细胞连接相关基因在大鼠肝再生中表达模式

细胞连接是组织、器官形成的基础。为在基因转录水平了解紧密连接、粘附连接、粘着斑和间隙连接相关基因在肝再生中作用,本文用搜集网站资料和查阅相关论文等方法获得上述基因,用Rat Genome 230 2.0芯片检测它们在大鼠再生肝中表达情况,将3次检验结果相同或相似、在肝再生中发生有意义表达变化、真手术组和假手术组表达差异显著的基因视为肝再生相关基因。初步证实上述4种细胞连接中79、53、109和53个基因与肝再生相关。其中,肝再生启动(部分肝切除后0.5～4h)、G0/G1过渡(PH后4～6h)、细胞增殖(部分肝切除后6～66h)、细胞分化和组织结构功能重建(部分肝切除后72～168h)等4个阶段起始表达的基因数和基因的总表达次数为124、43、122、10和249、145、957、306。表明相关基因主要在肝再生启动阶段起始表达,在不同阶段发挥作用。它们共上调972次,下调540次,表明肝再生中大多数细胞连接相关基因表达加强,少数基因表达降低。它们表达的相似性分为均上调、上调占优势、均下调、下调占优势、上调和下调相近等5类,涉及102、38、73、27和16个基因,它们表达的时间相关性分为0.5和1h、2h、4和6h、8和12h、16h、18和48h、24h、30和42h、36h、54和60h、66和72h、96h、120h、144和168h等14组,表明肝再生中细胞生理生化活动具有阶段性。它们的表达模式分为41类,表明肝再生中细胞生理生化活动具有多样性和复杂性。根据肝再生中基因表达变化和表达模式推测,肝再生早期和前期间隙连接形成增强,晚中期和后期间隙连接形成减少;早期、前期和后期粘着斑形成增强;紧密连接和粘附连接的形成贯穿于整个肝再生。     

Expression profiles of the genes associated with metabolism and transport of amino acids and their derivatives in rat liver regeneration

Summary. Amino acids (AA) are components of protein and precursors of many important biological molecules. To address effects of the genes associated with metabolism and transport of AA and their derivatives during rat liver regeneration (LR), we firstly obtained the above genes by collecting databases data and retrieving related thesis, and then analyzed their expression profiles during LR using Rat Genome 230 2.0 array. The LR-associated genes were identified by comparing the gene expression difference between partial hepatectomy (PH) and sham-operation (SO) rat livers. It was approved that 134 genes associated with metabolism of AA and their derivatives and 26 genes involved in transport of them were LR-associated. The initially and totally expressing number of these genes occurring in initial phase of LR (0.5–4 h after PH), G0/G1 (4–6 h after PH), cell proliferation (6–66 h after PH), cell differentiation and structure-function reconstruction of liver tissue (72–168 h after PH) were respectively 76, 17, 79, 5 and 162, 89, 564, 195, illustrating that these LR-associated genes were initially expressed mainly in initial stage, and functioned in different phases. Frequencies of up-regulation and down-regulation of them being separately 564 and 357 demonstrated that genes up-regulated outnumbered those down-regulated. Categorization of their expression patterns into 22 types implied the diversity of cell physiological and biochemical activities. According to expression changes and patterns of the above-mentioned genes in LR, it was presumed that histidine biosynthesis in the metaphase and anaphase, valine metabolism in the anaphase, and metabolism of glutamate, glutamine, asparate, asparagine, methionine, alanine, leucine and aromatic amino acid almost were enhanced in the whole LR; as for amino acid derivatives, transport of neutral amino acids, urea, γ-aminobutyric acid, betaine and taurine, metabolism of dopamine, heme, S-adenosylmethionine, thyroxine, and biosynthesis of hydroxyproline, nitric oxide, orinithine, polyamine, carnitine, selenocysteine were augmented during the entire liver restoration. Above results showed that metabolism and transport of AA and their derivates were necessary in liver regeneration. Authors’ address: Prof. Dr. C. S. Xu, College of Life Science, No. 46, Jianshe RD, Henan, Xinxiang 453007, China     

蛋白质代谢、折叠、运输、定位、装配相关基因在大鼠肝再生中表达变化

为在基因转录水平了解蛋白质代谢、折叠、运输、定位、装配相关基因在大鼠肝再生中表达情况和作用,本文用搜集网站资料和查阅相关论文等方法获得上述基因,用Rat Genome 2302.0芯片检测它们在大鼠再生肝中表达情况,用真、假手术比较方法确定肝再生相关基因。初步证实上述基因中1147个基因与肝再生相关。其中,参与蛋白质代谢、折叠、运输、定位和装配的基因以上调表达为主;参与蛋白质代谢的基因主要在部分肝切除（partial hepatectomy,PH）后0.5-1h和16-30h起始表达;0.5-12h表达的促进蛋白降解基因数多于促进蛋白积累基因数,而16-48h表达的促进蛋白质积累基因数显著多于促进蛋白质降解基因数;蛋白质合成相关基因在肝再生的16、24、42和66h表达上调较多,在42h最多;几乎在整个肝再生中蛋白质降解相关基因表达上调,在早、前期较多,在后期较少;蛋白质折叠相关基因在2、16-24、42、66、72和168h表达上调较多,在66h最多;蛋白质运输和定位相关基因在整个肝再生中表达上调,在66h表达上调最多;蛋白质装配相关基因在96h前均表达上调,其中,12h表达上调基因最多。根据上述结果推测,在肝再生中期蛋白质合成旺盛,几乎整个肝再生中蛋白质降解、折叠、运输定位和装配活动活跃。     

Duration of the Mitotic Cycle in Cell Cultures of Haplopappus gracilis

The duration of the cell cycle and its component phases in cell cultures of Haplopappus gracilis was estimated by means of pulse labelling with tritiated thymidine and subsequent autoradiographic techniques. The total duration of the mitotic cycle was found to be 22.0 hours. The average durations of the following component phases were: the synthetic period (S) 6.4 hours, the postsynthetic period (G₂) 4.86 hours, prophase (P) 0.64 hours, metaphase (M) 0.40 hours, anaphase + early telophase (AT) 0.36 hours, the presynthetic period (G₁) 9.34 hours. The results indicate that G₁ and G₂ are the phases, which are most prolonged in populations of cultivated cells when compared to the same phases in root lip cells from the same species.     

Analysis of spindle microtubule organization in untreated and taxol-treated PtK1 cells.

Taxol, a microtubule stabilizing agent, has been used to study changes in spindle microtubule organization during mitosis. PtK₁ cells have been treated with 5 μg/ml taxol for brief periods to determine its effect on spindle architecture. During prophase taxol induces microtubules to aggregate, particularly evident in the region between the nucleus and cell periphery. Taxol induces astral microtubule formation in prometaphase and metaphase cells concomitant with a reduction in spindle length. At anaphase taxol induces an increase in length in astral microtubules and reduces microtubule length in the interzone. Taxol-treated telophase cells show a reduction in the rate of furrowing and astral microtubules lack a discrete focus and are arranged more diffusely on the surface of the nuclear envelope. In summary, taxol treatment of cells prior to anaphase produces an increase in astral microtubules, a reduction in kinetochore microtubules and a decrease in spindle length. Brief taxol treatments during anaphase through early G₁ promotes stabilization of microtubules, an increase in the length of astral microtubules and a delayed rate of cytokinesis.     

CELL DIVISION IN SPIROGYRA. I. MITOSIS

Dividing cells of Spirogyra sp. were examined with both the light and electron microscopes. By preprophase many of the typical transverse wall micro-tubules disappeared while others were seen in the thickened cytoplasmic strands. Microtubules appeared in the polar cytoplasm at prophase and by prometaphase they penetrated the nucleus. They were attached to chromosomes at metaphase and early anaphase, and formed a sheath surrounding the spindle during anaphase; they were seen in the interzonal strands and cytoplasmic strands at telophase. The interphase nucleolus, containing 2 distinct zones and chromatinlike material, fragmented at prophase; at metaphase and anaphase nucleolar material coated the chromosomes, obscuring them by late anaphase. The chromosomes condensed in the nucleoplasm at prophase, moving into the nucleolus at prometaphase. The nuclear envelope was finally disrupted at anaphase during spindle elongation; at telophase membrane profiles coated the reforming nuclei. During anaphase and early telophase the interzonal region contained vacuoles, a few micro-tubules, and sometimes eliminated n ucleolar material; most small organelles, including swollen endoplasmic reticulum and tubular membranes, were concentrated in the polar cytoplasm. Quantitative and qualitative cytological observations strongly suggest movement of intact wall rnicrotubules to the spindle at preprophase and then back again at telophase.     

细胞外基质相关基因在大鼠肝再生中表达模式分析

细胞外基质具有维持细胞极性、调节细胞粘附、增殖、组织器官形态、发生、分化等功能。为了进一步在基因转录水平了解细胞外基质在大鼠肝再生中变化和作用, 用搜集网站资料和查阅相关论文等方法获得细胞外基质基因, 用Rat Genome 230 2.0芯片检测它们在大鼠再生肝中表达情况, 用真、假手术比较方法确定肝再生相关基因。初步证实上述97个基因与肝再生相关。其中, 肝再生启动(部分肝切除(parital hepatectomy, PH)后0.5~4 h)、G0/G1过渡(PH后4~6 h)、细胞增殖(PH后6~66 h)、细胞分化和组织结构功能重建(PH后72~168 h)等4个阶段起始表达的基因数为49、19、73、5, 基因总表达的次数为84、51、369、144, 表明相关基因主要在肝再生启动阶段起始表达, 在不同阶段发挥作用。它们表达的相似性分为均上调、上调占优势、均下调、下调占优势、上调和下调相近等5类, 涉及38、21、21、10和7个基因, 共上调411次, 下调186次, 分为24种表达模式, 表明肝再生中细胞生理生化活动具有阶段性、多样性和复杂性。根据细胞外基质相关基因在肝再生中表达变化推测, 肝再生前期纤粘连蛋白形成相关基因表达增强, 肝再生中期胶原形成相关基因表达增强。     

Localization of anti-topoisomerase IIα antibodies under normal conditions and after artificial chromosome condensation and decondensation

By means of immunofluorescence method, localization of DNA-topoisomerase IIα (Topo IIα) in interphase nuclei and chromosomes at different stages of mitosis was studied in situ under normal conditions and after treatment with condensing and decondensing solutions. In non-isolated mitotic M-HeLa cell chromosomes, Topo IIα was uniformly distributed along chromatids after fixation and permeabilization in situ. After treatment of cells with decondensing solutions (10 mM Tris; 0.1 mM CaCl₂ in 10 mM Tris; 0.3 mM CaCl₂ in 10 mM Tris; 15% DMEM; 75 mM KCl), Topo IIα was evenly distributed along chromatids in prophase, prometaphase and metaphase; its concentration was the highest in the pericentromere region. After treatment of cells with condensing solutions containing 0.7 mM, 1 mM, 2 mM or 3 mM CaCl₂ in 10 mM Tris, Topo IIα was not detected in prophase, metaphase and anaphase. However, in late telophase anti-Topo IIα antibodies were found in reforming nuclei under identical conditions. After sequential treatment with condensing and decondensing solutions, the distribution patterns of Topo IIα in chromosomes were the same as after treatment with only decondensing solutions. In anaphase and telophase, Topo IIα was evenly distributed along chromatids, while in prophase, prometaphase and metaphase it was predominantly localized in the pericentromere region. After the treatment of cells with condensing solutions chromosome staining was not observed, apparently due to “masking” of binding sites for anti-Topo IIα antibodies. Homogenous distribution of Topo IIα along chromatids in non-isolated chromosomes was preserved after the treatment of cells with hypotonic solutions; however, under these conditions Topo IIα concentration was higher in centromeres.     

Mitotic reversion in prophase of PTK1 cells induced by argon laser microirradiation

In this paper, we report the effects of laser microirradiation of prophase nucleoli and mitotic chromosomes in cells of female rat kangaroo kidney epithelial cell line PTK₁. When the laser power delivered to sample surface was 90–190 mW, irradiation of one of the two nucleoli in the prophase cell did not inhibit the mitotic progress, but resulted in the loss of the irradiated nucleolus in daughter cells. When the laser power was increased to 360–420 mW, either irradiation of the nucleolus or chromosome in midprophase caused a blockage of mitosis at terminal midprophase. The irradiated cells returned morphologically to early prophase. No mitotic reversion occurred in the case of irradiation of chromosomes at late prophase, prometaphase, metaphase, and anaphase. Irradiation of the cytoplasm in prophase cells caused a 50–70 min mitotic delay at prophase. However, the irradiated cells underwent successive mitotic divisions. The mechanism of laser-induced mitotic prophase reversion is discussed.     

温度对丙酮酸生物合成动力学、能荷和氧化-还原度的影响

在光滑球拟酵母(Torulopsis glabrata620)生产丙酮酸的过程中,温度对丙酮酸生物合成有着重要的影响。考察了不同发酵温度下基质消耗、细胞生长、丙酮酸合成及能荷水平和氧化-还原度等方面的差异。在恒温发酵中,维持较高的发酵温度可以增强糖耗,促进菌体生长,加速丙酮酸积累,但前期胞内能荷水平较高,菌体消耗较多葡萄糖合成菌体,后续产酸能力不足,导致丙酮酸得率降低;维持较低的发酵温度可以在发酵后期提供稳定的产酸能力,但菌体代谢缓慢,后期胞内NADH/NAD 水平较高,丙酮酸生产强度降低。因此仅仅采取单一的温度控制策略很难达到丙酮酸高产量、高产率和高生产强度的统一。     

Timing the phases of the microtubule cycles involved in cytoplasmic and nuclear divisions in cells of undisturbed onion root meristems

The duration of the different phases of the microtubule and chromosome cycles were estimated in the native diploid cell populations of Allium cepa L root meristems proliferating undisturbed, under steady state conditions, at the physiological temperature of 15°C. The cycles were coupled by considering their fitting in relation to the short process of nuclear envelope breakdown. In the cycle related to cytoplasmic division, the preprophase band which predicts the future position of the phragmoplast made its appearance, as a wide band, 16 mm before the G₂ to prophase transition, ie it was only present during the final 5% of the total G₂ timing (5 h 30 mm). The band became narrow only 6 mm after prophase had started and it was present in this form for the remaining prophase time (2 h 24 mm). Its disappearance occurred strictly coinciding with nuclear envelope breakdown, at the end of prophase. No microtubules related to cytoplasmic division were apparent until 9 mm after telophase had initiated. The two initial stages of phragmoplast formation which followed occupied, respectively, 27 mm and 54.5 mm of the 2-h long telophase. On the other hand, the third and last stage in phragmoplast formation covered both the final 35 mm of mitosis and the 6 initial mm of the G₁ of the next interphase. A very short (less than 4 mm) stage of microtubular nucleation around the nuclear envelope took place immediately afterwards, before the cortical array of microtubules appeared. The microtubule cycle related to nuclear division started with the apparent activation of the future spindle poles 7.4 mm before prophase was over. The mitotic spindle developed in the 5.6 mm long prometaphase. The spindle functioned in metaphase for the 42 mm it lasted, half spindles being separated for the 37 mm anaphase occupied in these cells.     

Differential expression of a phosphoepitope at the kinetochores of moving chromosomes

A phosphorylated epitope is differentially expressed at the kinetochores of chromosomes in mitotic cells and may be involved in regulating chromosome movement and cell cycle progression. During prophase and early prometaphase, the phosphoepitope is expressed equally among all the kinetochores. In mid-prometaphase, some chromosomes show strong labeling on both kinetochores; others exhibit weak or no labeling; while in other chromosomes, one kinetochore is intensely labeled while its sister kinetochore is unlabeled. Chromosomes moving toward the metaphase plate express the phosphoepitope strongly on the leading kinetochore but weakly on the trailing kinetochore. This is the first demonstration of a biochemical difference between the two kinetochores of a single chromosome. During metaphase and anaphase, the kinetochores are unlabeled. At metaphase, a single misaligned chromosome can inhibit further progression into anaphase. Misaligned chromosomes express the phosphoepitope strongly on both kinetochores, even when all the other chromosomes of a cell are assembled at the metaphase plate and lack expression. This phosphoepitope may be involved in regulating chromosome movement to the metaphase plate during prometaphase and may be part of a cell cycle checkpoint by which the onset of anaphase is inhibited until complete metaphase alignment is achieved.     

An indirect test for a role of the synaptonemal complex in the establishment of sister chromatid cohesiveness

The meiotic behavior of a special maize trisome was quantitatively observed at pachytene, metaphase I, anaphase I, prophase II, metaphase II and anaphase II. The data obtained are consistent with (but do not prove) the model that sister chromatid cohesiveness at anaphase I may be established during pachytene synapsis of the chromosome regions involved. The data suggest, however, that the normal prophase II maintenance of dyad integrity by cohesiveness of sister chromatid centromere regions does not depend upon prior synapsis of these regions, although monads separated from each other on the anaphase I spindle may be delivered to the same prophase II daughter nucleus. — The strands which some of the time connect sister chromatids which are separating equationally at anaphase I show a positive Feulgen staining reaction.     

Most of the homoeologous pairing at metaphase I in wheat-rye hybrids is not chiasmatic

The use of telomeric C-bands in wheat-rye hybrids has made it possible to distinguish three types of wheat-wheat (1B^L) and wheat-rye associations (a, end-to-end extremely distal; b, end-to-ed distal; and c, interstitial) between homoeologous chromosomes at different metaphase I stages (early, middle and late) and also to estimate the actual recombination frequencies for such associations at anaphase I. There was a decrease of the a and b association frequencies during the different metaphase I stages, whereas the c type remained without variation in all stages. A good fit between the frequencies of c associations at metaphase I and the number of recombinant chromosomes at anaphase I, assuming a maximum of one chiasma per bond, was found; however, there was no correspondence between metaphase I and anaphase I data when all associations (a + b + c) were considered. In addition, rye-rye homologous pairing was observed at metaphase I, but no evidence for rye-rye recombination was found at anaphase I. The results indicate that most of end-to-end (a and b) homoeologous and nonhomologous associations are actually nonchiasmatic and are a remnant of prophase pairing.     

Activation of the MKK/ERK Pathway during Somatic Cell Mitosis: Direct Interactions of Active ERK with Kinetochores and Regulation of the Mitotic 3F3/2 Phosphoantigen

The mitogen-activated protein (MAP) kinase pathway, which includes extracellular signal–regulated protein kinases 1 and 2 (ERK1, ERK2) and MAP kinase kinases 1 and 2 (MKK1, MKK2), is well-known to be required for cell cycle progression from G1 to S phase, but its role in somatic cell mitosis has not been clearly established. We have examined the regulation of ERK and MKK in mammalian cells during mitosis using antibodies selective for active phosphorylated forms of these enzymes. In NIH 3T3 cells, both ERK and MKK are activated within the nucleus during early prophase; they localize to spindle poles between prophase and anaphase, and to the midbody during cytokinesis. During metaphase, active ERK is localized in the chromosome periphery, in contrast to active MKK, which shows clear chromosome exclusion. Prophase activation and spindle pole localization of active ERK and MKK are also observed in PtK₁ cells. Discrete localization of active ERK at kinetochores is apparent by early prophase and during prometaphase with decreased staining on chromosomes aligned at the metaphase plate. The kinetochores of chromosomes displaced from the metaphase plate, or in microtubule-disrupted cells, still react strongly with the active ERK antibody. This pattern resembles that reported for the 3F3/2 monoclonal antibody, which recognizes a phosphoepitope that disappears with kinetochore attachment to the spindles, and has been implicated in the mitotic checkpoint for anaphase onset (Gorbsky and Ricketts, 1993. J. Cell Biol. 122:1311–1321). The 3F3/2 reactivity of kinetochores on isolated chromosomes decreases after dephosphorylation with protein phosphatase, and then increases after subsequent phosphorylation by purified active ERK or active MKK. These results suggest that the MAP kinase pathway has multiple functions during mitosis, helping to promote mitotic entry as well as targeting proteins that mediate mitotic progression in response to kinetochore attachment.     

Microinjection of mitotic cells with the 3F3/2 anti-phosphoepitope antibody delays the onset of anaphase

The transition from metaphase to anaphase is regulated by a checkpoint system that prevents chromosome segregation in anaphase until all the chromosomes have aligned at the metaphase plate. We provide evidence indicating that a kinetochore phosphoepitope plays a role in this checkpoint pathway. The 3F3/2 monoclonal antibody recognizes a kinetochore phosphoepitope in mammalian cells that is expressed on chromosomes before their congression to the metaphase plate. Once chromosomes are aligned, expression is lost and cells enter anaphase shortly thereafter. When microinjected into prophase cells, the 3F3/2 antibody caused a concentration-dependent delay in the onset of anaphase. Injected antibody inhibited the normal dephosphorylation of the 3F3/2 phosphoepitope at kinetochores. Microinjection of the antibody eliminated the asymmetric expression of the phosphoepitope normally seen on sister kinetochores of chromosomes during their movement to the metaphase plate. Chromosome movement to the metaphase plate appeared unaffected in cells injected with the antibody suggesting that asymmetric expression of the phosphoepitope on sister kinetochores is not required for chromosome congression to the metaphase plate. In antibody-injected cells, the epitope remained expressed at kinetochores throughout the prolonged metaphase, but had disappeared by the onset of anaphase. When normal cells in metaphase, lacking the epitope at kinetochores, were treated with agents that perturb microtubules, the 3F3/2 phosphoepitope quickly reappeared at kinetochores. Immunoelectron microscopy revealed that the 3F3/2 epitope is concentrated in the middle electronlucent layer of the trilaminar kinetochore structure. We propose that the 3F3/2 kinetochore phosphoepitope is involved in detecting stable kinetochore-microtubule attachment or is a signaling component of the checkpoint pathway regulating the metaphase to anaphase transition.     

Distinct developmental expression of two elongase family members in zebrafish

Despite the known importance of long-chained polyunsaturated fatty acids (LC-PUFA) during development, very little is known about their utilization and biosynthesis during embryogenesis. Combining the advantages of the existence of a complete range of enzymes required for LC-PUFA biosynthesis and the well established developmental biology tools in zebrafish, we examined the expression patterns of three LC-PUFA biosynthesis genes, Elovl2-like elongase (elovl2), Elovl5-like elongase (elovl5) and fatty acyl desaturase (fad) in different zebrafish developmental stages. The presence of all three genes in the brain as early as 24 hours post fertilization (hpf) implies LC-PUFA synthesis activity in the embryonic brain. This expression eventually subsides from 72 hpf onwards, coinciding with the initiation of elovl2 and fad expression in the liver and intestine, 2 organs known to be involved in adult fish LC-PUFA biosynthesis. Collectively, these patterns strongly suggest the necessity for localized production of LC-PUFA in the brain during in early stage embryos prior to the maturation of the liver and intestine. Interestingly, we also showed a specific expression of elovl5 in the proximal convoluted tubule (PCT) of the zebrafish pronephros, suggesting a possible new role for LC-PUFA in kidney development and function.     

Effects of cold treatment on synchronous Chinese hamster cells treated in mitosis

Chinese hamster mitotic cells (95% in mitosis with 85% in metaphase and 10% in anaphase) were shaken loose from monolayer asynchronous cultures and stored at 1°C for up to 28 hours. During this period, the mitotic index did not decrease and the cells remained cytologically normal. However, over a four-hour period, metaphase cells located within 1.8 minutes of anaphase, 10% of the metaphase population, were able to move into anaphase; this point of 1.8 minutes corresponds in time to that reported for the spindle activation marker. When the cells were warmed to 37°C, they were delayed in entering G₁ and S (35 and 70 minutes, respectively, after a 4-hour treatment). This delay of 70 minutes was maintained for three cell cycles, during which a high degree of synchrony was maintained. Cold treatment for 12 hours produced delays into G₁ and S of 50 and 110 minutes, respectively. A fraction of the metaphase cells (12 or 50% after treatments of 4 or 12 hours, respectively) evidenced chromosomal aggregation, were unable to complete cytokinesis, and appeared in the next division as tetraploid cells. These tetraploid cells were unable to survive and produce macroscopic colonies. It is concluded that this decrease in viability is caused by irreversible effects on the spindle and/or centriolar components in metaphase cells located prior to the spindle activation marker.     

CELL CYCLE-SPECIFIC CHANGES IN HISTONE PHOSPHORYLATION ASSOCIATED WITH CELL PROLIFERATION AND CHROMOSOME CONDENSATION

Preparative polyacrylamide gel electrophoresis was used to examine histone phosphorylation in synchronized Chinese hamster cells (line CHO). Results showed that histone f1 phosphorylation, absent in G₁-arrested and early G₁-traversing cells, commences 2 h before entry of traversing cells into the S phase. It is concluded that f1 phosphorylation is one of the earliest biochemical events associated with conversion of nonproliferating cells to proliferating cells occurring on old f1 before synthesis of new f1 during the S phase. Results also showed that f3 and a subfraction of f1 were rapidly phosphorylated only during the time when cells were crossing the G₂/M boundary and traversing prophase. Since these phosphorylation events do not occur in G₁, S, or G₂ and are reduced greatly in metaphase, it is concluded that these two specific phosphorylation events are involved with condensation of interphase chromatin into mitotic chromosomes. This conclusion is supported by loss of prelabeled ³²PO₄ from those specific histone fractions during transition of metaphase cells into interphase G₁ cells. A model of the relationship of histone phosphorylation to the cell cycle is presented which suggests involvement of f1 phosphorylation in chromatin structural changes associated with a continuous interphase "chromosome cycle" which culminates at mitosis with an f3 and f1 phosphorylation-mediated chromosome condensation.     

THE REPLICATION TIME AND PATTERN OF THE LIVER CELL IN THE GROWING RAT

Three-week-old male rats of the Wistar strain were given tritiated thymidine, 1 µc/gm body weight, intraperitoneally and were killed at intervals from 0.25 to 72 hours later. Autoradiographs were made from 5 µ sections, stained by the Feulgen method. The replication time and its component intervals were determined from the scoring of the labeling of interphase nuclei as well as of prophase, metaphase, anaphase, and telophase nuclei. Absorption of the intraperitoneally injected label is rapid and is attended by "flash" labeling during interphase. The results show that at any one time about 4 per cent of the liver cells are synthesizing DNA preliminary to cell division. These cells alternate with waves of other cells and it is estimated that about 10 per cent of the liver cell population is engaged in cell duplication. The replication time is about 21.5 hours, and its component intervals occupy the following times: DNA synthesis, 9 hours; post-DNA synthesis gap, 0.50 hour; prophase, 1.3 hours; metaphase, 1.0 hour; anaphase, 0.4 hour; telophase, 0.3 hour; postmitosis gap, 9.0 hours. A group of liver cells has been recorded in at least 3 successive replication cycles.