Reproduction: continuity or change

Reproduction is the biological process by which organisms produce new individuals of the same species. The main comparison here is straightforward, but it matters: asexual reproduction keeps successful gene combinations together, while sexual reproduction reshuffles them.

In asexual reproduction, one parent produces offspring without fusion of gametes. In eukaryotes, this usually depends on mitosis, so the offspring are genetically identical to the parent unless mutation has occurred. That can work well when an individual is already well adapted to a stable environment. The parent is, in effect, making more bodies with the same successful genotype.

Sexual reproduction involves haploid gametes fusing to form a genetically new individual. It takes longer and usually uses more energy, but it gives offspring new combinations of alleles. That variation provides the raw material for adaptation when environments change.

Key differences between asexual and sexual reproduction.

Avoid saying that asexual reproduction is always “better” because it is faster, or that sexual reproduction is always “better” because it produces variation. The advantage depends on the environment: continuity helps when conditions stay favourable; change helps when conditions shift.

Two opposite events in one life cycle

A sexual life cycle depends on two processes that balance each other. Meiosis is a nuclear division process that reduces chromosome number by producing haploid nuclei from a diploid nucleus. Fertilization is the fusion of male and female gametes to form a zygote.

Without meiosis, fertilization would make the chromosome number double in every generation. Meiosis halves the chromosome number; fertilization restores it. That’s the chromosome bookkeeping.

There’s also the genetic side. During meiosis, segregation and recombination break up parental combinations of alleles. When two gametes fuse, alleles from two parents come together in a new combination. That is why siblings produced sexually are usually genetically different from each other.

A gamete is a haploid reproductive cell that can fuse with another gamete during fertilization. A zygote is a diploid cell formed by fusion of gametes and is the first cell of a new sexually produced individual.

The prime difference: which gamete travels?

In many sexual organisms, the two gametes aren’t the same. Anisogamy is a reproductive condition in which male and female gametes differ in size and usually in mobility. The male gamete travels to the female gamete; from that one fact, most of the other differences follow.

A male gamete is a usually small reproductive cell that travels towards a female gamete and contributes genetic material with little stored food. A female gamete is a usually larger reproductive cell that remains relatively stationary and contains more cytoplasm and food reserves for early development.

Small male gametes can be made in very large numbers. Larger, more resource-rich female gametes are produced in smaller numbers. This difference affects reproductive strategies too: males often increase reproductive success by reaching more eggs, while females often invest more resources per gamete or offspring.

Comparison of male and female gametes in anisogamy.

Isogamy is a reproductive condition in which fusing gametes are similar in size and appearance. Some fungi show this, but animals and flowering plants are anisogamous.

Male-typical reproductive anatomy

The male-typical reproductive system produces sperm, adds fluid to make semen, and transfers semen into the female-typical reproductive tract.

Key structures and functions are:

Female-typical reproductive anatomy

The female-typical reproductive system produces eggs, receives sperm, provides a site for fertilization, supports pregnancy and allows birth.

When you draw these systems, use clear continuous outlines. Label lines should touch the structure being named, with positions and proportions kept sensible. For example, the sperm duct must connect from the epididymis towards the urethra, and the oviduct must lead from near the ovary towards the uterus.

The menstrual cycle is two linked cycles

The menstrual cycle is the repeating set of changes in the ovaries and uterus that prepares the body for a possible pregnancy. It includes the ovarian cycle, the repeated development and release of an egg from the ovary, and the uterine cycle, the repeated change in the uterus lining, or endometrium.

Day 1 is the first day of menstruation. In the ovary, the follicular phase is the part of the cycle when follicles develop. A follicle is an ovarian structure containing an oocyte surrounded by supporting cells. Usually, one follicle becomes dominant. Ovulation is the release of an egg from a mature follicle into the oviduct, usually around the middle of the cycle.

After ovulation, the wall of the follicle forms the corpus luteum, a temporary endocrine structure that secretes progesterone and some oestradiol. This part is the luteal phase. If pregnancy does not occur, the corpus luteum degenerates and hormone levels fall.

In the uterus, the endometrium is the inner lining. It thickens and becomes more vascular in preparation for implantation. If no embryo implants, the endometrium breaks down and is shed during menstruation.

Hormonal regulation and feedback

Follicle-stimulating hormone (FSH) is a pituitary protein hormone that stimulates follicle development and oestradiol secretion. Luteinizing hormone (LH) is a pituitary protein hormone that triggers ovulation and formation of the corpus luteum.

Oestradiol is an ovarian steroid hormone that stimulates repair and thickening of the endometrium and affects pituitary hormone secretion. At moderate levels, it inhibits FSH secretion. That is negative feedback, a control mechanism in which a change reduces the process that caused it. At high levels near mid-cycle, oestradiol stimulates LH secretion and produces positive feedback, a control mechanism in which a change increases the process that caused it. The result is the LH surge.

Progesterone is an ovarian steroid hormone that maintains the thickened endometrium and inhibits FSH and LH secretion. When progesterone falls near the end of the cycle, menstruation can occur and the next cycle can begin.

A useful data skill here is to graph hormone concentration against day of cycle. Real cycles vary in length, and hormone peaks do not occur on identical days in every person. The pattern to recognize is: FSH supports follicle growth, oestradiol rises before ovulation, LH surges at ovulation, and progesterone dominates the luteal phase.

From sperm-egg contact to the first mitosis

In humans, fertilization starts when a sperm reaches the egg, moves through the surrounding follicle cells, and penetrates the glycoprotein layer called the zona pellucida. Proteins in the sperm membrane bind to proteins in the egg membrane, and the two cell membranes fuse.

The sperm nucleus then enters the egg. The sperm tail and mitochondria are not inherited as functional parts of the embryo: the tail is left outside or destroyed, and sperm mitochondria are normally destroyed. That’s why mitochondrial inheritance is usually maternal.

Once the sperm nucleus has entered, the egg is activated. At first, the sperm and egg nuclei stay separate. Their nuclear membranes then dissolve before the first mitotic division. Condensed chromosomes from both nuclei attach to the same spindle and go through one joint mitosis, producing two diploid nuclei.

A careful detail: the zygote is not produced by the sperm “injecting a whole sperm cell” into the egg. For inheritance, the key event is entry of the sperm nucleus and participation of both parental chromosome sets in the first mitosis.

Hormonal control is deliberately overridden

In vitro fertilization (IVF) is a medical procedure in which eggs are fertilized by sperm outside the body under controlled laboratory conditions, and resulting embryos may be transferred to the uterus.

The first hormonal target is the person’s normal cycle. Drugs stop the pituitary from secreting its usual FSH and LH, so ovarian secretion of oestradiol and progesterone falls as well. Doctors can then control the timing of ovarian events instead of having to work around the natural cycle.

Artificial hormone doses are then given to induce superovulation, which is the development and release-ready maturation of more follicles than usual in one cycle. FSH injections make many follicles develop. Later, another hormone dose promotes the final maturation of the eggs before collection.

Eggs are collected from follicles and mixed with sperm in sterile culture conditions. If fertilization and early development occur, one or more embryos may be transferred to the uterus. Progesterone support is commonly used so that the endometrium remains suitable for implantation.

For the syllabus, the key point is not every clinical detail of IVF. Normal hormone secretion is suppressed first, then artificial hormone doses are used to produce more eggs than a natural cycle would usually provide.

Flowers are sexual organs

In flowering plants, sexual reproduction happens in flowers, even in a hermaphroditic flower or plant. A hermaphroditic flower is a flower that has both male and female reproductive structures; it can contribute male gametes and female gametes, but fertilization is still fusion of gametes after meiosis.

The male structures are stamens. Each is usually an anther held on a filament. An anther is a flower structure that produces pollen grains. Inside anthers, diploid cells undergo meiosis and form haploid cells, which then develop into pollen grains. Pollen is a male gametophyte structure that carries male gametes and can be transferred to a stigma.

The female structures are carpels, which include stigma, style and ovary. A stigma is a receptive surface that receives pollen. A style is a stalk-like structure through which a pollen tube grows towards the ovary. An ovary is the basal part of a carpel that contains ovules. An ovule is a structure inside the ovary that contains the female gamete and can develop into a seed after fertilization.

Pollination is the transfer of pollen from an anther to a stigma. Don’t confuse it with fertilization. After pollination, the pollen grain grows a pollen tube down the style. Male gametes travel through the pollen tube to an ovule. Fertilization occurs inside the ovule when a male gamete fuses with the egg. The zygote develops into an embryo with an embryo root, embryo shoot and one or two cotyledons.

This links neatly to interspecific relationships and reproduction. Many plants depend on animals to transfer pollen, while the animals may get nectar or pollen as food. That is a mutualism, an interspecific interaction in which both species benefit.

How structure guides the pollinator

An insect-pollinated flower is a flower with structural and chemical features that make it more likely that visiting insects transfer pollen between flowers of the same species.

Typical features include:

• large or brightly coloured petals that advertise the flower and guide insects;
• scent that attracts insects from a distance;
• nectar secreted by nectaries as an energy reward;
• nectaries positioned so the insect brushes past anthers and stigma;
• large, sticky or spiky pollen grains that attach to insect bodies;
• a sticky stigma that captures pollen carried by insects;
• anthers and stigma positioned in the insect’s path.

For a drawing of an insect-pollinated flower, a half-view diagram often works best, since it can show both the outside of the flower and the internal ovules. Annotate functions, not just labels: petals attract, sepals protect the bud, anthers produce pollen, filaments position anthers, stigma receives pollen, style provides the route for the pollen tube, ovary contains ovules, and ovules contain the female gamete and later become seeds.

Interspecific relationships are doing real biological work here. The plant gets pollen transfer; the insect gets food. Your diagram should show that relationship clearly, with the insect directed past the reproductive structures.

Avoiding self-pollination before fertilization

Cross-pollination is the transfer of pollen from an anther on one plant to a stigma on another plant of the same species. Because the male and female gametes come from different plants, it promotes new allele combinations.

Self-pollination is the transfer of pollen from an anther to a stigma on the same plant. In a hermaphroditic plant, this can cause inbreeding. Homozygosity increases, and vigour may fall if harmful recessive alleles are expressed.

Plants promote cross-pollination in several ways:

The evolutionary advantage is genetic variation. Cross-pollination makes it more likely that fertilization combines alleles from different individuals, which can improve vigour and provide variation for natural selection when environments change.

A genetic barrier to self-fertilization

Self-incompatibility is a genetic mechanism in flowering plants that blocks self-pollen, or pollen from genetically similar plants, from fertilizing ovules successfully. It can prevent pollen from germinating on the stigma or stop the pollen tube growing through the style.

What’s being avoided is inbreeding. Inbreeding means reproduction between closely related individuals, which raises the chance that offspring inherit identical alleles from a recent common ancestor. In plants, self-pollination is an extreme form of inbreeding. It tends to lower genetic diversity and can reduce vigour.

Self-incompatibility systems depend on alleles at one or more compatibility genes. When the pollen and stigma share certain self-incompatibility alleles, the plant rejects the pollen. If they differ enough, the pollen tube can grow and fertilization can occur. As a result, the male and female gametes that fuse during fertilization are more likely to come from different plants.

This has practical consequences for crops. An orchard planted with one self-incompatible variety may flower well but produce little fruit. Planting a second compatible variety nearby can provide pollen with different compatibility alleles, allowing fruit formation.

Dispersal is not pollination

Seed dispersal is the movement of seeds away from the parent plant to new sites. By moving away, seeds face less competition with the parent plant and with each other, and the species may spread into new habitats.

Pollination and seed dispersal happen at different stages in the flowering plant life cycle. Pollination moves pollen from anther to stigma before fertilization. Seed dispersal happens later, after fertilization and embryo development, when seeds move away from the parent plant.

Key differences between pollination and seed dispersal in flowering plants.

Fruits and seeds have adaptations for different dispersal methods. Dry fruits may split explosively, fleshy fruits may be eaten by animals, winged or feathery fruits may travel on the wind, and hooked fruits may catch on animal fur.

Germination is the resumption of growth of a seed embryo under suitable conditions. As germination begins, the embryo grows and develops by using food reserves stored in the seed. Enzymes mobilize these reserves, converting stored molecules into soluble products that can be transported to the growing embryo. The embryo root usually emerges first, followed by growth of the embryo shoot.

Investigations of seed dispersal work well for experimental design. For example, if you test how drop height affects the distance travelled by winged seeds, drop height is the independent variable and dispersal distance is the dependent variable. Control seed type, release method, wind conditions, surface, and measurement method, then repeat enough trials to judge the spread of results.

The hypothalamus starts the sequence

Puberty is the sequence of developmental changes that transforms a child into a sexually mature individual. It begins when the hypothalamus releases more gonadotropin-releasing hormone (GnRH), a peptide hormone that stimulates the pituitary gland to release gonadotropins.

As puberty starts, GnRH is released in stronger pulses during childhood. The pituitary gland then releases more LH and FSH. These hormones are called gonadotropins because they act on the gonads: testes or ovaries.

In typical male bodies, LH stimulates cells in the testes to secrete testosterone. FSH and LH also support sperm production and testis development. Testosterone is a steroid sex hormone that promotes development of male secondary sexual characteristics.

In typical female bodies, FSH stimulates follicle development and oestradiol secretion. LH supports ovulation and later corpus luteum formation. Oestradiol and progesterone help develop female secondary sexual characteristics and reproductive tissues.

Remember the chain like this: the hypothalamus increases GnRH, the pituitary increases FSH and LH, the gonads increase steroid sex hormones, and those steroid hormones produce the visible and functional changes of puberty.

Same broad stages, different outcomes

Gametogenesis is the production of gametes from germ-line cells. In humans, it involves mitosis, cell growth, two meiotic divisions and differentiation.

Spermatogenesis is gametogenesis in the testes that produces sperm. Germinal epithelium cells divide by mitosis and grow into primary spermatocytes. Each primary spermatocyte then completes meiosis I and meiosis II, producing four haploid spermatids. These spermatids differentiate into sperm by forming a tail, condensing the nucleus and reducing cytoplasm. The result is a large number of small, motile gametes.

Oogenesis is gametogenesis in the ovaries that produces eggs. Mitosis occurs before birth, and the cells enter meiosis but then pause. After puberty, some follicles begin developing during each cycle. Usually one oocyte completes part of meiosis and is ovulated. Because the divisions are unequal, one large egg gets most of the cytoplasm, while small polar bodies receive very little.

Learn the contrast clearly: spermatogenesis usually produces four functional sperm per meiosis, continuously from puberty, with each sperm containing little cytoplasm. Oogenesis usually produces one functional egg per meiosis, released cyclically, with abundant cytoplasm to support early development.

One sperm only

Polyspermy means fertilization of an egg by more than one sperm. In humans, that would give the embryo an abnormal chromosome number, so it is not viable. Two linked barriers keep polyspermy rare: the acrosome reaction and the cortical reaction.

The acrosome reaction

The acrosome reaction happens when a sperm binds to the zona pellucida and releases digestive enzymes from its acrosome. The acrosome is an enzyme-containing vesicle in the sperm head. The zona pellucida is a glycoprotein coat surrounding the egg. Acrosomal enzymes digest a route through the zona pellucida, allowing one sperm to reach the egg membrane and fuse with it.

The cortical reaction

The cortical reaction is the exocytosis of enzyme-containing cortical granules from the egg after the first sperm fuses with it. These enzymes harden the zona pellucida and alter sperm-binding glycoproteins, so no additional sperm can bind and pass through.

Barriers have a clear role here in living systems. Before fertilization, the zona pellucida acts as a selective barrier that the successful sperm must penetrate. Immediately after fertilization, that same layer is changed into a stronger barrier against all other sperm.

Early development to implantation

After fertilization, the zygote has one haploid nucleus from the sperm and one from the egg. Both replicate their DNA. In the first mitosis, the nuclear membranes break down, and chromosomes from both parents divide on the same spindle, producing two diploid nuclei.

The early embryo keeps dividing by mitosis. The original egg cytoplasm is shared out between more and more cells, so these early cells get smaller rather than growing much between divisions.

A blastocyst is a hollow early embryonic stage with an outer cell layer, an internal fluid-filled space and an inner cell mass. It forms several days after fertilization as cells divide, move and specialize. The blastocyst moves from the oviduct to the uterus.

Implantation is the attachment and embedding of the blastocyst into the endometrium. The outer cell layer grows into the uterine lining and starts exchange with maternal tissues, while the inner cell mass continues developing into the embryo.

You don't need to learn a list of extra embryonic stage names here. Focus on the blastocyst and implantation in the endometrium.

Detecting a hormone from the embryo or placenta

Human chorionic gonadotropin (hCG) is a protein hormone released by the early embryo and, later, by the developing placenta. In early pregnancy, hCG keeps the corpus luteum active, so it continues to secrete progesterone and maintain the endometrium.

Pregnancy tests detect hCG in urine with monoclonal antibodies, which are identical antibody molecules made from one clone of cells and bind to one specific antigen. On the test strip, mobile dye-labelled antibodies attach to hCG if it’s present. Farther along the strip, immobilized antibodies catch the hCG-antibody complex, producing a coloured test band.

The strip also needs a separate control band. This band forms whether or not hCG is present, showing that urine has moved along the strip and that the antibodies were working. Two bands therefore indicate hCG detected; one control band indicates no detectable hCG.

Exchange without mixing blood

The placenta forms from foetal and maternal tissues. During pregnancy, it lets materials pass between the foetus and the mother, so a human foetus can stay in the uterus to a more advanced stage than mammals that do not develop a placenta.

For structure, you only need one detail: placental villi provide a large surface area. These finger-like projections of foetal tissue are bathed by maternal blood, while foetal blood flows through capillaries inside the villi. The two blood supplies come very close, but they do not normally mix.

The placental barrier is a selectively permeable layer of cells between maternal blood and foetal blood. It allows exchange, while preventing direct blood mixing. Oxygen, glucose, amino acids, water and some antibodies move from mother to foetus. Carbon dioxide, urea and other wastes move from foetus to mother.

Small molecules such as oxygen and carbon dioxide cross by diffusion. Glucose moves by facilitated diffusion, water by osmosis, and some larger molecules such as antibodies by endocytosis.

The placenta is another useful biological barrier: close enough for rapid exchange, but separate enough to keep maternal and foetal circulations distinct.

Maintaining pregnancy

Progesterone maintains pregnancy. In the early stages, the corpus luteum secretes progesterone under the influence of hCG. Later, the placenta takes over progesterone secretion. Progesterone keeps the endometrium in place and inhibits contractions of the myometrium, the muscular wall of the uterus.

That continuous support matters. If progesterone support fails too early, the uterus lining and uterine muscle activity may no longer support pregnancy.

Triggering childbirth

Near the end of pregnancy, progesterone levels fall. With less inhibition of uterine contractions, secretion of oxytocin can occur; this pituitary hormone stimulates contraction of uterine smooth muscle.

Oxytocin causes contractions of the myometrium. As the baby’s head stretches the cervix, stretch receptors send signals that lead to more oxytocin release. More oxytocin makes contractions stronger, causing more stretch and then more oxytocin release. This is positive feedback.

The positive feedback loop stops once birth occurs and the stretch stimulus is removed. During childbirth, the cervix dilates, the amniotic sac ruptures, the baby is pushed through the cervix and vagina, and the placenta is expelled afterwards.

Correlation is not automatically causation

Hormone replacement therapy (HRT) is a medical treatment that supplements oestrogen, and sometimes progesterone, to reduce symptoms associated with menopause. Coronary heart disease (CHD) is a condition in which coronary arteries are narrowed or blocked, reducing blood supply to heart muscle.

Early epidemiological studies found a lower incidence of CHD in women using HRT. At first glance, it looked as if HRT might protect against CHD. Correlation alone, though, could not justify that conclusion.

Later randomized controlled trials showed that HRT led to a small increase in CHD risk. The earlier apparent protective effect came from a confounding variable: women receiving HRT tended, on average, to have higher socioeconomic status, and higher socioeconomic status is itself causally associated with lower CHD risk.

This is the nature of science point. Correlation is a relationship in which two variables vary together. Causation is a relationship in which a change in one variable produces a change in another. Epidemiological correlations can suggest hypotheses, but randomized controlled trials give stronger evidence for cause and effect because they reduce the influence of confounding variables.