Telophase

After the dramatic events of prophase, prometaphase, metaphase, and anaphase — chromatin condensation, spindle assembly, chromosome alignment, and the sudden separation of sister chromatids — telophase might seem like an afterthought: the untidy cleanup at the end of the show. It is anything but. Telophase is the stage during which the cell rebuilds two functional nuclei from their component parts, reconstituting the nuclear architecture that took prophase several minutes to dismantle. It is the stage during which chromosomes return from their most compact, transcriptionally silent form to the open chromatin structure that supports gene expression. And it is the stage during which cytokinesis begins — the physical division of one cell into two. Understanding telophase requires understanding not just what happens, but the precise molecular logic by which CDK1 inactivation simultaneously unleashes dozens of reversal reactions that together rebuild the interphase cell.

Telophase — Definition, Boundaries, and What Makes It Distinct from the Stages That Precede It

Telophase (from the Greek telos, end, and phasis, appearance) is the final stage of mitosis and meiosis during which the separated chromosome sets at each pole of the dividing cell are repackaged into functional daughter nuclei. It is defined operationally as beginning when the chromosomes arrive at the spindle poles following anaphase movement, and ending when two distinct, functional nuclei are present within the cell — at which point the cell typically undergoes cytokinesis to complete physical separation into two daughter cells.

The temporal boundary between anaphase and telophase is gradual rather than abrupt — there is no single molecular switch that instantaneously converts one phase to the other. Instead, telophase processes (nuclear envelope reformation, chromosome decondensation, spindle disassembly) begin as chromosomes are still moving toward the poles in late anaphase, and several telophase events (particularly cytokinesis) extend well beyond the completion of nuclear reformation. This overlap reflects the biochemical reality: the same molecular trigger — CDK1 inactivation via cyclin B degradation — simultaneously initiates multiple reversal reactions that proceed at different rates and complete at different times.

Telophase in the Full Context of Mitosis — What Came Before and What It Reverses

To understand telophase fully, it is necessary to appreciate that it is not simply the last stage of mitosis — it is the reversal stage. Most of what happens in telophase directly undoes what prophase and prometaphase accomplished. Understanding each telophase event therefore requires knowing what the corresponding prophase event was, why it was necessary for cell division, and what molecular mechanism drives its reversal when CDK1 activity falls.

The Four Simultaneous Events of Telophase — What Happens and in What Order

Telophase is characterised by four major events that occur simultaneously and in an interconnected manner, all triggered by the fall in CDK1 activity as cyclin B is degraded. They are not strictly sequential — all four begin within minutes of CDK1 inactivation and overlap significantly — but they can be conceptually separated and mechanistically understood individually before being synthesised into a coherent picture of telophase as a whole.

Chromosome Decondensation — How Condensins Are Inactivated and Chromatin Relaxes

One of the most visually striking events of telophase — observable under a light microscope — is the progressive decondensation of the compact mitotic chromosomes as they arrive at the poles. The rod-like chromosomes visible during metaphase and anaphase lose their distinct, individually identifiable form and blur into a mass of increasingly diffuse chromatin as the nucleus reforms around them. This decondensation reflects the inactivation of the condensin complexes that established and maintained chromosome compaction, and the concurrent removal of mitosis-specific histone modifications.

Condensin I and Condensin II — The SMC Complexes That Compact Mitotic Chromosomes

Chromosome compaction in mitosis is primarily driven by two condensin complexes — condensin I and condensin II — both members of the structural maintenance of chromosomes (SMC) family of ring-shaped protein complexes. Both complexes share two SMC subunits (SMC2 and SMC4) that form a V-shaped dimer via their hinge domains, with their ATPase head domains at the tips of the V. Three additional HEAT-repeat subunits — the kleisin subunit (CAP-H for condensin I, CAP-H2 for condensin II) and two HEAT subunits (CAP-D2/CAP-G for condensin I; CAP-D3/CAP-G2 for condensin II) — bridge the ATPase heads and confer regulatory specificity.

The mechanism of condensin-driven compaction is now understood to involve loop extrusion — a process in which the condensin ring progressively extrudes larger and larger chromatin loops, effectively reeling in DNA from both sides of the complex and compacting the chromosome axis. Single-molecule experiments have directly visualised condensin extruding DNA loops in vitro at rates of ~1.5 kb/s. In telophase, as CDK1 inactivation dephosphorylates condensin subunits, the ATPase activity driving loop extrusion decreases, newly extruded loops are not maintained, and existing loops progressively dissolve as the chromatin returns to its interphase organisation driven by cohesin-based loop domain structures that are re-established in G1.

Nuclear Envelope Reformation — Membrane Recruitment, Fusion, and Nuclear Pore Assembly

The nuclear envelope is a double membrane — inner nuclear membrane (INM) and outer nuclear membrane (ONM), continuous with the endoplasmic reticulum — perforated by nuclear pore complexes (NPCs) and supported internally by the nuclear lamina. During prophase, this elaborate structure was entirely dismantled: lamins depolymerised, integral membrane proteins dispersed into the ER, and nuclear pore complexes disassembled into subcomplexes distributed throughout the cytoplasm. Telophase must reconstitute this structure with precision and speed around the decondensing chromosomes at each pole.

Nucleolus Reassembly — rRNA Transcription, Processing, and Ribosome Biogenesis Resumption

The nucleolus is not a membrane-bounded organelle but a phase-separated liquid condensate that forms around the nucleolus organiser regions (NORs) — the chromosomal loci carrying the tandemly repeated ribosomal RNA (rRNA) genes on the short arms of human acrocentric chromosomes (chromosomes 13, 14, 15, 21, and 22). During prophase, the nucleolus dissolves as RNA Polymerase I and its associated transcription factors are displaced from rDNA by the condensing chromosomes and inhibited by CDK1-mediated phosphorylation. In telophase, nucleolus reassembly occurs as a consequence of rDNA transcription resuming — the transcription products themselves seed the condensation of the pre-rRNA processing machinery into nascent nucleoli.

Spindle Disassembly and the Midbody — From Anaphase Spindle to Cytokinesis Organiser

The mitotic spindle — assembled from hundreds of microtubules, dozens of motor proteins, and numerous non-motor microtubule-associated proteins — must be partially disassembled during telophase while simultaneously being reorganised into the central spindle structure that specifies the cytokinesis division plane. This is not a complete spindle collapse but a precisely controlled partial disassembly with simultaneous structural reorganisation: the kinetochore microtubules and astral microtubules depolymerise, while the interpolar (central spindle) microtubules are stabilised and compacted into the midbody that persists through cytokinesis.

Kinetochore and Astral Microtubule Disassembly

The kinetochore microtubules that powered chromosome movement during anaphase are no longer needed once chromosomes arrive at the poles. Their depolymerisation in telophase is driven by kinesin-13 family microtubule depolymerases (MCAK/KIF2C in human cells), which are active at chromosome-containing regions and at poles, and by the general reduction in microtubule stabilising factors as CDK1 activity falls. Aurora A kinase, which had stabilised centrosomal and astral microtubules during metaphase by phosphorylating MCAK and other destabilisers, is inactivated during mitotic exit, contributing to astral microtubule depolymerisation. The centrosomes become the future microtubule-organising centres (MTOCs) of the daughter cells — their structure is maintained through telophase and they nucleate the interphase microtubule cytoskeleton in G1.

ANAPHASE CENTRAL SPINDLE (midzone):
  Antiparallel microtubule bundles between separating chromosome masses
  PRC1/MAP65    — antiparallel MT bundler; cross-links overlapping MTs
  MKLP1/KIF23   — kinesin-6; slides antiparallel MTs apart; drives spindle elongation
  Aurora B       — centromere/midzone kinase; stabilises midzone MTs; controls cytokinesis
  CPC complex   — Borealin, Survivin, INCENP scaffold; positions Aurora B at midzone

TELOPHASE: MIDZONE COMPACTION INTO MIDBODY:
  Antiparallel MT overlap zone compacts and becomes electron-dense
  MKLP1 activity drives microtubule compaction
  CEP55 recruits ALIX and TSG101 to the midbody
  ESCRT-III (CHMP4B etc.) accumulates at constriction zones flanking the midbody

MIDBODY STRUCTURE:
  Dark zone      — electron-dense protein matrix; dense MT bundles
  Flanking zones — ESCRT-III spirals; membrane constriction occurs here
  Diameter      — ~1 μm; visible by phase contrast as 'dark bridge' between cells

FUNCTION:
  Positions the site of cytokinesis membrane ingression
  Scaffolds recruitment of abscission machinery
  Aurora B NoCut checkpoint: delays abscission if DNA bridges present

Cytokinesis in Animal Cells — The Contractile Actomyosin Ring and Cleavage Furrow

In animal cells, cytokinesis is accomplished by cleavage — the progressive ingression of the plasma membrane inward from the cell equator, driven by contraction of an actomyosin ring assembled beneath the plasma membrane. The position of the cleavage furrow is specified by the central spindle midzone during anaphase and telophase, ensuring that the division plane precisely bisects the space between the two sets of chromosomes at the two poles — placing one nucleus in each daughter cell.

How the Division Plane Is Specified

The central spindle midzone signals to the overlying cell cortex to assemble the cleavage furrow precisely at the cell equator through the action of RhoA GTPase. The GEF (guanine nucleotide exchange factor) Ect2 is activated at the midzone through interaction with MgcRacGAP and MKLP1, and activates cortical RhoA by converting it from GDP-bound (inactive) to GTP-bound (active) specifically at the cell equator. Active RhoA-GTP then recruits and activates the two effectors required for contractile ring assembly: mDia formin (which nucleates linear actin filaments) and Rho-kinase ROCK (which phosphorylates myosin light chain, activating myosin II ATPase and enabling actin-myosin interaction). The contractile ring assembles from approximately 105 actin filaments and myosin II bipolar filaments, forming a ring ~7–10 μm in diameter beneath the equatorial plasma membrane.

Cytokinesis in Plant Cells — The Phragmoplast and Cell Plate Formation

Plant cells cannot undergo cytokinesis by membrane ingression — their rigid cellulose cell wall prevents the inward deformation of the plasma membrane that drives cleavage furrow ingression in animal cells. Instead, plant cells divide by building a new cell wall from the inside out, using a plant-specific structure called the phragmoplast that delivers Golgi-derived vesicles containing cell wall precursors to the cell equator, where they fuse to form the cell plate — the precursor of the new cell wall separating the two daughter cells.

The Pre-Prophase Band and Division Plane Memory

A unique feature of plant cytokinesis is that the division plane is specified before mitosis even begins — not during anaphase as in animal cells. The pre-prophase band (PPB) is a cortical ring of microtubules and actin filaments that appears in late G2/early prophase at the future division site, persists through prophase, and then disappears before metaphase. Although the PPB is gone by the time cytokinesis begins, it leaves a “memory” in the cortex — a region of altered cortical protein composition (depleted of certain microtubule-stabilising proteins, marked by specific GTPase activating proteins called TAN line proteins) that persists through mitosis and guides the phragmoplast to fuse the cell plate with the parent wall at exactly the position marked by the PPB. This spatial memory mechanism, absent in animal cells, reflects the need for plant cells to precisely control where the new wall is inserted — errors in wall placement in plant tissues can have developmental consequences for cell identity and tissue organisation.

Abscission — Molecular Mechanism of the Final Cut

Abscission is the last irreversible event of cell division — the physical severing of the thin cytoplasmic bridge connecting two daughter cells at the end of cytokinesis. The molecular mechanism of abscission was elucidated relatively recently and revealed a surprising connection between cell division and the ESCRT (endosomal sorting complexes required for transport) pathway — the same machinery that forms multivesicular bodies, releases exosomes, and mediates HIV budding from infected cells. The topological equivalence is the key insight: all three processes involve membrane constriction leading to membrane scission from the cytoplasmic face — cutting a narrow membrane neck from inside rather than outside.

Molecular Regulation of Mitotic Exit — CDK1, APC/C, and the Phosphatase Switch

The molecular logic of telophase is essentially the logic of mitotic exit: the sharp, irreversible inactivation of CDK1 — which was the master activator of all prophase events — simultaneously releases all the CDK1-dependent phosphorylation that maintained the mitotic state, allowing constitutively active phosphatases to dephosphorylate CDK1 substrates and drive the cell back to an interphase state. Understanding this regulatory circuit is essential for understanding why telophase events are simultaneous rather than sequential, and why mitotic exit is normally irreversible once it begins.

THE CDK1 / CYCLIN B SWITCH:

High CDK1 activity (metaphase) — maintained by:
  - Cyclin B stabilised (APC/C-Cdc20 not yet active — SAC is on)
  - CDK1 phosphorylates: condensin, nuclear lamins, NEBD factors,
    Ect2, NPC components, PP1/PP2A inhibitory subunits
  - This keeps: chromosomes condensed, NE disassembled,
    spindle stable, phosphatases inactive

TRIGGER (anaphase — SAC silenced, all kinetochores attached):
  APC/C-Cdc20 activates → ubiquitinates SECURIN → Separase releases cohesin
  APC/C-Cdc20 also ubiquitinates CYCLIN B → proteasomal degradation
  CDK1 activity begins to fall as cyclin B is degraded

MITOTIC EXIT CASCADE (telophase):
  CDK1↓ → PP1 and PP2A-B55 activate (no longer inhibited by CDK1)
  PP1/PP2A-B55 dephosphorylate ALL CDK1 substrates simultaneously:
    → Condensins inactivated       → chromosome decondensation
    → Lamins dephosphorylated      → lamin polymerisation
    → INM proteins dephosphorylated → NE reformation
    → UBF dephosphorylated         → rDNA transcription resumes
    → Ect2 dephosphorylated        → RhoA activation for cytokinesis
    → NPC proteins dephosphorylated → NPC assembly resumes

LOCKING IN G1 (APC/C-Cdh1 takes over):
  APC/C-Cdh1 maintains low CDK1 activity through G1
  Degrades remaining cyclin B and cyclin A
  Only inactivated when S-phase CDK-cyclin complexes (CDK2-cyclin E/A) accumulate

Telophase I and Telophase II in Meiosis — Two Rounds of Division, Two Distinct Outcomes

Meiosis produces haploid gametes from diploid precursor cells through two successive divisions — meiosis I and meiosis II — separated by a brief interphase (interkinesis) that, critically, involves no DNA replication. Each division has its own telophase with distinct features reflecting the different chromosome separation events that preceded it. Understanding the difference between telophase I and telophase II — particularly what has been separated and therefore what chromosome content each daughter nucleus receives — is a recurrent examination topic in genetics and cell biology courses.

Telophase Variation Across Eukaryotes — Open, Closed, and Semi-Closed Mitosis

The version of telophase described above — involving the reformation of a completely broken-down nuclear envelope — is characteristic of animals, plants, and many fungi. This is called open mitosis, because the nuclear envelope fully opens (breaks down) during prometaphase. But a substantial proportion of eukaryotic diversity divides by closed mitosis, in which the nuclear envelope never breaks down, and consequently the “telophase” events of nuclear reformation do not occur in the same way.

The evolutionary transition between closed and open mitosis is an active area of research. Open mitosis has the advantage of allowing the spindle full access to chromosomes without having to penetrate the nuclear envelope, potentially increasing accuracy of chromosome segregation for large eukaryotic genomes with many chromosomes. However, it comes with the cost of having to completely dismantle and rebuild the nuclear envelope — a complex, error-prone process. The observation that nuclear envelope reformation errors (producing micronuclei) are common in cancer cells, and that these micronuclei can undergo DNA damage and chromothripsis, highlights one cost of open mitosis that is clinically significant.

Cancer Biology and Telophase — When Cell Division Completion Goes Wrong

Errors during or after telophase — in cytokinesis, nuclear envelope reformation, chromosome segregation completion, or abscission — are not merely interesting cell biological curiosities. They produce abnormalities — binucleated cells, micronuclei, polyploid cells — that are directly relevant to cancer initiation and progression. Understanding the connection between telophase failure and cancer provides a mechanistic explanation for some of the most characteristic features of cancer cell genomes: aneuploidy, chromosomal instability, and the catastrophic genome rearrangements called chromothripsis.

Frequently Asked Questions About Telophase