The Cell Cycle

Question 1

Binary Fission

Answer 1

Cell division in prokaryotes – a form of asexual reproduction.

• Initiated when DNA of bacterial chromosome is attached by proteins to the inside or plasma membrane
• DNA replication begins along origin of replication region of bacterial chromoosome
• Cells begin to elongate until two DNA attachment sites are at opposite ends of elongated cell
• DNA replication completed –> cell constricts at midpoint and synthesis of cell membrane and cell wall –> division into two identical daughter cells

Question 2

Mitosis in Fertilized Eggs

Answer 2

• Allows for a unicellular fertilized egg to develop into complex multicellular organism
• Early embryos have stem cells that reproduce indefinitely and differentiate into specialized cells
• Cell division contributes to continual renewal and repair of cells once organism is fully grown

Question 3

Adult vs Embryonic Stem Cells

Answer 3

• Adult stem cells are NOT able to give rise to all cell types in organism
• ARE able to replace non reproducing specialized cells
• E.g: Mammalian adult skeletal muscle has little cell division…. so when they undergo injury, stem cells activate and undergo proliferation, differentiation, and fusion of myoblasts (precursors) –> committed to forming mature myofibres
• Before activation, the satellite stem cells were in the G0 phase… after, they reenter cell cycle, allowing for myoblasts

Question 4

Prokaryotic vs Eukaryotic Cell Division

Answer 4

Eukaryotic cells require more regulated control as part as a larger cell cycle.

• Due to larger DNA, linear chromosomes, condensed into nucleus

Question 5

The Eukaryotic Cell Cycle

Answer 5

Interphase: Cells make preparations for division (replication of DNA, overall growth of cell size)

• G1 phase: Prepare the cell for DNA synthesis (cell growth and duplication of organelles)
• S phase: DNA synthesis (chromosomes are replicated)
• G2 phase: Prepare the cell for mitosis (growth, duplication, building protein machinery)

M phase: Mitosis and cytokinesis

Question 6

How Long Does it Take a Cell to Pass Through Cell Cycle?

Answer 6

Depends on the type of cell.

• E.g: Epithelial cells frequently divide, some cells may pause in the G0 phase

Question 7

G0 Phase

Answer 7

Phase in the cell cycle where cells are not actively dividing

• Between M and S phase
• Varies in time
• Permanent in nondividing cells (muscle cells, lens cells, neurons, etc.)

Question 8

Stages of Mitosis

Answer 8

1) Prophase
2) Prometaphase
3) Metaphase
4) Anaphase
5) Telophase

Question 9

Fleming’s Pioneering Work on Mitosis

Answer 9

Discovered the distinct stages of mitosis based on chromosomal position and features.

• Used stained developing salamander embryos

Question 10

Interphase

Answer 10

Start: Chromosomes are long, thin chromatin fibres

G1 Phase (First Gap): The cell grows physically larger, copies organelles, and makes molecular building blocks needed for later steps.

S Phase (Synthesis): The cell synthesizes a complete copy of its DNA (DNA replication) and duplicates the centrosome, which helps separate DNA during M phase.

• Sister chromatids: Two identical copies of a single replicated chromosome
• IMPORTANT: replication of the centrosome!!! there are two centrosomes!

G2 Phase (Second Gap): The cell grows more, makes proteins and organelles, and begins to reorganize its contents in preparation for mitosis.

Question 11

M Phase

brief overview

Answer 11

Mitotic phase = division of the cell

(1) Mitosis (nuclear division):

• Prophase → chromosomes condense, spindle forms
• Prometaphase → nuclear envelope breaks down, spindle fibers attach to kinetochores
• Metaphase → chromosomes align at the middle (metaphase plate)
• Anaphase → sister chromatids separate
• Telophase → nuclei reform

(2) Cytokinesis:

• Cytoplasm divides → 2 identical daughter cells

Question 12

Prophase

Answer 12

2 main sites:

1) Nucleus:

• Chromatin fibers contract, DNA condesnes by tightly coiling
• Chromosomes are visible and have two identical sister chromatids joined at centromeres

2) Cytoplasm:

• Mitotic spindle forms
• Assembly of microtubules begins in the centrosomes (animals) and microtubule organizing centre (plants)
• Centrosomes at opposite poles of the cell (animal)

Question 13

Prometaphase

Answer 13

• Fragmentation/breaking down of nuclear envelope
• Microtubule that extends from each centrosome attaches to kinetochores (regions on the centromeres)
• Chromosomes do not appear completely aligned

Question 14

Kinetochores

Answer 14

Specialized protein structures that associate with each one of the two sister chromatids on either side of the centromere.

• Kinetichor microtubules are essential to help pull chromosome to the poles of the cell during mitosis

Question 15

Polar Microtubules

Answer 15

• Microtubules secreted from centromeres
• Interact with eachother, helping push the poles of the cell away from eachother during mitosis
• Overlaps w/ eachother in the middle
• Help elongate the cell during mitosis

AKA nonkinetochore microtubule

Question 16

Metaphase

Answer 16

Chromosomes align in the centre of cell, in a region named “the metaphase plate”

• Kinetochore microtubules attached at kinetochors of each sister chromatid to assist w/ the movement of the chromosome

Question 17

Anaphase

Answer 17

• Kinechore microtubules shorten
• Binding proteins between sister chromatids break down
• Disjunctional Segragation: Centromeres of sister chromatids disjoin and segragate separate into individual chromosomes (NO LONGER CHROMATIDS!!!)
• Pulled towards opposite poles
• Polar (nonkinetochore) microtubules push against eachother to help elongate the cell

End: Two ends of the cell will have equal and complete sets of chromosomes

Question 18

Telophase

Answer 18

Two new daughter nuclei form in the cell.

• Nonkinetochore microtubules further elongate the cell
• Nuclear envelop reforms around chromosomes at opposite poles of the cell
• Chromosomes decondense
• Spindle microtubules are depolymerized (broken down)
• End of mitosis

Question 19

Cytokinesis

(animal cells)

Answer 19

Division of parent cell into two daughter cells.

• Division of the cytoplasm, therefore of the cell
• Formation of contractile ring made up of motor proteins that contract bundle of actin fibres along midline of cell
• Formation of defined cleavage furrow
• Separates cell into two daughter cells

^Animals, slime molds, fungi form a cleavage furrow

Question 20

Cytokinesis

(plant cells)

Answer 20

• Plant cells lay down newly developed cell wall along cell plate region in middle of dividing cell
• Forming cell wall fuses with existing cell wall, completing cytokinesis

Question 21

Mitosis Promoting Factor

Answer 21

• Cyclin
• Cyclin-dependent Kinase (CDK protein)

Together (Cyclin-CDK complex), control progression of cell cycle.

• Multitude of changes that occur during the various cell cycle events by phosphorylation of target proteins that promote cell division

Question 22

Hunt’s Experiments on Proteins in Mitosis

Answer 22

• Hunt used radioactively labelled methionine to detect changes in protein levels of developing sea urchin embryos
• Samples taken any 10 minutes, gel electrophoresis separates different protein types in sample
• Cyclin was found to have an oscillating pattern, and although function was not known, they knew it had something to do with cell cycle progression

Question 23

Kinase

Answer 23

Enzymes that activate or inactivate other proteins by phosphorylating key amino acids on the target proteins.

• Activated by binding to cyclin proteins
• Hence, activity dependent on cyclin

Question 24

Different Types of Cyclin-CDK Kinases

Answer 24

1) G1/S cyclin-CDK complex: prepares cell for DNA replication (G1 to S phase)

• Increasing expression of histones, etc…

2) S cyclin-CDK complex: Helps initiate DNA synthesis

3) M cyclin-CDK complex: Helps prepare cell for mitosis

• Phosphorylation of proteins needed for nuclear membrane breakdown, regulate assembly of microtubules

Question 25

Cell Cycle Checkpoints

Answer 25

Cellular surveillance. * Able to block cyclin-CDK activity if something is wrong in the progression of the cell cycle * Opportunity for damage to be repaired

Question 26

3 Major Checkpoints of Cell Cycle

Answer 26

1) DNA damage checkpoint: End of G1, Before S, "Is DNA damaged?" 2) DNA replication checkpoint: End of G2, before M, "Is all DNA replicated?" 3) Spindle Assembly Checkpoint: Before anaphase, "Are all chromosomes attached to the spindle?"

Question 27

DNA Damage Checkpoint

Answer 27

* Occurs at G1/S cyclin-CDK * Genes that normally inhibit cell cycle progression are normally turned off * Damage detected? --> Kinases phosphorylate **P53**, which acts as a T.F to turn on genes that inhibit cell cycle * Gives cell opportunity and time to repair damaged DNA

Question 28

P53

Answer 28

A protein that can inhibit the cell cycle when turned on. * Normally present in very low levels in the nucleus due to it being degraded quickly * Upon phosphorylated (by another protein), accumulates in nucleus and acts as a transcription factor to turn on genes that inhibit cell cycle * **CDK inhibitor protein** which binds and blocks activity of G1/S cyclin-CDK complex, arrest the cell cycle in G1 phase * Gives cell opportunity to repair damaged DNA

Question 29

Spindle Assembly Checkpoint

Answer 29

* Regulatory protein associated w/ spindle assembly checkpoint moniters degree to which sister chromatids are attached to microtubules of mitotic spindle at kinetochore regions * Unattached kinetochores create "WAIT" signal --> recruitment of **spindle assembly checkpoint proteins** * Proteins activated by a lack of tension in centromere area * SAC proteins block progression into anaphase until each sister chromatid is attached to kinetochore microtubule * Once this occurs, proteins removed from centromere region * **Separase** breaks sister chromatid attachments

Question 30

Why do Gap (G1, G2) Phases Exist?

Answer 30

* Ensure the parent cell is large enough in size and has required organelles before mitosis * Normal function of daughter cells

Question 31

Nonkinetochore Microtubules

Answer 31

* Forms a cage-like network * Facilitates activities of the cell cycle components * Assists in elongating the entire cell during anaphas

Question 32

Cyclins

Answer 32

Proteins that regulate cyclin dependent kinase (CDK) activity

Question 33

Cyclin Dependent Kinase (CDK)

Answer 33

* Protein kinase, which uses ATP * Adds phosphate groups to proteins * Induces conformational change * Activated by attaching to cyclin

Question 34

MPF Protein Complex

Answer 34

Mitotic Promoting Factor * Cyclin-CDK complex

Question 35

How does MPF complex turn itself off?

Answer 35

By activating a process that degrades cyclin. CDK is reused.