Reproduction, at a glance

Mitosis maintains chromosome number and produces genetically identical cells; it supports growth, repair and asexual reproduction.

• Asexual reproduction uses one parent and produces genetically identical offspring.
• Mitosis preserves chromosome number.
• Growth adds cells; repair replaces damaged cells; asexual reproduction makes another organism.

Keep in mind: DNA is copied before division. Each daughter cell receives a full matching set, so chromosome number is maintained.

Haploid gametes carry one chromosome set each; their nuclei fuse to form a diploid zygote.

• Sexual reproduction involves fusion of male and female gamete nuclei and produces genetically varied offspring.
• Haploid: one set. Diploid: two sets.
• Homologous chromosomes carry the same genes at corresponding positions but may carry different alleles.

Keep in mind: The chromosomes carry the same genes at corresponding positions, but their alleles can differ.

Meiosis separates homologous chromosomes and halves the chromosome number, producing genetically dissimilar haploid cells.

• Meiosis is a reduction division used in gamete formation.
• Homologous chromosomes separate so each resulting set contains one member of each pair.
• Mitosis maintains number; meiosis halves it.

Keep in mind: Normal meiosis halves the chromosome number consistently. The combinations of inherited information vary, not the required number of sets.

Stamens produce pollen; a carpel receives pollen and contains ovules. Petals attract pollinators and sepals protect the bud.

• Stamen = anther + filament. Carpel = stigma + style + ovary.
• Anthers produce pollen; the stigma receives it; ovules lie inside the ovary.
• Sepals protect the bud; conspicuous petals help attract insect pollinators.

Keep in mind: The ovary contains ovules. Fertilisation occurs inside an ovule, not throughout the whole ovary.

Pollination transfers pollen from anther to stigma. Self or cross depends on the plants involved, not simply the number of flowers.

• Self-pollination: transfer within one flower or between flowers on the same plant.
• Cross-pollination: transfer between different plants of the same species.
• Pollination happens before fertilisation; it is not fusion of nuclei.

Keep in mind: The insect may move within one flower or between flowers on one plant. Check the source and recipient plants.

Flower structures fit the pollen carrier: contact with an insect or exposure to moving air.

• Insect flowers often have conspicuous petals, scent or nectar and sticky pollen.
• Wind flowers often have exposed dangling anthers and large feathery stigmas.
• Wind pollen is often small, light, smooth and produced in large quantities.

Keep in mind: The stigma receives pollen. Its large exposed surface helps catch airborne grains; pollen release occurs at the anther.

A compatible pollen grain germinates on the stigma; its tube grows down the style into an ovule, delivering a male nucleus for fusion.

• Stigma -> style -> ovary -> ovule.
• The pollen tube enters the ovule through the micropyle.
• Fertilisation is fusion of male and female gamete nuclei, forming a zygote.

Keep in mind: The grain germinates on the stigma. A tube grows from it and carries the male nucleus towards the ovule.

Testes produce sperm; sperm ducts carry them towards the urethra, where gland fluids contribute to semen.

• Testes produce sperm and testosterone.
• The scrotum holds the testes outside the main body cavity at a suitable lower temperature.
• Sperm ducts transport sperm; the prostate contributes fluid; the urethra passes through the penis.

Keep in mind: Sperm are produced in the testes. The prostate contributes fluid to semen.

Ovaries release eggs; oviducts are the usual site of fertilisation; the uterus supports development after implantation.

• Ovaries produce eggs and the hormones oestrogen and progesterone.
• An oviduct carries the released egg towards the uterus.
• The cervix is the narrow neck of the uterus; the vagina opens to the outside.

Keep in mind: They have different roles: egg production and release, transport and usual fertilisation, then implantation and development.

Oestrogen rebuilds the lining; progesterone helps maintain it after ovulation. Falling levels allow menstruation if pregnancy is not established.

• Menstruation sheds uterine lining; ovulation releases an egg from an ovary.
• Oestrogen promotes repair and thickening of the lining; progesterone maintains it.
• Cycle length and ovulation timing vary. A calendar cannot guarantee an infertile day.

Keep in mind: Day 14 belongs to a simplified example. Timing varies, sperm can survive for several days, and the fertile phase extends around ovulation.

The zygote divides and implants in the uterine lining; the placenta exchanges materials while the amniotic sac and fluid protect development.

• Fertilisation -> zygote -> mitotic divisions -> ball of cells -> implantation.
• Amniotic sac encloses fluid that cushions the developing fetus and allows movement.
• Placenta: nutrients and oxygen towards the fetus; carbon dioxide and urea towards maternal blood.

Keep in mind: It connects the fetus to the placenta. Dissolved nutrients reach the placenta through maternal blood after digestion and absorption.

HIV transmission requires particular body fluids and an entry route; ordinary social contact does not transmit it.

• Routes include sexual exposure, contaminated blood/injecting equipment, and transmission during pregnancy, birth or breastfeeding.
• Condoms, sterile equipment, screened blood and effective HIV treatment reduce transmission.
• HIV is not transmitted by hugging, sharing food or ordinary classroom contact.

Keep in mind: HIV needs specific exposure routes. It is not spread by ordinary social contact, and sustained undetectable viral load on treatment prevents sexual transmission.