User:BlakeForrest/Zygote

Zygote

Zygote
Zygote formation: egg cell after fertilization with a sperm. The male and female pronuclei are converging, but the genetic material is not yet united.

A zygote (from Ancient Greek ζυγωτός (zygōtós) 'joined, yoked', from ζυγοῦν (zygoun) 'to join, to yoke') is a eukaryotic cell formed by a fertilization event between two gametes. The zygote's genome is a combination of the DNA in each gamete, and contains all of the genetic information of a new individual organism. There are some important aspects of zygotes like fungi (fusion of haploid cells), plants (polyploid), and totipotency (produce whole organisms).

In multicellular organisms, the zygote is the earliest developmental stage. In humans and most other anisogamous organisms, a zygote is formed when an egg cell and sperm cell come together to create a new unique organism. In single-celled organisms, the zygote can divide asexually by mitosis to produce identical offspring. Mitosis is when a single cell divides into two identical daughter cells, each having the same number of chromosomes as the parent cell.  This process is important for development as it produces new cells and replaces old or damaged cells. The daughter cells resulting from mitosis are called diploid cells means they contain a chromosomes from both the mother and father, making the genetic information passed down from each parent is given to its offspring.

During mitosis, the genetic material in the parent cell is replicated and separated into two identical sets, one for each daughter cell, including the condensation of chromosomes, the formation of a spindle apparatus, and the alignment and segregation of chromosomes.

The process of mitosis is critical for normal development and homeostasis in the body. It allows for the growth and repair of tissues, the production of new blood cells, and the formation of gametes in the reproductive system. It also plays a crucial role in the prevention of genetic abnormalities by ensuring that each daughter cell receives an exact copy of the parent cell's genetic material.

German zoologists Oscar and Richard Hertwig made some of the first discoveries on animal zygote formation in the late 19th century.

Humans

Main articles: Development of the human body and Human fertilization

In human fertilization, a released ovum (a haploid secondary oocyte with replicate chromosome copies) and a haploid sperm cell (male gamete) combine to form a single 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 blastomeres.

Between the stages of fertilization and implantation, the developing embryo is sometimes termed as a preimplantation-conceptus. This stage has also been referred to as the pre-embryo in legal discourses including relevance to the use of embryonic stem cells. 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 fallopian tube towards the uterus while continuing to divide without actually increasing in size, in a process called cleavage. After four divisions, the conceptus consists of 16 blastomeres, and it is known as the morula. Through the processes of compaction, cell division, and blastulation, the conceptus takes the form of the blastocyst by the fifth day of development, just as it approaches the site of implantation. When the blastocyst hatches from the zona pellucida, it can implant in the endometrial lining of the uterus and begin the gastrulation stage of embryonic development.

The human zygote has been genetically edited in experiments designed to cure inherited diseases.

Monozygotic (one egg) twins are identical twins form when one egg has been fertilized by one sperm and the zygote splits into two. This happens at the very earliest stage of development when the zygote is no more than a cluster of a few cells. Dividing this early in conception means that each baby has the same genetic information as the other.

Zygote cleavage is the process of cell division that occurs in the zygote, which is the single cell formed by the fusion of sperm and egg during fertilization. Cleavage begins shortly after fertilization and involves a rapid series of cell divisions that divide the zygote into smaller and smaller cells, called blastomeres.

Dizygotic twins, also known as fraternal twins, are twins that develop from two separate eggs fertilized by two different sperm. They are no more genetically similar than any other siblings and can be of different sexes. Dizygotic twins are more common than monozygotic (identical) twins and their occurrence is influenced by factors such as maternal age and genetics.

Infertility & IVF

Demonstration of IVF

Infertility can be caused by problems with the formation or function of the zygote.  If the egg or sperm is damaged, or if there are genetic abnormalities, it may not be able to form a viable zygote or may result in a zygote that cannot develop properly. This can lead to infertility or miscarriage.  In vitro fertilization (IVF) is a procedure that helps couples struggling with infertility to conceive a child. The process involves fertilizing an egg with a sperm outside of the woman's reproductive tract, sometimes in a laboratory dish or petri dish. This technology is also commonly known as "test tube baby" technology, although the term "test tube" is not entirely accurate. The process involves combining the egg and sperm in a special solution that mimics the conditions of the female reproductive tract. If fertilization occurs, a zygote is formed and undergoes several rounds of cell division, forming an embryo that is typically cultured in the laboratory for several days before being transferred to the woman's uterus.

Infertility can be caused by various factors such as hormonal irregularities, structural anomalies (including obstructions in the fallopian tubes or abnormalities in the uterus), problems with ovulation, and advancing age beyond 35.

Creating a zygote in a laboratory dish has revolutionized the field of reproductive medicine, making it possible for couples with fertility problems to conceive a child. Prior to IVF, couples with infertility had few options for having a biological child.

The zygote is typically formed in the woman's fallopian tubes during natural conception, in IVF, it can be artificially created and cultured in a petri dish in the laboratory. Doctors can control the conditions when fertilization occurs, increasing the chances of successful conception. Doctors can also screen the embryos for genetic abnormalities before implantation, reducing the risk of inherited diseases.

Fungi

In fungi, the sexual fusion of haploid cells is called 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 if fertilization occurs between meiotically unreduced gametes.

In land plants, the zygote is formed within a chamber called the 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.

Reprogramming to totipotency

The formation of a totipotent zygote with the potential to produce a whole organism depends on epigenetic reprogramming. DNA demethylation of the paternal 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 and possibly other DNA- repair- based mechanisms.

In other species

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

BlakeForrest/Zygote
Image of an Amoeba cell

In the 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.