Fertilisation and Implantation Questions in English

(C) Fertilization is the process of fusion of a sperm with an ovum. In humans,this process occurs in the $Ampulla$ of the fallopian tube (oviduct). Specifically,it often happens at the $Ampullary-isthmic$ junction,where the sperm and the ovum meet.

(A) Based on the provided diagram of the human foetus within the uterus:
$A$ represents the Umbilical cord,which connects the foetus to the placenta.
$B$ represents the Chorionic villi,which are finger-like projections that form the placenta.
$C$ represents the Plug of mucus in the cervix,which acts as a protective barrier during pregnancy.
Therefore,the correct identification is $A-$Umbilical cord,$B-$Chorionic villi,$C-$Plug of mucus in cervix.

(C) In the given figure,the developing embryo is shown at the $8$-celled and $16$-celled stages.
$A$ points to the individual cells formed by cleavage,which are known as blastomeres.
$B$ points to the outer protective glycoprotein layer surrounding the embryo,known as the zona pellucida.
Therefore,$A$ is a blastomere and $B$ is the zona pellucida.

(B) During the process of fertilization,the sperm typically enters the unfertilized egg through the animal pole. The animal pole is the region of the egg where the nucleus is located and is generally less dense in yolk compared to the vegetal pole.

(B) At the time of fertilization,the sperm secretes substances known as $sperm \ lysins$ to penetrate the egg membranes.
Specifically,the acrosome of the sperm releases enzymes such as $hyaluronidase$ and $acrosin$,which are collectively referred to as $sperm \ lysins$.
$Fertilizin$ is a glycoprotein secreted by the egg (ovum),not the sperm.
$Anti-fertilizin$ is a substance present on the surface of the sperm that reacts with $fertilizin$ to facilitate the attachment of the sperm to the egg.
Therefore,the secretions produced by the spermatozoa specifically for the purpose of penetrating the egg are $sperm \ lysins$,while $anti-fertilizin$ is a surface component involved in recognition.

(B) In humans,fertilisation is the process of fusion of a sperm with an ovum. This process takes place in the fallopian tube. Specifically,the sperm and the ovum are transported to the $Ampullary-isthmic$ junction,where the actual fertilisation occurs.

(C) The blastocyst gets implanted in the endometrium of the uterus.
This process is facilitated by the trophoblast cells,which attach to the endometrium.
After implantation,the chorionic villi and uterine tissue become interdigitated with each other and jointly form the placenta.
Implantation typically occurs about $7$ days after fertilization.
The inner cell mass differentiates into the embryo,not the trophoblast cells.

(C) During human embryonic development,the blastocyst consists of an outer layer of cells called the trophoblast and an inner group of cells attached to it called the inner cell mass. The trophoblast layer helps in the attachment of the blastocyst to the endometrium and eventually forms the chorionic villi and the foetal portion of the placenta,as well as protective and trophic membranes. The body of the developing embryo is formed exclusively by the inner cell mass.

(C) In the human female reproductive system,the blastocyst normally implants in the uterine wall.
$I$ represents the Endometrium,which is the inner glandular layer of the uterus where implantation occurs.
$II$ represents the Myometrium,the thick middle layer of smooth muscle.
$III$ represents the Infundibulum,the funnel-shaped part of the fallopian tube.
$IV$ represents the Fimbriae,which collect the ovum after ovulation.
$V$ represents the Cervical canal.
$VI$ represents the Vagina.
Therefore,the correct site of implantation is $I$ (Endometrium).

(C) Fertilization can only occur if the ovum and sperms are transported simultaneously to the ampullary-isthmic junction.
The lifespan of sperm in the female reproductive tract is approximately $48$ to $72$ hours.
The viability of the secondary oocyte after its release from the ovary is only about $24$ hours.
Therefore,if copulation does not occur during the fertile period when both gametes are viable,fertilization and subsequent pregnancy will not take place.

(B) The secondary oocyte remains arrested in the metaphase-$II$ stage of meiosis-$II$.
When the sperm comes in contact with the ovum,the acrosome releases enzymes that facilitate the penetration of the sperm through the zona pellucida and the plasma membrane.
The actual entry of the sperm into the cytoplasm of the ovum triggers the completion of the second meiotic division,resulting in the formation of a haploid ovum (ootid) and a second polar body.

(C) Based on the provided diagram of the ovum:
$A$ points to the $Zona$ $pellucida$,which is the thick,transparent glycoprotein layer surrounding the plasma membrane of the oocyte.
$B$ points to the $Corona$ $radiata$,which is the outermost layer of follicular cells surrounding the $Zona$ $pellucida$.
$C$ points to the $Perivitelline$ $space$,which is the space between the plasma membrane of the oocyte and the $Zona$ $pellucida$.
Therefore,the correct sequence is $A$: $Zona$ $pellucida$,$B$: $Corona$ $radiata$,$C$: $Perivitelline$ $space$. The correct option is $C$.

(C) The acrosome of the sperm contains several hydrolytic enzymes collectively known as sperm lysins.
$Acrosin$ is a proteolytic enzyme,also known as $zona \ lysin$,which specifically digests the $zona \ pellucida$ layer of the ovum to facilitate sperm entry.
$Hyaluronidase$ acts on the ground substance of the follicle cells (corona radiata).
$Corona \ penetrating \ enzyme$ $(CPE)$ also helps in the penetration of the corona radiata layer.

(A) During fertilization,the sperm contributes its nucleus and the proximal centriole to the secondary oocyte.
The proximal centriole is essential for the formation of the mitotic spindle apparatus,which is required for the first cleavage of the zygote.
The distal centriole is generally involved in the formation of the sperm tail (axoneme) and does not play a role in the first cleavage.

(A) During the cleavage process,the zygote undergoes rapid mitotic divisions to form a solid ball of cells called the morula.
In this stage,the total volume of the embryo does not increase because the cells divide without any growth in size,meaning the cytoplasm is partitioned into smaller cells called blastomeres.
Since the embryo is enclosed within the zona pellucida,which does not expand during these early divisions,the overall size remains constant until the blastocyst stage.
Therefore,both the Assertion and the Reason are correct,and the Reason explains why the cells do not increase in size (due to the constraint of the zona pellucida).

(A) The Assertion is correct because fertilisation is a chance event that requires the presence of both gametes at the same time and place.
Sperms are viable for $48$ to $72$ hours,while the ovum is viable for only about $24$ hours.
For fertilisation to occur,the ovum and sperms must be transported simultaneously to the ampullary-isthmic junction of the fallopian tube.
If this timing does not coincide,fertilisation will not occur,which explains why not all copulations result in pregnancy.

(A) Fertilizin is a glycoprotein present on the surface of the egg membrane.
Anti-fertilizin is present on the surface of the sperm.
During fertilization,the fertilizin of the egg reacts with the anti-fertilizin of the sperm,which is a species-specific interaction.
This interaction causes the agglutination of sperms around the egg,which is known as the agglutination reaction.
Therefore,both the Assertion and the Reason are correct,and the Reason is the correct explanation of the Assertion.

(C) $ZIFT$ stands for Zygote Intra Fallopian Transfer.
In this procedure,the zygote or early embryo up to $8$ blastomeres is transferred into the Fallopian tube.
Therefore,the statement that 'more than $8$ blastomeres are used' is false,as embryos with more than $8$ cells are typically used in $IUT$ (Intra Uterine Transfer).

(B) In vivo fertilisation refers to the process where the fusion of male and female gametes occurs within the female reproductive tract,specifically within the oviduct (fallopian tube).
This is the natural method of fertilisation,as opposed to in vitro fertilisation $(IVF)$,which occurs outside the body in a laboratory setting.

(D) In mammals,the ovum is surrounded by several layers. The outermost layer is the corona radiata,followed by the zona pellucida,which is a thick,non-cellular glycoprotein layer.
The zona pellucida contains specific receptors (such as $ZP3$ proteins) that are essential for the recognition and binding of the sperm.
This binding process triggers the acrosome reaction in the sperm,allowing it to penetrate the zona pellucida and reach the plasma membrane of the oocyte.
Therefore,the correct answer is $D$.

(B) $\rightarrow$ In land animals and some aquatic animals,the sperms are introduced by the male into the reproductive tract of the female during copulation. This process is known as internal fertilization.

(B) In the diagram of an ovum surrounded by sperms,the $Zona$ $Pellucida$ is the thick,transparent,non-cellular layer that surrounds the plasma membrane of the oocyte.
Based on standard biological diagrams of fertilization:
$P$ typically represents the corona radiata (the outer layer of follicular cells).
$Q$ represents the $Zona$ $Pellucida$ (the glycoprotein layer).
$R$ represents the perivitelline space.
$S$ represents the plasma membrane of the ovum.
Therefore,the correct label for $Zona$ $Pellucida$ is $Q$.

(C) Fertilization is the process of fusion of a sperm with an ovum.
In humans,this process occurs in the fallopian tube.
Specifically,the sperm and the ovum are transported to the $Ampullary-isthmic$ junction of the fallopian tube,where fertilization takes place.
If fertilization does not occur at this site,pregnancy cannot be established.

(B) The structure described,characterized by an outer layer of cells called the trophoblast and an inner group of cells known as the inner cell mass,is the $Blastocyst$.
$1$. $Zygote$ is a single-celled structure formed after fertilization.
$2$. $Morula$ is a solid ball of $8-16$ cells.
$3$. $Blastocyst$ is the stage that implants into the uterine wall.

(A) The uterus has three layers of tissue: the outer perimetrium,the middle myometrium,and the inner endometrium.
Implantation is the process by which the blastocyst attaches to and embeds itself into the uterine wall.
This process specifically occurs in the endometrium,which is the innermost glandular layer of the uterus that undergoes cyclic changes during the menstrual cycle to prepare for pregnancy.

(A) $ZIFT$ stands for $Zygote$ $Intra$ $Fallopian$ $Transfer$.
In this assisted reproductive technology,the zygote or early embryos (up to $8$ blastomeres) are transferred into the fallopian tube of the female.
Therefore,the correct option is $A$.

(B) Monozygotic (identical) twins are formed from a single zygote that splits into two,meaning they must have the same sex.
Since the twins in this case are a boy and a girl,they cannot be monozygotic.
Fraternal (dizygotic) twins are formed from two separate eggs fertilized by two separate sperm cells.
Because they arise from different fertilization events,they are genetically as similar as any other siblings,sharing approximately $50\%$ of their genes.
Therefore,a boy and a girl twin pair must be fraternal twins.

(A) The process of implantation involves the attachment of the embryo to the uterine wall.
After fertilization,the zygote undergoes cleavage to form a $2, 4, 8, 16$ celled stage called the morula.
The morula continues to divide and transforms into a blastocyst.
The blastocyst consists of an outer layer of cells called the trophoblast and an inner cell mass.
It is the blastocyst that gets embedded in the endometrium of the uterus,leading to pregnancy.

(B) Statement-$I$ is correct. After the formation of the blastocyst,the outer layer called the trophoblast attaches to the endometrium,and the inner cell mass differentiates into the embryo.
Statement-$II$ is incorrect. It is not the morula but the blastocyst that gets embedded (implanted) in the endometrium of the uterus.

(C) Fertilization is the process of fusion of a sperm with an ovum.
In humans,this process occurs in the $Ampulla$ region of the Fallopian tube (oviduct).
The ovum released from the ovary is transported to the $Ampulla$ where it meets the sperm.
Therefore,the correct site for fertilization is the $Ampulla$ of the Fallopian tube.

(B) In the given diagram of an ovum undergoing fertilization:
$A$ represents the corona radiata,which is the outer layer of follicular cells.
$B$ represents the secondary oocyte (ovum).
$C$ represents the zona pellucida,which contains $ZP-3$ receptors that facilitate sperm binding.
$D$ represents the perivitelline space,which is the space between the zona pellucida and the plasma membrane of the oocyte. Upon the entry of the first sperm,cortical granules release enzymes into this space,causing the zona pellucida to harden and preventing polyspermy.
Therefore,option $B$ is the correct description.

(C) The uterus wall consists of three layers of tissue: the outer perimetrium,the middle myometrium,and the inner endometrium.
During the process of implantation,the blastocyst attaches to and becomes embedded in the endometrium,which is the innermost glandular layer of the uterus.
The endometrium undergoes cyclic changes during the menstrual cycle to prepare for the implantation of the embryo.

(A) Cleavage is a series of rapid mitotic cell divisions of the zygote,which converts the single-celled zygote into a multicellular structure called a blastocyst.
It begins as the zygote moves through the isthmus of the oviduct towards the uterus.
Therefore,$A = \text{Mitosis}$ and $B = \text{Isthmus}$.

(A) The provided image shows a blastocyst.
$A$ represents the outer layer of cells known as the trophoblast,which attaches to the endometrium of the uterus.
$B$ represents the inner cell mass,which contains stem cells that differentiate into the embryo.
Therefore,$A$ is the trophoblast (attaches to the endometrium) and $B$ is the inner cell mass (differentiates into the embryo).

(D) Oxytocin is a hormone released by the posterior pituitary gland.
During sexual intercourse,the release of oxytocin causes rhythmic contractions of the smooth muscles of the uterus and the fallopian tubes.
These contractions facilitate the transport of sperms towards the ampulla,which is the site of fertilization in the fallopian tube.

(B) The correct sequence of events during fertilization is as follows:
$1$. Capacitation of sperms $(v)$: This is the physiological maturation of sperms in the female reproductive tract.
$2$. Fertilizin-antifertilizin reaction $(i)$: This is the chemical recognition between the sperm and the egg surface.
$3$. Acrosome reaction $(iii)$: The release of enzymes from the acrosome to penetrate the zona pellucida.
$4$. Cortical reaction $(ii)$: This prevents polyspermy by modifying the zona pellucida after the first sperm enters.
$5$. Karyogamy $(iv)$: The fusion of the male and female pronuclei to form a diploid zygote.
Therefore,the correct sequence is $v \rightarrow i \rightarrow iii \rightarrow ii \rightarrow iv$.

(A) Cleavage is a series of rapid mitotic divisions of the zygote,which converts the single-celled zygote into a multicellular structure called a blastocyst (or blastula).
Meiosis does not occur during cleavage; only mitosis takes place to increase the number of cells without increasing the total cytoplasmic volume.
Therefore,the statement claiming that cleavage involves meiotic divisions is incorrect.

(C) In the human reproductive system, after ejaculation, sperm travel through the cervix and uterus to reach the ampulla of the fallopian tube. The viability of sperm in the female reproductive tract is generally considered to be about $24$ to $48$ hours, although some studies suggest they can remain fertile for up to $72$ hours. Among the given options, the range $12-23$ hours is the most scientifically plausible approximation for the duration of viability in the ampulla before fertilization occurs.

(A) $Statement I$ is correct because the sperm penetration involves the acrosomal reaction,which is a chemical process involving the release of enzymes.
$Statement II$ is correct because the acrosome of the sperm secretes hyaluronidase,an enzyme that digests the hyaluronic acid present in the corona radiata cells surrounding the ovum,allowing the sperm to penetrate.

(B) During the process of fertilization,a specific chemical interaction occurs between the gametes.
The ovum (egg cell) secretes a glycoprotein substance known as fertilizin.
The sperm secretes a protein substance known as antifertilizin.
These two substances are species-specific and interact with each other to facilitate the recognition and binding of the sperm to the ovum,which is essential for successful fertilization.

(D) Fertilization is a species-specific process.
This specificity is mediated by chemical interactions between the sperm and the ovum.
The ovum secretes a chemical substance called 'fertilizin' (a glycoprotein),while the sperm surface contains a complementary substance called 'antifertilizin'.
For successful fertilization,the fertilizin of the ovum must be compatible with the antifertilizin of the sperm.
If the sperm of species $X$ cannot fertilize the ovum of species $Y$,it is because the antifertilizin of species $X$ and the fertilizin of species $Y$ are not specific to each other,preventing the necessary binding.

(B) During the process of fertilization,the sperm enters the secondary oocyte.
This entry triggers the completion of $Meiosis \ II$ in the secondary oocyte,which was previously arrested at the metaphase $II$ stage.
Additionally,the sperm contributes its centriole to the ovum,which is essential for the formation of the spindle apparatus required for the first mitotic division of the zygote.
Therefore,the introduction of centrioles into the ovum is a critical post-fertilization event.

(C) Fertilization in humans typically occurs in the ampulla region of the fallopian tube,not the womb.
Zygote is formed by the fusion of a haploid sperm and a haploid egg,resulting in a diploid $(2n)$ cell.
The fertilization membrane is a specialized layer formed after the entry of the first sperm into the egg,which prevents the entry of additional sperm,a process known as preventing polyspermy.
Therefore,the statement regarding the fertilization membrane is correct.

(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 this process,the zygote undergoes successive divisions to form $2, 4, 8,$ and finally $16$ daughter cells known as blastomeres.
The stage consisting of $8$ to $16$ blastomeres is typically referred to as the morula.
Therefore,the morula is characterized as a $16$-celled stage.

(A) Fertilizin is a glycoprotein substance secreted by the mature ovum (egg cell).
It plays a crucial role in the process of fertilization by interacting with the antifertilizin present on the surface of the sperm.
This interaction is species-specific,ensuring that the sperm of the same species can fertilize the ovum.
Therefore,the correct option is $A$.

(B) During fertilization, a sperm comes in contact with the $Zona$ $\text{pellucida}$ layer of the ovum and induces changes in the membrane that block the entry of additional sperm.
This ensures that only one sperm can fertilize the ovum, a process known as polyspermy block.
Therefore, the $Zona$ $\text{pellucida}$ acts as a primary barrier to prevent polyspermy.

(A) The diagram represents a blastocyst.
In a blastocyst,the blastomeres are arranged into an outer layer called the trophoblast and an inner group of cells attached to the trophoblast called the inner cell mass.
'$X$' points to the outer layer of cells,which is the trophoblast.

(A) Fertilization is the fusion of the male and female gametes. In humans,this process occurs in the ampullary region of the fallopian tube. For fertilization to be successful,both the ovum and the sperm must be transported to the ampullary region simultaneously. If they do not reach this specific site at the same time,fertilization cannot occur.

(B) In humans,fertilisation is the process of fusion of a sperm with an ovum. This process occurs in the $Ampullary$ region of the fallopian tube. The ovum released by the ovary is transported to the $Ampullary$ region where it meets the sperm. Fertilisation can only occur if the ovum and sperms are transported simultaneously to the $Ampullary$ region. Therefore,the correct option is $B$.