Unit 4 (Cell Cycle)

Cell Cycle

The life of a cell from its formation until it divides

• Cell division allows for the reproduction of cells, growth of cells, and tissue repair

Organization of DNA

• Cells must organize and package their DNA before division

• DNA wraps around proteins called histones to form nucleosomes

• Strings of nucleosomes form chromatin

• Chromatin is in a non-condensed form until the cell begins to divide

• After DNA replication, chromatin condenses to form a chromosome (densely packed for easier division)

• Since DNA was replicated, each chromosome has a duplicated copies that join together to form sister chromatids

Centromere

The region on each sister chromatid where they are most closely attached

Kinetochore

Proteins attached to the centromere that link to each sister chromatid to the mitotic spindle

Genome

All of a cells genetic information (DNA)

• Prokaryotes: singular, circular DNA

• Eukaryotes: One or more linear chromosomes

• Every Eukaryote has a specific number of chromosomes

• Humans: 46

• Chimps: 48

• Elephants: 56

Homologous chromosomes

Two chromosomes (one from mom and dad) are the same length, have the same centromere position, and carry genes controlling the same characteristics

Somatic cells

• Body cells

• Diploid (2n): 2 sets of chromosomes, one set from each parent

• Mitosis

• Humans: 2n = 46

Gametes

• Reproductive cells (egg/sperm)

• Haploid (n): One set of chromosomes

• Meiosis

• Humans: n = 23

Cell cycle order

interphase (G₁ → S → G₂) → mitosis → cytokinesis

Interphase

• 90% of the cell cycle

• G₁ (First gap phase): The cell grows and carries out normal functions

• S (Synthesis phase): DNA replication and chromosome duplication occurs

• G₂ (Second gap phase): Final growth and preparation for mitosis

M phase

• Mitosis: Nucleus divides

• Cytokinesis: Cytoplasm divides

• Results in 2 identical diploid daughter cells

Phases of Mitosis

1. Prophase

2. Prometaphase

3. Metaphase

4. Anaphase

5. Telophase and cytokinesis

Prophase

• Chromatin condenses

• Nuclei disappear

• Duplicated chromosomes appear as sister chromatids

• Mitotic spindles begin to form

• Centrosomes move away from eachother

Prometaphase

• Nuclear envelope fragments

• Microtubules enter nuclear area and some attach to kinetochores

• Microtubules guide chromosome movement

Metaphase

• Centrosomes are at opposite poles

• Chromosomes line up at the metaphase plate

• Microtubules are attached to each kinetochore (anchor chromosomes to the spindle fiber)

Anaphase

• Sister chromatids separate and move to opposite ends of the cell due to the microtubules shortening

• Cell elongates

Telophase and cytokinesis

• Two daughter nuclei form

• Nuclei reappear

• Chromosomes became less condensed

Cytokinesis

• Animals: Cleavage furrow appears due to a contractile ring of actin filaments

• Plants: Vesicles produced by the Golgi travel to the middle of the cell and form a cell plate

Regulation of the cell cycle

• Control points

• Cells receive stop and go signals

G₁ Checkpoint

• Most important

• Checks for cell size, growth factors, and DNA damage

• Go: cell completes the whole cell cycle

• Stop: cell enters a non-dividing state (Quiescent) aka G₀

G₀

• Cells aren’t preparing to divide and just doing their job

• Some cells stay in G₀ forever (muscle/nerve cells) and some can be called back to the cell cycle (response to injury/stress)

G₂ Checkpoint

• Checks for completion of DNA replication and DNA damage

• Go: cell proceeds to mitosis

• Stop: cell cycle stops and the cell will attempt to repair damage

• If damage cannot be repaired, Apoptosis will occur (programmed cell death)

M (Spindle) Checkpoint

• Checks for microtubule attachment to chromosomes at the kinetochores of metaphase

• Go: cell proceeds to anaphase and completes mitosis

• Stop: cell will pause mitosis to allow for spindles to finish attaching to chromosomes

Internal Cell Cycle Regulators

Involves an internal control system that consists of

• Cyclin (proteins)

• Concentration varies

• Synthesized and degraded at specific stages of the cell cycle

• Kinases/CDKs (cyclin-dependent enzymes)

• Concentration remains constant

• Active only when its specific cyclin is present

• CDK complexes phosphorylate target proteins, which help regulate key events in the cell cycle

External Cell Cycle Regulators

• Growth Factors

• Hormones that are released by cells that stimulate cell growth

• The signal transduction pathway is initiated

• CDKS are activated, leading to progression through the cycle

• Contact (or density) inhibition

• Cell surface receptors recognize contact with other cells

• Initiates a signal transduction pathway that stops the cell cycle in the G₁ phase

• Anchorage dependence

• Cells rely on attachment to other cells or the extracellular matrix to divide

Cancer prevention

• Do not smoke: Nicotine causes mutations in cells at high rates

• Eat healthy and drink water: Fatty foods and dehydration can affect the functions of cells

• Protect your skin from the sun by using SPF: Sun is damaging to skin cells and can cause mutations to occur after exposure