Cambridge International A Level Biology Revision Guide

Cambridge International A Level Biology
Each month, one secondary oocyte is released into
the oviduct from one of the ovaries. If it is fertilised, it
continues its division by meiosis, and can now be called
an ovum. The chromosomes of the spermatozoan and the
ovum join together to form a single diploid nucleus, and
the cell that is made by this process is called a zygote. The
zygote can now divide repeatedly by mitosis to form first
an embryo, and then a fetus.
Gametogenesis in flowering plants
Figure 16.12 shows the structure of a typical flower. Male
gametes are produced in the anthers, and female gametes
in the ovules.
a
pollen mother cell (2n)
meiosis
4 haploid cells (n)
mitosis
young pollen grains
containing two haploid
nuclei (n)
b
mature pollen grains (n)
petal
372
sepal
stigma
style
ovary
ovule
carpel
flower stalk
anther
filament
nectary
stamen
receptacle
Figure 16.12 The structure of a flower.
Inside the anthers, diploid pollen mother cells divide
by meiosis to form four haploid cells. The nuclei of each
of these haploid cells then divide by mitosis, but the cell
itself does not divide (cytokinesis does not take place),
resulting in cells that each contain two haploid nuclei.
These cells mature into pollen grains, each surrounded
by a protective wall made up of a tough exine and thinner
intine (Figure 16.13). One of the haploid nuclei is called
the tube nucleus, and the other is the generative nucleus.
These are the male gametes.
Inside each ovule, a large, diploid, spore mother cell
develops. This cell divides by meiosis to produce four
haploid cells. All but one of these degenerates, and the one
surviving haploid cell develops into an embryo sac
(Figure 16.14).
Figure 16.13 a The development of pollen grains from pollen
mother cells. b Mature pollen grains (× 1500).
The embryo sac grows larger, and its haploid nucleus
divides by mitosis three times, forming eight haploid
nuclei. One of these becomes the female gamete.
Fertilisation occurs when a male gamete from a pollen
grain fuses with a female gamete inside an ovule. This
forms a diploid zygote, which grows into an embryo plant.
Note that in plants, unlike animals, the gametes are not
formed directly by meiosis. Instead, meiosis is used in the
production of pollen grains and the embryo sac and the
gametes are then formed inside these structures by
mitotic divisions.