Page 26 - PRE-U STPM BIOLOGY TERM 1

Biology Term 1 STPM Chapter 2 Structure of Cells and Organelles

The centromere is the primary constriction of the chromosome.
Besides the centromere, there may be another constriction which
is called secondary constriction. Some chromosomes may have
secondary constriction. Other chromosomes may be of longer or
shorter length. All these determine the shape of chromosomes.

2

Chromosome with Chromosome with
secondary constriction extra length
Figure 2.15 Two types of abnormal chromosomes
(x) The structural organisation of chromosome is as follows:
• Each chromosome is made up of two sister chromatids which are
identical. Each chromatid is made up of one molecule of DNA as
a result of replication. Each of the molecules becomes a chromatid
and they are attached together by the centromere.
• During prophase, each DNA molecule winds around a group of
8 histone molecules, forming a complex unit called nucleosome.
During interphase, a certain amount of DNA forms nucleosomes,
and the genes are inactivated.
• 6 such nucleosomes may coil regularly to form a secondary
structure, which may be folded or coiled to become the compact
chromatid.
• Such regularity may not be present in all species of organisms
especially in bees, whose chromosomes with DNA and nucleosomes
are folded, rather than neatly coiled.
• The centromere is a constricted portion of the chromatid where
protein keeps the two chromatids together. At the end of mitotic
metaphase, the centromere divides, chromatids separate and are
pulled by spindle fibres to the opposite poles.

Secondary
coiled
DNA Histone Nucleosome

Chromatin
fibre

Tertiary coiled Coiling
Figure 2.16 Organisation of chromosomal structure
71

02[STPM Bio T1].indd 71 3/29/18 5:08 PM