Fertilisation and Implantation Questions in English

(C) The zygote undergoes repeated mitotic divisions called cleavage as it moves through the isthmus of the oviduct towards the uterus.
These divisions result in the formation of $8$ to $16$ daughter cells called blastomeres.
The embryo with $8-16$ blastomeres is called a morula.
The morula continues to divide and transforms into a blastocyst (a type of blastula) as it moves further into the uterus.
Therefore,the solid,mulberry-like stage is known as the morula.

(D) 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 sperm travels through the vagina and uterus to reach the fallopian tube,where it meets the ovum released from the ovary.

(D) Polyspermy refers to the entry of more than one sperm into an ovum during fertilization.
To prevent this,the ovum undergoes specific changes immediately after the first sperm enters.
The cortical granules present beneath the plasma membrane of the ovum release their contents into the perivitelline space.
These contents harden the zona pellucida,forming a fertilization membrane.
This membrane acts as a physical barrier that prevents any other sperm from entering the ovum,thereby ensuring monospermy.

(B) Cleavage is a series of rapid mitotic divisions of the zygote which converts the single-celled zygote into a multicellular structure called a blastocyst.
As the zygote moves from the ampulla towards the uterus through the oviduct (Fallopian tube),it undergoes these mitotic divisions.
The process of cleavage specifically occurs while the zygote is passing through the isthmus of the oviduct towards the uterus.

(B) The process by which the blastocyst becomes embedded in the endometrium of the uterus is called implantation.
$1$. Fertilization is the fusion of male and female gametes to form a zygote.
$2$. Implantation is the attachment of the blastocyst to the uterine wall,which leads to pregnancy.
$3$. Gestation is the period of development of the fetus in the uterus.
$4$. Parturition is the process of giving birth to the young one.

(B) Cleavage is a series of rapid mitotic divisions of the zygote,which converts the single-celled zygote into a multicellular structure called a morula.
During cleavage,the total volume of the embryo remains constant because there is no growth phase ($G_1$ or $G_2$) between divisions.
As the number of cells increases,the size of each individual cell (blastomere) decreases because the cytoplasm is partitioned into smaller units without an increase in total cytoplasmic mass.

(A) In humans,fertilization occurs in the ampullary region of the Fallopian tube.
Following fertilization,the zygote undergoes a series of mitotic divisions known as cleavage.
Cleavage begins while the zygote is still moving through the Fallopian tube towards the uterus.
Therefore,the process of cleavage starts in the Fallopian tube.

(B) Fertilizin is a glycoprotein substance secreted by the mature ovum (egg).
It acts as a chemical attractant for sperm of the same species.
It interacts with the antifertilizin present on the surface of the sperm,facilitating the recognition and binding of the sperm to the egg during the process of fertilization.

(A) In mammals,the head of the sperm contains an acrosome,which is filled with digestive enzymes known as sperm lysins. The most important sperm lysin is $Hyaluronidase$. This enzyme acts on the ground substances of the follicle cells (corona radiata) surrounding the ovum,allowing the sperm to penetrate and fertilize the egg. Therefore,the correct answer is $Hyaluronidase$.

(D) Identical twins,also known as monozygotic twins,are produced 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 genetic makeup.
This process is distinct from fraternal twins,which result from the fertilization of two separate eggs by two separate sperms.

(B) Fertilization is the process of fusion of male and female gametes.
It is classified into two types based on the location where the fusion occurs:
$1$. External fertilization: Occurs outside the body of the organism (e.g.,in water).
$2$. Internal fertilization: Occurs inside the body of the organism.
This classification is fundamentally based on the site where the fusion of gametes takes place,which is often correlated with the site of embryonic development. In many organisms,the site of fertilization determines the subsequent site of embryonic development.

(A) The process of fusion of the nuclei of a sperm and an ovum is known as $Karyogamy$.
$Karyogamy$ is a critical step in the process of $Syngamy$ (fertilization),where the haploid nuclei of the male and female gametes fuse to form a diploid $Zygote$.
$Parthenogenesis$ is the development of an embryo from an unfertilized egg.
$Vitellogenesis$ is the process of yolk formation in the oocyte.
$Oogenesis$ is the process of formation of female gametes (ova).

(B) In most animal species, the sperm enters the ovum through the $animal \text{ } pole$. The $animal \text{ } pole$ is the region of the egg that contains the nucleus and is generally less dense in yolk, making it the site where fertilization typically occurs. The $vegetal \text{ } pole$ is typically rich in yolk and is not the primary site for sperm entry.

(A) The ovum secretes a chemical substance known as $Fertilizin$ (a glycoprotein) from its surface. This substance acts as a chemoattractant that attracts the sperm,which possesses $Antifertilizin$ on its surface. The interaction between $Fertilizin$ and $Antifertilizin$ is a species-specific reaction that facilitates the binding of the sperm to the ovum during the process of fertilization.

(D) Fertilizin is a glycoprotein present on the surface of the egg (ovum),while antifertilizin is a protein present on the surface of the sperm.
During the process of fertilization,the fertilizin of the egg interacts with the antifertilizin of the sperm.
This interaction is species-specific,which ensures that the sperm of the same species can bind to and fertilize the egg of that specific species.
Therefore,the primary function of the fertilizin-antifertilizin interaction is the binding of gametes of the same species.

(B) During the process of fertilization,the sperm approaches the ovum. The ovum is surrounded by several layers,including the corona radiata and the zona pellucida. The sperm first comes into contact with the $Zona \ pellucida$ layer of the ovum. This contact triggers the acrosome reaction in the sperm,which releases enzymes that help the sperm penetrate the $Zona \ pellucida$ to reach the plasma membrane of the ovum.

(B) Fertilization is the process of fusion of a sperm with an ovum. In the human female reproductive system,this process takes place in the $Ampulla$ region of the fallopian tube (oviduct). The sperm and ovum are transported simultaneously to the $Ampulla$ region where fertilization occurs.

(D) Fertilization is the process of fusion of a haploid male gamete (sperm) with a haploid female gamete (ovum/egg).
Both the sperm and the egg contain $n$ number of chromosomes (haploid).
Upon fusion,they form a diploid zygote $(2n)$,which contains the full set of chromosomes ($2n$ = $46$ in humans).
Therefore,the correct statement is that a haploid sperm and a haploid egg fuse to form a diploid zygote.

(B) In the human female reproductive cycle,if fertilization does not occur,the corpus luteum cannot be maintained because the human chorionic gonadotropin $(hCG)$ hormone is not produced by the embryo.
As a result,the corpus luteum undergoes degeneration and transforms into a white fibrous scar tissue known as the corpus albicans.
This degeneration leads to a sharp decline in the levels of progesterone and estrogen,which triggers the shedding of the uterine endometrium,resulting in menstruation.

(B) During the process of fertilization in many organisms,the sperm enters the ovum specifically through the animal pole. The animal pole is the region of the egg where the nucleus is located and where the cytoplasm is less dense with yolk,facilitating the entry of the sperm head. The vegetal pole,conversely,is typically rich in yolk and is not the site of sperm entry.

(D) Cleavage is a series of rapid mitotic cell divisions of the zygote following fertilization.
These divisions occur without any significant growth of the daughter cells.
The resulting daughter cells formed during the cleavage process are known as $Blastomeres$.

(D) The life of all sexually reproducing multicellular organisms begins from a single cell called the $Zygote$.
This $Zygote$ is formed by the fusion of male and female gametes during the process of fertilization.
It represents the starting point of the development of an individual organism.

(C) The formation of a zygote occurs through the process of fertilization,which involves the fusion of two haploid gametes.
Gametes are specialized reproductive cells.
In humans,the male gamete is the sperm cell and the female gamete is the egg cell (ovum).
Since both sperm and egg cells are types of reproductive cells,they both participate in the formation of the zygote.
However,the most comprehensive answer that includes both types of gametes is 'Reproductive cells'.

(C) The process of fusion of a male gamete (sperm) with a female gamete (ovum) is known as fertilization.
This fusion results in the formation of a diploid cell called a $Zygote$.
The $Zygote$ contains the combined genetic material from both parents,representing the first cell of a new organism.

(D) The development of an adult individual begins with a single cell called the $Zygote$.
$Zygote$ is formed by the fusion of male and female gametes during the process of fertilization.
This $Zygote$ undergoes repeated mitotic cell divisions to form an embryo,which eventually develops into a complete adult organism.

(C) The process of fertilization involves the fusion of a male gamete and a female gamete. This fusion results in the formation of a diploid cell known as the zygote. Therefore,the zygote is formed by the union of male and female gametes.

(B) Assertion $(A)$ is true because all somatic cells in an organism are derived from the zygote through repeated mitotic cell divisions. Since mitosis maintains the same chromosome number and genetic content,every cell contains the same genetic material as the original zygote.
Reason $(R)$ is also true because the zygote is indeed formed by the fusion of haploid male and female gametes during fertilization.
However,the reason $(R)$ explains the origin of the zygote,but it does not explain why every cell in the body retains the same genetic material as the zygote (which is due to the process of mitosis). Therefore,$R$ is not the correct explanation of $A$.

(C) In sexual reproduction,the fusion of male and female gametes is called fertilization. The product of this fusion is a diploid cell known as the $Zygote$. The $Zygote$ serves as the vital link that ensures the continuity of the species between organisms of one generation and the next.

(A) The process of fusion of a male gamete with a female gamete is known as fertilization. This fusion results in the formation of a diploid cell called a zygote. Therefore,fertilization is the event that leads to the formation of a zygote.

(C) The process of fusion of a male gamete (sperm) and a female gamete (ovum) is called fertilization.
This fusion results in the formation of a diploid cell known as the $Zygote$.
The $Zygote$ is the first cell of a new organism.

(C) The correct answer is $C$. Capacitation is the physiological maturation process that spermatozoa undergo within the $female$ $reproductive$ $tract$.
During this process, secretions from the female genital tract remove coating substances (such as cholesterol and glycoproteins) deposited on the surface of the sperm, particularly over the acrosome.
This removal exposes the receptor sites on the acrosome, which allows the sperm to bind to the zona pellucida of the egg and become active enough to penetrate it.
Therefore, capacitation is essential for fertilization and occurs only after the sperm enters the female reproductive system.

(D) : The fusion of a haploid male gamete (sperm) and a haploid female gamete (ovum) to form a diploid zygote is called fertilisation.
In human beings,fertilisation takes place only when the ovum and sperms are transported simultaneously to the ampullary-isthmic junction of the Fallopian tube (oviduct).

(D) : Ectopic pregnancy is a complication of pregnancy in which implantation of the embryo takes place at a site other than the uterus.
Signs and symptoms include abdominal pain and vaginal bleeding.
Most ectopic pregnancies $(90\%)$ occur in the Fallopian tube,which are known as tubal pregnancies.

(C) : After fertilization,the zygote undergoes rapid mitotic divisions,called cleavage,which is characterized by the absence of growth of daughter cells. This leads to the conversion of a single-celled zygote into a multicellular structure called a blastocyst. Implantation or embedding of the zygote into the endometrium of the uterus occurs in the blastocyst stage. The blastocyst comes in contact with the endometrium in the region of the embryonal knob or embryonic disc and adheres to it. The surface cells of the trophoblast secrete lytic enzymes which cause corrosion of the endometrial lining. They also give rise to finger-like outgrowths called chorionic villi. Chorionic villi and uterine tissue become interdigitated. Villi not only help in fixation but also in the absorption of nourishment.

(A) The process of fusion of a sperm with an ovum is called fertilization.
During fertilization,a sperm comes in contact with the zona pellucida layer of the ovum and induces changes in the membrane that block the entry of additional sperms.
Thus,it ensures that only one sperm can fertilize an ovum.
The secretions of the acrosome help the sperm enter into the cytoplasm of the ovum through the zona pellucida and the plasma membrane.
In contact with the surface of the egg covering,the acrosome releases its contained hydrolytic enzymes,also called sperm lysins.
This is known as the acrosomal reaction.
The acrosomal reaction results in the dissolving of corona cells and the degeneration of the zona pellucida,which helps in sperm penetration.

(D) The correct answer is $(d)$.
Implantation in the endometrial uterine wall takes place at the blastocyst stage of embryonic development.
Before implantation,the blastomeres of the early blastocyst arrange themselves into an outer layer called the trophoblast and an inner group of cells attached to the trophoblast called the inner cell mass.
It is the trophoblast layer through which the blastocyst attaches to the endometrium,and the inner cell mass differentiates into the embryo.

(D) The correct answer is $D$. In a regularly cycling human female,the most common physiological reason for the cessation of menstruation is pregnancy,which occurs following the fertilization of the ovum.
$1$. Upon fertilization,the embryo secretes $hCG$ (human Chorionic Gonadotropin),which maintains the corpus luteum.
$2$. The corpus luteum continues to secrete high levels of progesterone and estrogen,which prevent the shedding of the endometrial lining.
$3$. Consequently,the menstrual cycle is suspended to support the developing embryo.

(A) : Cleavage is a series of rapid mitotic cell divisions by which a single fertilized egg cell (zygote) is transformed into a multicellular structure called the blastula.
During cleavage,there is no significant growth in the size of the embryo,and the shape remains largely unchanged except for the formation of a central cavity known as the blastocoel.
The ratio of nuclear material $(DNA)$ to cytoplasm increases with each division.
The pattern or mode of cleavage is primarily determined by the amount of yolk present in the egg and its distribution (pattern of yolk deposition).
Based on the yolk content,cleavage is classified as holoblastic (total) or meroblastic (partial).

(B) : Assisted reproductive technologies $(ART)$ include a number of special techniques which assist infertile couples to have children.
An important technique of $ART$ is the test tube baby programme.
The baby produced by conceiving in a culture dish and nursing in the uterus is called a test tube baby.
This method involves in vitro fertilization $(IVF)$, $i.e.$, fertilization of male and female gamete outside the body in conditions similar to those in the body, followed by embryo transfer $(ET)$.
Zygote or early embryo up to $8$ blastomeres is transferred into the Fallopian tube $(ZIFT - \text{Zygote Intra Fallopian Transfer})$, and an embryo with more than $8$ blastomeres is transferred to the uterus to complete its further development.

(D) : In the in vitro fertilization method,popularly known as the test-tube baby programme,ova from the wife/donor (female) and sperms from the husband/donor (male) are collected and induced to form a zygote under simulated conditions in the laboratory.
The zygote or early embryos (with up to $8$ blastomeres) can then be transferred into the Fallopian tube,which is known as Zygote Intra Fallopian Transfer $(ZIFT)$.
Embryos with more than $8$ blastomeres are transferred into the uterus,known as Intra Uterine Transfer $(IUT)$,to complete their further development.

(A) The sex of a human baby is determined at the time of fertilization.
In humans,the female produces gametes with $22 + X$ chromosomes,while the male produces two types of sperm: $22 + X$ and $22 + Y$.
When a sperm carrying an $X$ chromosome fertilizes the ovum,the resulting zygote is $44 + XX$ (female).
When a sperm carrying a $Y$ chromosome fertilizes the ovum,the resulting zygote is $44 + XY$ (male).
Since this genetic combination is established the moment the sperm fuses with the egg,the sex is determined at fertilization.

(D) Fertilization is the process of fusion of a haploid male gamete (sperm) with a haploid female gamete (ovum).
Both the sperm and the ovum contain $n$ number of chromosomes (haploid).
Upon fusion,they form a diploid zygote which contains $2n$ number of chromosomes.
Therefore,the correct statement is that a haploid sperm fuses with a haploid ovum to form a diploid zygote.

(D) In mammals, fertilization is the process where the sperm and the ovum fuse to form a zygote.
This process typically occurs in the ampulla region of the $Fallopian \text{ tube}$ (oviduct).
The ovum is released from the ovary and travels into the $Fallopian \text{ tube}$, where it meets the sperm.
Therefore, the correct site for fertilization is the $Fallopian \text{ tube}$.

(C) After ovulation,the ruptured follicle transforms into a structure called the corpus luteum.
The corpus luteum primarily secretes large amounts of progesterone,which is essential for the maintenance of the endometrium.
It also secretes some amount of estrogen.
Since progesterone is the primary hormone secreted to support pregnancy,it is the most appropriate answer.

(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 finally $16$ daughter cells.
These daughter cells are known as blastomeres.
The embryo consisting of $8$ to $16$ blastomeres is called a Morula.
Therefore,the $16$-celled stage is specifically referred to as the Morula stage.

(B) During the process of fertilization,a sperm comes in contact with the $zona$ $pellucida$ layer of the ovum and induces changes in the membrane.
These changes in the $zona$ $pellucida$ layer are crucial because they block the entry of additional sperms into the ovum.
This mechanism ensures that only one sperm can fertilize the ovum,a phenomenon known as polyspermy prevention.
Therefore,the correct event that occurs due to these changes is the blocking of additional sperms.

(A) Cleavage is the series of mitotic divisions of the zygote.
$1$. $Holoblastic$ cleavage refers to complete cleavage where the entire egg is divided.
$2$. $Meroblastic$ cleavage refers to incomplete or partial cleavage,which occurs in eggs with a large amount of yolk (e.g.,birds,reptiles).
Therefore,incomplete cleavage is synonymous with $Meroblastic$ cleavage.