What is a Zygote?
While answering the question what is a zygote, one can state that the zygote is a eukaryotic cell formed by the fertilization of two gametes. Thus, an egg cell or ovum fertilized by a sperm cell is a zygote. Zygote formation in human or another multicellular organism is the first developmental stage in the process of reproducing a complete individual. This happens as a given zygote is totipotent i.e. it has the potential to completely create a new offspring owing to all the necessary genetic information for the making of a complete organism.
It contains the genetic material of both the parents in cases of sexual reproduction except in cases of single-celled organisms where the zygote divides asexually by mitosis to produce identical offspring. Although usually diploid in nature, it can be a polyploid in certain cases of plant reproduction where the gametes are unreduced meiotically.
Zygote Development
Zygote meaning as stated above is a reference to a cell right after fertilization of the gametes. This cell or zygote will further divide into a mass of cells known as blastocyst which will transform into an embryo which will further transform into a fetus and over a period of months the fetus will transform into an offspring. Generally, the division of the zygote occurs mitotically, but there are certain exceptions in fungi where the division can occur meiotically depending on the life cycle. During this mitotic division, the zygote does not change in size but splits into different cells via cleavage. Initially, the cleavage of the zygote leads to the two-cell stage, then to the four-cell stage. During these stages, they are known as blastomeres. These blastomeres are divided into a large number of cells then it is known as a blastocyst. This is clearly shown in the zygote picture moving further.
Taking a well-known example of zygote formation in human it is understandable that the sperm cell has to penetrate the egg cell for fertilisation. The fertilisation event that leads to zygote formation can happen through either sexual intercourse or medically assisted fertilisation such as in-vitro fertilisation (IVF). Then the DNA in the two haploid nuclei which come from the parents now known as pronuclei, undergo replication temporarily taking the number of chromosomes to 4n diploid in a zygote. Over a period of 30 hours from fertilization during zygote development the fusion of the two pronuclei takes place and there is an immediate mitotic division of the zygote into two 2n diploid cells known as blastomeres.
The zygote picture is clearly given below along with the cycle of stages from zygote development to blastocyst formation:
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There is an interesting fact about zygote meaning. The etymology of zygote meaning has its roots in the Greek language where it means ‘to join’. This zygote meaning correctly describes the zygote formation after the fertilization event and the fusion of the pronuclei during zygote development.
Events Following Zygote Formation in Human and Zygote Development
Scientific studies employ the use of zygotes of different species to understand the entire process of regeneration and reproduction. A very common zygote example is that of a rat that is used in scientific research. The zygote example of rats helps in understanding the process of zygote formation in human and zygote development because these processes are quite similar in both.
The cleavage of the zygote occurs between the events of fertilisation and implantation. While the zygote moves down the uterus for implantation it continues to divide mitotically into different cells leading to the formation of blastomeres and then to blastocyst, as is clear from the zygote picture given above. The period of zygote formation and zygote development with continuous division lasts for four days. This is known as the germinal period and lasts up to fourteen days. After fourteen days, the embryonic period begins. The embryonic period lasts from two weeks to eight weeks after conception where the developing organism is known as an embryo. Following the embryonic period is the fetal period where the embryo now transformed into fetus develops until the birth of the child.
Thus, this entire cycle of events takes about nine months in humans for reproduction from the starting point of conception. As fascinating and highly complex, it is also a marvel of the evolutionary processes and development of organisms from a single cell - the zygote.
It is quite well known that there is considerable scientific development with regards to curing disease. In order to cure inherited diseases, there has been a successful attempt by scientists to genetically edit human zygote. The zygote formation in human is a vital stage for such genetic editing because even though zygote contains all the genetic information for developing a complete organism, the process of expression of these genes does not start until the formation of blastocyst. Therefore, the curing of inheritable diseases is possible at the zygote stage.
FAQs on Zygote
1. What is a zygote as per the Class 12 Biology syllabus?
A zygote is the first diploid eukaryotic cell formed by the fusion of male and female gametes during sexual reproduction. In humans, this occurs when a haploid sperm cell fertilises a haploid ovum (egg cell). This single cell contains the complete genetic blueprint, with half the chromosomes from each parent, required to develop into a new individual organism.
2. How is a zygote formed in humans?
In humans, a zygote is formed through the process of fertilisation, which typically occurs in the ampullary region of the fallopian tube. A sperm cell penetrates the layers of the ovum, leading to the fusion of their haploid nuclei. This event restores the diploid number of chromosomes (46) and marks the beginning of a new human life, initiating the first stage of embryonic development.
3. What is the difference between a zygote and an embryo?
The key difference lies in their cellular stage and development.
- A zygote is a single-celled entity that exists from the moment of fertilisation until it undergoes its first mitotic division.
- An embryo is the multicellular stage that develops from the zygote. After fertilisation, the zygote undergoes rapid cell divisions (cleavage) to form a solid ball of cells (morula), which then develops into a blastocyst. Once implantation occurs, the developing organism is referred to as an embryo.
4. Is a zygote haploid or diploid, and why is this significant?
A zygote is diploid (2n). This is highly significant because it restores the normal number of chromosomes for the species. The parent gametes (sperm and egg) are haploid (n), each containing half the set of chromosomes. Their fusion during fertilisation combines these two sets, ensuring the resulting organism has the correct amount of genetic information for proper growth and development.
5. What does it mean for a zygote to be a totipotent cell?
Totipotency is the unique ability of a single cell to differentiate and develop into a complete organism, including all embryonic and extra-embryonic tissues (like the placenta). The zygote is the ultimate totipotent cell because it has the potential to give rise to every single cell type in the body, from nerve cells to muscle cells, eventually forming a fully functional individual.
6. How is the biological sex of a human zygote determined at the moment of fertilisation?
The biological sex is determined by the sex chromosome carried by the sperm cell that fertilises the egg. All egg cells contain one X chromosome. Sperm cells can carry either an X or a Y chromosome.
- If a sperm with an X chromosome fertilises the egg, the resulting zygote will be XX (female).
- If a sperm with a Y chromosome fertilises the egg, the resulting zygote will be XY (male).
7. What are the first developmental steps a zygote undergoes after formation?
Immediately after formation, the zygote begins a series of rapid mitotic divisions known as cleavage while it travels down the fallopian tube. This process divides the large single-celled zygote into smaller cells called blastomeres. These divisions form a 2-cell stage, then a 4-cell stage, an 8-cell stage, and so on, eventually forming a solid ball of cells called the morula, which then develops into the blastocyst before implantation.
8. How does zygote formation in flowering plants differ from that in humans?
While both involve the fusion of gametes, the process in flowering plants, known as double fertilisation, is unique. Two male gametes from a pollen grain are involved. One male gamete fuses with the egg cell to form the diploid zygote (2n), which develops into the plant embryo. The second male gamete fuses with the two polar nuclei in the central cell to form the triploid primary endosperm nucleus (3n), which develops into the endosperm, a nutritive tissue for the embryo. This dual fusion event does not occur in humans.
