Page 24 - PRE-U STPM BIOLOGY TERM 1

Biology Term 1 STPM Chapter 2 Structure of Cells and Organelles

(iii) The shape of the nucleolus is usually spherical, but it can be
oval and it can change its shape.
(iv) There is usually one nucleolus per nucleus, but in rare cases,
there can be two per nucleus.
(v) The structure of nucleolus as interpreted from the electron
micrograph is shown in Figure 2.13.
2

Region with chromosome/DNA
Region with fibrils
Region with granules
Figure 2.13 Schematic interpretation of the nucleolus structure
(vi) It has a region with chromosome where one or two chromosomes
or DNA are found. In fact, it is the genes (organisers) found in
the DNA that will start the process of nucleolus formation.
The genes code the rRNA and protein of the ribosome. The
nucleolus is the structure that is involved in the process of
making ribosomes.
(vii) It has another region with fibrils where the transcription of
genes forms rRNA. Some of the RNA act like mRNA, move
out into the cytoplasm and are translated into proteins by
ribosomes there. The others are rRNA and combined with the
proteins moved in from cytoplasm to form coarser fibrils before
they coil to form the ribosome subunits.
(viii) It has a third region with granules where rRNA and protein
interact, coil and fold to form two types of ribosome subunits.
One type is larger than the other. The larger is the 60S type and
the smaller is the 40S type. These granular ribosome subunits
will move away from the nucleus through the nuclear pores
into the cytoplasm.
(ix) There is a cyclic change for the nucleolus like the nuclear
membrane. It disappears during prophase of cell division and
reappears later at telophase. This is because RNA and protein
can be hydrated at prophase. The cells have to form ribosomes
after cell division.

(d) Chromosomes (and its organisation)
(i) Chromosomes are structures that are formed from DNA and
histone during metaphase of mitosis. The DNA of the nucleus
at other times can also be called chromosomes.
(ii) Chromosomes have no shape and they are not organised during
interphase. They exist as chromatin, long DNA molecules with
certain parts attached with histone. The part of DNA that is not
coiled around histone contains active genes. The genes can later
be transcribed to form proteins.

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