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BIOL 200 (Section 921)
Lecture # 3, June 21, 2006
• Reading for unit 3 on Interphase
nucleus (Lecture 3):
ECB 2nd edition, Chap 5 pp. 177-191, Chap 15 pp. 502-506, Chap
17 p. 578, Chap 19 pp 651-2. Good questions: 5-1; 5-4 a, b, c; 5-5;
5-7; 5-10; 5-11; 5-12; 5-13; 5-17.
• Learning Objectives:
1. Understand chromatin structure. Explain how proteins and DNA
interact to form chromosomes, starting with the 2nm naked DNA
molecule. Be able to explain the structure of nucleosomes and the
forces stabilizing these structures, then proceed to the, 10 nm fibre,
higher order structures, chromosomal loops, euchromatin and
heterochromatin.
2. Understand the organization of chromosomes; describe centromere
and telomere.
3. Describe the organization of the interphase nucleus, and the
transport of macromolecules into and out of the nucleus
Eukaryotic DNA is packaged into chromosomes
in plants (Fig. 5-1) and human (Fig. Fig. 5-13)
Knobs show ribosomal RNA encoding DNA (rDNA)
Replication and segregation of chromosomes
during cell cycle [Fig. 5-17]
Three DNA sequence elements are needed to
produce a eukaryotic chromosome that can be
replicated and segregated [Fig. 5-18]
The bacterial Nucleoid: (A) A bacterial cell with
a distinct nucleoid (B) Release of chromosomal
DNA from ruptured bacterial cells
[Becker et al. The World of the Cell, 6th ed.]
DNA in mitotic chromosomes is more compact
than in interphase chromosomes
The interphase nucleus [Fig. 5-19]
Two types of chromatin
– Euchromatin
• diffuse in appearance by EM
• relatively uncondensed
• contains transcriptionally active genes
– Heterochromatin
• condensed in appearance by EM
• contains mostly inactive genes
• constitutive heterochromatin is permanently
condensed
• facultative heterochromatin is specifically
inactivated, and depends on the cell type.
Fig.
8-10,
p.253
Naked DNA
Beads on String
(nucleosomes
and spacer DNA)
30 nm Interphase
chromatin (packed
nucleosomes)
ONLY during cell
division, chromatin
is further packaged
Fig. 5-24, p.186
Nucleosomes=
DNA+histone proteins
Fig. 5-21, p.184
A nucleosome core particle showing DNA
tightly wrapped around a histone core
Fig. 5-21, p.184
30 nm
chromatin
fibre
Nucleosomes
In beads on a
string
Nucleosomes (basic units of chromatin structure) contain DNA
wound around a protein core of eight histone molecules
Gel electrophoresis
Gel electrophoresis
Chromatin remodeling complexes change
the nucleosome structure
Interphase nucleus
Nuclear envelope
• Is made up of two lipid bilayer membranes, the inner
nuclear membrane and the outer nuclear membrane
• The outer membrane is continuous with the endoplasmic
reticulum (ER) membrane
• The space between the inner and outer membranes is
known as the perinuclear space.
• The nuclear envelope forms a barrier between the
cytoplasm and the nucleus and controls the passage
through nuclear pores of macromolecules in and out of
the nucleus.
Macromolecular transport into and out of the
Nucleus
[Becker et al. The World of the Cell, 6th ed.]
Nuclear transport of macromolecules
• Macromolecules transported into the nucleus:
–
–
–
–
–
–
–
DNA replication enzymes,
nuclear lamina proteins,
nuclear matrix proteins,
ribosomal proteins,
chromatin proteins (histones),
transcription factors,
structural proteins.
• Macromolecules transported out of the nucleus :
– messenger RNA,
– transfer RNA,
– ribosomal subunits.
Nuclear pores
• Are protein complexes that
span both nuclear
membranes,
• Regulate the passage of
macromolecules between
cytoplasm and nucleoplasm
• Allow movement by
diffusion of small molecules
• Provide for regulated active
transport of macromolecules
and macromolecular
assemblies such as
ribosomal subunits.
• The pore complex is 70-90
nm in diameter and the
opening is thought to be
about 9 nm in diameter.
Nuclear pore complexes allow transport of
molecules into and out of the nucleus
Transport of proteins through nuclear
pores
• Active transport –
requires GTP
hydrolysis
• Prospective nuclear
proteins have a
nuclear localization
signal (NLS): -ProLys-Lys-Lys-ArgLys-Val-
Intermediate filaments of the nuclear lamina support
and strengthen the nuclear envelope [Fig. 17-8]
The nuclear envelope breaks down and
reforms during mitosis [Fig. 19-18]
Nuclear lamina (nuclear cortex) - an example
of a non-chromatin protein in the nucleoplasm.
• The inner surface of the nuclear membrane is
asociated with a protein complex called the
nuclear lamina.
• The lamina supplies an internal structure for the
nucleus
• The interphase chromosomes bind to the
nuclear lamina.
• The lamina proteins help the nucleus to
reorganize after mitosis
• The lamina binds other membrane receptors in
the inner nuclear membane
Interphase Nucleus
Heterochromatin
Euchromatin
Nuclear Envelope
Nucleolus
Nucleolus:
Heterochromatin:
Nuclear
Euchromatin:
site
ofcondensed
ribosomal
dark,
Envelope:
lighter,
assembly
and
DNA
that ais
Contains
double
transcriptionally
rRNA
transcription
transcriptionally
membrane
active
DNA
inactive
during
surrounding
the
interphase.
nucleus. Contains
nulear pores
throughout.
Nucleolus
• Is a unique region within the nucleus
• It consists of chromosomal regions containing
the ribosomal RNA transcription units
• It is the site of ribosomal RNA synthesis
• It is the site of assembly of ribosomes
• It consists of a fibrous component (DNA and
RNA transcripts) and a granular region
consisting of developing ribosomal subunits.
Nucleoplasm - (inside nucleus,
lumen of the nucleus)
• Semifluid matrix
• Made up of extended chromatin (DNA and
protein complexes) and non-chromatin proteins
(i.e. nuclear matrix, nuclear lamina)
• Its function is to contain the genetic material and
serve as a site for DNA replication and for
transcription of the genetic material.