Presentation on theme: "The Cell Nucleus and the Control of Gene Expression"— Presentation transcript:
1The Cell Nucleus and the Control of Gene Expression CHAPTER 12The Cell Nucleus and the Control of Gene Expression
2IntroductionAll 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.
412.1 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.
5The Nucleus of a Eukaryotic Cell (2) The Nuclear EnvelopeThe 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.
10The Nucleus of a Eukaryotic Cell (4) The Structure of the Nuclear Pore Complex and its Role in Nucleocytoplasmic ExchangeProteins 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.
15The 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.
16Importing proteins from the cytoplasm into the nucleus
17Importing proteins from the cytoplasm into the nucleus (continued)
18The Nucleus of a Eukaryotic Cell (6) Chromosomes and ChromatinPackaging the GenomeChromosomes consist of chromatin fibers, composed of DNA and associated proteins.Each chromosome contains a single, continuous DNA molecule.
19The Nucleus of a Eukaryotic Cell (7) Nucleosomes: The Lowest Level of Chromosome OrganizationThe protein component of chromosomes include histones, a group of highly conserved proteins.Histones have a high content of basic amino acids.
20The 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.
24The 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.
25The Nucleus of a Eukaryotic Cell (10) Higher Levels of Chromatin StructureA 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.
29The Nucleus of a Eukaryotic Cell (11) Heterochromatin and EuchromatinEuchromatin 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.
30The 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.
32The Nucleus of a Eukaryotic Cell (13) The Histone Code and Formation of HeterochromatinThe 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.
34The 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.
35Example of proteins that bind selectively to modified histones
36Correlation between transcriptional activity and histone acetylation
37Model showing possible events during formation of heterochromatin
38The Nucleus of a Eukaryotic Cell (15) The Structure of a Mitotic ChromosomeChromatin 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.
41The Nucleus of a Eukaryotic Cell (16) TelomeresThe 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.
43The 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.