Biology of Disease

poles of the spindle. During anaphase, the chromatids of each chromosome
are pulled apart and each chromatid moves towards the poles of the spindles
reaching the ends by telophase. In telophase, the cell undergoes division to
two daughter cells by the plasma membrane constricting and cutting across
the spindle equator. The spindle breaks down and nuclear envelopes form
around each separated grouped of chromatids, now called chromosomes. The
nucleoli also become apparent in the new nuclei and the chromosomes return
to the nonvisible forms typical of cells in interphase. Hence the parental cell
has divided to form two daughter cells that are genetically identical to the
parent.

DNA REPLICATION AND THE CELL CYCLE

CZhhVg6]bZY!BVjgZZc9Vlhdc!8]g^hHb^i]:YLddY )&.

Interphase

Prophase

Metaphase

Anaphase

Telophase

Interphase

Figure 15.7 An outline of mitosis as described

in the main text. For simplicity only two

chromosomes, each consisting of two

chromatids, are shown.

B)

Figure 15.8 (A) The structure of taxol,

which is obtained from (B) the leaves of

the pacific yew tree (Taxus brevifolia).

Meiosis occurs prior to reproduction during the formation of gametes
(Chapter 7). The parental cell has a diploid number of chromosomes, whereas
the daughter cells are now gametes with the corresponding haploid number
of chromosomes. During meiosis, the number of chromosomes is halved and
the daughter cells receive only one of each type of chromosome and, for this
reason, meiosis is sometimes called reduction division.

Meiosis may be thought of as consisting of two separate divisions (Figure
15.9). In the first meiotic division (prophase I through to telophase I) the
parent cell divides into two cells each of which receives one of each pair of
homologous chromosomes. Each of these chromosomes consists of two
chromatids. The second meiotic division (prophase II through to telophase II)
results in each chromosome being separated into chromatids, with the result
that four daughter cells each with a haploid chromosome complement are
formed. In prophase I, the chromosomes contract and the nucleolus shrinks
in size. Homologous chromosomes lie side by side in pairs, a situation called
synapsis. Each member of the pair is bivalent. It is at this stage that genetic
recombination or crossing over occurs. While they are paired, the nonsister
chromatids, that is one maternal and one paternal chromatid, of a homologous
pair are broken at equivalent positions and exchange homologous pieces of
material (Figure 15.9). The crossed strands of the chromatids formed during
recombination are called chiasmata (singular chiasma). Recombination

N

O O

O

O

O

O

O

O

O

O

OH

OH

OH

C 6 H 5

C 6 H 5

H 5 C 6

H

H

O

CH 3

CH 3

CH 3

CH 3

CH 3

CH 3

A)