D3.1 Reproduction

Reproduction covers cloning, human and plant reproductive anatomy, cycles, fertilization, pregnancy, seed development and hormonal coordination across sexual life cycles.

Learning objectives

D3.1.1Sexual vs. asexual reproduction• Asexual reproduction uses mitosis without gametes or fertilization, producing clones• Sexual reproduction uses meiosis and fertilization to generate genetic variation D3.1.2Role of meiosis and gamete fusion• Meiosis produces haploid gametes and prevents chromosome doubling each generation• Random fertilization fuses gametes to form unique diploid zygotes D3.1.3Male vs. female sexes• Male gametes are small, numerous, and usually motile• Female gametes are larger, fewer, and contain resources for early development D3.1.4Human reproductive system anatomy• Male structures include testes, epididymis, sperm duct, glands, urethra, and penis• Female structures include ovaries, oviducts, uterus, endometrium, cervix, vagina, and vulva D3.1.5Ovarian and uterine cycles• FSH, LH, oestradiol, and progesterone coordinate ovarian and uterine cycles• Follicle growth, ovulation, corpus luteum, endometrium build-up, and menstruation are linked D3.1.6Fertilization in humans• Fertilization occurs in the oviduct after sperm reaches the egg• Sperm and egg nuclei fuse so paternal and maternal chromosomes form the zygote genome D3.1.7In vitro fertilization (IVF)• IVF uses hormones to stimulate superovulation and control egg maturation• Eggs are collected, fertilized outside the body, and embryos transferred to the uterus D3.1.8Sexual reproduction in flowering plants• Flowering plants produce male gametes in pollen and female gametes in ovules• Pollination, pollen-tube growth, and fertilization produce embryos inside seeds D3.1.9Insect-pollinated flower features• Insect-pollinated flowers often have petals, scent, nectar, sticky pollen, and sticky stigma• Floral structures position pollinators to transfer pollen from anther to stigma D3.1.10Promoting cross-pollination• Cross-pollination increases variation by transferring pollen between different plants• Mechanisms include dioecy, self-incompatibility, and different maturation times D3.1.11Self-incompatibility mechanisms• Self-incompatibility prevents pollen from fertilizing ovules of the same plant• Recognition systems block self-pollen growth and reduce inbreeding D3.1.12Seed dispersal and germination• Seed dispersal reduces competition with parent plants and spreads offspring• Germination uses water uptake, enzyme activation, and food reserve mobilization D3.1.13(HL)—Puberty control• GnRH from the hypothalamus stimulates pituitary FSH and LH release• Oestradiol and testosterone drive primary and secondary sexual development D3.1.14(HL)—Gametogenesis• Spermatogenesis produces four sperm by equal divisions and differentiation• Oogenesis produces one ovum and polar bodies by unequal cytokinesis D3.1.15(HL)—Preventing polyspermy• Acrosome enzymes allow sperm penetration of the zona pellucida• Cortical granule release changes the zona pellucida to block polyspermy D3.1.16(HL)—Blastocyst and implantation• Cleavage divisions form a morula and then a blastocyst with inner cell mass• The blastocyst implants in the endometrium and begins placental development D3.1.17(HL)—Pregnancy testing• Early embryo/placenta secretes hCG to maintain the corpus luteum• Pregnancy tests use monoclonal antibodies to detect hCG in urine D3.1.18(HL)—Placenta role• Placental villi provide large surface area for exchange without mixing blood• The placenta transfers gases, nutrients, wastes, antibodies, and hormones D3.1.19(HL)—Hormonal control• Progesterone and oestradiol maintain endometrium and inhibit further ovulation• Oxytocin and prostaglandins create positive feedback during childbirth contractions D3.1.20(HL)—Hormone replacement therapy (HRT)• HRT replaces declining oestradiol/progesterone after menopause to reduce symptoms• Evaluate benefits against risks including coronary heart disease and other side effects

Feature	Asexual reproduction	Sexual reproduction
gametes and fertilization	absent	present
main nuclear divisions	mitosis	meiosis makes gametes; mitosis grows offspring
parentage	usually one parent	genetic contribution from two gametes
genetic outcome	clones, except for new mutations	new allele combinations

Asexual: binary fission in Amoeba, budding in yeast or Hydra, runners in strawberry, bulbs and tubers.
Sexual: fusion of sperm and egg in animals, or male and female gametes delivered through pollen and ovules in flowering plants.

Asexual reproduction can increase numbers rapidly when a successful genotype suits stable conditions. Sexual reproduction is slower and depends on gamete transfer, but variation increases the chance that some offspring suit changed conditions.

Clones are not guaranteed to remain absolutely identical: mutation and environmental effects can still create differences.

diploid germ cell (2n) → meiosis → haploid gametes (n) → fertilization → diploid zygote (2n) → mitosis → multicellular diploid organism

Meiosis halves the chromosome-set number before gametes fuse. Fertilization then adds one haploid set from each gamete, restoring diploidy rather than doubling the species' chromosome number every generation.

Haploid means one chromosome set and diploid means two sets. The human counts 23 and 46 are examples, not definitions for every species.

A diploid organism makes haploid gametes by meiosis, two gametes fuse into a diploid zygote, and mitosis produces the next diploid organism.

Event	What is randomized	Result
crossing over in meiosis I	DNA segments exchanged between homologous non-sister chromatids	recombinant chromatids
independent assortment	maternal and paternal homologues sent to gametes in different combinations	many chromosome combinations
random fertilization	which genetically distinct sperm and egg fuse	a unique zygote combination

Meiosis creates genetically varied haploid gametes; fertilization combines two of those independently varied genomes. Mutation can add new alleles, but recombination mainly rearranges alleles already present in the parents.

Variation is a population advantage under changing conditions, not a promise that every individual offspring will be better adapted.

In anisogamous species, the sex producing small gametes is defined as male and the sex producing large gametes as female.

Gamete feature	Sperm / male gamete	Egg / female gamete
size	small	large
number produced	very many	relatively few
usual movement	motile or carried to the egg	usually non-motile
cytoplasm and reserves	little	abundant, supporting early development
shared requirement	haploid nucleus capable of fusion	haploid nucleus capable of fusion

Producing many small gametes increases the chance that at least one reaches an egg. Investing more cytoplasm and resources in each egg supports the zygote before it can obtain resources independently.

Sex is defined here by gamete type, not by stereotypes about whole-organism behaviour or parental care.

seminiferous tubules in testes → epididymis → sperm duct → fluids added by seminal vesicles and prostate → urethra → penis

Structure	Essential function
testes in scrotum	make sperm and testosterone at a temperature below body core
epididymis	completes sperm maturation and stores sperm
sperm duct	transports sperm during ejaculation
seminal vesicles and prostate	add nutrient-containing, alkaline seminal fluid
urethra and penis	conduct and deliver semen outside the body

Semen is sperm plus glandular fluid. It is not produced entirely in the testes, and semen production is not the same process as spermatogenesis.

Paired male and female human reproductive-system diagrams label the main organs and connecting ducts.

ovary releases secondary oocyte → oviduct receives and transports it; fertilization usually occurs in the upper oviduct → embryo reaches uterus → implantation occurs in endometrium → cervix and vagina form the exit route at birth

Structure	Essential function
ovary	produces oocytes, oestradiol and progesterone
oviduct	transports oocyte or embryo; usual fertilization site
uterus	muscular organ supporting embryo and foetus
endometrium	vascular lining for implantation and early support
cervix	muscular neck between uterus and vagina
vagina and vulva	canal and external structures for intercourse and birth

Draw the oviduct opening into the uterine cavity, not ending inside the uterine wall. The ovary lies near the funnel-shaped end but is not directly joined by a sealed tube.

Approximate time	Ovary	Dominant signal	Endometrium
days 1–5	new follicles begin developing	FSH rises	menstruation
days 5–13	one follicle becomes dominant	rising oestradiol	rebuilds
around day 14	ovulation	LH surge	remains prepared
days 15–28	corpus luteum active, then degenerates if no pregnancy	progesterone rises, then falls	maintained, then shed

Twenty-eight days and day 14 are useful averages, not fixed dates for every person or every cycle. Interpret event order and hormone relationships before relying on a calendar number.

A 28-day timeline aligns FSH, LH, oestradiol and progesterone curves with follicle growth, ovulation, corpus luteum and endometrial change.

FSH from anterior pituitary → follicle growth → follicle secretes oestradiol → endometrium proliferates

Oestradiol condition	Feedback effect	Functional consequence
rising during follicle development	supports further follicle responsiveness and oestradiol production	one follicle becomes dominant
high before mid-cycle	suppresses FSH but stimulates strong LH release	limits additional follicles and prepares ovulation

FSH primarily stimulates follicle development. LH is not a substitute name for FSH; its surge triggers the later ovulation transition.

high oestradiol → LH surge → ovulation → ruptured follicle becomes corpus luteum

corpus luteum → progesterone plus oestradiol → endometrium maintained and FSH/LH inhibited

If pregnancy does not begin: corpus luteum degenerates → progesterone and oestradiol fall → endometrium is no longer maintained → menstruation; loss of inhibition allows FSH to rise for a new cycle.

Ovulation is caused by the LH surge. Menstruation is caused later by loss of ovarian steroid support; the two events are not simultaneous.

Human fertilization normally begins in the upper oviduct when a sperm reaches the secondary oocyte and their plasma membranes fuse.

sperm reaches and penetrates oocyte coverings → sperm and oocyte membranes fuse → sperm nucleus enters while most sperm mitochondria and tail remain outside → oocyte completes meiosis II → male and female haploid nuclear material forms the diploid zygote genome

The zygote receives one homolog of each chromosome pair from each parent. Nearly all cytoplasmic organelles, including mitochondria, come from the egg.

Fertilization occurs in the oviduct; implantation occurs later in the uterus. These are separate events at different locations.

Control and stimulation: suppress the natural cycle when the protocol requires it → inject FSH to develop multiple follicles → give an hCG trigger to coordinate final oocyte maturation.

Collection and culture: retrieve oocytes → combine with prepared sperm or inject one sperm → culture fertilized embryos and assess early development.

Transfer and support: place selected embryo or embryos in the uterus → provide progesterone support to maintain a receptive endometrium → test for pregnancy.

Transferring more embryos can increase the chance that one implants but also increases multiple-pregnancy risk. Modern protocols balance success with maternal and foetal safety.

meiosis halves chromosome-set number and reshuffles alleles → specialized haploid gametes form → random fertilization restores diploidy → mitosis grows the genetically unique zygote

Transition	Main location	Key control or structure
sperm production and maturation	testis → epididymis	FSH/testosterone; seminiferous tubules
follicle growth and ovulation	ovary	FSH, oestradiol and LH surge
fertilization	upper oviduct	gamete encounter and membrane fusion
implantation preparation	endometrium	oestradiol and progesterone
IVF intervention	clinic/laboratory → uterus	controlled stimulation, retrieval, culture and transfer

When explaining a reproductive event, name the chromosome state, the anatomical location and the controlling signal. Mixing these three levels is the main source of fragmented answers.

pollen transferred from anther to receptive stigma (pollination) → compatible pollen germinates → pollen tube grows through style → male nuclei enter ovule through micropyle → one male nucleus fuses with egg nucleus (fertilization) → diploid zygote develops into embryo

Before fertilization	After fertilization
ovule contains female gamete	ovule develops into seed
ovary contains one or more ovules	ovary develops into fruit
egg nucleus is haploid	zygote nucleus is diploid

Pollination ends at the stigma. Pollen does not travel intact to the ovary; a pollen tube carries male nuclei from the stigma through the style.

A pollen grain germinates on the stigma and grows a pollen tube through the style into an ovule where male and female nuclei fuse.

Whorl or organ	Parts	Reproductive function
sepals	calyx	protect the developing flower bud
petals	corolla	advertise to animal pollinators
stamen	anther + filament	produce and present pollen
carpel	stigma + style + ovary	receive pollen, guide tube and contain ovules

Anther produces pollen → filament presents anther → stigma receives pollen → style provides pollen-tube path → ovary contains ovules where fertilization occurs.

A labelled insect-pollinated flower shows petals, nectary, anther, filament, stigma, style, ovary and ovules along a pollinator's contact route.

Feature	Effect on pollination
conspicuous petals and scent	attract insects from a distance
nectar	rewards entry and repeated visits
anthers placed on the route	dust visiting insect with pollen
sticky or sculptured pollen	adheres to animal body
stigma placed on the route	contacts pollen carried from another flower
sticky stigma	retains deposited pollen

The features work as a system: attraction brings the insect, reward draws it into a predictable position, and flower geometry makes its body contact anthers or stigmas.

The plant gains gamete transfer; the pollinator gains nectar or pollen food. This mutualism increases transfer efficiency but does not guarantee that every visit causes cross-pollination.

Cross-pollination transfers pollen from an anther on one plant to a stigma on a different plant of the same species, increasing opportunities to combine different parental alleles.

Mechanism	How self-pollination is reduced
anthers and stigma mature at different times	self-pollen is absent when the stigma is receptive
anthers and stigma are at different heights	a visitor contacts them on different body regions
separate male and female flowers or plants	one flower or plant cannot supply both gametes
pollinator movement or wind	carries pollen between individuals

Cross-pollination can increase genetic variation, but it also depends on pollen reaching a compatible plant. The mechanism reduces selfing; it does not ensure successful fertilization.

Self-incompatibility is a genetically controlled recognition system in which a carpel rejects pollen carrying an incompatible self identity.

pollen lands on stigma → pollen and carpel recognition alleles or proteins are compared → incompatible self pollen fails to hydrate, germinate or extend its tube → male nucleus cannot reach ovule → self-fertilization is prevented

Rejecting genetically similar pollen promotes outcrossing, reduces inbreeding and lowers the chance that identical harmful recessive alleles meet in offspring.

Self-incompatibility acts after pollen arrives. It is different from temporal or spatial separation, which reduces the chance that self pollen reaches the stigma.

Flower structure	Mature product
zygote	embryo root, shoot and cotyledon or cotyledons
fertilized ovule	seed containing embryo and food reserve
ovule wall	protective seed coat or testa
ovary	fruit surrounding or carrying seeds

Wind, water, animals or explosive fruit mechanisms carry seeds away from the parent. Separation reduces competition for light, water, minerals and space and can colonize new sites.

Pollination moves pollen before fertilization. Seed dispersal moves a fertilized, developing offspring after seed formation.

Seeds move away from a parent plant and later take up water, mobilize food reserves and emerge as seedlings.

Dormancy is an internal block to growth in a viable seed; germination is the resumed growth of the embryo using stored food.

Dormancy may persist because the embryo is immature, inhibitory abscisic acid remains, the testa is impermeable, or a species-specific chilling period has not occurred.

External condition	Why germination needs it
water	rehydrates cells, softens testa and enables metabolic reactions
oxygen	supports aerobic respiration and ATP production
suitable temperature	permits enzyme-catalysed reserve mobilization and growth

Light is required for germination in some species but not all. Water, oxygen and a suitable temperature are the general conditions emphasized here.

water uptake activates embryo → embryo releases gibberellin → signal reaches food-storage tissue → hydrolytic enzymes such as amylase are produced → starch becomes maltose and then glucose; proteins become amino acids → soluble products move to growing root and shoot

Mobilized product	Immediate use
glucose	respiration for ATP and carbon skeletons for biosynthesis
amino acids	synthesis of enzymes and structural proteins

The seedling depends on stored reserves until leaves expand and photosynthesis supplies enough organic material for independent growth.

pollen production → transfer to stigma → compatibility check → pollen-tube delivery → gamete fusion in ovule → seed and fruit formation → dispersal → dormancy release → germination and reserve mobilization

Transition problem	Adaptation
pollen must reach a conspecific stigma	pollinator attraction, wind transfer and flower geometry
selfing can reduce variation	timing, spatial separation, separate sexes and self-incompatibility
offspring compete with parent	seed dispersal
growth begins in poor conditions	dormancy and environmental germination requirements

Pollination is gamete delivery, fertilization is nuclear fusion, and dispersal moves the resulting seed. Keep those transitions separate while explaining how each supports successful outcrossing and establishment.

hypothalamus increases pulsatile GnRH → anterior pituitary releases FSH and LH → gonads increase gametogenesis and steroid-hormone secretion → reproductive organs mature and secondary sexual characteristics develop

Shared level	Testis-dominant response	Ovary-dominant response
pituitary gonadotropins	FSH supports sperm production; LH stimulates testosterone secretion	FSH supports follicle development; LH supports ovulation and ovarian steroid secretion
gonadal steroids	testosterone supports reproductive maturation and male-typical secondary traits	oestradiol and progesterone support reproductive maturation and female-typical cyclic changes

Sex steroids also influence bone growth, muscle protein synthesis and changes in body composition. Puberty is therefore a coordinated endocrine transition, not a single anatomical event.

FSH and LH operate in all sexes; their names reflect how they were first studied, not hormones found only in ovaries.

Spermatogenesis begins at puberty in the germinal epithelium of seminiferous tubules and continues toward the tubule lumen with support from Sertoli cells.

Cell stage	Division or change	Ploidy / product
spermatogonium	mitosis and growth	primary spermatocyte, 2n
primary spermatocyte	meiosis I	two secondary spermatocytes, n
secondary spermatocytes	meiosis II	four spermatids, n
spermatids	differentiation, no further division	four mature sperm, n

Sertoli cells nourish and organize developing germ cells. Interstitial cells between tubules secrete testosterone; the epididymis completes sperm maturation and stores the cells after they leave the testis.

Spermatids are the products of meiosis; sperm are the differentiated cells. Adding seminal-vesicle and prostate fluid later produces semen, not more sperm.

Oogenesis begins before birth. Primary oocytes remain arrested in follicles until cycles after puberty recruit a small group for further development.

oogonium divides by mitosis and grows → primary oocyte begins meiosis → meiosis I completes with unequal cytokinesis → large secondary oocyte plus first polar body → secondary oocyte begins meiosis II and is ovulated while arrested → sperm entry triggers completion → ovum plus second polar body

Unequal cytokinesis keeps almost all cytoplasm, organelles and stored materials in one cell while polar bodies remove extra chromosome sets. The products are genetically haploid but not equal in developmental capacity.

The cell released at ovulation is normally a secondary oocyte, not yet a completed ovum. Meiosis II finishes only if fertilization begins.

Dimension	Spermatogenesis	Oogenesis
begins	puberty	before birth, resumes after puberty
production pattern	continuous after puberty	cyclic recruitment from finite primary-oocyte pool
cytokinesis	approximately equal	highly unequal
products per primary cell	four functional sperm	one functional ovum plus polar bodies
meiosis II	completes before sperm differentiation	completes only after sperm entry

Both begin with diploid germ cells, include a multiplication and growth phase, use meiosis I to separate homologues and meiosis II to separate sister chromatids, and produce haploid nuclei.

The contrast explains anisogamy: sperm production maximizes the number of motile gametes, whereas oogenesis concentrates resources into one large cell capable of supporting the earliest embryo.

Parallel spermatogenesis and oogenesis routes track mitosis, meiosis I, meiosis II, four sperm and one ovum with polar bodies.

GnRH → pituitary FSH and LH → gonadal steroid secretion plus gametogenesis → sexually mature reproductive systems

Question	Sperm route	Oocyte route
where?	seminiferous tubules	ovarian follicles
when?	continuous from puberty	begins before birth; resumes cyclically
how many functional products?	four per primary spermatocyte	one per primary oocyte
why different?	maximize delivery probability	concentrate cytoplasm and reserves

Track cell name, chromosome-set number, cytoplasm and timing together. Those four dimensions distinguish the routes more reliably than memorizing isolated stage lists.

sperm passes between follicle cells → sperm surface interacts with zona pellucida → acrosomal membrane releases hydrolytic enzymes → enzymes digest a local path through zona → sperm reaches and fuses with oocyte plasma membrane

The acrosome is a membrane-bound enzyme compartment at the sperm head. Its reaction is localized: it enables one sperm to cross the extracellular coat rather than dissolving the entire zona pellucida.

The acrosome reaction permits penetration. It does not itself prevent polyspermy; the block is triggered after sperm–oocyte membrane fusion.

A sperm acrosome releases enzymes to cross the zona pellucida, then sperm–oocyte fusion triggers cortical granule release that blocks other sperm.

Fusion of the first sperm and oocyte plasma membranes triggers a rapid calcium-mediated release of cortical-granule contents by exocytosis.

first membrane fusion → cortical granules release enzymes outside oocyte → zona-pellucida proteins are modified and hardened → additional sperm can no longer bind or penetrate effectively → only one paternal nucleus enters

If two sperm nuclei fused with the oocyte nucleus, the zygote would receive an abnormal extra chromosome set. Blocking later sperm therefore protects the diploid chromosome complement required for normal development.

The block must follow the first successful fusion quickly. A barrier acting before any fusion would prevent fertilization altogether.

zygote → 2-cell and 4-cell cleavage stages → compact morula → fluid-filled blastocyst

During cleavage, repeated mitoses partition the original zygote cytoplasm into progressively smaller blastomeres. Cell number rises, but the embryo has not yet gained substantial mass.

Cilia and smooth-muscle activity move the dividing embryo down the oviduct so that blastocyst formation and uterine arrival are coordinated.

A zygote divides through cleavage and morula stages to a blastocyst with inner cell mass and outer trophoblast before implantation.

Blastocyst region	Early role
inner cell mass	gives rise to the embryo proper and later foetus
outer trophoblast cells	attach to and invade endometrium; contribute to placenta
fluid-filled cavity	creates the blastocyst organization rather than a solid morula

about day 7: blastocyst contacts receptive endometrium → trophoblast attaches and invades → embryo embeds over the following days → maternal tissue and embryonic membranes begin forming the placenta

Implantation anchors the embryo and establishes access to endometrial nutrients before the mature placental exchange system is complete.

The blastocyst is not simply a larger morula. Its cells have reorganized into an inner mass, outer layer and cavity with different developmental roles.

Without a pregnancy signal, the corpus luteum degenerates, progesterone falls and the endometrium is shed. An implanted embryo must interrupt that normal reset.

trophoblast / early placenta secretes hCG → hCG maintains corpus luteum → corpus luteum continues progesterone secretion → endometrium remains thick and vascular → early pregnancy is sustained

As the placenta develops, it becomes an endocrine organ and increasingly supplies progesterone and oestradiol itself, so pregnancy no longer depends on the corpus luteum indefinitely.

The signal must be produced early because the non-pregnant cycle is already programmed to reduce luteal hormone support.

A lateral-flow pregnancy test detects hCG in urine with specific monoclonal antibodies and produces separate test and control lines.

urine moves by capillary action → hCG binds mobile coloured anti-hCG antibody → complex is captured by immobilized antibody at test line → coloured test line appears

excess mobile coloured antibody continues along strip → a different immobilized antibody captures it at control line → coloured control line confirms that liquid flowed and reagents functioned

Lines visible	Interpretation
control + test	hCG detected
control only	no hCG detected above test threshold
no control line	invalid test, regardless of test-line appearance

Placental feature	Exchange advantage
many branching villi and microvilli	large surface area
thin barrier between maternal blood space and foetal capillaries	short diffusion distance
continuous maternal and foetal blood flow	maintains concentration gradients
extensive foetal capillary network	rapidly carries exchanged substances away or toward surface

Maternal blood bathes the outside of villi; foetal blood remains inside villus capillaries connected to the umbilical cord. Substances cross the placental barrier, but the blood cells normally do not mix.

Separation reduces immune and pressure incompatibilities while still permitting efficient molecular exchange across a small distance.

Placental villi containing foetal vessels project into maternal blood spaces while the two blood supplies remain separated by a thin exchange barrier.

Material	Main direction	Typical mechanism or significance
oxygen	mother → foetus	diffusion down maintained gradient
carbon dioxide	foetus → mother	diffusion
glucose	mother → foetus	facilitated diffusion
amino acids and some ions	mother → foetus	active transport contributes
water	both directions as gradients require	osmosis
urea and other wastes	foetus → mother	diffusion / transport for maternal excretion
maternal antibodies	mother → foetus	receptor-mediated transfer gives passive immunity

The placenta is a selective exchange organ, not a perfect shield. Many bacteria are excluded, but some viruses, drugs, alcohol and other small molecules can cross and affect development.

The foetus does not breathe air or eat directly. Maternal respiratory, digestive and excretory systems supply and remove materials through the placental interface.

Early pregnancy: embryonic hCG maintains corpus luteum → corpus luteum secretes progesterone and oestradiol → endometrium remains intact.

Later pregnancy: placenta becomes main source of progesterone and oestradiol → uterine lining and pregnancy conditions continue after corpus luteum declines.

Hormonal effect	Reproductive consequence
progesterone maintains endometrium and suppresses uterine contraction	implantation site remains supportive
progesterone and oestradiol inhibit FSH and LH	further follicle development and ovulation are suppressed

The source changes from ovary to placenta, but hormone output overlaps so support is not interrupted during the handover.

Near birth, reduced progesterone influence removes inhibition of uterine muscle, allowing oxytocin and prostaglandins to promote coordinated contractions.

uterine contractions push foetal head against cervix → cervix stretches → sensory signals reach hypothalamus → posterior pituitary releases more oxytocin → contractions become stronger and more frequent → cervix stretches further

The loop is positive because each response strengthens the original stimulus. Birth removes the foetal head from the cervix, ending stretch and therefore breaking the loop.

Positive feedback is useful here because labour needs a decisive endpoint. It is not the usual stabilizing negative feedback used to maintain a physiological variable near a set point.

During menopause, ovarian responses and secretion of oestradiol and progesterone decline, menstrual cycles cease and loss of steroid-hormone effects can produce symptoms.

Hormone replacement therapy supplies oestradiol, with a progestogen when protection of an intact endometrium is required, to reduce symptoms such as hot flushes and other effects of hormone loss.

Benefit and risk depend on the individual, hormone formulation, dose, route, timing and duration. The biology course uses HRT to examine evidence, not to prescribe one treatment for everyone.

Replacing hormones can reduce symptoms without restoring fertility or reversing ovarian ageing.

Study design	Observation	Main inference limit
early observational studies	HRT users had lower CHD incidence	users differed in socioeconomic status, health-service use and other risk factors
randomized controlled trials	treatment groups showed a small CHD-risk increase in the studied populations	result depends on participants, regimen and timing but better isolates treatment effect

A confounding variable is associated with both the exposure and the outcome. Higher socioeconomic status could increase HRT uptake while independently lowering coronary risk, creating a protective-looking correlation.

To claim causation, ask whether groups were comparable before treatment, whether allocation was randomized, whether outcome measurement was blinded, and whether the effect is consistent with biological mechanism and other trials.

Correlation can motivate a hypothesis, but it cannot by itself show that HRT caused the lower—or higher—disease rate.

acrosome opens a local path → cortical reaction closes entry to later sperm → cleavage builds cell number → blastocyst opens an interface with endometrium → hCG keeps hormonal support open → placenta maintains selective exchange → oxytocin feedback drives birth to completion

Transition	Enabling mechanism	Protective limit
fertilization	acrosomal penetration and membrane fusion	cortical polyspermy block
implantation	trophoblast attachment and invasion	organized endometrial interface
foetal exchange	thin, vascular placental villi	maternal and foetal blood remain separate
pregnancy to birth	progesterone support followed by oxytocin amplification	loop ends when cervical stretch ends
hormone therapy	replacement of lost signalling	evidence-based benefit–risk selection

Successful reproduction is not one event. Each stage depends on a signal or structure that enables the next transition while preventing an incorrect place, time, cell or causal conclusion.

3 marks

Outline natural methods of cloning in some eukaryotes.

4 marks

Explain the need for both fusion of gametes and meiosis in a sexual life cycle.

6 marks

Draw a labeled diagram of the female reproductive system.

8 marks

Explain the roles of specific hormones in the menstrual cycle, including positive and negative feedback mechanisms.

6 marks

Describe the process of fertilization in humans.

9 marks

Embryos that are produced by in vitro fertilization can be screened for genetic disease. Outline the process of in vitro fertilization, including one example of a situation when it is used.

4 marks

Outline pollination, fertilization and seed dispersal.

4 marks

Draw a half-view of an animal-pollinated flower.

2 marks

Outline how cross-pollination can be promoted by flowering plants.

1 mark

Cherry trees (Prunus avium) have two self-incompatibility alleles. What benefit do self-incompatibility alleles have?

6 marks

Outline the metabolic processes that occur in starchy seeds during germination.

1 mark

What controls the developmental changes during puberty?

I. Increased release of gonadotropin-releasing hormone (GnRH) by the hypothalamus
II. Luteinizing hormone (LH) leading to increased sex hormone production
III. Gonadotropin-releasing hormone (GnRH) triggering the onset of increased luteinizing hormone (LH)

8 marks

Compare and contrast the processes of spermatogenesis and oogenesis.

2 marks

Explain the mechanism that prevents polyspermy during fertilization.

1 mark

What is a blastocyst?

1 mark

What is a function of human chorionic gonadotropin (HCG)?

8 marks

Explain how the structure and functions of the placenta maintain pregnancy.

7 marks

Describe the hormone feedback mechanisms that help to prepare a woman's body for pregnancy, sustain the pregnancy and then give birth.

Next Topic

—D3.2 InheritanceRead

Sexual vs. asexual reproduction

Role of meiosis and gamete fusion

Human reproductive system anatomy

Ovarian and uterine cycles

Fertilization in humans

In vitro fertilization (IVF)

Sexual reproduction in flowering plants

Insect-pollinated flower features

Promoting cross-pollination

Self-incompatibility mechanisms

Seed dispersal and germination

Puberty control

Gametogenesis exam focus

Preventing polyspermy

Blastocyst and implantation

Pregnancy testing

Placenta role

Hormonal control