Cambridge International A Level Biology Revision Guide

Cambridge International A Level Biology
a
If chromosome number is not halved, the number of
chromosomes doubles every generation:
c
n
n
2n
*
2n
*
*
2n
2n
4n
*
4n
haploid sperm (white
speck on left) fertilising
haploid egg
gametes zygote adult gametes zygote adult
* mitosis
b
If chromosome number is halved, the number of
chromosomes stays the same every generation:
n
n
• n
2n * 2n *
•
2n 2n
n
gametes zygote adult gametes zygote adult
diploid zygote
* mitosis
• meiosis occurs, which halves the number of chromosomes
two-cell diploid embryo
Figure 16.7 A life cycle in which the chromosome number is a not halved; b halved; c life cycle stages in a sea urchin .
368
A diploid cell is one that possesses two complete sets of
chromosomes; the abbreviation for diploid is 2n.
A haploid cell is one that possesses one complete set of
chromosomes; the abbreviation for haploid is n.
Meiosis
The process of meiosis is best described by means of
annotated diagrams (Figure 16.8). An animal cell is shown
where 2n = 4, and different colours represent maternal and
paternal chromosomes. The associated behaviour of the
nuclear envelope, cell surface membrane and centrosomes
is also shown. Remember, each centrosome contains a pair
of centrioles (Chapters 1 and 5).
Unlike mitosis (page 97), meiosis involves two divisions,
called meiosis I and meiosis II. Meiosis I is a reduction
division, resulting in two daughter nuclei with half the
number of chromosomes of the parent nucleus. In meiosis
II, the chromosomes behave as in mitosis, so that each of
the two haploid daughter nuclei divides again. Meiosis
therefore results in a total of four haploid nuclei. Note that
it is the behaviour of the chromosomes in meiosis I that is
particularly important and contrasts with mitosis.
Figure 16.8 summarises the process of meiosis
diagrammatically. Figure 16.9 shows photographs of the
process as seen with a light microscope.
Two of the events that take place during meiosis help
to produce genetic variation between the daughter cells
that are produced. These are independent assortment
of the homologous chromosomes, and crossing over,
which happens between the chromatids of homologous
chromosomes. When these genetically different gametes
fuse, randomly, at fertilisation, yet more variation is
produced amongst the offspring. In order to understand
how these events produce variation, we first need to
consider the genes that are carried on the chromosomes,
and the way in which these are passed on from parents to
offspring. This branch of biology is known as genetics.