Mitosis: Understanding Anaphase A and Onset

• Anaphase is a critical stage of mitosis characterized by the separation of sister chromatids and their movement to opposite poles of the cell

Anaphase Onset

• Anaphase begins abruptly with the separation of sister chromatids, marking the transition from metaphase
• This separation is triggered by the anaphase-promoting complex/cyclosome (APC/C), a ubiquitin ligase
• APC/C targets securin for degradation, which releases separase, a protease
• Separase cleaves cohesin, the protein complex holding sister chromatids together
• Once cohesin is cleaved, the sister chromatids are free to move towards opposite poles

Anaphase A

• Anaphase A involves the movement of chromosomes towards the spindle poles
• This movement is driven by the depolymerization of microtubules at the kinetochore
• Kinetochores are protein structures on the centromeres of chromosomes where microtubules attach
• As microtubules shorten, the chromosomes are pulled towards the poles
• Motor proteins, such as dynein, at the kinetochore play a role in this process

Anaphase B

• Anaphase B involves the elongation of the spindle and the movement of the spindle poles further apart
• This is achieved through two main mechanisms
  • Sliding of interpolar microtubules: Interpolar microtubules from opposite poles overlap at the spindle midzone and are pushed apart by motor proteins
  • Pulling forces acting on astral microtubules: Astral microtubules extend from the spindle poles to the cell cortex, and pulling forces exerted by motor proteins at the cortex pull the poles apart
• These processes contribute to the overall separation of the chromosomes

Microscopic Observation of Anaphase

• Under a microscope, anaphase can be identified by the following characteristics
  • Sister chromatids abruptly separate and move towards opposite poles\
  • The characteristic V-shape of the separating chromatids as they are pulled by the centromeres
  • The spindle elongates as the poles move further apart
• Different staining techniques, such as Giemsa staining or immunofluorescence, can be used to visualize the chromosomes and spindle fibers

Common Issues in Anaphase

• Chromosome Lagging
  • Occurs when one or more chromosomes fail to properly attach to the spindle or move correctly during anaphase
  • Lagging chromosomes can result in aneuploidy (abnormal chromosome number) in daughter cells
• Anaphase Bridges
  • Form when DNA becomes entangled or when dicentric chromosomes (chromosomes with two centromeres) are present
  • These bridges can lead to chromosome breakage and genomic instability
• Spindle Defects
  • Problems with spindle formation or function can disrupt chromosome segregation during anaphase
  • Incorrect spindle positioning may cause unequal chromosome distribution

Significance of Anaphase

• Ensures accurate segregation of replicated chromosomes
• Maintenance of correct chromosome number in daughter cells
• Prevents aneuploidy, which can lead to genetic disorders
• Essential for faithful cell division and organismal development