Biology 12

558 MHR • Appendix 4

are produced that help the early embryo orient

and develop as it grows. Once stimulated,

meiosis I continues and one small polar body

is produced. This polar body contains one set

of duplicated chromosomes and very little

cytoplasm. The oocyte continues through

meiosis II and stops at metaphase II until it is

fertilized. Fertilization by a sperm cell, and the

arrival of the sperm pronucleus, lead to an

increase of calcium inside the oocyte. This

stimulates the oocyte to complete meiosis,

producing one haploid pronucleus and a

second polar body at the other centrosome pole.

The zygote has two pronuclei that move

directly to the S phase. Since the egg stored

quantities of mRNA and key proteins, the early

embryo skips phases G 1 and G 2 and alternates

between the S phase and M phase during

mitotic cleavage. As the two pronuclei prepare

for their first M stage, their membranes

disintegrate and the chromosomes merge to

form the nucleus of the new individual.

growth of oocyte

hormones

synapsis of

homologues

preparation for

metaphase I

meiosis begins

metaphase I

first polar body

second

polar body

preparation for

metaphase II

stasis until

fertilization

fertilization

pronuclei prepare

for mitosis

chromatids

separate

contains one

copy of each

homologue

male pronucleus

enters egg

Figure A4.4 Oogenesis is a specialized form of meiosis. Cytoplasm is not evenly

distributed. Hormones initiate meiosis, which arrests in metaphase II. Meiosis is

completed after fertilization.