14.4 The Cell Cycle

183

Table 14.1 Successive events in the eukaryotic cell cycle

PhaseSuperscript normal a^a

Process

Feature(s)

M

Prophase

Chromosome condensation

M

Metaphase

Centrosomes separate and form two asteriated poles at

opposite ends of the cell

M

Prometaphase

The nuclear envelopeSuperscript normal b^b is degraded, microtubules from the

centrosomes seek the chromosomes

M

Metaphase

Microtubules from the centrosomes find the chromosomes

M

Anaphase A

The two arms of each chromosome are separated and

drawn towards the centrosomes

M

Anaphase B

Centrosomes move further away from each other together

with their half-chromosomes

M

Telophase

The cell divides

G1

Decondensation

Chromosomes disappear, nuclear envelope reforms

around the DNA, microtubules reappear throughout the

cytoplasm

S

Interphase

Cell growth

G2Superscript normal c^c

Interphase

DNA duplication

Superscript normal a^aSee Fig. 14.2

Superscript normal b^bThe nuclear envelope is a bilayer lipid membrane in which proteins are embedded

Superscript normal c^cMitosis (see Sect. 14.4.1) is considered to begin at the end of G2 and last until the beginning of G1

Differences Between Prokaryotes and Eukaryotes (1)

Prokaryotes undergo neither meiosis nor mitosis (their DNA is segregated as it repli-

cates), their chromosomes are not organized into chromatin (although there is a region

called the nucleoid in which the genetic material is concentrated), nor does the DNA

spend much of its time inside a special compartment, the nucleus (although the chro-

mosome is usually visible as the nucleoid). Chromosome replication typically starts

from a single site in prokaryotes (the origin of replication, ori, which may comprise a

few hundred bases) but from many sites (thousands) in eukaryotes—otherwise repli-

cation, proceeding at about 50 bases per second, would take far too long. As it is,

the human genome takes about 8 hours to be replicated. Prokaryotic DNA is circular

(and hence does not require telomeres), ¹⁹ whereas eukaryotic DNA is linear.

Differences Between Protozoans and Metazoans

In a single-celled protozoan, the germline is the soma (body). The metazoan is quite

different because its germline (a single cell) must divide and multiply in order to

create the soma. All cells have the same genes (with some specialized exceptions,

19 There are some exceptions; for example, Streptomyces coelicolor has a linear genome.