Term 1 review: support, hormones, reproduction and immunity

Chapter 3 is one story told many times. Asexual reproduction copies a parent by mitosis. Sexual reproduction uses meiosis to make haploid (N) gametes, and fertilization restores 2N. In one life cycle a haploid generation that makes gametes alternates with a diploid generation that makes spores, as in the fern. In a flowering plant the anther makes microspores (N) from spore mother cells (2N), and the ovule makes an embryo sac with 8 nuclei. A pollen tube brings two male nuclei: one fuses with the egg to make the zygote (2N) and one fuses with the two polar nuclei to make the endosperm nucleus (3N). The ovule becomes the seed and the ovary becomes the fruit. In humans, spermatogenesis makes four sperm from one primary spermatocyte, while oogenesis makes one ovum and polar bodies from one primary oocyte. In the menstrual cycle FSH makes a follicle grow and make oestrogen, which builds the endometrium; at about day 14 LH makes the follicle release the egg and become the corpus luteum, which makes oestrogen and progesterone. If there is no fertilization the hormones fall and the lining breaks down. If there is, the zygote divides into a morula and then a blastocyst, which implants at the end of the first week. The chorion, the amnion, the placenta and the umbilical cord then support the embryo for nine months. Twins come from two ova (fraternal) or from one zygote that splits (identical).