The Cell Cycle Flashcards
What best distinguishes living vs non-living?
The ability to reproduce
Key Function of Cell Division (3):
1) Asexual Reproduction
2) Growth and Development
3) Cell Replacement
Asexual Reproduction
Unicellular organisms divide to form individual organisms
–> Prokaryotes, unicellular eukaryotes (yeast, amoebas, etc.)
Cell Division’s Role in Growth and Development
Multicellular organisms divide to grow –> Enables organisms to develop from a single cell to a complex cell system
Cell Division’s Role in Cell Replacement
Tissue renewal/repair:
Replacing cells that die from accidents or normal wear and tear
What must be highly coordinated for proper cell division?
The timing of DNA replication and the actual cell division
Genome
All of a cell’s DNA (all the genetic info)
Eukaryote vs Prokaryote Genome
Eukaryotes = Genome containing multiple DNA molecules
Prokaryotes = Genome containing usually only one DNA molecule
Chromosomes
DNA wrapped in protein –> DNA-Protein Complex
–> Carry genes which specify an organism’s inherited traits
Gene
A segment of DNA
Replicated vs UN-replicated chromosomes
Replicated Chromosome = “X Shape” –> TWO DNA molecules
UN-Replicated Chromosome = NOT “X shape”
–> ONE DNA molecule
When in the cell cycle do we see replicated chromosomes?
During cell division (mainly mitosis)
–> DNA is usually kept in its unreplicated form
What do the chromosome proteins do?
Control chromosome structure (folding) and function (gene expression)
Chromatin
All the DNA and proteins of chromosomes
–> (kind of referring to all chromosomes)
Every species has a ________________ ______________ of chromosomes
1) Characteristic
2) Number
of chromosomes DOES NOT EQUAL
sophistication
Human Somatic Cells Chromosomes
46 Total Chromosomes –> 23 pairs (one from mom and the other from dad to form the pairs)
Gamete Chromosomes
1/2 the number of chromosomes as a somatic cell
Form of DNA when NOT dividing
OR
when DNA is REPLICATING!
Each chromosome is in the form of a long, thin chromatin fiber (uncondensed/uncoiled)
–> Cannot see using light microscope
Form of DNA AFTER Replication
OR
When cell is dividing
The chromosomes condense; each chromatin fiber becomes densely coiled and folded (“X shape”)
–> CAN see using a light microscope
Sister Chromatids
Copies of the same chromosome that are joined together (make up the classing “X shape’)
What holds sister chromatids together?
Cohesins
Cohesins
Proteins that hold sister chromatids together along their length
Centromere
A specialized sequence of DNA where sister chromatids most CLOSELY attach/tether
A centromere is NOT a _______________
A centromere IS a _________ _______________
1) Location
2) DNA sequence
How do centromeres cause greater tethering between the sister chromatids?
Due to proteins that recognize and bind to centromere DNA
Importance of centromere
Important for proper splitting of the sister chromatids during segregation
Chromosome Arm
The portion of a chromosome to either side of the centromere
Cell Cycle
The life of a cell from the time it is first formed during division of a parent cell until its own division into 2 daughter cells
Two main processes must alternate to allow for proper cell division:
1) Doubling the genome –> Chromosome replication
2) Separation of the duplicated genome into exactly one half –> Chromosome segregation
The cell cycle is split into two overall phases
1) Interphase
2) M-Phase
Steps of Interphase
1) G1 phase
2) S phase
3) G2 phase
Purpose of Interphase
Sets up and readies the cell to undergo mitosis and cell division
–> Accounts for around 90% of the cell cycle
M-Phase
1) Mitosis
2) Cytokinesis
G1 Phase
“First Gap” AKA First Growth
–> Cell grows, producing proteins and cytoplasmic organelles
S Phase
“Synthesis” phase
–> DNA/chromosome replication: the cell duplicates its chromosomes
G2 Phase
“Second Gap” AKA Second Growth
–> Cell continues to grow AND completes preparations for the cell to divide
–> ** The centrosome duplicates**
MTOC
Microtubule Organizing Centers
–> Structure from which microtubules emerge
What is the Microtubule Organizing Centers (MTOC) in animals?
Centrosome
Centrosome
A microtubule organizing center for the formation of the mitotic spindle
–> Made of TWO centrioles that are at a 90 degree angle to each other
How many centrosomes are needed in cell division?
Two
When does the centrosome duplicate?
In Interphase: G2 Phase
Mitosis
Nuclear division (separation of replicated chromosomes)
Steps to Mitosis:
1) Prophase
2) Pro-metaphase
3) Metaphase
4) Anaphase
5) Telophase
(PpMAT)
Prophase (3)
3 main events take place:
1) Centrosomes begin separating die to the lengthening of the microtubules between them (formation of an early mitotic spindle)
2) Chromosomes condense (into their classic “X shape”)
3) Nucleoli disappear
Is the nuclear envelope still present in prophase?
Yes
What causes mass cell reorganization in prophase?
The depolymerization of cytoskeletal microtubules to release tubulin that is then recruited for formation of the mitotic spindle
Pro-Metaphase (3)
3 main events take place:
1) Nuclear envelope fragments
2) Kinetochores form at each centromere of the chromosomes (2 per chromosome: 1 on each sister chromatid)
3) Microtubules from the spindle begin attaching to the kinetochores (progressing towards alignment in the middle)
Kinetochores
A protein complex that assembles on the centromere of each sister chromatid
How do kinetochores bond to the centromere?
Some proteins of the kinetochore are DNA binding proteins that recognize the centromere DNA sequence, making the kinetochores from there
Kinetochore Analogy
Kinetochores are like “landing pads” for microtubules
Metaphase (3)
3 main events take place:
1) Centrosomes are now at EXACTLY opposite poles
2) Chromosomes have all arrived at the metaphase plate (middle)
3) All kinetochores are attached to kinetochore microtubules
Metaphase Plate
An imaginary plane exactly midway between the poles of the cell
What causes the chromosomes to align at the metaphase plate?
A tug of war
–> Microtubules are pulling and pushing which puts OPPOSING FORCES on the chromosomes attached
What conditions of the “tug of war” must be met in order for the chromosomes to align in the middle?
The opposing forces placed on the chromosomes by the microtubules must be EQUAL –> The chromosomes align in the middle and metaphase is “reached”
Anaphase (2)
2 main events take place:
1) Sister chromatids separate suddenly and get pulled to opposite sides of the cell
2) Non-Kinetochore microtubules elongate which causes the cell to also elongate overall
–> The SHORTEST step of mitosis
What begins anaphase?
The Anaphase Promoting Complex (APC)
Anaphase Promoting Complex (APC)
A protein complex that has PROTEASE activity that, when activated, cleaves (cuts) COHESINS
What activates APC?
TENSION
–> Kinetochore-microtubule interactions must be oriented specifically to achieve a level of tension that in turn ACTIVATES APC
What is the correct kinetochore interaction with microtubules?
Microtubules from each respective side connect to the kinetochore closest to the side of their origin (BOTH kinetochores must be bonded to a microtubule)
–> “Back to back” attachment
–> Produces the sufficient tension that activates APC
How does the APC protect the cell?
Anaphase will be delayed until all chromosome kinetochores have been properly attached to the microtubules
–> This ensures proper chromosomal segregation and that there are no errors when pulling the chromatids apart
Protects against improper segregation of the DNA
What does APC do once activated?
APC proteases CLEAVE the cohesins holding the sister chromatids together which then allows the microtubules to pull them apart and to opposite poles
Telophase (4)
4 main events take place:
1) 2 daughter nuclei begin to form (along with their nuclear envelopes)
2) Nucleoli Reappear
3) Chromatin becomes less tightly coiled
–> **Cannot be seen under light microscope at this point
4) Removal of the mitotic spindle: Any remaining spindle microtubules DEPOLYMERIZE
–> MITOSIS IS COMPLETE AT THIS POINT
Cytokinesis
Cytoplasmic Division –> Leading to (usually) 2 daughter cells
(Process is slightly different between animals and plants)
When does cytokinesis begin?
Generally begins during ANAPHASE or TELOPHASE
What marks the beginning of cytokinesis?
Formation of the cleavage furrow (in animals)
Around when does the cleavage furrow begin to form?
Around LATE ANAPHASE
Cleavage Furrow
A shallow groove in the cell surface near the old metaphase plate
–> A CONTRACTILE RING OF ACTIN MICROFILAMENTS
What causes the cleavage furrow to contract/pinch in?
The contractile ring of actin microfilaments associate with myosin protein molecules allowing the ring to contract
What happens as the cleavage furrow ring contracts?
The cleavage furrow deepens until the parent cell is pinched into 2 = 2 daughter cells
Cytokinesis Proces in PLANTS
1) During TELOPHASE, vesicles derived from the golgi apparatus move along microtubules to the middle of the cell
2) Vesicles COALESCE in the middle of the cell = CELL PLATE
3) Cell plate enlargens until the membrane surrounding the plate fuses with the membrane along the perimeter of the cell
= 2 daughter cells with individual plasma membranes
–> Cell wall then forms between the two cells
In plant cytokinesis, what do the vesicles carry?
Materials used for building the cell plate and wall
Binary Fission
= “Division in half”
A type of asexual reproduction typically observed in prokaryotes and a select few unicellular eukaryotes
Binary Fission in Eukaryotes vs Prokaryotes
Eukaryotes = Involves mitosis
Prokaryotes = NO mitosis
Bacterial Binary Fission
1) Chromosome replication begins at the origin = two copies of the origin, one of the copies moves to the other end of the cell
2) Origins are at 2 opposite ends of the cell
–> Replication continues and cell continues to grow
3) Replication finishes and the cell is roughly double its original size = Plasma membrane pinches inwards (by a tubulin like protein)
4) New cell wall is deposited and cell division is complete
Bacterial chromosome is _________ and ________…
circular and attached to the cell membrane
In bacterial binary fission, all newly synthesized DNA is…
Attached to the plasma membrane
Mitotic Spindle
A microtubule based machine:
Provides MECHANICAL FORCE that drives the alignment of chromosomes during Metaphase AND the separation of sister chromatids during anaphase
Origins of the mitotic spindle
centrosomes
What is the mitotic spindle made up of?
Consists of fibers made of microtubules and associated proteins
3 types of microtubules in the mitotic spindle:
1) Astral MT
2) Kinetochore MT
3) Overlap (“Non-kinetochore”) MT
Astral Microtubules
A radial array of short microtubules that extend out from each centrosome (Gives the “star” appearance of the aster)
Kinetochore Microtubules
Microtubules that actually attach to chromosome kinetochores
Overlap Microtubules
AKA Non-Kinetochore MT
–> Microtubules that DO NOT bind to kinetochores but elongate to overlap and interact with each other
Mitotic Spindle Through the Phases
Interphase === Centrosome duplicates
Early Prophase === 2 centrosomes are adjacent on the nuclear membrane
Prophase === Microtubules grow –> Centrosomes pushed apart
Metaphase === Each mitotic spindle is at opposite ends of the cell
Telophase === Mitotic spindle deforms
