Essentials of Anatomy and Physiology

cells, each with the haploid number(half the usual
number) of chromosomes. In women, meiosis takes
place in the ovaries and is called oogenesis. In men,
meiosis takes place in the testes and is called sper-
matogenesis. The differences between oogenesis and
spermatogenesis will be discussed in Chapter 20, The
Reproductive Systems.
The egg and sperm cells produced by meiosis have

the haploid number of chromosomes, which is 23 for

humans. Meiosis is sometimes called reduction divi-

sion because the division process reduces the chromo-

some number in the egg or sperm. Then, during

fertilization, in which the egg unites with the sperm,

the 23 chromosomes of the sperm plus the 23 chro-

mosomes of the egg will restore the diploid number

of 46 in the fertilized egg. Thus, the proper chromo-

62 Cells

Interphase

Nucleus

Prophase

Centrioles

Pair of chromatids

Centromere

Equator of cell

Spindle

fibers

Metaphase

Anaphase

Nuclear membrane re-forms

Telophase

Cytoplasm divides

Figure 3–5. Stages of mitosis in a cell with the diploid number of four. See Table 3–4 for

description.

QUESTION:In prophase, what is a pair of chromatids made of?