Lecture 20 - Cell Cycle and Cell Cycle Regulation

Where do cells arise from?

• pre-existing cells

What is the purpose of mitosis and the cell cycle in eukaryotes?

• Asexual reproduction in unicellular organisms

• Growth

• Wound Repair

The last 2 are involved with multicellular eukaryotes

Meiosis vs Mitosis

Stages of the cell cycle in eukaryotes

• Cell Cycle: alternation of interphase (non-dividing) and M-phase (dividing) stages

• Non-dividing cells (most in your body) are arrested in G₀

Phases of interphase

• G₁ and G₂, and S-phase

G₁ and G₂

• Organelles are replicated and cells grow in size

S-phase

• Chromosomes are replicated

• Centrosomes are duplicated

Centrosomes

• Centriole + pericentriolar material

• microtubule organizing center (MTOC)

Chromosomes during S-phase

• are replicated

Sister chromatids

• join at the centromere and are exact copies of the same genetic information

Prophase

• Chromosomes condense: become shorter and thicker

• Transcription slows or stops

• Spindle apparatus begins to form

Prometaphase

• Nuclear envelope breaks down

• Microtubules contact chromosomes at kinetochores

M

Metaphase

• chromosomes line up in the middle of the cell (AKA metaphase plate)

Anaphase

• sister chromatids separate and are pulled to opposite poles of the cells

Telophase

• chromosomes decondense

• nuclear envelope reforms

Cytokinesis in animals

• a ring of actin and myosin filaments contracts inside the cell membrane, causing it to pinch inward in a cleavage furrow

• cytoplasm is divided, two daughter cells form

Cytokinesis in plants

• vesicles are transported from the Golgi apparatus to the middle of the dividing cell along microtubules. These vesicles fuse to form a cell plate

• cytoplasm is divided, two daughter cells form

How to bacteria do cell division?

• Binary Fission

Microtubules during interphase

• form a meshwork throughout the cytoplasm, serving as tracks for motor proteins to transport vesicles and organelles

M

Microtubules during mitosis

• form the spindle apparatus to separate chromosomes

Microtubules

• hollow tubes formed from tubulin dimers

Dynamic Instability

• cycles of growth follow by rapid shrinkage at the + ends

• used to explore cellular space

Cellular Space

• if the growing microtubule does not “find” a chromosome, it shrinks back

• if the growing microtubule does “find” a chromosome, it is stabilized

Sister Chromatids and Microtubules

• microtubules must have a sister chromatid attached to each pole for stability

How do the chromatids move to the poles of the cell during anaphase?

• microtubules shorten at the kinetochore end

Variation of Length of S-phase and M-phase

• generally a fixed length for a given organism

Variation of Length of G₁ and G₂

• eliminated in rapidly dividing cells

Variation of Length of Non-dividing cells

• are arrested in G₀. most cells in the adult body are G₀

Rate of cell division

• can vary in response to specific conditions

Cyclins and Cyclin-dependent kinases (CDK)

Cyclin and CDK levels in the cell cycle

• The concentration of the kinase subunit does not change during the cell cycle; but the concentration of cyclin changes during the cell cycle

MPF

• M-phase promoting factor

• consists of M-cyclin and a CDK

What does MPF phosphorylate?

• chromosomal proteins

• nuclear lamins

• microtubule associated proteins

• ubiquitin-ligase, leading to the degradation of M-cyclin

How is MPF deactivated?

• M-cylcin is ubiquitinated by an enzyme that is activated during anaphase of mitosis

• The ubiquitin tag targets cyclin to the proteosome where it is destroyed

Is there only one cyclin-CDK that reguated every stage of the cell cycle?

• no, there are different cyclin-cdk complexes (M-cyclin — CDK, S-cyclin — CDK, G₁/S-cyclin — CDK)