AP Bio: The Cell Cycle

Cell theory

All cells come from pre-existing cells

1 parent cell → 2 daughter cells

Genome

All of a cells DNA

Amount varies based on species

Chromatin

Uncoiled DNA that exists for most of a cells life

Chromosomes

Condensed DNA present during division

One long DNA strand from chromatin wraps around DNA-organizing proteins (histones) → wrap around each other → form thicker strands of DNA/proteins

After copying: each chromosome has 2 identical sister chromatids attached @ centromere by cohesions & will separate into 2 daughter cells (each with a copy of parent cell’s genetic info)

Centrosomes

Region near side of animal cell containing 2 centrioles

Produce spindle fibers used to move things around during cell division

Ex: microtubule-organizing center (MTOC)

Mitotic Spindle

Network of microtubule fibers & proteins

AKA spindle fibers

Extends from centrosome and attaches to chromosomes to separate them

Kinetochore

Proteins @ centromere (middle region of chromosome) where spindles attach

Order of Cell Cycle

1. Interphase

   1. G1

   2. S

   3. G2

2. Mitosis

   1. Prophase

   2. Prometaphase

   3. Metaphase

   4. Anaphase

   5. Telophase/Cytokinesis

G1 Phase

Acquiring nutrients

Growing in size

Organelles replicate

Preparation for division

S Phase

Synthesis

Replication of DNA

Makes uncondensed sister chromatids

G2 Phase

Growing in size

Preparing for division

Making enzymes

Prophase

DNA starts condensing into chromosomes: prevents DNA tangling & allows for accurate separation later)

Mitotic spindles begin forming as centrosomes move to opposite poles

Nucleolus disappears

Prometaphase (AKA late prophase)

Nuclear envelope starts to break down

DNA finishes condensing

Centrosomes have reached opposite poles

Spindle fibers attach to kinetochores, non-kinetochore microtubules reach across circumference

Metaphase

Spindle fiber “tug of war” moves chromosomes to middle

Anaphase

Cohesions are cleaved (split) by enzymes

Sister chromatids separate

Kinetochore microtubules shorten  → pull chromosomes toward centrioles @ each pole

Now daughter chromosomes

Telophase

Nuclei begin to reform

DNA condenses into chromatin

Spindles break down

Cytoplasm begins dividing

Cytokinesis

Division of cytoplasm

Animal cells: form cleavage furrow through inward pinching of microfilaments in membrane

Plant cells: form cell plate through building new cell wall inside using vesicles from Golgi

Binary Fission

Division in half

Division in Prokaryotes 

1. DNA replication

2. Cell elongates (growth)

3. Plasma membrane pulls inward → starts building new cell wall

4. 2-daughter cells

Which Division is More Error-Prone?

Prokaryotic

Regulatory Proteins

Trigger & coordinate order of cell cycle

Checkpoints

Moment in cycle when regulatory proteins control a process

G1, G2, M

End of G1 Checkpoint

Checks cell-size, nutrient availability, growth factors, any DNA damage

If something wrong: enter G0 non dividing phase (only possible in G1)

Quiescence

Wait to re-enter cycle after entering G0

Senescence

No longer can divide after entering G0 → apoptosis

Apoptosis

Programmed cell death

Cells break into blebs (big vesicles)

Enzymes break contents down

Digested by cells of immune system

Protects neighboring cells from damage

End of G2 Checkpoint

Regulatory proteins check that DNA replicated

If something wrong: pause & repair → apoptosis if irreparable

M Checkpoint @ Transition Through Metaphase

Regulatory proteins check if spindle fibers attach

If something wrong: pause & repair → apoptosis if irreparable