Quick Revision

DNA copying (replication) is essential in reproduction because: (1) DNA carries genetic information from parents to offspring. (2) It ensures the new organism has the same body design and characteristics. (3) Minor variations during DNA copying lead to evolution and survival advantages. (4) If DNA is not copied, the new cells would not have instructions for protein synthesis and cell function.

Hydra reproduces asexually by budding. A bud develops as an outgrowth on the parent body, grows, develops tentacles, and eventually detaches to become a new individual.

Placenta is a disc-like tissue embedded in the uterine wall, connected to the embryo via umbilical cord. Functions: (1) Provides nourishment — glucose, oxygen from mother's blood to embryo. (2) Removes waste — CO₂, urea from embryo to mother's blood. (3) Acts as a barrier — prevents mixing of mother's and foetus's blood. (4) Produces hormones (like progesterone and estrogen) to maintain pregnancy.

Advantages of sexual reproduction: (1) Creates genetic variation due to combination of DNA from two parents — promotes diversity. (2) Variations help species adapt to changing environments — better survival. (3) New combinations of traits may be beneficial. (4) Promotes evolution of species. Asexual reproduction produces clones (genetically identical offspring) — no variation, so the entire population can be wiped out by a single disease or environmental change.

Pollination: Transfer of pollen grains from anther to stigma of a flower. Can be self-pollination or cross-pollination. Carried out by wind, water, insects, birds. Fertilisation: Fusion of male gamete (from pollen) with female gamete (egg/ovule) to form a zygote. Occurs after pollen tube grows down the style to reach the ovule. Pollination precedes fertilisation.