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CHAPTER 12. The Cell Nucleus and the Control of Gene Expression. Introduction. All cells in a multicellular organism contain the same complement of genes. Cells express their genetic information selectively. Gene expression is controlled by regulatory machinery in the cell nucleus.
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CHAPTER 12 The Cell Nucleus and the Control of Gene Expression
Introduction • All cells in a multicellular organism contain the same complement of genes. • Cells express their genetic information selectively. • Gene expression is controlled by regulatory machinery in the cell nucleus.
12.1 The Nucleus of a Eukaryotic Cell (1) • The contents of the nucleus are enclosed by the nuclear envelope. • A typical nondividing nucleus includes: • Chromosomes as extended fibers of chromatin. • Nucleoli for rRNA synthesis. • Nucleoplasm as the fluid where solutes are dissolved. • The nuclear matrix, which is the protein-containing fibrillar network.
The Nucleus of a Eukaryotic Cell (2) • The Nuclear Envelope • The nuclear envelope is a structure that divides the nucleus from its cytoplasm. • It consists of two membranes separated by a nuclear space. • The two membranes are fuses at sites forming a nuclear pore. • The inner surface of the nuclear envelope is lined by the nuclear lamina.
The Nucleus of a Eukaryotic Cell (3) • The nuclear lamina • Support the nuclear envelope. • It is composed of lamins. • The integrity of the nuclear lamina is regulated by phosphorylation and dephosphorylation.
The Nucleus of a Eukaryotic Cell (4) • The Structure of the Nuclear Pore Complex and its Role in Nucleocytoplasmic Exchange • Proteins and RNA are transported in and out of the nucleus. • Nuclear pores contain the nuclear pore complex (NPC) that appears to fill the pore like a stopper. • NPC is composed of ~30 proteins called nucleoporins.
The Nucleus of a Eukaryotic Cell (5) • Proteins synthesized in the cytoplasm are targeted for the nucleus by the nuclear localization signal (NLS). • Proteins with an NLS stretch bind to an NLS receptor (importin). • Conformation of the NPC changes as the protein passes through. • RNAs move through the NPCs as RNPs and carry NES (nuclear export signals) to pass through.
Importing proteins from the cytoplasminto the nucleus (continued)
The Nucleus of a Eukaryotic Cell (6) • Chromosomes and Chromatin • Packaging the Genome • Chromosomes consist of chromatin fibers, composed of DNA and associated proteins. • Each chromosome contains a single, continuous DNA molecule.
The Nucleus of a Eukaryotic Cell (7) • Nucleosomes: The Lowest Level of Chromosome Organization • The protein component of chromosomes include histones, a group of highly conserved proteins. • Histones have a high content of basic amino acids.
The Nucleus of a Eukaryotic Cell (8) • DNA and histones are organized into repeating subunits called nucleosomes. • Each nucleosome includes a core particle of supercoiled DNA and histone H1 serving as a linker. • DNA is wrapped around the core complex. • The histone core complex consists of two molecules each of H2A, H2B, H3, and H4 forming an octamer.
The Nucleus of a Eukaryotic Cell (9) • Histone modification is one mechanism to alter the character of nucleosomes. • DNA and histones are held together by noncovalent bonds.
The Nucleus of a Eukaryotic Cell (10) • Higher Levels of Chromatin Structure • A 30-nm filament is another level of chromatin packaging, maintained by histone H1. • Chromatin filaments are organized into large supercoiled loops. • The presence of loops in chromatin can be seen: • In mitotic chromosomes form which histones have been extracted. • In meiotic lampbrush chromosomes from amphibian oocytes.
The Nucleus of a Eukaryotic Cell (11) • Heterochromatin and Euchromatin • Euchromatin returns to a dispersed state after mitosis. • Heterochromatin is condensed during interphase. • Constitutive heterochromatin remains condensed all the time. • Facultative heterochromatin is inactivated during certain phases of the organism’s life.
The Nucleus of a Eukaryotic Cell (12) • Constitutive heterochromatin : • Found mostly around centromeres and telomeres. • Consists of highly repeated sequences and few genes. • Facultative heterochromatin: • Is found in one of the X chromosomes as a Barr body through X inactivation. • X inactivation is a random process, making adult females genetic mosaics.
The Nucleus of a Eukaryotic Cell (13) • The Histone Code and Formation of Heterochromatin • The histone code hypothesis states that the activity of a chromatin region depends on the degree of chemical modification of histone tails. • Histone tail modifications influence chromatin in two ways: • Serve as docking sites to recruit nonhistone proteins. • Alter the way in which histones of neighboring nucleosomes interact with one another.
The Nucleus of a Eukaryotic Cell (14) • Heterochromatin has many methylated H3 histones, which stabilize the compact nature of the chromatin. • Small RNAs and specific enzymes play a role in histone methylation.
Example of proteins that bind selectivelyto modified histones
Correlation between transcriptional activityand histone acetylation
Model showing possible events during formation of heterochromatin
The Nucleus of a Eukaryotic Cell (15) • The Structure of a Mitotic Chromosome • Chromatin of a mitotic cell exists in its most highly condensed state. • Staining mitotic chromosomes can provide useful information. • A karyotype is a preparation of homologous pairs ordered according to size. • The pattern on a karyotype may be used to screen chromosomal abnormalities.
The Nucleus of a Eukaryotic Cell (16) • Telomeres • The end of each chromosome is called a telomere and is distinguished by a set of repeated sequences. • New repeats are added by a telomerase, a reverse transcriptase that synthesizes DNA from a DNA template.
The Nucleus of a Eukaryotic Cell (17) • Telomeres (continued) • Telomeres are required for the complete replication of the chromosome because they protect the ends from being degraded. • Telomerase activity is thought to have major effects on cell life.
