scholarly journals The Golgi and Endoplasmic Reticulum Remain Independent during Mitosis in HeLa Cells

1998 ◽  
Vol 9 (3) ◽  
pp. 623-635 ◽  
Author(s):  
Stephen A. Jesch ◽  
Adam D. Linstedt
Keyword(s):  
Endoplasmic Reticulum ◽  
Hela Cells ◽  
Velocity Gradient ◽  
Brefeldin A ◽  
Breakdown Product ◽  
Interphase Cells ◽  
M Phase ◽  
Gradient Fractionation ◽  
Mitotic Cells

Partitioning of the mammalian Golgi apparatus during cell division involves disassembly at M-phase. Despite the importance of the disassembly/reassembly pathway in Golgi biogenesis, it remains unclear whether mitotic Golgi breakdown in vivo proceeds by direct vesiculation or involves fusion with the endoplasmic reticulum (ER). To test whether mitotic Golgi is fused with the ER, we compared the distribution of ER and Golgi proteins in interphase and mitotic HeLa cells by immunofluorescence microscopy, velocity gradient fractionation, and density gradient fractionation. While mitotic ER appeared to be a fine reticulum excluded from the region containing the spindle-pole body, mitotic Golgi appeared to be dispersed small vesicles that penetrated the area containing spindle microtubules. After cell disruption, M-phase Golgi was recovered in two size classes. The major breakdown product, accounting for at least 75% of the Golgi, was a population of 60-nm vesicles that were completely separated from the ER using velocity gradient separation. The minor breakdown product was a larger, more heterogenously sized, membrane population. Double-label fluorescence analysis of these membranes indicated that this portion of mitotic Golgi also lacked detectable ER marker proteins. Therefore we conclude that the ER and Golgi remain distinct at M-phase in HeLa cells. To test whether the 60-nm vesicles might form from the ER at M-phase as the result of a two-step vesiculation pathway involving ER–Golgi fusion followed by Golgi vesicle budding, mitotic cells were generated with fused ER and Golgi by brefeldin A treatment. Upon brefeldin A removal, Golgi vesicles did not emerge from the ER. In contrast, the Golgi readily reformed from similarly treated interphase cells. We conclude that Golgi-derived vesicles remain distinct from the ER in mitotic HeLa cells, and that mitotic cells lack the capacity of interphase cells for Golgi reemergence from the ER. These experiments suggest that mitotic Golgi breakdown proceeds by direct vesiculation independent of the ER.

Ability to organize microtubules in taxol-treated mitotic PtK2 cells goes with the SPN antigen and not with the centrosome

1992 ◽  
Vol 102 (1) ◽  
pp. 91-102 ◽  
Author(s):  
M. Kallajoki ◽  
K. Weber ◽  
M. Osborn
Keyword(s):  
Plasma Membrane ◽  
Hela Cells ◽  
Nuclear Matrix ◽  
Essential Role ◽  
Soluble Form ◽  
Microtubule Nucleation ◽  
Brain Microtubules ◽  
Mitotic Cells

The SPN antigen plays an essential role in mitosis, since microinjection of antibodies causes mitotic arrest. Here we show, by examination of the relative locations of SPN antigen, the centrosomal 5051 antigen and tubulin in normal mitotic, and in taxol-treated mitotic cells, that the SPN antigen is involved in organizing the microtubules of the spindle. The 210 kDa protein defined as SPN antigen relocates from the nuclear matrix to the centrosome at prophase, remains associated with the poles at metaphase and anaphase, and dissociates from the centrosomes in telophase. In taxol-treated mitotic cells, SPN staining shows a striking redistribution while 5051 antigen remains associated with centrosomes. SPN antigen is seen at the plasma membrane end of the rearranged microtubules. SPN antigen is always at the center of the multiple microtubule asters (5 to 20 per cell) induced by taxol, whereas 5051 again remains associated with the centrosomal complex (1 to 2 foci per cell). Microtubule nucleation is associated with the SPN antigen rather than with the 5051 antigen. Microinjection of SPN-3 antibody into taxol-treated mitotic PtK2 cells causes disruption of the asters as judged by tubulin staining of the same cells. Finally, SPN antigen extracted in soluble form from synchronized mitotic HeLa cells binds to, and sediments with, pig brain microtubules stabilized by taxol. This association of SPN antigen with microtubules is partially dissociated by 0.5 M NaCl but not by 5 mM ATP. Thus SPN antigen binds to microtubules in vitro and seems to act as a microtubular minus-end organizer in mitotic cells in vivo.

scholarly journals Spectrin redistributes to the cytosol and is phosphorylated during mitosis in cultured cells.

1992 ◽  
Vol 119 (6) ◽  
pp. 1559-1572 ◽  
Author(s):  
V M Fowler ◽  
E J Adam
Keyword(s):  
Cell Body ◽  
Hela Cells ◽  
Plasma Membranes ◽  
Cultured Cells ◽  
Beta Subunit ◽  
Actin Binding ◽  
Culture Dish ◽  
Cell Rounding ◽  
Interphase Cells ◽  
Mitotic Cells

Dramatic changes in morphology and extensive reorganization of membrane-associated actin filaments take place during mitosis in cultured cells, including rounding up; appearance of numerous actin filament-containing microvilli and filopodia on the cell surface; and disassembly of intercellular and cell-substratum adhesions. We have examined the distribution and solubility of the membrane-associated actin-binding protein, spectrin, during interphase and mitosis in cultured CHO and HeLa cells. Immunofluorescence staining of substrate-attached, well-spread interphase CHO cells reveals that spectrin is predominantly associated with both the dorsal and ventral plasma membranes and is also concentrated at the lateral margins of cells at regions of cell-cell contacts. In mitotic cells, staining for spectrin is predominantly in the cytoplasm with only faint staining at the plasma membrane on the cell body, and no discernible staining on the membranes of the microvilli and filopodia (retraction fibers) which protrude from the cell body. Biochemical analysis of spectrin solubility in Triton X-100 extracts indicates that only 10-15% of the spectrin is soluble in interphase CHO or HeLa cells growing attached to tissue culture plastic. In contrast, 60% of the spectrin is soluble in mitotic CHO and HeLa cells isolated by mechanical "shake-off" from nocodazole-arrested synchronized cultures, which represents a four- to sixfold increase in the proportion of soluble spectrin. This increase in soluble spectrin may be partly due to cell rounding and detachment during mitosis, since the amount of soluble spectrin in CHO or HeLa interphase cells detached from the culture dish by trypsin-EDTA or by growth in spinner culture is 30-38%. Furthermore, mitotic cells isolated from synchronized spinner cultures of HeLa S3 cells have only 2.5 times as much soluble spectrin (60%) as do synchronous interphase cells from these spinner cultures (25%). The beta subunit of spectrin is phosphorylated exclusively on serine residues both in interphase and mitosis. Comparison of steady-state phosphorylation levels of spectrin in mitotic and interphase cells demonstrates that solubilization of spectrin in mitosis is correlated with a modest increase in the level of phosphorylation of the spectrin beta subunit in CHO and HeLa cells (a 40% and 70% increase, respectively). Two-dimensional phosphopeptide mapping of CHO cell spectrin indicates that this is due to mitosis-specific phosphorylation of beta-spectrin at several new sites. This is independent of cell rounding and dissociation from other cells and the substratum, since no changes in spectrin phosphorylation take place when cells are detached from culture dishes with trypsin-EDTA.(ABSTRACT TRUNCATED AT 400 WORDS)

scholarly journals A novel, dynein-independent mechanism focuses the endoplasmic reticulum around spindle poles in dividing Drosophila spermatocytes

2019 ◽  
Author(s):  
Darya Karabasheva ◽  
Jeremy T. Smyth
Keyword(s):  
Endoplasmic Reticulum ◽  
Sudden Onset ◽  
Animal Cells ◽  
Specific Mechanism ◽  
Spindle Poles ◽  
Spindle Apparatus ◽  
Independent Mechanism ◽  
M Phase ◽  
Dividing Cells

AbstractIn dividing animal cells the endoplasmic reticulum (ER) concentrates around the poles of the spindle apparatus by associating with astral microtubules (MTs), and this association is essential for proper ER partitioning to progeny cells. The mechanisms that associate the ER with astral MTs are unknown. Because astral MT minus-ends are anchored by centrosomes at spindle poles, we tested the hypothesis that the MT minus-end motor dynein mediates ER concentration around spindle poles. Live in vivo imaging of Drosophila spermatocytes undergoing the first meiotic division revealed that dynein is required for ER concentration around centrosomes during interphase. In marked contrast, however, dynein suppression had no effect on ER association with astral MTs and concentration around spindle poles in early M-phase. Importantly though, there was a sudden onset of ER-astral MT association in Dhc64C RNAi cells, revealing activation of an M-phase specific mechanism. ER redistribution to spindle poles also did not require non-claret disjunctional (ncd), the other known Drosophila MT minus-end motor, nor Klp61F, a MT plus-end motor that generates spindle poleward forces. Collectively, our results suggest that a novel, M-phase specific mechanism of ER-MT association that is independent of MT minus-end motors is required for proper ER partitioning in dividing cells.

scholarly journals Glucocappasalin Induces G2/M-Phase Arrest, Apoptosis, and Autophagy Pathways by Targeting CDK1 and PLK1 in Cervical Carcinoma Cells

2021 ◽  
Vol 12 ◽  
Author(s):  
Guangya Xu ◽  
Xueling Yan ◽  
Zhongjia Hu ◽  
Lulu Zheng ◽  
Ke Ding ◽  
...  
Keyword(s):  
Cell Cycle ◽  
Cervical Cancer ◽  
Cervical Carcinoma ◽  
Hela Cells ◽  
Carcinoma Cells ◽  
Dependent Manner ◽  
Cycle Arrest ◽  
Phase Arrest ◽  
M Phase

Glucocappasalin (GCP), a natural product derived from the seeds of Descurainia sophia (L.) Webb. ex Prantl, exhibits potential antitumor activity in HeLa cervical carcinoma cells. In this study, we investigated the anti-cervical cancer property of GCP through the induction of cell cycle arrest, apoptosis, and autophagy in vitro and in vivo, and elucidated the underlying molecular mechanisms. We demonstrated that treatment with GCP inhibited the growth of HeLa, Siha, and Ca Ski cell lines in a dose-dependent manner, with HeLa cells displaying particular sensitivity to the GCP treatment. Subsequently, the expression of cyclin-dependent kinase 1 (CDK1) and polo like kinase 1 (PLK1) were evaluated in HeLa cells using the CDK1 kinase assay kit, the fluorescence polarization assay, real-time quantitative PCR, and western blotting. Our results demonstrate that GCP could be employed to attenuate the expression of CDK1 and PLK1 in a dose- and time-dependent manner. The complementary results obtained by flow cytometry and western blotting allowed us to postulate that GCP may exhibit its antitumor effects by inducing G2/M cell cycle arrest. Moreover, HeLa cells treated with GCP exhibited a loss in mitochondrial membrane potential, together with the activation of caspases 3 and 9, and poly ADP-ribose polymerase (PARP). Additionally, we found that GCP could increase the formation of acidic vesicular organelles (AVOs), as well as the levels of Beclin1, LC3-II, p62, and Atg5 proteins in HeLa cells. Further studies indicated that GCP triggered autophagy via the suppression of the PI3K/AKT/mTOR signaling pathways. The autophagy inhibitor 3-methyladenine (3-MA) was used to determine whether autophagy affects the apoptosis induced by GCP. Interestingly, the inhibition of autophagy attenuated apoptosis. In vivo anti-tumor experiments indicated that GCP (60 mg/kg, i.p.) markedly reduced the growth of HeLa xenografts in nude mice without apparent toxicity. Taken together, we demonstrate that GCP induces cell cycle G2/M-phase arrest, apoptosis, and autophagy by acting on the PI3K/AKT/mTOR signaling pathways in cervical carcinoma cells. Thus, GCP may represent a promising agent in the eradication of cervical cancer.

scholarly journals Transport of Bacterial Lipopolysaccharide to the Golgi Apparatus

1999 ◽  
Vol 190 (4) ◽  
pp. 523-534 ◽  
Author(s):  
Nathalie Thieblemont ◽  
Samuel D. Wright
Keyword(s):  
Endoplasmic Reticulum ◽  
Golgi Apparatus ◽  
Hela Cells ◽  
Brefeldin A ◽  
Cellular Responses ◽  
Epithelial Cell Line ◽  
Cholera Toxin B Subunit ◽  
B Subunit ◽  
Cell Line Hela ◽  
Tetramethylrhodamine Isothiocyanate

Addition of lipopolysaccharide (LPS) to cells in the form of LPS–soluble (s)CD14 complexes induces strong cellular responses. During this process, LPS is delivered from sCD14 to the plasma membrane, and the cell-associated LPS is then rapidly transported to an intracellular site. This transport appears to be important for certain cellular responses to LPS, as drugs that block transport also inhibit signaling and cells from LPS-hyporesponsive C3H/HeJ mice fail to exhibit this transport. To identify the intracellular destination of fluorescently labeled LPS after its delivery from sCD14 into cells, we have made simultaneous observations of different organelles using fluorescent vital dyes or probes. Endosomes, lysosomes, the endoplasmic reticulum, and the Golgi apparatus were labeled using Texas red (TR)–dextran, LysoTracker™ Red DND-99, DiOC6(3), and boron dipyrromethane (BODIPY)–ceramide, respectively. After 30 min, LPS did not colocalize with endosomes, lysosomes, or endoplasmic reticulum in polymorphonuclear leukocytes, although some LPS-positive vesicles overlapped with the endosomal marker, fluorescent dextran. On the other hand, LPS did appear to colocalize with two markers of the Golgi apparatus, BODIPY–ceramide and TRITC (tetramethylrhodamine isothiocyanate)–labeled cholera toxin B subunit. We further confirmed the localization of LPS in the Golgi apparatus using an epithelial cell line, HeLa, which responds to LPS–sCD14 complexes in a CD14-dependent fashion: BODIPY–LPS was internalized and colocalized with fluorescently labeled Golgi apparatus probes in live HeLa cells. Morphological disruption of the Golgi apparatus in brefeldin A–treated HeLa cells caused intracellular redistribution of fluorescent LPS. These results are consistent with the Golgi apparatus being the primary delivery site of monomeric LPS.

scholarly journals Regulation of Ca2+ influx during mitosis: Ca2+ influx and depletion of intracellular Ca2+ stores are coupled in interphase but not mitosis.

1991 ◽  
Vol 2 (11) ◽  
pp. 915-925 ◽  
Author(s):  
S F Preston ◽  
R I Sha'afi ◽  
R D Berlin
Keyword(s):  
Hela Cells ◽  
Membrane Receptors ◽  
Rapid Rise ◽  
Cell Responses ◽  
Initial Release ◽  
Interphase Cells ◽  
Strong Stimulation ◽  
Two Phases ◽  
Stimulation Of ◽  
Mitotic Cells

Activation of a wide variety of membrane receptors leads to a sustained elevation of intracellular Ca2+ ([Ca2+]i) that is pivotal to subsequent cell responses. In general, in nonexcitable cells this elevation of [Ca2+]i results from two sources: an initial release of Ca2+ from intracellular stores followed by an influx of extracellular Ca2+. These two phases, release from intracellular stores and Ca2+ influx, are generally coupled: stimulation of influx is coordinated with depletion of Ca2+ from stores, although the mechanism of coupling is unclear. We have previously shown that histamine effects a typical [Ca2+]i response in interphase HeLa cells: a rapid rise in [Ca2+]i followed by a sustained elevation, the latter dependent entirely on extracellular Ca2+. In mitotic cells only the initial elevation, derived by Ca2+ release from intracellular stores, occurs. Thus, in mitotic cells the coupling of stores to influx may be specifically broken. In this report we first provide additional evidence that histamine-stimulated Ca2+ influx is strongly inhibited in mitotic cells. We show that efflux is also strongly stimulated by histamine in interphase cells but not in mitotics. It is possible, thus, that in mitotics intracellular stores are only very briefly depleted of Ca2+, being replenished by reuptake of Ca2+ that is retained within the cell. To ensure the depletion of Ca2+ stores in mitotic cells, we employed the sesquiterpenelactone, thapsigargin, that is known to affect the selective release of Ca2+ from intracellular stores by inhibition of a specific Ca(2+)-ATPase; reuptake is inhibited. In most cells, and in accord with Putney's capacitative model (1990), thapsigargin, presumably by depleting intracellular Ca2+ stores, stimulates Ca2+ influx. This is the case for interphase HeLa cells. Thapsigargin induces an increase in [Ca2+]i that is dependent on extracellular Ca2+ and is associated with a strong stimulation of 45Ca2+ influx. In mitotic cells thapsigargin also induces a [Ca2+]i elevation that is initially comparable in magnitude and largely independent of extracellular Ca2+. However, unlike interphase cells, in mitotic cells the elevation of [Ca2+]i is not sustained and 45Ca2+ influx is not stimulated by thapsigargin. Thus, the coupling between depletion of intracellular stores and Ca2+ influx is specifically broken in mitotic cells. Uncoupling could account for the failure of histamine to stimulate Ca2+ influx during mitosis and would effectively block all stimuli whose effects are mediated by Ca2+ influx and sustained elevations of [Ca2+]i.

scholarly journals Molecular Cloning and Characterization of Phocein, a Protein Found from the Golgi Complex to Dendritic Spines

2001 ◽  
Vol 12 (3) ◽  
pp. 663-673 ◽  
Author(s):  
Gilbert Baillat ◽  
Abdelaziz Moqrich ◽  
Francis Castets ◽  
Agnès Baude ◽  
Yannick Bailly ◽  
...  
Keyword(s):  
Golgi Complex ◽  
Hela Cells ◽  
Brefeldin A ◽  
Calmodulin Binding ◽  
Adult Rat ◽  
Adult Rat Brain ◽  
Clathrin Adaptor

Phocein is a widely expressed, highly conserved intracellular protein of 225 amino acids, the sequence of which has limited homology to the ς subunits from clathrin adaptor complexes and contains an additional stretch bearing a putative SH3-binding domain. This sequence is evolutionarily very conserved (80% identity betweenDrosophila melanogaster and human). Phocein was discovered by a yeast two-hybrid screen using striatin as a bait. Striatin, SG2NA, and zinedin, the three mammalian members of the striatin family, are multimodular, WD-repeat, and calmodulin-binding proteins. The interaction of phocein with striatin, SG2NA, and zinedin was validated in vitro by coimmunoprecipitation and pull-down experiments. Fractionation of brain and HeLa cells showed that phocein is associated with membranes, as well as present in the cytosol where it behaves as a protein complex. The molecular interaction between SG2NA and phocein was confirmed by their in vivo colocalization, as observed in HeLa cells where antibodies directed against either phocein or SG2NA immunostained the Golgi complex. A 2-min brefeldin A treatment of HeLa cells induced the redistribution of both proteins. Immunocytochemical studies of adult rat brain sections showed that phocein reactivity, present in many types of neurons, is strictly somato-dendritic and extends down to spines, just as do striatin and SG2NA.

scholarly journals Cytocortical organization during natural and prolonged mitosis of mouse 8-cell blastomeres

Development ◽  
1988 ◽  
Vol 102 (1) ◽  
pp. 143-158 ◽  
Author(s):  
M.H. Johnson ◽  
S.J. Pickering ◽  
A. Dhiman ◽  
G.S. Radcliffe ◽  
B. Maro
Keyword(s):  
Polar Region ◽  
Mitotic Cell ◽  
Cytochalasin D ◽  
Surface Polarity ◽  
Time Points ◽  
Interphase Cells ◽  
M Phase ◽  
Cell Diversity ◽  
Polar Area ◽  
Mitotic Cells

Late 8-cell blastomeres were harvested within the first 45 min after entering mitosis. Some mitotic cells were analysed within the ensuing 2 h for the organization of their surface in relation to their progress through mitosis. Whereas in most late interphase cells microvilli were restricted to a discrete polar region, in mitotic cells at all stages from early metaphase to immediately postcytokinesis microvilli were found to be present over more of the cell surface. Other mitotic cells were placed in nocodazole to arrest them in M-phase for up to 10 h. They were found to show an even more extensive distribution of microvilli over the whole surface, the longer periods of incubation yielding more extended coverage such that many cells no longer appeared to have any residual surface polarity. Removal from nocodazole at all time points from 1 to 10 h resulted in most cells completing mitosis to yield pairs of cells which, in most cases, resembled pairs derived from nonarrested blastomeres and in which a defined polar area of microvilli was restored. However, the percentage of differentiative divisions decreased after 6 h arrest. If, instead of removing cells from nocodazole, they were placed in both nocodazole and cytochalasin D (CCD) for periods of up to 3 h, most microvilli retracted to reveal a tight polar zone of CCD-resistant microvilli. This result suggests that a heterogeneity of cytocortical organization may still exist within the arrested mitotic cell. We propose a model to explain the origin of this heterogeneity of organization and its relationship to the generation of cell diversity.

scholarly journals Arachidonic acid mobilization is suppressed during mitosis: role of cytosolic phospholipase A2 activation

1995 ◽  
Vol 309 (1) ◽  
pp. 91-97 ◽  
Author(s):  
R D Berlin ◽  
S F Preston
Keyword(s):  
Arachidonic Acid ◽  
Phospholipase A2 ◽  
Hela Cells ◽  
Arachidonic Acid Release ◽  
Cytosolic Phospholipase ◽  
Interphase Cells ◽  
Acid Release ◽  
Arachidonate Release ◽  
Stimulation Of ◽  
Mitotic Cells

In interphase HeLa cells, incubation with histamine or thapsigargin led to the rapid release of arachidonic acid. The release was absolutely dependent on Ca2+, consistent with the activation of an 85 kDa cytosolic phospholipase A2 (cPLA2). In metaphase-arrested HeLa cells, by contrast, the stimulation of arachidonate release by these agents was inhibited by more than 90%. The lack of arachidonic acid release by mitotic cells was at least partly expected, since histamine- or thapsigargin-induced Ca2+ influx and elevations of cytosolic free Ca2+ are known to be strongly inhibited during mitosis [Preston, Sha'afi and Berlin (1991) Cell Regul. 2, 915-925]. Indeed, incubation of interphase cells with the Ca2+ ionophore A23187 alone induced a high level of arachidonate release. However, even A23187 failed to elicit release from mitotic cells. Since the Ca(2+)-dependent release of arachidonate by many cell types is promoted by preincubation with ligands that activate receptors of the tyrosine kinase class, and tumour promoters that lead to the phosphorylation of cPLA2, we determined if the responses of mitotic HeLa cells could be modified by this ‘priming’ process. We first established that epidermal growth factor and phorbol 12-myristate 13-acetate were effective priming agents in interphase cells: cells preincubated with the hormone or tumour promoter showed a 2-fold stimulation of thapsigargin- or A23187-induced arachidonic acid release. However, none of the priming agents reversed the lack of mitotic cell response. This refractoriness was not caused by destruction of cPLA2 during mitosis: by Western blotting, cPLA2 of interphase and mitotic cells was shown to be present in comparable amounts. Moreover, cPLA2 activities measured in extracts of interphase and mitotic cells were also comparable. Surprisingly, mitotic cPLA2 appeared to be constitutively phosphorylated in non-hormone-treated (control) cells. The results indicate a novel mechanism of regulation by cPLA2 activity in mitotic cells.

scholarly journals RIBONUCLEIC ACID AND PROTEIN SYNTHESIS IN MITOTIC HELA CELLS

1965 ◽  
Vol 27 (3) ◽  
pp. 565-574 ◽  
Author(s):  
Terry C. Johnson ◽  
John J. Holland
Keyword(s):  
Protein Synthesis ◽  
Rna Polymerase ◽  
Hela Cells ◽  
Messenger Rna ◽  
Rna Synthesis ◽  
Mitotic Cell ◽  
Interphase Cell ◽  
Cell Extracts ◽  
Interphase Cells ◽  
Mitotic Cells

HeLa cells arrested in mitosis were obtained in large numbers, with only very slight interphase cell contamination, by employing the agitation method of Terasima and Tolmach, and Robbins and Marcus. Protein synthesis and RNA synthesis were almost completely suppressed in mitotic cells. Active polyribosomes were nearly absent in mitotic cells as compared with interphase cells treated in the same way. Cell-free protein synthesis and RNA polymerase activity were also greatly depressed in extracts of metaphase cells. The deoxyribonucleoprotein (DNP) of condensed chromosomes from mitotic cells was less efficient as a template for Escherichia coli RNA polymerase than was DNP from interphase cells, although isolated DNA from both sources was equally active as a primer. Despite very poor endogenous amino acid incorporation by extracts of metaphase cells, polyuridylate stimulated phenylalanine incorporation by a larger factor in mitotic cell extracts than it did in interphase cell extracts. These results suggest that RNA synthesis is suppressed in mitotic cells because the condensed chromosomes cannot act as a template, and that protein synthesis is depressed at least in part because messenger RNA becomes unavailable to ribosomes. This conclusion was supported by the demonstration that cells arrested in metaphase supported multiplication of normal yields of poliovirus, thereby showing that the mitotic cell is capable of considerable synthesis of RNA and protein.

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