrosis occur?
When cells are damaged by toxins or starved of essential nutrients or oxygen
What happens when cells are damaged by toxins or starved of essential nutrients/oxygen?
The cells swell up and burst, releasing contents into the extracellular environment
What does the swelling and bursting of cell during necrosis lead to?
Inflammation
What is apoptosis?
The genetically programmed series of events that result in cell death
What are the two reasons for a cell initiating cell death?
- The cell is no longer needed by the organism
- The longer cells live, the more prone to genetic damage, leads to cancer.
Give an example of apoptosis due a cell no longer being needed.
Human fetus- web-like hands with connective tissue between fingers. This undergoes apoptosis as development proceeds.
What cells under go apoptosis more commonly because of they are prone to genetic damage?
Blood and epithelia lining organs such as the intestine
What is blood and epithelia lining organs more prone to genetic damage?
They are exposed to high levels of toxic substances
How often is blood/ epithelia lining organs replaced?
Days or weeks
What events occur in apoptosis?
Cell become isolated from neighbors
Cell cuts up chromatin into nucleosome sized pieces
Forms membranous lobes/ ‘blebs’ that break into cell fragments
What happens after a cell has undergone apoptosis?
Surrounding living cells ingest the remains of the dead cell
How is the cell death cycle controlled?
Signals inside or outside of the cell
What type of signals initiate apoptosis?
- Lack of mitotic signal / growth factor
- Recognition of damaged DNA
How do external signals, or lack of them, initiate apoptosis?
Receptor protein in the plasma membrane changes shape and activates a class of enzymes called caspases
How are caspases activated by internal signals?
Internal signals cause mitochondria release molecules which activate caspases
What do caspases do?
Hydrolyze proteins of the nuclear envelope, nucleosomes and plasma membranes
What is it important to understand how cells die?
Drugs used to treat diseases of cell proliferation (such as cancer) work through apoptosis signals
What cancer treatment drug blocks the synthesis of thymine?
5-flurouracil
What cancer treatment drug blocks the function of microtubules?
Paclitaxel
How does radiation treat cancer?
Extensive DNA damage promotes apoptosis
