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A zygote (, ) is a
eukaryotic Eukaryotes () are organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy (biology), tax ...

eukaryotic
cell formed by a
fertilization Fertilisation or fertilization (see American and British English spelling differences#-ise.2C -ize .28-isation.2C -ization.29, spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes ...

fertilization
event between two
gamete A gamete ( /ˈɡæmiːt/; from Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the foll ...
s. The zygote's
genome In the fields of molecular biology and genetics, a genome is all genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The genome includes both the genes (the coding regions) and the noncodin ...

genome
is a combination of the DNA in each gamete, and contains all of the genetic information necessary to form a new individual organism. In multicellular organisms, the zygote is the earliest developmental stage. In humans and most other
anisogamous Anisogamy (also called heterogamyHeterogamy is a term applied to a variety of distinct phenomena in different scientific domains. Usually having to do with some kind of difference, "hetero", in reproduction, "gamy". See below for more specific sen ...

anisogamous
organisms, a zygote is formed when an
egg cell The egg cell, or ovum (plural ova), is the female reproductive Reproduction (or procreation or breeding) is the biological process by which new individual organisms – "offspring" – are produced from their "parent" or parents. Reproducti ...
is fertilized by a
sperm cell Sperm is the male reproductive cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, "female" reproductive cell and a smaller, "male" one). Animals produce motile sperm with a tail known as a flagellu ...

sperm cell
. In single-celled organisms, the zygote can divide asexually by
mitosis In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical proce ...

mitosis
to produce identical offspring. German zoologists
Oscar Oscar, OSCAR, or The Oscar may refer to: People * , an Irish- and English-language name also used in other languages; the article includes the names Oskar, Oskari, Oszkár, Óscar, and other forms. * , legendary figure, son of Oisín and gran ...
and
Richard Hertwig Richard Wilhelm Karl Theodor Ritter von Hertwig (23 September 1850 in Friedberg, Hesse Hesse (, , ) or Hessia (, ; german: Hessen ), officially the State of Hessen (german: links=no, Land Hessen), is a state of the Germany, Federal Republic of ...
made some of the first discoveries on animal zygote formation in the late 19th century.


Fungi

In fungi, the sexual fusion of haploid cells is called
karyogamy Karyogamy is the final step in the process of fusing together two haploid eukaryotic cells, and refers specifically to the fusion of the two cell nucleus, nuclei. Before karyogamy, each haploid cell has one complete copy of the organism's genome. In ...

karyogamy
. The result of karyogamy is the formation of a diploid cell called the zygote or zygospore. This cell may then enter meiosis or mitosis depending on the life cycle of the species.


Plants

In plants, the zygote may be
polyploid Polyploidy is a condition in which the biological cell, cells of an organism have more than two paired (Homologous chromosome, homologous) sets of chromosomes. Most species whose cells have Cell nucleus, nuclei (eukaryotes) are diploid, meaning t ...
if fertilization occurs between meiotically unreduced gametes. In
land plants The Embryophyta (), or land plants, are the most familiar group of green plant Plants are predominantly photosynthetic Photosynthesis is a process used by plants and other organisms to Energy transformation, convert light energy into ...

land plants
, the zygote is formed within a chamber called the
archegonium 240px, Diagram of archegonium anatomy An archegonium (pl: archegonia), from the ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 3 ...

archegonium
. In seedless plants, the archegonium is usually flask-shaped, with a long hollow neck through which the sperm cell enters. As the zygote divides and grows, it does so inside the archegonium.


Humans

In human fertilization, a released ovum (a haploid secondary
oocyte An oocyte (, ), oöcyte, ovocyte, or rarely ocyte, is a female gametocyte A gametocyte is a eukaryotic Eukaryotes () are organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any indiv ...
with replicate chromosome copies) and a haploid
sperm Sperm is the male reproductive Cell (biology), cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce motile sperm with a tail known as ...

sperm
cell (
male Male (symbol: ♂) is the sex of an organism that produces the gamete (sex cell) known as sperm, which fuses with the larger female gamete, or ovum, in the process of fertilization. A male organism cannot sexual reproduction, reproduce sexually ...

male
gamete)—combine to form a single 2n diploid cell called the zygote. Once the single sperm fuses with the oocyte, the latter completes the division of the second meiosis forming a haploid daughter with only 23 chromosomes, almost all of the cytoplasm, and the male pronucleus. The other product of meiosis is the second polar body with only chromosomes but no ability to replicate or survive. In the fertilized daughter, DNA is then replicated in the two separate pronuclei derived from the sperm and ovum, making the zygote's chromosome number temporarily 4n diploid. After approximately 30 hours from the time of fertilization, a fusion of the pronuclei and immediate mitotic division produce two 2n diploid daughter cells called
blastomere In biology, a blastomere is a type of cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or religious recluse l ...
s. Between the stages of
fertilization Fertilisation or fertilization (see American and British English spelling differences#-ise.2C -ize .28-isation.2C -ization.29, spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes ...

fertilization
and
implantation Implantation may refer to: * Implantation (human embryo), in which the human embryo adheres to the wall of the uterus * Implant (medicine), insertion of implants * Endometrial transplantation, as part of the Endometriosis, theory of retrograde mens ...
, the developing human is a ''preimplantation
conceptus A conceptus (Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the Roman Rep ...

conceptus
''. There is some dispute about whether this conceptus should no longer be referred to as an
embryo An embryo is the early stage of development of a multicellular organism A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms ar ...

embryo
, but should now be referred to as a
proembryoProembryo, or pro-embryo, in a flowering plant The flowering plants, also known as Angiospermae (), or Magnoliophyta (), are the most diverse group of Embryophyte, land plants, with 64 Order(biology), orders, 416 Family (biology), families, approx ...
, which is the terminology that traditionally has been used to refer to plant life. Some ethicists and legal scholars make the argument that it is incorrect to call the conceptus an ''embryo'' because it will later differentiate into both intraembryonic and extraembryonic tissues, and can even split to produce multiple embryos (identical twins). Others have pointed out that so-called extraembryonic tissues are really part of the embryo's body that are no longer used after birth (much as milk teeth fall out after childhood). Further, as the embryo splits to form identical twins – leaving the original tissues intact – new embryos are generated, in a process similar to that of cloning an adult human. In the US the National Institutes of Health has determined that the traditional classification of pre-implantation embryo is still correct. After fertilization, the conceptus travels down the
oviduct The passageway from the ovaries The ovary is an organ found in the female reproductive system that produces an ovum. When released, this travels down the fallopian tube The Fallopian tubes, also known as uterine tubes, salpinges (singular salp ...
towards the
uterus The uterus (from Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the ...

uterus
while continuing to
divide
divide
mitotically
mitotically
without actually increasing in size, in a process called
cleavage Cleavage may refer to: Science * Cleavage (crystal), in mineralogy and materials science, a process of splitting a crystal * Cleavage (geology), the foliation perpendicular to stress as a result of ductile deformation * Cleavage (embryo), in embr ...
. After four divisions, the conceptus consists of 16 blastomeres, and it is known as the
morula A morula (Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the Roman Republic, it bec ...
. Through the processes of compaction, cell division, and blastulation, the conceptus takes the form of the
blastocyst The blastocyst is a structure formed in the early development of mammal Mammals (from Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally spoken in th ...

blastocyst
by the fifth day of development, just as it approaches the site of implantation. When the blastocyst hatches from the
zona pellucida The zona pellucida (plural zonae pellucidae, also egg coat or pellucid zone) is a glycoprotein Glycoproteins are protein Proteins are large s and s that comprise one or more long chains of . Proteins perform a vast array of functions ...

zona pellucida
, it can implant in the endometrial lining of the uterus and begin the embryonic stage of development. The human zygote has been genetically edited in experiments designed to cure inherited diseases.


Reprogramming to totipotency

The formation of a
totipotent Cell potency is a cell's ability to differentiate into other cell types. The more cell types a cell can differentiate into, the greater its potency. Potency is also described as the gene activation potential within a cell, which like a continuum, ...
zygote with the potential to produce a whole organism depends on
epigenetic In biology, epigenetics is the study of heritability, heritable phenotype changes that do not involve alterations in the DNA sequence. The Ancient Greek, Greek prefix ''wikt:epi-, epi-'' ( "over, outside of, around") in ''epigenetics'' implies f ...
reprogramming.
DNA demethylation For molecular biology in mammals, DNA demethylation causes replacement of 5-methylcytosine (5mC) in a DNA sequence by cytosine (C) (see figure of 5mC and C). DNA demethylation can occur by an active process at the site of a 5mC in a DNA sequence ...
of the paternal
genome In the fields of molecular biology and genetics, a genome is all genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The genome includes both the genes (the coding regions) and the noncodin ...

genome
in the zygote appears to be an important part of epigenetic reprogramming. In the paternal genome of the mouse, demethylation of DNA, particularly at sites of methylated cytosines, is likely a key process in establishing totipotency. Demethylation involves the processes of
base excision repair Base excision repair (BER) is a cellular mechanism, studied in the fields of biochemistry Biochemistry or biological chemistry, is the study of es within and relating to living s. A sub-discipline of both and , biochemistry may be divided ...
and possibly other DNA- repair- based mechanisms.


In other species

A ''
Chlamydomonas alt=, 329x329px, Cross section of a ''Chlamydomonas reinhardtii'' cell ''Chlamydomonas'' is a genus of green algae consisting of about 325 speciesSmith, G.M. 1955 ''Cryptogamic Botany Volume 1. Algae and Fungi'' McGraw-Hill Book Company Inc a ...

Chlamydomonas
'' zygote contains chloroplast DNA (cpDNA) from both parents; such cells are generally rare, since normally cpDNA is inherited uniparentally from the mt+ mating type parent. These rare biparental zygotes allowed mapping of chloroplast genes by recombination.


In protozoa

In the
amoeba An amoeba (; less commonly spelt ameba or amœba; plural ''am(o)ebas'' or ''am(o)ebae'' ), often called an amoeboid, is a type of cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Cl ...

amoeba
, reproduction occurs by cell division of the parent cell: first the nucleus of the parent divides into two and then the cell membrane also cleaves, becoming two "daughter" Amoebae.


See also

* Breastfeeding and fertility *
Fertilization Fertilisation or fertilization (see American and British English spelling differences#-ise.2C -ize .28-isation.2C -ization.29, spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes ...

Fertilization
*
ProembryoProembryo, or pro-embryo, in a flowering plant The flowering plants, also known as Angiospermae (), or Magnoliophyta (), are the most diverse group of Embryophyte, land plants, with 64 Order(biology), orders, 416 Family (biology), families, approx ...


References

{{Authority control Developmental biology Reproductive system Human female reproductive system