Autoradiographic and microphotometric studies of desoxyribose nucleic acid during microgametogenesis in Lilium longiflorum

Caught in the Act: Live-Cell Imaging of Plant Meiosis

Live-cell imaging is a powerful method to obtain insights into cellular processes, particularly with respect to their dynamics. This is especially true for meiosis, where chromosomes and other cellular components such as the cytoskeleton follow an elaborate choreography over a relatively short period of time. Making these dynamics visible expands understanding of the regulation of meiosis and its underlying molecular forces. However, the analysis of meiosis by live-cell imaging is challenging; specifically in plants, a temporally resolved understanding of chromosome segregation and recombination events is lacking. Recent advances in live-cell imaging now allow the analysis of meiotic events in plants in real time. These new microscopy methods rely on the generation of reporter lines for meiotic regulators and on the establishment of ex vivo culture and imaging conditions, which stabilize the specimen and keep it alive for several hours or even days. In this review, we combine an overview of the technical aspects of live-cell imaging in plants with a summary of outstanding questions that can now be addressed to promote live-cell imaging in Arabidopsis and other plant species and stimulate ideas on the topics that can be addressed in the context of plant meiotic recombination.

Looking at plant cell cycle from the chromatin window

The cell cycle is defined by a series of complex events, finely coordinated through hormonal, developmental and environmental signals, which occur in a unidirectional manner and end up in producing two daughter cells. Accumulating evidence reveals that chromatin is not a static entity throughout the cell cycle. In fact, there are many changes that include nucleosome remodeling, histone modifications, deposition and exchange, among others. Interestingly, it is possible to correlate the occurrence of several of these chromatin-related events with specific processes necessary for cell cycle progression, e.g., licensing of DNA replication origins, the E2F-dependent transcriptional wave in G1, the activation of replication origins in S-phase, the G2-specific transcription of genes required for mitosis or the chromatin packaging occurring in mitosis. Therefore, an emerging view is that chromatin dynamics must be considered as an intrinsic part of cell cycle regulation. In this article, we review the main features of several key chromatin events that occur at defined times throughout the cell cycle and discuss whether they are actually controlling the transit through specific cell cycle stages.

SYNTHESIS OF DEOXYRIBONUCLEIC ACID BY MICRO- AND MACRONUCLEI OF TETRAHYMENA PYRIFORMIS

Evidence as to the times of DNA synthesis in micronucleate Tetrahymena pyriformis (mating type II, variety 1) has been obtained by briefly exposing individuals of different ages to tritiated thymidine, returning them to non-radioactive medium, fixing at division, and preparing autoradiographs. A variable length of interphase, ranging from a few minutes to about 2 hours, has been found to precede the initiation of macronuclear DNA synthesis. Once begun, however, the period of synthesis appears to be similar in all cells, regardless of generation time, and has been estimated at 1 to 1(1/2) hours. Under the conditions of these experiments, the time elapsing between the end of synthesis and subsequent division into daughter cells ranges from approximately 1(1/2) to 2(1/2) hours in generation times long enough to allow such variability. Division of the micronucleus occurs shortly before the cell begins to divide; its DNA synthesis starts immediately and continues after cell division for a total period estimated at about an hour.

EVIDENCE For THE CYTOPLASMIC SYNTHESIS OF NUCLEAR HISTONE DURING SPERMIOGENESIS IN THE GRASSHOPPER CHORTOPHAGA VIRIDIFASCIATA (DE GEER)

Histone synthesis during spermiogenesis in the grasshopper Chortophaga viridifasciata was studied using autoradiographic and cytochemical methods. It was found that meiosis is followed by a cessation of RNA synthesis, an elimination of RNA from the nucleus, and, during the cytoplasmic sloughing accompanying the initial cytoplasmic elongation, a loss of most of the RNA from the cell. The initial phase of cell elongation results in a long spermatid headed by a spherical RNA-less nucleus bounded by a thin RNA-containing layer of cytoplasm. Subsequent nuclear elongation is accompanied by a replacement of the typical histones by others rich in arginine. This replacement is the result of synthesis of new protein. Incorporation of arginine is first seen to occur in the thin cytoplasmic layer surrounding the nucleus. This layer was shown by staining and electron microscopy to contain aggregations of ribosome-like particles. These observations support the conclusion that the histone is synthesized in association with the RNA granules in the cytoplasm, then migrates into the nucleus where it combines with the DNA.

A STUDY OF THE PENETRATION OF MAMMALIAN CELLS BY DEOXYRIBONUCLEIC ACIDS

Tritium-labeled deoxyribonucleic acid (DNA) from pneumococci and from human leukocytes was added to growing cultures of HeLa cells at 37°C. Autoradiography revealed an extensive localization of tritium in the nuclear regions. The label could not be removed by treatment with ribonuclease or dilute perchloric acid, but quantitative removal from the cells could be effected with deoxyribonuclease. Chemical and radioactivity determinations on nucleic acids isolated from the exposed HeLa cells revealed the presence of tritium in all 4 DNA bases. About 12 µg. of tritiated DNA was recovered from 6 x 10⁶ HeLa cells which had been exposed for 24 hours to 240 µg. of the human DNA. From this, it is concluded that the amount of DNA, or its degradation products, taken up by the cells was equivalent to at least 10 per cent of the normal HeLa cell complement.

A microphotometric study of the syntheses of desoxyribonucleic acid and nuclear histone

1. The fast green stain of Alfert and Geschwind for nuclear basic protein is shown to obey the Beer-Lambert laws when used on purified histone. Interference from acid substances other than nucleic acids as a possible source of error is indicated. 2. Use of this technique after a modified Feulgen stain enables determination of relative amounts of desoxyribonucleic acid and histone in the same individual cells. 3. DNA and histone are shown to have the same distribution in formalin-fixed nuclei. 4. The syntheses of DNA and histone proceed simultaneously resulting in the doubling of both these substances prior to cell division. 5. The standard error for histone values is greater than that for DNA; however, the source of this variability is not known.

The metabolism of chromosomal ribonucleic acid in Drosophila salivary glands and its relation to synthesis of desoxyribonucleic acid

Incorporation of adenine-8-C(14) into chromosomal nucleic acids of Drosophila salivary glands has been observed with the autoradiographic technique. RNA-C(14) and DNA-C(14) were detected as the fractions extractable by ribonuclease digestion and resistant to ribonuclease, respectively. Extractions with desoxyribonuclease and acids were also used to identify the nucleic acids. Time-course curves were determined from grain counts. RNA-C(14) concentration reached a maximum in 2 hours, and decreased after the 4th hour. DNA-C(14) concentration reached its maximum within 8 hours, and showed no decreases during a 48-hour experiment. In the latter part of the period of observation, morphological differentiation of the gland occurred, the decrease in RNA-C(14) became very rapid, and a large increase in DNA-C(14) was observed. Marked decrease in RNA-C(14) and increase in DNA-C(14) were detectable in a few hours when isotope was administered shortly before visible differentiation of the gland. Measurements of nuclear size indicated no significant decreases in RNA-C(14) amount prior to the period of differentiation. During this later period, a large decrease in RNA-C(14) amount occurs suddenly, and the same amount of C(14) is added simultaneously to the DNA fraction, as expected if RNA-C(14) is utilized in the synthesis of DNA.

The accumulation of soluble deoxyribosidic compounds in relation to nuclear division in anthers of Lilium longiflorum

Intervals of sporogenous DNA synthesis in lily anthers are associated with marked accumulations of methanol- and acid-soluble deoxyribosidic compounds. There are differences in solubility between compounds accumulating at meiosis and those accumulating at mitosis. The formation of these compounds appears to be related to a breakdown of somatic DNA.

Functional significance of the crystal cells in the larva of Drosophila melanogaster

Cytoplasmic crystalline inclusions are found in some larval haemocytes of Drosophila melanogaster. Blackening can be experimentally induced in these cells, and previously it was suggested that either the substrate or enzyme for the tyrosine-tyrosinase system leading to melanin production in Drosophila larvae is found in these inclusions in the crystal cells. The present report is an attempt to further localize the enzyme and substrate. Larvae have been fed on food containing α-C¹⁴-tyrosine and autoradiographs of the blood cells taken from these larvae subsequently prepared. The C¹⁴ activity in the crystal cells is restricted to the crystal inclusions in the cells and is significantly higher than that found in the other type of haemocytes, the plasmatocytes. When samples of the blood cells are incubated in DOPA solution, the extra-crystalline cytoplasm becomes blackened while the crystals themselves remain colorless. These observations are consistent with the hypothesis that the substrate is localized in the crystal inclusions whereas enzyme is found in the surrounding cytoplasm. An in vivo structural isolation would serve to separate enzyme and substrate rather than an inhibition by dehydrogenases as postulated by previous authors. In vitro examination with the phase microscope has shown that the crystal cells rupture easily and the crystals dissolve in the haemolymph. Therefore any treatment which tends to disrupt the structural integrity of the cell will allow the enzyme and substrate to come together. Humoral factors preceding metamorphosis might account for the in vivo release of the enzymatic reaction by initially altering the permeability of the cell.

The metabolic characteristics of nucleolar, chromosonal, and cytoplasmic ribonucleic acid of Drosophila salivary glands

Incorporation and retention of adenine-8-C¹⁴ and of P³²O₄ by nucleolar, chromosomal, and cytoplasmic RNA have been studied. Radioisotope concentrations were determined from autoradiographs, by grain counting, and RNA concentrations by microphotometry after basic staining. The relation between rates of RNA accumulation and rates of adenine incorporation was used to determine if synthesis was used to replace RNA which was lost from a fraction, and to obtain estimates of turnover rate. Nucleolar incorporation patterns indicate its incorporation is independent of growth, and there is complete turnover of the fraction in an hour or less. Nucleolar turnover is attributed to degradation of RNA within the nucleolus rather than to movement of intact molecules from the nucleolus. Chromosomal RNA reaches a much lower maximum specific activity than nucleolar, and a slightly higher maximum than cytoplasmic RNA. It showed faster incorporation than cytoplasmic RNA while accumulating RNA at the same rate as the cytoplasm, suggesting chromosomal RNA turnover. No evidence of cytoplasmic RNA turnover was found: rate of incorporation and rate of growth were correlated, and retention studies detected no decrease in amount of RNA-C¹⁴, RNA-P³², or RNA. Different ultimate precursors are indicated for nucleolar and non-nucleolar RNA by the observation that the nucleolar precursor is labeled before the precursor of non-nucleolar RNA.

Is there "metabolic" DNA in the mouse seminal vesicle?

This study was designed to answer the question: Is H³-thymidine uptake by nuclei of the mouse seminal vesicle evidence for DNA synthesis and mitosis, or does it signify some "metabolic" function of DNA unrelated to chromosome duplication? Mice were given an intraperitoneal injection of H³-thymidine. Six hours later Feulgen squashes of the seminal vesicle epithelium were made and covered with autoradiographic stripping film. The silver grains above labeled nuclei were counted, and the Feulgen dye contents of these same nuclei were determined photometrically after removal of the grains from the emulsion. Unlabeled nuclei were also measured. The dye contents of non-radioactive nuclei form a unimodal distribution, indicating that polyploidy is absent from this tissue. The radioactive nuclei fall into two groups. In the first, the average dye content is the same as that of the cold nuclei (2C). In the second, the values range from 2C to 4C. In the 2C to 4C group the grain count is proportional to the dye content, showing that incorporation is correlated with synthesis. The radioactive 2C nuclei arose by mitosis during the course of the experiment. This is shown by the following facts: (1) They frequently occur in pairs. (2) They average smaller than unlabeled 2C nuclei. (3) Their average grain count is approximately half that of the 4C nuclei. (4) Labeled division figures are found. (5) A mitotic rate estimated from the number of labeled 2C nuclei accords reasonably well with one based on the number of observed mitoses. Since the incorporation of thymidine accompanies DNA synthesis and precedes mitosis, there is no reason to postulate a special "metabolic" DNA in this tissue.

Schedule of spermatogenesis in the pulmonate snail Helix aspersa, with special reference to histone transition

The schedule of spermatogenesis is determined from the times necessary for cells labeled with tritium thymidine during premeiotic DNA synthesis to pass through the successive spermatogenic stages. A transition from a typically somatic histone rich in lysine, to a histone rich in arginine is shown to occur during spermatid stages. A later shift to a protamine is observed in the maturing sperm. These changes are characterized by the use of in situ staining methods. The transition to an arginine-rich histone is accompanied by incorporation of tritium-labeled arginine, hence reflects synthesis of new protein. Comparison of the timing of arginine and thymidine incorporation, and independent measurements of DNA, show that in contrast to the case of premitotic chromosome duplication, the histone synthesis in the spermatid is unaccompanied by DNA synthesis. During the initial histone change, fine filaments are formed within the nucleus, which aggregate to form lamellae. This fine structure is lost during maturation of the sperm.

Meiotin-1: the meiosis readiness factor?

Meiotin-1 is a protein found in developing microsporocytes of Lilium longiflorum, and immunological assays indicate that cognates exist in both mono- and dicotyledonous plants. Its temporal and spatial expression pattern, coupled with its unusual distribution in chromatin and the properties it shares with histone H1, encourages speculation that it is involved in regulating meiotic chromatin structure. Molecular analyses provide support for the hypothesis that meiotin-1 arose from histone H1 by an exon shuffling mechanism, as meiotin-1 is an H1-like protein that lacks the amino-terminal domain shared by H1 molecules. We have proposed that meiotin-1 serves to limit chromatin condensation in order to foster the unique cytological and molecular events which occur during meiotic prophase. As such, meiotin-1 fits the role of a 'meiosis readiness factor', and its accumulation to a threshold level may commit mitotically dividing progenitor cells to differentiate into meiocytes.

The effects of polyploidy on meiotic duration and pollen development in cereal anthers

The anther is useful for the study of development because it provides comparisons between mitotic and meiotic divisions; between nuclear behaviour during the sporophyte and gametophyte stages; and between cell cycles and divisions involving cells with different nuclear DNA contents. The duration of the period from the immediately premeiotic mitosis until the start of leptotene at 20 °C was estimated to be about 48 h in hexaploid Chinese Spring wheat ( Triticum aestivum ), and about 42 h in diploid Petkus Spring rye ( Secale cereale ). Comparisons of the durations of meiosis and of pollen maturation in wheat, in rye and in Triticale genotypes showed that in all three the durations of these stages of development decreased as ploidy level increased. Within each ploidy level, genotypes with higher nuclear DNA content had longer meiotic durations. Differences in both meiotic duration and pollen maturation resulted from proportional differences in the duration of all component stages. These results obtained from comparisons of a closely related group of species in the Gramineae are similar to the results obtained previously (Bennett 1971) from comparisons of plant species taken from widely different families. Data in animals showing positive linear relationship between meiotic duration and the duration of spermatogenesis are collected. Possible causes of the faster rates of development during meiosis and pollen maturation in polyploid cereal species, and of the constant proportions between the durations of all their constituent stages, are discussed.

The duration of meiosis in pollen mother cells of wheat, rye andTriticale

The duration of meiosis and its stages at 20 °C has been determined in wheat (Triticum aestivum2n= 6x= 42), in rye (Secale cereale2n= 14) and inTriticale(2n= 8x= 56) by sampling methods and by timing the intervals between the pre-meiotic DNA synthesis and meiotic stages following the incorporation of tritiated thymidine. The results from all the methods used were in general agreement. Meiosis takes about 24 h in wheat, 21 h inTriticaleand about 51 h in rye. The lengths of the meiotic stages relative to that of the division correspond reasonably well in the three forms studied but zygotene and pachytene were much longer in rye than in wheat andTriticale.

THE RELATION BETWEEN DNA SYNTHESIS AND CHROMOSOME STRUCTURE AS RESOLVED BY X-RAY DAMAGE

Vicia faba root tip cells were treated for short periods with tritiated thymidine, either immediately before or after exposure of roots to x-rays, and autoradiograph preparations were analysed in an attempt to test the hypothesis that chromatid type (B') aberrations are induced only in those chromosome regions that have synthesized DNA prior to x-irradiation, whereas chromosome type (B'') aberrations are induced only in unduplicated chromosome regions. Studying the relation between presence or absence of label at loci involved in aberrations, in cells irradiated at different development stages, and the pattern of labelling in cells carrying both types of aberration leads to the conclusion that B'' aberrations are induced only in unreplicated chromosome regions. Following replication, only B' aberrations are induced, but these aberrations are also induced in chromosome regions preparing to incorporate DNA. It is suggested that the doubled response of the chromosome to x-rays prior to DNA incorporation might reflect a physical separation of replicating units prior to replication. The aberration yields in damaged cells which were irradiated in G₁S, and early G₂ were in the ratio of 1.0:2.0:3.2. The data indicate that the increased yield of B' in early G₂ relative to S cells may be a consequence of changes in the spatial distribution of the chromosomes within the nucleus.

Correlations between gametophytic (pollen) and sporophytic (seed) generations for polyunsaturated fatty acids in oilseed rape Brassica napus L

Lipids were extracted from the diploid seed and haploid pollen of Brassica napus L. Two fractions of pollen lipids, namely the diploid-specified pollen-coat and the haploid-specified internal cytoplasmic lipids were obtained. Significant correlations exist between pollen and seed generations for linoleic (18∶2) and linolenic (18∶3) acids. In pollen internal storage lipids, the level of 18∶3 is positively correlated and the level of 18∶2 is negatively correlated with the level of 18∶3 in seed lipids. Evidence is presented that strongly supports the hypothesis that lipid biosynthesis occurs within the pollen and that synthesis is specified by haploid genes. These data support the concept of pollen selection, so that selecting among living pollen grains for superior individuals has potential as a new plant breeding tool for improving seed oil quality.

Viability of rat spermatogenic cells in vitro is facilitated by their coculture with Sertoli cells in serum-free hormone-supplemented medium

Spermatogenic cells from 20- to 35-day-old rats were grown in vitro in the presence of Sertoli cells maintained in serum-free hormone/growth factor-supplemented medium and alternating high/low concentrations of follicle-stimulating hormone in the medium. In cell reaggregation experiments, spermatogenic cells reassociate with Sertoli cells but not with peritubular cells or cell-free substrate. Autoradiographic experiments using [3H]thymidine as a labeled precursor for DNA synthesis show that spermatogonia and preleptotene spermatocytes, connected by cytoplasmic bridges, have a synchronous S phase. [3H]Thymidine-labeled preleptotene spermatocytes progress until later stages of meiotic prophase. Time-lapse cinematographic studies of Sertoli/spermatogenic cell cocultures show three major movement patterns. While Sertoli cell cytoplasmic processes between adjacent cells display tensional forces, spermatogonia are engaged in oscillatory cell movements different from the nuclear rotation observed in meiotic prophase spermatocytes. Results of this study show that the proliferation of premeiotic cells and the differentiation of meiotic prophase cells do occur in vitro in association with Sertoli cells maintained in a medium that allows differentiated cell functions.

Cytophotometric study of nuclear differentiation during pollen development in Tradescantia paludosa

During pollen development the dry weight, total protein, histone, DNA, arginine, and lysine content were analysed by cytophotometric methods in partially isolated nuclei. The amount of analysed substances increased from the end of the meiosis to the mitosis of the microspores to the double of the initial values. After mitosis the ratio histone/DNA remained almost unchanged in both vegetative and generative nuclei. On the other hand a large difference in the ratio non-histone protein/DNA could be observed, the vegetative nucleus containing more non-histone protein than the generative nucleus. The rate of RNA synthesis being higher in the vegetative nuclei, these non-histone proteins may have some function in nuclear activation. The DNA of the generative nucleus is duplicated before anthesis, whilst in the vegetative nucleus the DNA content remains constant.

Meiosis in protists. Some structural and physiological aspects of meiosis in algae, fungi, and protozoa

Images

The time and duration of preleptotene chromosome condensation stage in Lilium hybrid cv. Black Beauty

A period of chromosome condensation followed by decondensation occurs between premeiotic interphase and leptotene inLiliumhybrid cv. ‘Black Beauty’. The duration of this period was estimated to be about 1.16 days in pollen mother cells (p. m. cs) of plants grown at 20 °C. Preleptotene chromosome condensation and contraction also occurred in embryo sac mother cells (e. m. cs) of ‘Black Beauty’. However, the maximum degree of chromosome contraction at the preleptotene condensation stage was much greater in p. m. cs than in e. m. cs. Moreover, the preleptotene condensation and decondensation stage was apparently shorter in e. m. cs (about 0.7 days) than in p. m. cs. The developmental behaviour of p. m. c. chromosomes during pre­leptotene condensation and decondensation was essentially the same as that described by Walters (1970, 1972) for the corresponding stages inLilium longiflorumcv. ‘Croft’. A brief illustrated description is given of the appearance of p. m. cs at various stages of preleptotene chromosome condensation in both methylene blue stained anther sections, and in Feulgen stained anther squashes. ‘Black Beauty’ p. m. cs entered preleptotene chromosome condensation stage from G2 of premeiotic interphase having completed DNA synthesis. All the p. m. cs within an anther loculus underwent preleptotene chromo­some condensation stage with a degree of synchrony not less than that found at early first meiotic prophase. At maximum chromosome contration the diploid chromosome number (2n= 24) could often be counted, but no evidence of association of chromosomes was seen. At this stage the appearance of the chromosomes was indistinguishable from that of late prophase or prometaphase chromosomes at mitosis. Thereafter chromo­somes underwent decondensation and elongation and eventually passed into leptotene without first entering any other stage. The nature and possible significance of preleptotene chromosome condensation and decondensation are discussed. The appearance and behaviour of chromosomes at preleptotene condensation stage is the same as that displayed by chromosomes at mitotic prophase. Similarly, the appearance and behaviour of chromosomes at preleptotene decondensation is indistinguishable from that normally seen at mitotic telophase. It is suggested, therefore, that preleptotene condensation and decondensation represent a true mitotic reversion in which metaphase and anaphase are omitted. In normalLiliumgenotypes meiosis is initiated by p. m. cs at G2 of premeiotic interphase. In ‘Black Beauty’, however, the control regu­lating the initiation of meiosis apparently is effective later in the cell cycle and acts only after p. m. cs have entered mitotic prophase. It is concluded that preleptotene chromosome condensation stage has no effect on normal meiotic chromosome pairing behaviour and probably has no significant function. Two parallel threads (chromatids) are visible in chromosomes at preleptotene condensation stage while, in chromo­somes at leptotene, invariably only a single thread is seen. It is suggested, therefore, that chromosomes at maximum preleptotene condensation can­not enter a meiotic sequence without first undergoing a within-chromosomal reorganization of their chromatin, and that preleptotene chromo­some decondensation stage represents such a reorganization.

The time and duration of female meiosis in Lilium

The time and duration of meiosis in ovules and anthers was estimated in plants of two Lilium hybrids (cultivars ‘Sonata’ and ‘Black Beauty’) grown under controlled conditions. Within each flower bud meiosis did not start in the embryo sac mother cell (e.m.c.) until about the time when meiosis in pollen mother cells (p.m.cs) was ended. In both cultivars meiosis lasted about 50% longer in e.m.cs than in p.m.cs. Thus, at a mean temperature of 20 ± 1 °C meiosis in ‘Sonata’ took 7.5 days in p.m.cs and 10.5 days in e.m.cs, while in ‘Black Beauty’ it took 10.5 days in p.m.cs and 16.0 days in e.m.cs.

Meiotic DNA synthesis during mouse spermatogenesis

The incorporation of radioactivity into various cells in the sequence of spermatogenesis was measured by preparing highly purified spermatozoan nuclei from the cauda epididymidis of mice at daily intervals after injection of (3H)thymidine. The stages of differentiation of these sperm at the time of thymidine administration were calculated from the kinetics of spermatogenesis. The procedure for purification of sperm nuclei included sonication, mechanical shearing, and treatment with trypsin, DNase, Triton X-100, 2M NaC1, and sodium dodecyl sulfate. DNA was isolated from these nuclei by treatment with dithiothreitol and pronase, followed by phenol extraction and ethanol precipitation. The levels of radioactivity in the epididymal sperm head preparations were low (less than 13 dpm/mouse) for 27 days after injection, and then rose dramatically to over 4 times 104 dpm/mouse. Further experiments demonstrated that the 11 dpm of 3H radioactivity contained in sperm heads at 21 or 26 days after injection of (3H)TdR was significantly above background and contamination levels from other cells or other sources. Most of the radioactivity was in the sperm DNA and represented incorporation of tritium from (3H)TdR into the nuclear DNA of meiotic cells at 0.002 percent of the rate of incorporation into S-phase cells. Little, if any, (3H)TdR was incorporation into the DNA of spermatids. The levels of DNA synthesis during the meiotic prophase in the mouse appear to be much lower than those reported for other organisms.

Chromosome replication during meiosis: identification of gene functions required for premeiotic DNA synthesis

Recent comparisons of chromosome replication in meiotic and mitotic cells have revealed significant differences in both the rate and pattern of DNA synthesis during the final duplication preceding meiosis. These differences suggested that unique gene functions might be required for premeiotic replication that were not necessary for replication during growth. To provide evidence for such functions, we isolated stage-specific mutants in the yeast Saccharomyces cerevisiae which permitted vegetative replication but blocked the round of replication before meiosis. The mutants synthesized carbohydrate, protein, and RNA during the expected interval of premeiotic replication, suggesting that their lesions preferentially affected synthesis of DNA. The mutations blocked meiosis, as judged by a coincident inhibition of intragenic recombination and ascospore formation. The lesions were characterized as recessive nuclear genes, and were designated mei-1, mei-2, and mei-3; complementation indicated that the relevant gene products were not identical.