HUMAN BIOLOGY

370 Chapter 18

expLoreon your oWn

suMMary

section 18.1 In reproduction, a parent

cell produces a new generation of cells,

or parents produce a new individual.

Reproduction is part of a life cycle, a

recurring sequence in which individuals

grow, develop, and reproduce. Each cell of a

new generation must receive a copy of the parental DNA and

enough cytoplasm to start up its own operation.

Mitosis maintains the diploid number of chromosomes in

the two daughter nuclei.

A chromosome is a DNA molecule and its associated

proteins. The sum of the chromosomes in a given cell type

is the chromosome number. Human somatic cells have a

diploid chromosome number of 46, or two copies of 23 types

of chromosomes. Except for the sex chromosomes, pairs of

homologous chromosomes are the same length and carry

similar genes.

section 18.2 The cell cycle begins when

a new cell is produced and ends when that

cell divides. The longest part of the cycle is

interphase, which includes a growth stage

(G1), when the cell roughly doubles the

number of organelles and other components

in its cytoplasm. In the S stage the cell’s

chromosomes are duplicated. There is also a final, short growth

stage (G2). The duration of the cell cycle varies in different types

of cells.

Human cells divide the cell nucleus by either mitosis or

meiosis. Mitosis is the division mechanism in somatic cells—

body cells that are not specialized to make gametes. It functions

in growth and tissue repair. Meiosis divides the nucleus in

germ cells, cells in the gonads that give rise to gametes.

An unduplicated chromosome consists of one DNA mole-

cule and proteins. A duplicated chromosome consists of two

DNA molecules that are temporarily attached to each other

as sister chromatids. When the nucleus divides, microtubules

attach to a centromere on the paired chromatids. The

microtubules are part of a spindle that moves chromosomes

during nuclear division.

sections 18.3, 18.4 Mitosis has four

stages:

1. Prophase. Duplicated, threadlike chro-
   mosomes condense into rodlike structures;
   new microtubules start to assemble in orga-
   nized arrays near the nucleus; they will form a
   spindle. The nuclear envelope disappears.


2. Metaphase. Spindle microtubules orient the sister
   chromatids of each chromosome toward opposite spindle
   poles. The chromosomes line up at the spindle equator.


3. Anaphase. Sister chromatids of each chromosome
   separate. Both are now independent chromosomes, and they
   move to opposite poles.


4. Telophase. Chromosomes become threadlike again and
   a new nuclear envelope forms around the two clusters of
   chromosomes. Mitosis is completed. Cytokinesis divides the
   cytoplasm; the result is two diploid cells.
   Division of the cytoplasm, called cytokinesis, occurs toward
   the end of mitosis or shortly afterward.
   sections 18.6, 18.7 Meiosis reduces
   the total number of chromosomes in daughter
   cells. Two consecutive divisions of a germ
   cell cut the parental diploid chromosome
   number in half. Meiosis I distributes the pairs
   of homologous chromosomes. Meiosis II
   separates sister chromatids. The result, after
   cytokinesis, is four haploid cells.

Section 16.2 explains that oogenesis begins when a female is still a

developing embryo. Meiosis I begins in the immature eggs (primary oocytes) of a

female embryo but is arrested in prophase I. Meiosis I won’t resume until the female

undergoes puberty. From then until menopause, just before an egg is ovulated, it will

undergo the remaining stages of meiosis I. As the egg is traveling down an oviduct,

meiosis II begins. This stage also is arrested, in metaphase II. Only if the egg is fertil-

ized will all the stages of meiosis finally be completed (Figure 18.15).

Knowing this sequence, you can calculate fairly accurately how long it took for the

egg that helped make you to pass through all the stages of meiosis. You only need to

know the month and year your mother was born and the month and year you were

conceived (or born). As a starting point, remember that a female’s primary oocytes

form during the third month of embryonic development, about 6 months before birth.

If you have siblings, do the same calculation for them.

Figure 18.15 This photograph shows an egg

with a sperm that will penetrate and fertilize

it—marking the end of meiosis in the egg and

the beginning of bodily growth by mitosis.

Don W. Fawcett/Science Source

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