C H R O M O S O M EP A I R I N G
The buddy system
in
chromosomes
d homologous
abovelookidentical.
h
rl
il
t
h
I
I
mes by producing gametes
one set of chromosomes
of two sets.
some organisms, such as plants,
of haploid cells are formed
. Gametes are formed at one
'd a plant's life cycle and special
f, cells called spores are formed
stage. Sporesgive rise to a
organism. In other organisms,
animals, only gametes are
br- meiosis. In all organisms,
keeps the number of chromosable over generations.
rnu can see in Figure 5.9, the
of haploid gametes (n + n)
oftpring with diploid cells
the offspring develops into an
@loid body cells are produced
repeated rounds of mitotic
bn. Mitotic cell division
ins the diploid chromosome
,2n). Eventually,a diploid
ir-e cell undergoes meiosis to
next generation of haploid
rn).
s that reproducesexually
r have both diploid and haploid
some stage of their life
ffimrever, the timing of meiosis
can vary among different
orqanisms.
You already know that all human cells,
except for sex cells, contain 46 chromosomes. These 46 chromosomes are
arranged in 23 pairs. Each of the 23
pairs is called a homologous pair.
With one exception, each pair (of the
23 pairs) contains chromosomes that
are exactly alike in size, location of the
centromere, and the dark-and-light
banding pattern seen after staining
with dyes. See Figure 5.70. Only one
pair can differ. In females, this pair is
alike and is called )C(. In males, this
pair is dissimilar in appearance and is
called XY.
During meiosis, chromosomes
in each homologous pair replicate and
a single copy from each pair is distributed to each daughte-rnucleus. In
humans, a cell with 46 chromosomes
rvtmYuAY
The Chickenand
the E$$
E€€s, the $ametes
produced by hens
a.nd other fowl, are
a common part of
the human diet.
Most eggs produced
for huma;r consumption are not fertilized. and therefore
cannot develoP into
offspring.
Calcr"rlating Haploid and Diploid Numbers
Haploid and diploid are designated by the algebraic nota'
tions n and 2n, respectively. Either number can be
calculated when the other number is known. For example,
if the haploid number (n) is 3, the diploid number €n) is
2x3, or 6. lf the diploid number Pn) is 12' the haploid
number (2) is /.2+2, or 6. The table below shows the hap'
loid or diploid numbers of a variety of organisms. Gopy the
table onto a separate sheet of paper and complete it. Then
use the table to answer the following questions'
I Whatarethe haploidnumbers
for the two plantslistedin the
table?
, h i c ho r g a n i s m s '
2 I n t h et a b l e w
to
numbers
areclosest
diploid
thatof a human?
n h ya d i p l o i dn u m b e r
3 E x p l a iw
is alwayseven.
dnd
4 W h i c ho r g a n i s mh' sa p l o i a
do youfind most
diploidnumbers
Why?
surprising?
0rganism
Haplold Dlplold
Number Number
ameba
25
chimpanzee
24
earthworm
18
1010
fern
hamster
22
honeybee
56
human
46
onron
16