Anaphase in Mitosis & Meiosis: Processes & Significance

Introduction

Anaphase is the phase that separates duplicate genetic material carried in the nucleus of the parent cell into two sets destined for daughter cells. It follows metaphase where chromosomes are aligned at the equator.

In this article you will learn what happens during anaphase, how anaphase-promoting complex (APC/C), securin, separase and cohesin trigger separation, the role of different microtubules, anaphase in mitosis with anaphase A and B, and anaphase I and anaphase II in meiosis with protection by shugoshin.

What is Anaphase?

Anaphase is the third stage of mitosis after prophase and metaphase and before telophase. During previous metaphase, sister chromatids or replicated chromosomes are aligned along the cell's equator on the metaphase plate.

During anaphase, each pair of chromosomes separates into two identical but independent chromosomes. Each separated chromosome is pulled by mitotic spindles known as microtubules attached at kinetochores toward opposite poles.

The function of anaphase is to ensure that each daughter cell receives identical sets of chromosomes before final phase, telophase, and cytokinesis.

What Happens During Anaphase?

Role of Anaphase-Promoting Complex

Anaphase begins when anaphase-promoting complex/cyclosome (APC/C), an E3 ubiquitin ligase, terminates metaphase.

APC/C tags securin, a protein that inhibits separase and helps in metaphase to anaphase transition, for destruction by adding ubiquitin. Securin acts as inhibitory chaperone of separase, a cysteine protease.

When securin is destroyed by proteasome, separase enzyme is released. Separase then cleaves cohesin protein complex, especially subunit Scc1/Rad21, which holds sister chromatids together.

Microtubules Involved in Force Generation

Several unique microtubules create forces required for separation:

• Kinetochore microtubules: Attach to kinetochores at centromeres and shorten, pulling chromatids to poles.
• Interpolar microtubules: Non-kinetochore microtubules from opposite poles overlap and push centrosomes apart, elongating cell.
• Astral microtubules: Radiate from centrosomes to cell cortex, anchoring and orienting spindle and contributing to cell shaping into oval form.

Chromosome Movement and Shapes

This leads to splitting of centromere, pulling sister chromatids to poles by kinetochore microtubules. Separated sister chromatids then form V or Y shape at either pole because centromeres lead and arms trail.

Separation means single sister chromosomes now contain same genetic information but function independently as future daughter nuclei. Successful completion leads to next phase, telophase.

Anaphase in Mitosis

Mechanism in Mitotic Cells

Anaphase in mitosis is triggered by separation of sister chromatids with help of separase. Separase breaks cohesion that binds sister chromatids as microtubules pull them toward opposite poles.

Astral and interpolar microtubules play major role in lengthening cell which takes oval shape.

Anaphase A and Anaphase B

Modern understanding divides mitotic anaphase into two overlapping processes:

• Anaphase A: Kinetochore microtubules shorten at kinetochore and at pole, moving chromosomes to poles. Chromosome-to-pole movement.
• Anaphase B: Interpolar microtubules slide apart via kinesin motors like Kinesin-5, and astral microtubules pull poles outward, causing pole-to-pole separation and cell elongation.

Both ensure accurate segregation.

Anaphase in Meiosis

Anaphase of meiosis is made up of two consecutive divisions, anaphase I and anaphase II. Since there is no DNA replication between meiosis I and meiosis II, diploid cell with two alleles for each gene gets reduced to haploid cells containing single allele at each gene.

Anaphase I

Homologous Chromosome Separation

Generally, anaphase I involves separating homologous chromosomes, each still consisting of two sister chromatids attached at centromere, to opposite poles while still attached to microtubules.

During this phase, kinetochore microtubules shorten pulling homologous chromosomes to opposite poles. Non-kinetochore microtubules lengthen pushing centrosomes apart. Cell elongates as it prepares for division.

Protection by Shugoshin

Cohesins around centromere remain protected by protein known as Shugoshin, meaning guardian spirit in Japanese. Shugoshin with protein phosphatase 2A protects centromeric cohesin from cleavage, preventing sister chromatids from separating while homologs are segregated. This ensures sister chromatids stay together in meiosis I.

Anaphase II

Sister Chromatid Separation

This is phase after metaphase II whereby remaining centromeric cohesins which are no longer protected by shugoshin are cleaved by separase.

This allows separation of sister chromatids which are then singly referred to as sister chromosomes. They move toward opposite poles of cells.

Mechanism is similar to mitotic anaphase: securin degradation, separase activation, cohesin cleavage, and poleward movement via kinetochore microtubules.

Comparison Table

Significance and Checkpoints

Anaphase is controlled by spindle assembly checkpoint ensuring all kinetochores are properly attached before APC/C activation. Errors cause aneuploidy. Proper timing ensures equal distribution of genome, essential for growth, repair, and formation of gametes with correct chromosome number.

3. Key Takeaways

• Anaphase separates duplicated genome: sister chromatids in mitosis and anaphase II, homologous chromosomes in anaphase I.
• Trigger is APC/C-mediated ubiquitination and destruction of securin, releasing separase which cleaves cohesin subunit Scc1/Rad21 at centromere.
• Three microtubule types participate: kinetochore microtubules shorten for chromosome-to-pole movement in anaphase A, interpolar and astral microtubules push poles apart in anaphase B elongating cell.
• Separated chromosomes show V or Y shape with centromere leading toward pole.
• In meiosis I centromeric cohesin is protected by shugoshin and PP2A preventing premature sister separation, allowing homolog separation and reduction to haploid.
• In meiosis II protection is removed, sister chromatids separate like mitosis producing four genetically different haploid cells.
• Spindle assembly checkpoint ensures anaphase onset only after correct bi-orientation, preventing aneuploidy.

4. Scientific References

1. Alberts B, Johnson A, Lewis J, Morgan D, Raff M, Roberts K, Walter P → Molecular Biology of the Cell → 6th Edition → Garland Science → Chapter 17 Mitosis – Anaphase and Mitotic Exit.
2. Lodish H, Berk A, Kaiser CA, Krieger M, Bretscher A, Ploegh H, Amon A, Scott MP → Molecular Cell Biology → 8th Edition → W.H. Freeman → Chapter 19 Cell Cycle – Anaphase Promoting Complex and Spindle.
3. Cooper GM, Hausman RE → The Cell: A Molecular Approach → 8th Edition → Oxford University Press → Chapter 14 Mitosis – Sister Chromatid Cohesion and Separation.
4. Karp G, Iwasa J, Marshall W → Karp's Cell Biology → 8th Edition → Wiley → Chapter 14 Cell Division – Anaphase A and Anaphase B Mechanisms.
5. Morgan DO → The Cell Cycle: Principles of Control → 2nd Edition → New Science Press → Chapter 6 Anaphase-Promoting Complex, Securin and Separase.
6. NCERT → Biology Textbook for Class XI → Reprint 2023-24 → National Council of Educational Research and Training, India → Chapter 10 Cell Cycle and Cell Division – Anaphase in Mitosis and Meiosis.
7. Peters JM → The anaphase promoting complex/cyclosome: a machine designed to destroy → Nature Reviews Molecular Cell Biology → 2006 → Volume 7, Issue 9, Pages 644-656.
8. Uhlmann F, Lottspeich F, Nasmyth K → Sister-chromatid separation at anaphase onset is promoted by cleavage of the cohesin subunit Scc1 → Nature → 1999 → Volume 400, Pages 37-42.
9. Kitajima TS, Kawashima SA, Watanabe Y → The conserved kinetochore protein shugoshin protects centromeric cohesion during meiosis → Nature → 2004 → Volume 427, Pages 510-517.
10. Marston AL, Amon A → Meiosis: cell-cycle controls shuffle and deal → Nature Reviews Molecular Cell Biology → 2004 → Volume 5, Issue 12, Pages 983-997.