Links

Principles Module 32

Objectives

• Detail the differences between division in prokaryotic and eukaryotic cells
• Define the cellular structures involved in mitosis and cytokinesis
• Describe the eukaryotic cell cycle and phases of mitosis

Key Points

Bacterial cell division

• division is different in prokaryotes and eukaryotes by necessity
• binary fission is simple division in bacteria
• single circular chromosome replicated at same time as division
• proteins control partitioning of DNA into daughter cells
• replication begins at ori site, goes bidirectionally around
• septum separates the 2 new cells
• formation is regulated by proteins
• forms at midpoint of cell, FtsZ protein forms ring

Eukaryotic chromosomes

• packaging
  • 40% DNA, 60% protein called histones
  • allow for coiling of DNA, packaging to hi density
  • ~ 200 bp = 8 histones = nucleosome
  • nucleosomes wrapped into solenoids, form 30 nm fiber
• ploidy
  • haploid is the number of chromosomes a species has
    • human haploid # is 23
    • Arabidopsis haploid # is 5
  • many species have 2 versionsof each chromosome, are diploid
    • versions called homologues, encode same genes at same places
    • slight differences in each from parents (alleles)
• replication
• results in each chromosome having exact duplicate copy
• copies held together as sister chromatids

Overview of eukaryotic cell cycle

• interphase
  • G1 (gap) growth phase, majority of most cells’ cycle
  • S synthesis of exact copy of genome
  • G2 (gap 2) growth, organelle replication, prep for mitosis
• M (mitosis) separation of 2 daughter genomes
• cytokinesis partitioning of cytoplasm

Structures

• centromere is a constriction on chromosome
  • made up of repeat sequences of DNA
• kinetochore a disk shaped collection of proteins attached to centromere
  • microtubules attach here during mitosis
• centrioles
  • MT organizing centers at poles of cell
• spindle is MT network that pulls chromosomes apart

Mitosis

• prophase
  • condensation of chromosomes
  • spindle apparatus forms
  • nuclear envelope disintegrates
• prometaphase
  • chromosomes attach to spindle at kinetochore
  • congression pulls chromosomes to center of cell
    • recall the growth of MTs by adding and subtracting tubulins
• metaphase
  • chromosomes aligned at cell equator, called the metaphase plate
• anaphase
  • centromeres split, sister chromatids separate from each other
  • kinetochores pulled toward poles by removal of tubulins
  • poles move apart
• telophase
• spindle breaks down, nuclear envelope reconstitutes
• chromosomes de-condense

Cytokinesis

• cells w/out walls
  • actin forms a belt, constricts to pinch cell, form cleavage furrow
• cells w/ walls
  • deposits of membrane, new wall carbohydrates in middle, cell plate

In-class Activities

Questions for Practice

• Compare and contrast prokaryotic and eukaryotic division. What are some reasons why this process should be different?
• What is the mitotic spindle? What is it made of? When does it form?
• List and briefly describe each of the major phases of mitosis.