Page 25 - PRE-U STPM BIOLOGY TERM 1

Biology Term 1 STPM Chapter 2 Structure of Cells and Organelles

(iii) The number of chromosomes per cell ranges from 8 – 100. The
chromosomes in diploid organisms exist in pairs. For example,
each human somatic (non-gametic) cell has 46 chromosomes,
in 23 pairs. In gametes, the number is halved.
(iv) Homologous chromosomes are identical in structure, though
not necessarily in allelic constitution.
2 (v) Maternal chromosomes are the set of chromosomes which
originates from the mother through the ovum. Paternal
chromosomes are the set that originates from the father through
the spermatozoon.
(vi) Sex chromosomes determine the sex of the organism. For
example, females have a pair of homologous X chromosomes
whereas the males have an X chromosome and a non-
homologous and much smaller Y chromosome. A gene called
the t factor in the Y chromosome determines the formation
of testes during the formation of the sex organ in the foetus.
Otherwise, the foetus will be a female.
(vii) Chromosomes other than the sex chromosomes are called
autosomes. They are usually in larger numbers. For example,
the human cells have 22 pairs of autosomes and one pair of sex
chromosomes.
(viii) The size of the chromosomes varies between species. The
average size of the human chromosome is 6 µm. The largest
human chromosome is labelled as chromosome 1 and the
smallest as chromosome 22 and chromosome Y.
Plants usually have larger chromosomes than animals. Birds and
fungi are among the organisms with the smallest chromosomes.
(ix) The chromosome shapes vary during the cell cycle. When
we refer to the chromosome shapes, we refer to their shapes
during metaphase of mitosis. Chromosomes consist of two
strands of cylindrical chromatids, which are attached together
by a structure called centromere. Therefore, the shapes of
chromosomes are determined by the position of the centromere,
which can be metacentric, telocentric or acrocentric, as shown
below.

Metacentric Telocentric Acrosentric
Figure 2.14 Types of chromosomes

70

02[STPM Bio T1].indd 70 3/29/18 5:08 PM