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Zygote
Zygote PM Wassarman, Mount Sinai School of Medicine, New York, NY, USA © 2013 Elsevier Inc. All rights reserved.
Glossary Fertilization The union of male and female germ cells, sperm and egg, respectively, to form a single cell that will ultimately give rise to a new individual that exhibits all of the characteristics of the species. Meiosis A process specific to germ cells that involves exchange of genetic material (crossing over and recombination of homologous chromosomes) as well as two reductive nuclear divisions, taking oocytes in females and spermatocytes in males from a diploid (contains homologous pairs of chromosomes) to a haploid (contains single copies of each chromosome) state. Mitosis A process that involves at least four steps, called ‘prophase’, ‘metaphase’, ‘anaphase’, and ‘telophase’, during which chromosomes are duplicated and the cell doubles in mass and volume. This is followed by cellular
Fertilization, Meiosis, and Mitosis The process of fertilization was first recognized and described in 1876, simultaneously by the Swiss biologist Herman Fol working with starfish and the German biologist Oscar Hertwig working with sea urchins. The fertilized egg is a single cell produced by fusion of female and male germ cells, unferti lized egg and sperm, respectively. Since germ cells undergo meiotic divisions to a haploid state (n) during oogenesis and spermatogenesis, fusion of unfertilized egg and sperm (fertili zation) restores a diploid (2n) number of chromosomes to the fertilized egg (46 chromosomes in humans, 23 pairs, and 40 chromosomes in mice, 20 pairs). For mammalian eggs, the first meiotic division (separation of homologous chromosomes and the formation of the first polar body) occurs at the time of ovulation, and the second meiotic division of the egg, with separation of chromatids (each chromosome consists of two chromatids) and the forma tion of the second polar body, occurs shortly after fusion with sperm. Fertilization results in activation of the egg and, at an appropriate time, the fertilized egg begins to divide mitotically, eventually giving rise to a multicellular organism that exhibits all of the characteristics of the species.
division (cytokinesis) producing two new cells that have identical properties of the original cell. Oogenesis A process of cellular differentiation specific to females that begins during fetal development with the formation of oogonia (mitotic), that in turn give rise to oocytes (meiotic), which ends during adulthood with two nuclear divisions (in mammals, one during ovulation and one as a consequence of fertilization) and the formation of unfertilized eggs (haploid). For female mammals, all germ cells present in the ovary at birth are nongrowing oocytes arrested in meiosis. Spermatogenesis A process of cellular differentiation specific to males by which spermatogonia (mitotic) give rise to spermatocytes (meiotic), that by two reductive nuclear divisions yield spermatids (haploid), which ends with the formation of mature spermatozoa capable of motility and species-restricted fertilization.
fertilized egg called a zygote (Figure 1). The timing of nuclear formation varies greatly from one species to another; for exam ple, it takes less than 1 h in sea urchins and more than 12 h in mice. In fact, in mice, pronuclei approach each other, but do not actually fuse together to become a diploid nucleus. Rather, pronuclear membranes disappear and chromosomes assemble on a spindle composed of microtubules. Replication of DNA
Zona pellucida
Pronuclei
Fertilized human egg
Pronuclei and Zygote Formation Nuclei contributed by unfertilized egg and sperm are called ‘female and male pronuclei’, respectively. In mice, the female pronucleus forms at about 7.5 h and the male pronucleus at about 5.5 h after fusion of unfertilized egg and sperm. The two pronuclei must come together near the center of the fertilized egg and form a single diploid nucleus (process of karyogamy). Only when the pronuclei become a single nucleus is the
400
Figure 1 Photomicrograph of a fertilized human egg. The positions of the zona pellucida and pronuclei are indicated. The two pronuclei are clearly seen in the center of the fertilized egg. Modified photomicrograph: Provided by Dr. Richard Sherbahn of the Advanced Fertility Center of Chicago.
Brenner’s Encyclopedia of Genetics, 2nd edition, Volume 7
doi:10.1016/B978-0-12-374984-0.01670-3
Zygote occurs at about 14.5 h after fertilization, as the pronuclei migrate toward the center of the egg. In mice, the first mitotic cleavage division occurs at about 20 h after fertilization, when chromosomes are assembled on a spindle.
Centrioles and Mitochondria In many animals, sperm contribute a centriole (bundles of microtubules) to the fertilized egg and this organelle helps to organize the first mitotic spindle on which the chromosomes are arranged. In this respect, the sperm centriole acts as a microtubule-organizing center (MTOC) in eggs. On the other hand, sperm contribute very few of the large number of mito chondria found in fertilized eggs (less that 0.01%), ensuring that mitochondrial DNA is maternally inherited.
Gene Expression The zygote is inactive with respect to nascent transcription of genomic DNA (synthesis of RNA), although translation of maternal transcripts (synthesis of protein using maternal RNA) takes place. The onset of nascent transcription is delayed until after the first cleavage division in mammals and until after the first 12 cleavage divisions in some nonmammalian species. Presumably, this period of transcriptional inactivity exhibited by the zygote provides time to remodel parental chromosomes.
maternally derived allele of a particular gene is active, whereas, in others, only the paternally derived allele is active. Such behavior is referred to as ‘genetic imprinting’. Some of these genes are absolutely essential for normal development. Apparently, as a result of the nonequivalence of pronuclei, parthenogenetic (bimaternal), gynogenetic (bimaternal), and androgenetic (bipaternal) mammalian zygotes cannot give rise to normal fetuses and live births.
See also: Diploidy; Haploid Number; Meiosis; Mitosis; Spermatogenesis, Mammals.
Further Reading Alberts B, Johnson A, Lewis J, et al. (2008) Molecular Biology of the Cell, 5th edn., pp. 1268. New York, NY: Garland Science. Austin CR and Short RV (1982) Reproduction in Mammals: Germ Cells and Fertilization, vol. 1, 2nd edn., pp. 177, London: Cambridge University Press. Austin CR and Short RV (1982) Reproduction in Mammals: Embryonic and Fetal Development, vol. 2, 2nd edn., pp. 190. London: Cambridge University Press. Gilbert SF (2006) Developmental Biology, 8th edn., pp. 817. Sunderland, MA: Sinauer Associates, Inc. Nagy A, Gertsenstein M, Vintersten K, and Behringer R (2002) Manipulating the Mouse Embryo: A Laboratory Manual, 3rd edn., pp. 764. Cold Spring Harbor, NY: Cold Spring Harbor Press. Wassarman PM and DePamphilis ML (1993) Guide to Techniques in Mouse Development, Methods in Enzymology, vol. 225, pp. 1021. New York: Academic Press.
Genetic Imprinting Relevant Websites In mammals, genomes derived from the unfertilized egg and sperm appear not to be equivalent. In some cases, only the
401
http://en.wikipedia.org – Wikipedia: Zygote
Glossary Fertilization The union of male and female germ cells, sperm and egg, respectively, to form a single cell that will ultimately give rise to a new individual that exhibits all of the characteristics of the species. Meiosis A process specific to germ cells that involves exchange of genetic material (crossing over and recombination of homologous chromosomes) as well as two reductive nuclear divisions, taking oocytes in females and spermatocytes in males from a diploid (contains homologous pairs of chromosomes) to a haploid (contains single copies of each chromosome) state. Mitosis A process that involves at least four steps, called ‘prophase’, ‘metaphase’, ‘anaphase’, and ‘telophase’, during which chromosomes are duplicated and the cell doubles in mass and volume. This is followed by cellular
Fertilization, Meiosis, and Mitosis The process of fertilization was first recognized and described in 1876, simultaneously by the Swiss biologist Herman Fol working with starfish and the German biologist Oscar Hertwig working with sea urchins. The fertilized egg is a single cell produced by fusion of female and male germ cells, unferti lized egg and sperm, respectively. Since germ cells undergo meiotic divisions to a haploid state (n) during oogenesis and spermatogenesis, fusion of unfertilized egg and sperm (fertili zation) restores a diploid (2n) number of chromosomes to the fertilized egg (46 chromosomes in humans, 23 pairs, and 40 chromosomes in mice, 20 pairs). For mammalian eggs, the first meiotic division (separation of homologous chromosomes and the formation of the first polar body) occurs at the time of ovulation, and the second meiotic division of the egg, with separation of chromatids (each chromosome consists of two chromatids) and the forma tion of the second polar body, occurs shortly after fusion with sperm. Fertilization results in activation of the egg and, at an appropriate time, the fertilized egg begins to divide mitotically, eventually giving rise to a multicellular organism that exhibits all of the characteristics of the species.
division (cytokinesis) producing two new cells that have identical properties of the original cell. Oogenesis A process of cellular differentiation specific to females that begins during fetal development with the formation of oogonia (mitotic), that in turn give rise to oocytes (meiotic), which ends during adulthood with two nuclear divisions (in mammals, one during ovulation and one as a consequence of fertilization) and the formation of unfertilized eggs (haploid). For female mammals, all germ cells present in the ovary at birth are nongrowing oocytes arrested in meiosis. Spermatogenesis A process of cellular differentiation specific to males by which spermatogonia (mitotic) give rise to spermatocytes (meiotic), that by two reductive nuclear divisions yield spermatids (haploid), which ends with the formation of mature spermatozoa capable of motility and species-restricted fertilization.
fertilized egg called a zygote (Figure 1). The timing of nuclear formation varies greatly from one species to another; for exam ple, it takes less than 1 h in sea urchins and more than 12 h in mice. In fact, in mice, pronuclei approach each other, but do not actually fuse together to become a diploid nucleus. Rather, pronuclear membranes disappear and chromosomes assemble on a spindle composed of microtubules. Replication of DNA
Zona pellucida
Pronuclei
Fertilized human egg
Pronuclei and Zygote Formation Nuclei contributed by unfertilized egg and sperm are called ‘female and male pronuclei’, respectively. In mice, the female pronucleus forms at about 7.5 h and the male pronucleus at about 5.5 h after fusion of unfertilized egg and sperm. The two pronuclei must come together near the center of the fertilized egg and form a single diploid nucleus (process of karyogamy). Only when the pronuclei become a single nucleus is the
400
Figure 1 Photomicrograph of a fertilized human egg. The positions of the zona pellucida and pronuclei are indicated. The two pronuclei are clearly seen in the center of the fertilized egg. Modified photomicrograph: Provided by Dr. Richard Sherbahn of the Advanced Fertility Center of Chicago.
Brenner’s Encyclopedia of Genetics, 2nd edition, Volume 7
doi:10.1016/B978-0-12-374984-0.01670-3
Zygote occurs at about 14.5 h after fertilization, as the pronuclei migrate toward the center of the egg. In mice, the first mitotic cleavage division occurs at about 20 h after fertilization, when chromosomes are assembled on a spindle.
Centrioles and Mitochondria In many animals, sperm contribute a centriole (bundles of microtubules) to the fertilized egg and this organelle helps to organize the first mitotic spindle on which the chromosomes are arranged. In this respect, the sperm centriole acts as a microtubule-organizing center (MTOC) in eggs. On the other hand, sperm contribute very few of the large number of mito chondria found in fertilized eggs (less that 0.01%), ensuring that mitochondrial DNA is maternally inherited.
Gene Expression The zygote is inactive with respect to nascent transcription of genomic DNA (synthesis of RNA), although translation of maternal transcripts (synthesis of protein using maternal RNA) takes place. The onset of nascent transcription is delayed until after the first cleavage division in mammals and until after the first 12 cleavage divisions in some nonmammalian species. Presumably, this period of transcriptional inactivity exhibited by the zygote provides time to remodel parental chromosomes.
maternally derived allele of a particular gene is active, whereas, in others, only the paternally derived allele is active. Such behavior is referred to as ‘genetic imprinting’. Some of these genes are absolutely essential for normal development. Apparently, as a result of the nonequivalence of pronuclei, parthenogenetic (bimaternal), gynogenetic (bimaternal), and androgenetic (bipaternal) mammalian zygotes cannot give rise to normal fetuses and live births.
See also: Diploidy; Haploid Number; Meiosis; Mitosis; Spermatogenesis, Mammals.
Further Reading Alberts B, Johnson A, Lewis J, et al. (2008) Molecular Biology of the Cell, 5th edn., pp. 1268. New York, NY: Garland Science. Austin CR and Short RV (1982) Reproduction in Mammals: Germ Cells and Fertilization, vol. 1, 2nd edn., pp. 177, London: Cambridge University Press. Austin CR and Short RV (1982) Reproduction in Mammals: Embryonic and Fetal Development, vol. 2, 2nd edn., pp. 190. London: Cambridge University Press. Gilbert SF (2006) Developmental Biology, 8th edn., pp. 817. Sunderland, MA: Sinauer Associates, Inc. Nagy A, Gertsenstein M, Vintersten K, and Behringer R (2002) Manipulating the Mouse Embryo: A Laboratory Manual, 3rd edn., pp. 764. Cold Spring Harbor, NY: Cold Spring Harbor Press. Wassarman PM and DePamphilis ML (1993) Guide to Techniques in Mouse Development, Methods in Enzymology, vol. 225, pp. 1021. New York: Academic Press.
Genetic Imprinting Relevant Websites In mammals, genomes derived from the unfertilized egg and sperm appear not to be equivalent. In some cases, only the
401
http://en.wikipedia.org – Wikipedia: Zygote