The Eukaryotic Cell Cycle

The Eukaryotic Cell Cycle

• Cells are the fundamental units of life
• The continuity of life relies on genetic material transfer between cells

Why Do Cells Divide?

• Reproduction
• Growth and development
• Cell replacement and tissue renewal

Types of Cell Division in Humans

• Asexual (genetic clones):
  • Mitosis conserves chromosome number
• Sexual (genetic variation):
  • Meiosis occurs only in gamete (sex cell) production
  • Reduces chromosome number

Eukaryotes and Chromosomes

• Human somatic cells have 23 pairs (46 total) of chromosomes
  • Autosomes: 22 pairs
  • Sex chromosomes: 1 pair (XX or XY)
    • Diploid (2n)
• Human gametes have 1 member of each chromosome pair (23 total) - Haploid (n)

Homologous Chromosomes

• Members of a chromosome pair are called homologs (homologous chromosomes)
• They are nearly identical in size and genetic composition
• Example: Both carry eye color genes, but one may have brown and the other blue

Sex Chromosomes

• Not true homologs
• X and Y chromosomes differ significantly in size and genetic composition

Two Types of Nuclear Division

• Mitosis and Meiosis
• Mitosis leads to identical diploid daughter cells for growth and repair.
• Meiosis leads to haploid gametes for sexual reproduction.
• Diagrams of the cell cycle shown.

Genome and Chromosomes

• Genome usually exists as chromatin
• Chromosomes condense only to prepare for division
• DNA is organized into multiple genes along chromosomes
• Centromere: is the narrow region where replicated chromosomes join

Karyotype

• Karyotype reveals chromosome number, size, and form in dividing cells

Cell Cycle (Life Cycle of a Cell) Phases

• Interphase:
  • G1 (first gap): Cell growth and development; commitment to division at G1 checkpoint
  • S (synthesis): DNA replication
  • G2 (second gap): Protein and structure synthesis for mitosis
  • G0: cells postponing division or never dividing again
• M (mitosis and cytokinesis)

S Phase

• Each chromosome replicates
• Replicated chromosomes are called sister chromatids
• Sister chromatids remain attached at the centromere
• Human cells have 46 chromosomes in G1 phase and 92 sister chromatids in G2 phase

G2 Phase

• Synthesizes proteins and structures needed during mitosis and cytokinesis

Mitosis

• Nuclear division, separation of sister chromatids
• Followed by cytokinesis to form two daughter cells

Control of Cell Division

• External factors: environmental conditions, signaling molecules, determines if a cell divides
• Internal factors: cell cycle control molecules, checkpoints.
  • Internal checkpoints: G1/S, G2/M, Metaphase

Internal Checkpoints

• G1/S checkpoint (restriction point): determines if conditions are favorable and if DNA is undamaged
  • Cells that fail checkpoint exit cycle to G0
• G2/M checkpoint: Checks DNA replication; monitors protein levels needed for mitosis
• Metaphase (spindle) checkpoint: Ensures all chromosomes are attached to the spindle apparatus

Checkpoint proteins

• Cyclins and cyclin-dependent kinases (cdks) advance cells through checkpoints.
• Cyclin levels vary; Cdks are constant but require binding to a cyclin to be active
• Checkpoint proteins act as sensors determining if the cell is ready for cell division

Mitosis Stages

• Interphase
• Prophase
• Prometaphase
• Metaphase
• Anaphase
• Telophase
• Cytokinesis

Mitotic Spindle

• Crucial for segregation of chromosomes into daughter cells - Ensures each daughter cell receives correct # chromosomes
• Mitotic spindle apparatus/spindle is responsible for organizing and sorting the chromosomes during mitosis.
• Centrosomes are microtubule organizing centers

Spindle Microtubules

• Comprised of tubulin proteins
• Three types
  • Astral microtubules
  • Polar microtubules
  • Kinetochore microtubules.

Chromosome movement during mitosis

• Motor proteins attached to chromosomes move during mitosis by de-polymerizing at kinetochore ends.

Cytokinesis

• Division of cytoplasm to form two new cells
  • Animals: Cleavage furrow constriction
  • Plants: Cell plate forms cell wall.