Fertilisation and Implantation Questions in English

(B) In humans,after fertilization,the zygote undergoes cleavage and develops into a blastocyst. This blastocyst gets implanted in the endometrium of the uterus. The uterus provides the necessary environment,nutrition,and protection for the developing embryo and fetus throughout the gestation period. Therefore,the developing embryo remains in the uterus.

(A) The $trophoblast$ is the outer layer of the blastocyst that attaches to the endometrium of the uterus.
It differentiates into two layers: the outer $syncytiotrophoblast$ and the inner $cytotrophoblast$.
The $trophoblast$ cells contribute to the formation of the embryonic part of the $placenta$ and the extra-embryonic membranes,specifically the $chorion$.
Therefore,the $placenta$ is the structure that develops from the $trophoblast$ in association with the maternal uterine tissue.

(C) Implantation is the process by which the blastocyst attaches to the endometrium of the uterus.
Following fertilization in the fallopian tube,the zygote undergoes cleavage to form a blastocyst.
This process typically takes about $6$ to $7$ days to reach the uterus.
Therefore,implantation generally occurs between the $7^{th}$ and $9^{th}$ days after ovulation.

(B) The correct answer is $B$. When the acrosome of the spermatozoa touches the surface of the egg,the cytoplasm of the egg bulges forward to form a structure known as the receptive cone or fertilization cone. This is the specific site through which the sperm enters the egg cell.

(D) Fraternal twins,also known as dizygotic twins,are produced when two separate ova are released from the ovaries and are fertilized by two different sperms simultaneously. This results in two distinct zygotes,each developing into an individual with a unique genetic makeup.

(C) The correct answer is $C$.
Antifertilizin is a proteinaceous substance found on the surface of the sperm head.
It has a molecular weight of approximately $1000$ and consists of acidic amino acids.
During the process of fertilization,antifertilizin of the sperm reacts with fertilizin of the egg,which facilitates the attachment of the sperm to the egg surface through a chemotactic interaction.

(B) During gametogenesis,the process of meiosis reduces the chromosome number to half,resulting in haploid $(n)$ gametes (sperm and ovum).
Fertilization is the fusion of a haploid male gamete $(n)$ and a haploid female gamete $(n)$.
This fusion results in the formation of a diploid $(2n)$ zygote.
Thus,fertilization restores the diploid condition from the haploid state.

(D) Polyspermy is a condition in which more than one sperm enters an ovum during the process of fertilization. Normally,the egg membrane undergoes rapid changes (depolarization) immediately after the entry of the first sperm to prevent the entry of additional sperms,ensuring that only one sperm fertilizes the egg.

(A) The interaction between fertilizin and antifertilizin is highly species-specific.
Fertilizin is present on the surface of the egg,while antifertilizin is present on the surface of the sperm.
These two molecules react only when they belong to the same species to facilitate fertilization.
Therefore,the statement that fertilizin of one species reacts with the antifertilizin of another species is incorrect.

(A) Fertilization is the process of fusion of a male gamete with a female gamete.
$1$. It brings the male and female gametes together to form a diploid zygote.
$2$. By fusing the haploid male and female gametes $(n + n)$,it restores the diploid condition $(2n)$,which is known as $Euploidy$.
$3$. During fertilization in humans,only the head of the sperm (containing the nucleus) enters the egg,while the tail remains outside. Therefore,statement $C$ is incorrect.
Since statement $C$ is incorrect,'All of these' cannot be the answer. The most accurate description among the choices is that it restores $Euploidy$ and brings gametes together. However,based on standard biological definitions,restoring $Euploidy$ is the primary biological consequence.

(B) During cleavage,the zygote undergoes rapid mitotic divisions to form a multicellular structure called a morula.
Unlike ordinary mitosis,there is no growth phase ($G_1$ or $G_2$) between the divisions.
As a result,the total volume of the embryo remains constant,and the number of cells increases while their individual size decreases.
Therefore,the cells do not grow in size during this process.

(A) Cleavage is a series of rapid mitotic divisions of the zygote.
During these divisions,the $G_1$ and $G_2$ phases of the cell cycle are significantly shortened or absent.
Because the cell cycle is accelerated to allow for rapid cell proliferation,$DNA$ replication occurs much faster than in typical somatic cell mitosis to keep up with the rapid division rate.

(A) In mammals,the embryo at the blastula stage is specifically referred to as a $Blastocyst$.
This structure consists of an outer layer of cells called the $Trophoblast$ and an inner cell mass.
Unlike the blastula of amphibians (like frogs),fish,or birds,which typically form a hollow sphere or a disc-like structure $(Blastoderm)$,the mammalian $Blastocyst$ is specialized for implantation into the uterine wall.
Therefore,the $Blastocyst$ is the characteristic blastula stage found in placental mammals.

(B) Cleavage is the series of rapid mitotic divisions of the zygote.
Depending on the amount and distribution of yolk,cleavage is classified into two main types:
$1$. Holoblastic cleavage: This is a total or complete cleavage where the entire egg divides.
$2$. Meroblastic cleavage: This is a partial or incomplete cleavage where only a portion of the egg cytoplasm undergoes division,usually because the yolk is too dense to be divided.
Therefore,meroblastic cleavage refers to partial division of the egg.

(B) The process of attachment of the blastocyst to the endometrium of the uterus is called implantation.
This process typically occurs about $6-7$ days after fertilization.
Once attached,the blastocyst becomes embedded in the uterine wall,which leads to pregnancy.

(A) Identical twins (monozygotic twins) are formed when a single fertilized egg (zygote) undergoes early cleavage and the resulting blastomeres separate into two independent embryos.
Since they originate from the same zygote,they share the same genotype and phenotype and are always of the same sex.

(A) zygote is formed by the fusion of a male gamete $(n)$ and a female gamete $(n)$ during the process of fertilization.
Since both gametes are haploid $(n)$,their fusion results in a diploid cell $(2n)$.
Therefore,the chromosome set in a zygote is $2n$.

(D) . Test tube baby refers to a baby born through the process of In Vitro Fertilization $(IVF)$. In this procedure,the ovum is fertilized by sperm outside the mother's body in a laboratory setting. Once fertilization occurs and the embryo reaches the appropriate stage,it is implanted into the mother's uterus for further growth and development until birth.

(C) surrogate mother is a woman who carries and gives birth to a child for another person or couple who will become the child's legal parents.
This process involves the implantation of an embryo,often created through $IVF$ (In Vitro Fertilization),into the uterus of the surrogate mother.
Therefore,the surrogate mother acts as a host for the development of the embryo until birth.

(A) The chemical substance secreted by the egg (ovum) that attracts sperm is called $Fertilizin$.
$Fertilizin$ is a glycoprotein present on the surface of the egg.
It interacts with the $Antifertilizin$ present on the surface of the sperm,which facilitates the recognition and binding of the sperm to the egg during the process of fertilization.
Therefore,the correct option is $A$.

(C) The $Corpus$ $luteum$ is a temporary endocrine structure in female ovaries that is involved in the production of relatively high levels of progesterone and moderate levels of estradiol and inhibin $A$. It develops from the ovarian follicle after ovulation. Therefore,it is a structure formed after the release of the ovum from the follicle.

(A) The pattern of cleavage in an embryo is significantly influenced by the amount and distribution of yolk in the egg.
Yolk acts as a physical barrier to the cleavage furrow.
In eggs with little yolk (isolecithal),cleavage is holoblastic (complete).
In eggs with a large amount of yolk (telolecithal or centrolecithal),cleavage is meroblastic (incomplete) because the cleavage furrow cannot penetrate the dense yolk.
Therefore,the distribution of yolk determines whether the cleavage will be holoblastic or meroblastic.

(A) The process of fertilization was first observed in sea urchins by Oscar Hertwig in $1875$. He observed the fusion of the male and female pronuclei within the egg,which provided the first clear evidence that fertilization involves the union of two gametes.

(D) Cleavage is a series of rapid mitotic divisions of the zygote that converts the single-celled zygote into a multicellular structure called a blastula.
Key characteristics of cleavage include:
$1$. Rapid cell divisions without significant growth in the total size of the embryo.
$2$. The size of individual cells (blastomeres) decreases with each division.
$3$. The interphase is very short,lacking the $G_1$ and $G_2$ phases.
$4$. Differentiation of blastomeres does not occur during the cleavage stage; it occurs later during gastrulation and organogenesis.
Therefore,differentiation of blastomeres is not a characteristic of cleavage.

(C) The corona radiata cells are held together by a matrix rich in hyaluronic acid.
Hyaluronic acid is a type of glycosaminoglycan,which is also known as a mucopolysaccharide.
It acts as a cementing substance that provides structural integrity to the layer of follicular cells surrounding the ovum.

(C) Fertilizin is a glycoprotein secreted by the mature egg (ovum). It acts as a chemical attractant for sperm of the same species. When fertilizin molecules on the surface of the egg interact with antifertilizin molecules on the surface of the sperm,it leads to the agglutination (clumping) of sperm cells around the egg,which facilitates the process of fertilization. Therefore,the primary function of fertilizin is to attract and agglutinate sperm.

(A) During the process of cleavage,the zygote undergoes repeated mitotic divisions as it moves through the isthmus of the oviduct towards the uterus.
These divisions result in the formation of $2, 4, 8,$ and eventually $16$ daughter cells known as blastomeres.
The embryo containing $8$ to $16$ blastomeres is specifically referred to as a Morula.
After the morula stage,it continues to divide and transforms into a blastocyst.

(B) After implantation,the corpus luteum continues to secrete high levels of progesterone to maintain the endometrium for pregnancy.
High levels of progesterone exert negative feedback on the anterior pituitary and hypothalamus,inhibiting the secretion of $FSH$ and $LH$.
Since $LH$ is required for ovulation,its inhibition prevents further ovulation during pregnancy.

(A) During the process of fertilization,the sperm comes in contact with the zona pellucida layer of the ovum. The acrosome of the sperm releases enzymes (sperm lysins) that help the sperm penetrate the egg. Upon contact,the plasma membrane of the sperm fuses with the plasma membrane of the ovum. The head of the sperm,containing the nucleus,and the centrioles enter the cytoplasm of the ovum. The middle piece (containing mitochondria) and the tail usually remain outside or degenerate. Therefore,only the head enters the ovum.

(C) Fertilization is the process of fusion of a male gamete (sperm) and a female gamete (ovum) to form a diploid zygote.
While the fusion of the nuclei (syngamy) is the core event,the term 'fertilization' broadly refers to the fusion of the two gametes themselves.
Therefore,the most accurate and fundamental definition is the fusion of two gametes.

(B) Cleavage is the series of rapid mitotic divisions that the zygote undergoes as it moves through the fallopian tube towards the uterus.
Fertilization occurs in the ampullary region of the fallopian tube.
As the zygote travels through the isthmus of the fallopian tube towards the uterus,it begins to undergo cleavage to form a $2, 4, 8, 16$ daughter cells called blastomeres.
Therefore,the cleavage of the zygote begins in the fallopian tube.

(A) The movement of sperm towards the ovum is primarily guided by chemical signals released by the ovum and its surrounding follicular cells. This directed movement in response to a chemical stimulus is known as $Chemotaxis$. While other factors like $Rheotaxis$ (movement in response to fluid flow) may play a role in the female reproductive tract,$Chemotaxis$ is the scientifically recognized term for the guidance of sperm towards the egg.

(B) In rabbits,as in other mammals,fertilization is an internal process. The sperm meets the ovum in the fallopian tube (oviduct). The fusion of the male and female gametes results in the formation of a diploid zygote within the fallopian tube.

(A) In the context of human embryonic development,the inner cell mass $(ICM)$ of the blastocyst consists of cells that are primarily columnar in shape. These cells are pluripotent and eventually differentiate into the three germ layers: ectoderm,endoderm,and mesoderm.

(B) During the process of fertilization,the sperm head releases enzymes from the acrosome,which is known as the acrosomal reaction.
One of the key enzymes released is $Hyaluronidase$.
This enzyme acts on the ground substances of the follicle cells (corona radiata) surrounding the ovum.
It dissolves the hyaluronic acid present in the egg membrane,allowing the sperm to penetrate and reach the zona pellucida to fertilize the egg.

(C) Fertilization is the process of fusion of a haploid sperm $(n)$ and a haploid ovum $(n)$ to form a diploid zygote $(2n)$.
This process restores the diploid number of chromosomes in the organism.
Therefore,the correct option is $C$.

(A) The blastocyst is an early developmental stage of the embryo.
It consists of an outer layer of cells known as the trophoblast,which attaches to the endometrium.
The inner group of cells attached to the trophoblast is known as the inner cell mass $(ICM)$,which eventually differentiates into the embryo proper.
Therefore,$A$ is the trophoblast and $B$ is the inner cell mass.

(A) Cleavage in the human zygote is holoblastic,meaning the entire zygote undergoes division. Because human eggs are alecithal (containing very little yolk),the cleavage furrows pass through the entire egg. Furthermore,the cleavage is equal,resulting in blastomeres of similar size during the initial stages of development.

(D) In placental mammals,fertilization is the process where the sperm and the ovum fuse to form a zygote. This process occurs in the ampullary region of the fallopian tube (oviduct). The ovum is released from the ovary and travels into the fallopian tube,where it meets the sperm. Therefore,the correct site for fertilization is the fallopian tube.

(B) The $Corpus Luteum$ is a temporary endocrine structure in female mammals that is involved in the production of relatively high levels of progesterone and moderate levels of estradiol and inhibin $A$.
It is formed from the ruptured follicle after ovulation in the ovary.
Therefore, the correct organ is the ovary.

(D) Polyspermy refers to the entry of more than one sperm into an egg during fertilization.
In many species,such as sea urchins,polyspermy is a common phenomenon that occurs naturally.
However,these organisms have developed mechanisms to prevent the fusion of multiple sperm nuclei with the egg nucleus,ensuring that only one sperm nucleus successfully fertilizes the egg.
In contrast,humans and many other mammals have strict blocks to prevent the entry of multiple sperm (polyspermy) into the egg.

(B) In humans,the zygote undergoes holoblastic cleavage,which is unequal. The first cleavage division results in two blastomeres of different sizes: one slightly larger and one slightly smaller. This is due to the distribution of cytoplasmic components and the nature of the isolecithal egg.

(B) In mammals,fertilization is the process where the sperm and the ovum fuse to form a zygote. This process typically takes place in the ampullary region of the Fallopian tube (oviduct). The ovum is released from the ovary and travels into the Fallopian tube,where it meets the sperm. Therefore,the correct site for fertilization is the Fallopian tube.

(A) The process of rapid mitotic cell division in the zygote,which converts the single-celled zygote into a multicellular structure called the blastocyst,is known as cleavage. Unlike normal mitosis,cleavage does not involve cell growth between divisions,resulting in smaller daughter cells called blastomeres.

(C) In humans,fertilization occurs in the ampullary region of the fallopian tube.
Cleavage is the series of mitotic divisions of the zygote.
Cleavage in humans is holoblastic (total) and equal,and it begins while the zygote is still moving through the fallopian tube towards the uterus.

(C) The acrosome reaction is a critical step in fertilization where the sperm releases hydrolytic enzymes (lysins) to penetrate the zona pellucida of the ovum.
This process is primarily triggered by the binding of the sperm to the zona pellucida proteins (specifically $ZP3$ receptors).
However,at the cellular level,this binding leads to an influx of $Ca^{2+}$ ions and an increase in intracellular $pH$,which facilitates the fusion of the acrosomal membrane with the sperm plasma membrane.
Among the given options,the influx of ions (often associated with $Ca^{2+}$ and $Na^+$) is the physiological trigger for the exocytosis of acrosomal contents.

(B) The process of cleavage in a zygote follows a geometric progression where the number of blastomeres after $n$ divisions is given by the formula $2^n$.
Here,we need to find the number of divisions $(n)$ required to reach $16$ blastomeres.
Setting up the equation: $2^n = 16$.
Since $16 = 2^4$,we have $2^n = 2^4$.
Therefore,$n = 4$.
Thus,$4$ cleavage divisions are required to form $16$ blastomeres.

(B) In the human embryo, the blastocyst consists of an outer layer of cells called the $Trophoblast$ and an inner group of cells attached to it called the $Inner \text{ } cell \text{ } mass$. The $Trophoblast$ layer gets attached to the endometrium, and the cells of the $Trophoblast$ differentiate to form the extra-embryonic membranes (such as the amnion, chorion, yolk sac, and allantois) which protect and nourish the developing embryo. Therefore, the correct answer is $B$.

(C) In sexually reproducing multicellular organisms,the process of development begins with the formation of a single-celled $zygote$.
This $zygote$ is formed by the fusion of male and female gametes during the process of fertilization.
Following fertilization,the $zygote$ undergoes a series of mitotic divisions (cleavage) to form a multicellular embryo,eventually leading to the development of the entire organism.

(A) Antifertilizin is a chemical substance found on the surface of the sperm. It is a proteinaceous substance,specifically a glycoprotein. During the process of fertilization,the antifertilizin of the sperm reacts with the fertilizin present on the surface of the egg (ovum). This interaction is species-specific and ensures that the sperm of the same species fertilizes the egg.