Reproduction

Asexual and sexual reproduction

Asexual reproduction produces offspring without the fusion of gametes. Binary fission, budding, fragmentation and vegetative propagation are examples. Offspring are often genetically very similar to the parent, apart from mutation and other sources of variation. Sexual reproduction combines genetic material through gamete fusion.

Meiosis reduces chromosome-set number, and fertilisation restores diploidy in a typical diploid life cycle. Sexual reproduction introduces variation through recombination, independent assortment and the combination of gametes from different parents.

• Vegetative propagation uses structures such as stems, roots or leaves to generate new plants.
• Asexual reproduction can be rapid and retain a useful genotype, while sexual reproduction produces new combinations.
• The presence of flowers alone does not tell you whether a particular seed arose through ordinary sexual reproduction or an alternative process such as apomixis.

Male and female gametophytes in flowering plants

The anther contains microsporangia in which microspore mother cells undergo meiosis to form haploid microspores. A microspore develops into a pollen grain, the male gametophyte. The generative cell ultimately produces two male gametes; the vegetative cell supports pollen-tube growth.

Inside an ovule, a megaspore mother cell usually undergoes meiosis. In the common monosporic pattern, one functional megaspore undergoes mitotic divisions to produce an eight-nucleate embryo sac. This mature female gametophyte is typically seven-celled, with an egg apparatus, three antipodals and a central cell containing two polar nuclei.

• An ovule is not an egg cell: it contains tissues surrounding the female gametophyte.
• An eight-nucleate embryo sac need not be eight-celled because two nuclei occupy the central cell.

Pollination and double fertilisation

Pollination transfers pollen from an anther to a stigma. Autogamy occurs within one flower, geitonogamy between flowers on the same plant, and xenogamy between different plants of the same species. Geitonogamy uses a pollinating agent but is genetically similar to self-pollination.

After compatible pollen germinates, the pollen tube delivers two male gametes. One fuses with the egg to form a diploid zygote. The other fuses with the two polar nuclei or their fused secondary nucleus, producing the usually triploid primary endosperm nucleus. Together these events constitute double fertilisation.

• The ovule develops into a seed, and the ovary commonly develops into the fruit.
• Endosperm nourishes the developing embryo. Its persistence in the mature seed varies among plants.
• Apomixis produces seeds without the usual fertilisation sequence; polyembryony describes more than one embryo in a seed.

Track the ploidy

In the common diploid angiosperm pattern, sperm n + egg n gives a 2n zygote. The second sperm n combines with two polar nuclei n + n to form 3n endosperm. The embryo and endosperm therefore have different chromosome-set numbers.

Human reproductive systems and gametogenesis

Testes contain seminiferous tubules where sperm develop. Sertoli cells support developing germ cells, and Leydig cells produce androgens. Sperm mature functionally in the epididymis and travel through ducts, mixing with accessory gland secretions.

Ovaries contain developing follicles. Oviducts receive the released secondary oocyte and provide the usual site of fertilisation. The uterus supports implantation and development, while the cervix connects it with the vagina.

Spermatogenesis involves mitotic multiplication, meiosis and differentiation into sperm. Human primary oocytes begin meiosis before birth and remain arrested in prophase I. A secondary oocyte is normally released after completion of meiosis I and remains arrested in metaphase II until fertilisation triggers completion.

• Unequal cytokinesis in oogenesis preserves most cytoplasm in one functional cell and produces small polar bodies.
• In the typical human pattern, meiosis in one primary spermatocyte generates four haploid spermatids.
• The cell released at ovulation is normally a secondary oocyte, rather than a cell that has already completed meiosis II.

The menstrual cycle and hormone coordination

The menstrual cycle coordinates ovarian events with changes in the uterine endometrium. FSH supports follicular development. Growing follicles release oestrogens, which help the endometrium proliferate. Sustained high oestrogen near the end of the follicular phase can produce positive feedback that contributes to the LH surge.

The LH surge triggers ovulation. The remaining follicular tissue forms the corpus luteum, which secretes progesterone and oestrogens. Progesterone supports the secretory endometrium. If pregnancy does not establish, corpus-luteum regression lowers these hormones and menstruation follows.

• A 28-day cycle is a teaching model; real cycle length and ovulation timing vary.
• FSH and LH are anterior pituitary hormones. Oestrogens and progesterone are produced primarily by ovarian structures during the cycle.
• Negative and positive feedback occur at different stages; do not apply a single feedback rule to the entire cycle.

Fertilisation, implantation and development

Human fertilisation normally occurs near the ampullary–isthmic region of the oviduct. Fusion of gametes restores the diploid chromosome number and triggers events that limit entry of additional sperm. Cleavage produces increasingly numerous, smaller cells as the early embryo travels toward the uterus.

The blastocyst includes an inner cell mass and an outer trophoblast. Implantation involves attachment and embedding in the endometrium. Placental tissues support exchange of gases, nutrients and wastes and secrete hormones. Maternal and fetal blood normally exchange substances across a barrier rather than mixing freely.

Early embryonic trophoblast-derived hCG helps maintain the corpus luteum. Later, the placenta becomes a major source of hormones supporting pregnancy. Birth involves coordinated hormonal and mechanical feedback, including oxytocin-driven uterine contractions. Prolactin supports milk synthesis, while oxytocin supports milk ejection.

Reproductive health: concepts for study

Reproductive health includes biological wellbeing, informed choices and prevention of infection. In examination questions, distinguish barrier, hormonal, intrauterine and surgical contraceptive mechanisms. A method that prevents pregnancy does not necessarily prevent sexually transmitted infections.

Assisted reproductive technologies support particular problems in conception using different procedures. In vitro fertilisation involves fertilisation outside the body followed by transfer of an embryo. Learn the process and terminology rather than treating every assisted method as identical.

• Fertilisation and implantation are separate events occurring at different stages.
• Contraception prevents pregnancy through specified mechanisms; it is not the same concept as treatment of infertility.