CBSE Class 12 Biology Human Reproduction Worksheet Set A

Very Short Answer Type Questions

Question. Given below are the events in human reproduction. Write them in correct sequential order. Insemination, gemetogenesis, fertilisation, parturition, gestation, implanation.
Answer : The reproductive events in humans include
(i) Gametogenesis Formation of gemetes (sperm in males, ova in females).
(ii) Insemination Transfer of sperm into female reproductive tract.
(iii) Fertilisation Fusion of male and female gemetes.
(iv) Implantation Formation, development and then attachment of blastocyst to the uterine wall.
(v) Gestation Embryonic development inside female body.
(vi) Parturition Delivery of the baby.

Question. The path of sperm transport is given below. Provide the missing steps in blank boxes.

Answer : The seminiferous tubules of the testes open into the vasa efferentia through rete testis. The vasa efferentia leave the testes and open into epididymis located along the posterior surface of each testes. The epididymis leads to vas deferens that ascends to the abdomen and loops over the urinary bladder.
It receives a duct from seminal vesicle and opens into urethra as the ejaculatory duct. These ducts store and transport the sperms from the testes to the outside through urethra.

Question. During reproduction, the chromosome number (2n) reduces to half (n) in the gametes and again the original number (2n) is restored in the offspring. What are the processes through which these events take place? 
Answer : The meiotic cell division reducers the chromosome numbers to half during gametogenesis and diploid (2n) number of chromosome is restored by the union of male and female gamete through process of fertilisation.

Question. What is the difference between a primary oocyte and a secondary oocyte?
Answer : Primary oocyte is a diploid cell formed in foetal ovary when the gamete mother cell, oogonia is arrested at prophase-I of meiosis. Secondary oocyte is the haploid cell formed
from primary oocyte that completes its first meiotic division, during puberty and produces the female gamete ova(n).

Question. What is the role of cervix in the human female reproductive system?
Answer : (i) The fundus is the broad, curved, dome-shaped upper area that receives the Fallopian tubes.
(ii) The body is the main part of the uterus, that starts directly below the level of the Fallopian tubes and continues downward until the uterine walls and cavity begin to narrow.
(iii) The isthmus is the lower, narrow neck region.
(iv) The cervix is the lowest part that extends downward from the isthmus until it opens into the vagina.

Question. Give the names and functions of the hormones involved in the process of spermatogenesis. Write the names of the endocrine glands from where they are released.
Answer : Hormones involved in spermatogenesis are   

Question. Mention the importance of LH surge during menstrual cycle.
Answer : Rapid secretion of LH leading to its maximum level during the mid menstrual cycle (14th day) called LH surge induces rupture of Graafian follicle and thereby the release of ovum (ovulation). The ovulation (ovulatory phase) is followed by the luteal phase during which the remaining parts of the Graafian follicle transform as the corpus luteum. The corpus luteum secretes large amounts of progesterone which is essential for maintenance of the endometrium. Such an endometrium is necessary of implantation of the fertilised ovum and other events of pregnancy.

Question. Which type of cell division forms spermatids from the secondary spermatocytes?
Answer : The secondary spermatocytes undergo the second meiotic division to produce four equal, haploid spermatids.
Note Secondary spermatocytes are produced, when the primary spermatocytes undergo the first meiotic division (reduction division).

Question. The mother germ cells are transformed into a mature follicle through series of steps. Provide the missing steps in the blank boxes.
Answer : The germinal epithelial cells divide repeatedly until many diploid oogonia are formed. The oogonia grow to form primary oocytes. Each primary oocyte then gets surrounded by a layer of granulosa cells and then called the primary follicle. The primary follicles get surrounded by more layers of granulosa cells and called secondary follicles.
The secondary follicle soon transforms into a tertiary follicle which is characterised by a fluid filled cavity called antrum. The primary oocyte within the tertiary follicle undergoes
meiotic division to become a secondary oocyte and a first polar body (haploid). The tertiary follicle further changes into the mature follicle or Graafian follicle. The Graafian follicle now ruptures to release the secondary oocyte (ovum) from the ovary by the process called ovulation.

Question. Why are menstrual cycles absent during pregnancy?
Answer : The menstrual flow results due to breakdown of endometrial lining of the uterus and its blood vessels which forms liquid that comes out through vagina. Menstruation only occurs if the released ovum is not fertillised. During pregnancy, all events of the menstrual cycle stop and the corpus luteum secretes large amounts of progesterone which is essential for the maintenance of the endometrium. These changes lead to no menstruation, during pregnancy.
Note Lack of menstruation may be indicative of pregnancy. However, it may also be caused due to some other underlying causes like stress, poor health, etc.

Question. Female reproductive organs and associated functions are given below in column I and II. Fill the blank blanks.
Answer : The female reproductive system consists of a pair of ovaries, a pair of Fallopian tubes (oviducts), uterus, vagina, external genitalia and mammary glands.
The ovaries have both an exocrine function (production of ova) and an endocrine function (secretion) of female sex hormones.
The oviduct (Fallopian tube) conveys the egg from the ovary to the uterus, and also provides the appropriate environment for its fertilisation. The uterus (womb) is a large, inverted,
pear-shaped, elastic sac specialised for the development of the embryo.
The vagina is adapted for receiving the penis during copulation, allowing menstrual flow and serving as the birth canal during parturition.

Question.  From where the parturition signals arise-mother or foetus? Mention the main hormone involved in parturition.
Answer : Parturition is induced by a complex neuroendocrine mechanism. The signals for parturition originate from the fully developed foetus and the placenta which induce mild uterine contractions called foetal ejection reflex.
The hormone involved in parturition is oxytocin that acts on the uterine muscle and causes stronger uterine contractions. This leads to the expulsion of the baby out of the uterus through the birth canal.

Question. What is the significance of epididymis in male fertility?
Answer : The epididymis helps the sperm in attaining maturity, acquiring increased motility and fertilising capacity. It also stores sperms for a short period before entering the vasa
deferens. The epididymis shows peristatic and segmenting contractions at intervals to push the sperm away from the testis.

Question. What is the significance of ampullary-isthmic junction in the female reproductive tract?
Answer : The act of fertilisation takes place in the female genital track that is at the junction of the isthmus and ampulla (ampullary - isthmic junction) of the Fallopian tube.
Note Fertilisation can only occur if the ovum and sperms are transported simultaneously to the ampullary - isthmic junction. All copulations do not lead to fertilisation and pregnancy.

Question. How does zona pellucida of ovum help in preventing polyspermy?
Answer : When a sperm penetrates ovum, it induces changes in the membrane that make the zona pellucida layer impenetrable to additional sperms. Thus, it ensures that only one sperm can fertilise an ovum and stops polyspermy.

Short Answer Type Questions

Question. A human female experiences two major changes, menarche and menopause during her life. Mention the significance of both the events.
Answer : In human beings, initiation of menstruation at puberty (between age 9-15 years) is called menarche. While, menstrual cycles ceases around 50 years of age, that is termed as menopause. Menarche signifies the maturation and readiness of the female reproductive system for child bearing. It marks the capability of the ovaries to produce mature oocyte (female gamete) that can now be fertilised by the sperm and also that the uterus is capable of supporting the foetal growth and development.
Menopause signifies the end of child bearing age. At this age supply of healthy eggs is very low, the levels of the hormones secreted by the ovaries decline and the menstruation stops.
The uterus no longer remains conductive for foetal growth.

Question. What is foetal ejection reflex? Explain how it leads to parturition?
Answer : Foetal ejection reflex encompasses the mild uterine contractions in response to the signals that originate from the fully developed foetus and the placenta. This triggers release of oxytocin from maternal pituitary. Oxytocin acts on the uterine muscle and causes stronger contractions, which in turn stimulates further secretion of oxytocin.
The stimulatory reflex between the uterine contraction and oxytocin secretion continues resulting in stronger and stronger contractions leading to the expulsion of baby out of uterus
through birth canal.

Question. Give a schematic labelled diagram to represent oogenesis (without descriptions).
Answer : 

Question. What are the changes in the oogonia during the transition of a primary follicle to Graafian follicle?
Answer : The germinal epithelial cells divide repeatedly until many diploid oogonia are formed. The oogonia grow to form primary oocytes. Each primary oocyte then gets surrounded by a layer of granulosa cells and then called the primary follicle.
The primary follicles get surrounded by more layers of granulosa cells and called secondary follicles. The secondary follicle soon transforms into a tertiary follicle which is characterised
by a fluid filled cavity called antrum.
The primary oocyte within the tertiary follicle undergoes meiotic division to become a secondary oocyte and a first polar body (haploid). The tertiary follicle further changes into the mature follicle or Graafian follicle that ruptures to release the secondary oocyte (ovum) from the ovary by the process called ovulation.

Question. Except endocrine function, what are the other functions of placenta.
Answer : Placenta is structural and functional unit between developing embryo (foetus) and maternal body.
Placenta acts as an endocrine tissue and produces several hormones like human Chorionic Gonadotropin (hCG), human Placental Lactogen (hPL), estrogens, progesterones, etc.
Other than the endocrine function, placenta also facilitates the supply of oxygen and nutrients to the embryo and removes carbon dioxide and excretory/waste materials produced by the developing faetus..

Question. Why doctors recommend breast feeding during initial period of infant growth?
Answer : The milk produced during the initial few days of lactation is called colostrum which contains several antibodies (especially 1gA) essential to develop resistance in the new-born babies against diseases. Breast-feeding during the initial period of infant growth is recommended by doctors for bringing up a healthy baby.

Question. (a) How many spermatozoa are formed from one secondary spermatocyte?
(b) Where does the first cleavage division of zygote take place?
Answer : (a) A primary spermatocyte completes the first meiotic division (reduction division) leading to formation of two equal, haploid cells called secondary spermatocytes (n = 23 chromosomes each). The secondary spermatocytes undergo the second meiotic division to produce four equal, haploid spermatids (n)., each spermatids produce spermatazoa.
(b) The mitotic division called cleavage starts 30 h after fertilisation when as to the zygote moves through the isthmus the Fallopian tube (oviduct) towards the uterus and forms blastomeres.

Question. Corpus luteum in pregnancy has a long life. However, if fertilisation does not take place, it remains active only for 10-12 days. Explain.
Answer : The ruptured Graafian follicle transform into the corpus luteum and secretes large amounts of progesterone which is essential for the maintenance of the endometrium. Such an endometrium is required for the implantation of fertilised ovum (blastocyst) and other events of pregnancy.
That’s why corpus luteum in pregnancy has a long life. But in the absence of fertilisation, maintenance of endometrium is not required. Therefore, corpus luteum degenerates with in 10-12 days.

Question. What are the events that take place in the ovary and uterus during follicular phase of the menstrual cycle.
Answer : The major events of the menstrual cycle are menstrual phase, follicular phase, ovulatory phase and luteal phase.
The follicular phase follows the menstrual phase. During this phase, the primary follicles in the ovary grow to become a fully mature Graafian follicle and simultaneously the endometrium of uterus regenerates through proliferation. These changes in the ovary and the uterus are induced by changes in the levels of pituitary and ovarian hormones.
The secretion of gonadotropins (LH and FSH) increases gradually during the follicular phase and stimulates follicular development as well as secretion of estrogens by the growing follicles.
Both LH and FSH attain a peak level in the middle of cycle (about 14th day). This rapid secretion of LH leading to its maximum level induces rupture of Graafian follicle to release ovum.

Question. Given below is a flow chart showing ovarian changes during menstrual cycle. Fill in the spaces giving the name of the hormones responsible for the events shown.
Answer : The secretion of gonadotropins (LH and FSH) increases gradually during the follicular phase and stimulates follicular development as well as secretion of estrogens. Rapid secretion of LH leading to its maximum level during the midcycle (14th day) of menstrual cycle induces rupture of Graafian follicle to release ovum.
The remaining parts of the Graafian follicle transform into the corpus luteum. The corpus luteum secretes large amounts of progesterone which is essential for maintenance of the
endometrium during pregnancy.  

Long Answer Type Questions

Question. What role does pituitary gonadotropins play during follicular and ovulatory phases of menstrual cycle? Explain the shifts in steroidal secretions.
Answer : Menstrual Phase (1-5 days)
Endrometrium breaks down the cell of endometrium secretions unfertilised ovum constitute menstrual flow. Progesteron production is reduced
Follicular Phese (6-13 days)
Endometrium rebuilds, FSH and oestrogen secretion is increased.
Ovulatory Phase (14-16 days)
Both LH and FSH attain peak level. Estrogen level is also high. It leads to ovulation.
Luteal Phase (16-28 days)
In absence of fertilisation corpus luteum secretes progesterone. Endometrium Thickens and uterine glands become secretary.
The menstrual cycle starts with the menstrual phase, when menstrual flow occurs and it lasts for 3-5 days. It results due to breakdown of endometrial lining of the uterus and its blood vessels. Follicular Phase The menstrual phase is followed by the follicular phase.
During this phase, the primary follicles in the ovary grow to become a fully mature Graafian follicle and simultaneously the endometrium of uterus regenerates through proliferation.
These changes in the ovary and the uterus are induced by changes in the levels of pituitary and ovarian hormones.
The secretion of gonadotropins (LH and FSH) increases gradually during the follicular phase and stimulates follicular development as well as secretion of estrogens by the growing
follicles. Both LH and FSH attain a peak level in the middle of cycle (about 14th day). Rapid secretion of LH leading to its maximum level during the mid-cycle called LH surge induces rupture of Graafian follicle and thereby the release of ovum (ovulation). The ovulation (ovulatory phase) is followed by the luteal phase during which the remaining parts of the Graafian follicle transform as the corpus luteum. The corpus luteum secretes large amounts of progesterone which is essential for maintenance of the endometrium. Such an endometrium is necessary for implantation of the fertilised ovum and other events of pregnancy. In the absence of fertilisation, the corpus luteum degenerates.This causes disintegration of the endometrium leading to menstruation.

Question. Meiotic division during oogenesis is different from that in spermatogenesis.Explain how and why?
Answer : Oogenesis is different from that of spermatogenesis in the following aspects   

Reasons
(i) Unequal cell division makes the ovum much larger than the other three polar bodies.
Because ovum has more cytoplasm and more organelles, it has a better chance of surviving.
(ii) The male makes millions of tiny sperms while, the female makes only one egg per month that also waits for second meiotic division, until just before fertilisation. This is a
way of conserving energy.
(iii) Sperm is smaller and motile as it has to move out of male system to female reproductive system. Larger egg has abundant reserve food so that embryo starts developing right
after fertilisation.

Question. The zygote passes through several developmental stages till implantation.
Describe each stage briefly with suitable diagrams.
Answer : The zygote passes through the following stages till implantations (i) The embryo with 8-16 blastomeres is called a morula.
(ii) The morula continues to divide and transforms into blastocyst as it moves further into uterus.
(iii) The blastomeres in the blastocyst are arranged into a surface layer called trophoblast and a cluster of interior cells attached to trophoblast are called the inner cell mass.
(iv) The trophoblast layer then gets attached to the endometrium and inner mass cells get differentiated as embryo.
(v) After attachment, the uterine cells divide rapidly to cover the blastocyst.
(vi) The blastocyst becomes embedded in the uterine endometrium. This is called implantation.

Question. Draw a neat diagram of the female reproductive system and label the parts associated with the following (a) production of gamete, (b) site offer tilisation (c) site of implantation and (d) birth canal.
Answer : The female reproductive system consists of a pair of ovaries along with a pair of oviducts, uterus, cervix, vagina and the external genitalia located in pelvic region.  (Image 65)
Labels
(i) Ovary (production of gamete)
(ii) Isthmus-ampullary junction(site of fertilisation)
(iii) Uterine endometrium (site of implantation)
(iv) Cervix and vagina (birth canal)

Question. With a suitable diagram, describe the organisation of mammary gland.
Answer : The description of mammary gland is as follows
(i) The mammary glands are paired structures (breasts) that contain undeveloped duct system and adipose tissue (fat containing cells).
(ii) During pregnancy, in response to oestrogen and progesterone, a glandular system develops for milk production.
(iii) Glandular tissue of each breast develops mammary lobes containing clusters of cells called alveoli.
(iv) The cells of alveoli secrete milk, which is stored in the cavities (lumens) of alveoli (v) The alveoli open into mammary tubules. The tubules of each lobe join to form a mammary duct.
(vi) Several mammary ducts join to form a wider mammary ampulla which is connected to lactiferous duct through which milk is sucked out by the baby.  (Image 65)

Question. Explain the role of pituitary and sex hormones in the process of spermatogenesis.
Answer : During spermatogenesis, gonadotropin releasing hormone (GnRH) is secreted by the hypothalamus, which stimulates the anterior pituitary gland to secrete luteinising hormone (LH) and follicle stimulating hormone (FSH). LH acts on the Leydig’s cells of the testes to secrete testosterone while FSH acts on Sertoli cells of the seminiferous tubules of the testes to secrete androgen binding protein (ABP) and inhibin. ABP concentrates testosterone and inhibin suppresses FSH synthesis. FSH also acts on spermatogonia to stimulate sperm production.

Question. Mention the event of meiosis that occurs in the tertiary follicle in a human ovary.
Answer : When  the secondary follicle transforms  into tertiary  follicle, the  primary  oocyte  within  the  tertiary follicle grows in size and completes its first meiotic division. It is an unequal division resulting in the formation of a large haploid secondary oocyte and a tiny first polar body.

Question. Write the function of each of the following : (a) Middle piece in human sperm.
(b) Luteinising hormone in human males.
Answer : (a) The middle  piece in  human  sperm possesses numerous  mitochondria,  which produce energy for the movement of tail that facilitate sperm motility essential for fertilisation.

Question. Differentiate between gametogenesis in human males and females on the basis of
(a) time of initiation of the process.
(b) Products formed at the end of the process.
Answer : Gametogenesis in human males is called spermatogenesis while it is called oogenesis in human females.
(a) Spermatogenesis is initiated at the time of puberty and oogenesis is initiated at the stage of embryonic development.
(b) In spermatogenesis, four spermatozoa are formed from one spermatogonia while in oogenesis one ovum is formed from one oogonium.

Question. Explain the steps in the formation of an ovum from an oogonium in humans.
Answer : The process of formation of a mature female gamete (ovum) is called oogenesis. It occurs in the ovaries. It consists of three phases : multiplication, growth and maturation.
(i) Multiplication phase : In the foetal development, certain cells in the germinal epithelium of the ovary of the foetus are larger than others. These cells divide by mitosis, producing a couple of million egg mother cells or oogonia in each ovary of the foetus. The oogonia multiply by mitotic divisions forming
(ii) Growth phase : This phase of the primary oocyte is very long. The oogonium grows into a large primary oocyte by taking food from the surrounding follicle cells.
(iii) Maturation phase : Each primary oocyte undergoes two maturation divisions, first meiotic and the second meiotic
In the first, meiotic division, the primary oocyte divides into two very unequal haploid daughter cells - a large secondary oocyte and a very small first polar body or polocyte. In the second maturation division, the first polar body may divide to form two second polar bodies. The secondary oocyte again divides into unequal daughter cells, a large ootid and a very small second polar body. The ootid grows into a functional haploid ovum. Thus from one oogonium, one ovum and three polar bodies are formed. The polar bodies take no part in reproduction and, hence, soon degenerate. In humans, ovum is released from the ovary in the secondary oocyte stage, this process is called ovulation.

Question. Describe in sequence the process of spermatogenesis in human.
Answer : Spermatogenesis is the process of formation of haploid spermatozoa from diploid spermatogonia inside the testes of the male. It includes following three phases :
(i) Multiplication phase – At sexual maturity, the undifferentiated primordial germ cells divide several times by mitosis to produce a large number of spermatogonia or sperm mother cells. Spermatogonia (2N) are of two types : type A spermatogonia and type B spermatogonia. Type A spermatogonia serve as the stem cells which divide to form second type of spermatogonia whenever required. Type B spermatogonia are progenitor cells which function as precursors of spermatozoa.
(ii) Growth phase - Each type B spermatogonium actively grows to a larger primary spermatocyte by obtaining nourishment from the nursing cells.
(iii) Maturation phase - Each primary spermatocyte undergoes two successive divisions, called maturation divisions. The first maturation division is reductional or meiotic. Hence, the primary spermatocyte divides into two haploid daughter cells called secondary spermatocytes. Both secondary spermatocytes now undergo second maturation division which is an ordinary mitotic division to form four haploid spermatids, by each primary spermatocyte.

Question. Mention the target cells of luteinising hormone in human males and females. Explain the effect and the changes which the hormone induces in each case. 
Answer : In males, luteinising hormone (LH) secreted by anterior lobe of pituitary gland acts on the Leydig’s cells of the testes to secrete testosterone. Testosterone brings about the growth of the secondary sex organs and development of secondary sexual characters.
In females, luteinising hormone induces the rupture of the mature graafian follicle and thereby the release of secondary oocyte (ovulation).

Question. Explain the development of a secondary oocyte (ovum) in a human female from the embryonic stage upto its ovulation. Name the hormones involved in this process.
Answer : The hormones involved in this process are luteinising hormone (LH) and follicle stimulating hormone (FSH), both secreted by anterior pituitary gland under the influence of GnRH from hypothalamus. FSH stimulates the growth of Graafian follicle and also the development of egg (secondary oocyte) within the follicle. LH induces the rupture of mature Graafian follicle and thereby the release of secondary oocyte (ovulation).

Question. (a) How is ‘oogenesis’ markedly different from
‘spermatogenesis’ with respect to the growth till puberty in the humans?
(b) Draw a sectional view of human ovary and label the different follicular stages, ovum and corpus luteum.
Answer :  (a) In spermatogenesis, the growth phase is very short. The spermatogonium actively grows into a larger primary spermatocyte by obtaining
In oogenesis, the growth phase is very long. It may extend over many years. The oogonium grows into a large primary oocyte. It then gets surrounded by alayer of granulosa cells to form primary follicle. A large number of these follicles degenerate during the period from birth to puberty. So, at puberty only 60,000 -
80,000 primary follicles are left in each ovary.

Question. When does oogenesis start in humans? Name the three hormones and their source linked with oogenesis. How do they influence the process of oogenesis? Explain
Answer : Oogenesis starts in embryonic developmental stage. The three hormones related with oogenesis are GnRH (Gonadotropin releasing hormone), luteinising hormone (LH) and follicle stimulating hormone (FSH). 

Question. Name the phase of menstrual cycle when a Graafian follicle transforms into an endocrine structure. Write its action thereafter.
Answer : Graafian follicle changes into an endocrine structure, called corpus luteum during luteal phase of menstrual cycle. Corpus luteum secretes progesterone which stimulates the uterine glands to produce increased amount of watery mucus and is also essential for maintenance of the endothelium

Question. Write the physiological reason, why a woman generally cannot conceive a child after 50 years of age?
Answer : A woman generally cannot conceive a child after 50 years of age as at this age menopause occurs. It is a phase in a woman’s life when ovulation and menstruation stops.

Question. Write the effect of the high concentration of LH on mature Graafian follicle.
Answer : High concentration of LH cause rupturing of Graafian follicle and thereby the release of ovum. This is called ovulation.

Question.Differentiate between major structural changes in the human ovary during the follicular and luteal phase of the menstrual cycle.
Answer : The major structural changes in human ovary during the follicular and luteal phase of menstrual cycle are as follows :

Question. Differentiate between menarche and menopause. 
Answer : Beginning of menstruation or first menstruation is called menarche that occurs at puberty. When menstrual cycle ceases around the age of 50 years, it is called as menopause.

Question. At what stage of menstrual cycle is corpus luteum formed in human females? When does it regress? 
Answer : Corpus luteum is formed during the luteal phase of the menstrual cycle. It regresses in the absence of fertilisation.

Question. Name the muscular and the glandular layers of human uterus. Which one of these layers undergoes cyclic changes during menstrual cycle? Name the hormone essential for the maintenance of this layer. 
Answer : The muscular layer of human uterus is myometrium and glandular layer is endometrium. The endometrium undergoes cyclical changes during menstrual cycle. Progesterone is essential for the maintenance of endometrium.

Question. Name the hormones influencing
(a) ovulation,
(b) development of corpus luteum.
Answer : a) On the 14th day of the menstrual cycle, surge in LH causes rupturing of Graafian follicle and release of ovum (ovulation).
(b) After ovulation, the remaining cells of the ovarian follicles are stimulated by LH to develop corpus luteum.

Question. Mention the role of gonadotropins in menstrual cycle. On what day of the menstrual cycle do the gonadotropins reach a peak?
Answer : LH and FSH secreted by pituitary gland are called gonadotropins.
The secretion of FSH and LH increase gradually during the follicular phase and stimulates the development of follicles as well as secretion of estrogens by the growing follicles. Both gonadotropins reach a peak level in the middle of the cycle (14th day). LH surge induces rupture of Graafian follicle and thereby the release of ovum (ovulation). This ovulatory phase is followed by luteal phase during which LH stimulates the remaining cells of the Graafian follicle to transform into corpus luteum which secretes large amounts of progesterone needed for maintenance of endometrium. In the absence of fertilisation, LH production gets reduced leading to degeneration of corpus luteum. This causes disintegration of endometrium leading to menstruation.