微英语细胞生物学英文试卷(I)

2021年1月20日发(作者：celebration)
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细胞生物学双语教案
4
（细胞核结构与功能）

Structure and Function of the Nucleolus
The nucleolus is organized from the
chromosomes. A number of chromosomes get together and transcribe ribosomal
RNA at this site. The nuclear organizing (NO) regions are seen as circular areas
(pale)
surrounded
by
a
rim
of
electron
dense
filaments.
These
filaments
collectively
are
called
the
pars
fibrosa
(PF).
This
is
formed
from
newly
transcribed ribosomal RNA. This diagram shows the parts of the nucleolus.

After the ribosomal RNA is transcribed, it is linked to proteins and one can see
accumulation of ribonucleoprotein particles in the pars granulosa (PG). These
particles form the two types of ribosomal subunits (large and small) which are
then
transported
out
of
the
Nuclear
Pores
separately.
The
pores
do
not
accommodate
the
assembled
ribosomes,
therefore,
they
cannot
reenter.
This
means that translation of RNA and synthesis of proteins must occur outside the
nucleolus. For more information follow the link to ribosomes

After the ribosomal subunits are in the cytoplasm, they are connected and then,
they
may
attach
to
endoplasmic
reticulum .
They
provide
an
attachment
and

reticulum, they form a pore that moves the newly synthesized proteins into the
cisterna of the rough endoplasmic reticulum .


The
above
figure
shows
another
electron
micrograph
of
a
nucleolus
with
the
nuclear
organizing
region,
the
pars
fibrosa
and
the
pars
granulosa.
Nucleoli
increase in number and enlarge when the cell is stimulated to produce proteins.
This is a sign that the cell has been stimulated or is actively involved in protein
synthesis.
Nucleoli
disappear
during
cell
division
and
then
reform
at
the
chromosomal nucleolar organizing centers.

One
can
study
synthesis
of
ribosomal
RNA
with
the
electron
microscope.
A
preparation
of
nucleoli
is
dissociated
and
spread
on
a
liquid
surface
(in
a
manner similar to the spreading of chromatin). Then, the synthesis of ribosomal
RNA
is
stimulated
and
after
a
period
of
time,
the
DNA
from
the
nucleolar
organizing region begins to look like a Christmas tree. The top of the tree is the
start
site.
As
you
move
down
the

the
branches
appear
longer.
Each
branch
is
a
growing
strand
of
ribosomal
RNA.
The
DNA
code
is
being
transcribed and the nucleotides added to the growing RNA strand.

For more information about the process, see the web site describing ribosomal
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