10_16_lecture.pptx

Further Packing of Chromatin  The 30-nm fiber seems to be packed together in an irregular, three-dimensional zigzag structure  These fibers fold into DNA loops 50,000–100,000 bp in length, stabilized by cohesin protein  The loops are spatially arranged through attachment to nonhistone proteins th...

Further Packing of Chromatin  The 30-nm fiber seems to be packed together in an irregular, three-dimensional zigzag structure  These fibers fold into DNA loops 50,000–100,000 bp in length, stabilized by cohesin protein  The loops are spatially arranged through attachment to nonhistone proteins that form a chromosomal scaffold © 2016 Pearson Education, Inc. © 2016 Pearson Education, Inc. Transcription and Packaging  Transcriptionally active DNA is less tightly packed than inactive DNA  A “looser” packaging would allow easy access by proteins involved in gene transcription © 2016 Pearson Education, Inc. Changes in Histones and Chromatin Remodeling Proteins Can Alter Chromatin Packing  Cells can tightly regulate the portions of chromatin that are active or inactive, through altering histones  Each histone has a protruding tail that can be tagged by the addition of methyl, acetyl, phosphate, or other groups  Various combinations of these tags create a histone code © 2016 Pearson Education, Inc. Histone Methylation and Acetylation  One tagging reaction is the methylation of lysine via histone methyltransferase  Methylation can serve as a signal for activation or repression of transcription, depending on the lysine involved  Acetylation of histone side chains is accomplished by histone acetyltransferases (HATs)  The opposite function is catalyzed by histone deacetylase (HDAC) © 2016 Pearson Education, Inc. Chromosomal DNA Contains Euchromatin and Heterochromatin  Sections of chromatin so highly compacted that they show up as dark spots in micrographs are called heterochromatin  More loosely packed, diffuse chromatin is called euchromatin  Active cells have euchromatic, but in preparation for cell division all the chromatin becomes highly compacted  After replication, each chromosome is composed of two identical chromatids © 2016 Pearson Education, Inc. Some Heterochromatin Plays a Structural Role in Chromosomes  Facultative heterochromatin can be converted to euchromatin, and vice versa  Some heterochromatin is permanently compacted; as constitutive heterochromatin  Serves structural functions © 2016 Pearson Education, Inc. Centromeres  Centromeres appear as constriction of chromosomes  Centromere DNA is bound by a complex of proteins and serves important functions  Centromeres maintain sister chromatid cohesion during mitosis and meiosis  They also serve as sites of kinetochores, crucial for attaching spindle microtubules to chromosomes during meiosis and mitosis © 2016 Pearson Education, Inc. Centromere Sequences  Centromeres are characterized by highly repetitive DNA sequences (CEN sequences)  Eukaryotes have their own CEN regions, which are not very similar from one organism to the next © 2016 Pearson Education, Inc. Telomeres  Telomeres are found at the tips of chromosomes  They contain highly repetitive DNA sequences  Telomeres protect chromosome ends from degradation during each round of DNA replication  All vertebrates studied so far have the same repeat sequence (TTAGGG) © 2016 Pearson Education, Inc. Chromosomes Can Be Identified by Unique Banding Patterns  Mitotic chromosomes viewed under the light microscope can be distinguished by size and position of the centromere  Similar-sized chromosomes can be distinguished by their banding patterns in response to stains  A common stain is Giemsa, which causes light- and dark-staining chromosome bands called G bands © 2016 Pearson Education, Inc. © 2016 Pearson Education, Inc. Eukaryotic Chromosomes Contain Large Amounts of Repeated DNA Sequences  In the 1960s, Roy Britten and David Kohne discovered repeat DNA sequences  They broke DNA into small fragments, denatured them by heating, and allowed them to renature  The rate of renaturation depends on the concentration of each kind of DNA sequence— those found in high concentration reanneal more quickly © 2016 Pearson Education, Inc. © 2016 Pearson Education, Inc. Bacterial versus Mammalian DNA  When mammalian and bacterial DNA were tested, it was expected that bacterial DNA, having fewer types of DNA sequences, should reanneal much faster  The results were not as expected; the calf DNA consisted of two classes of sequences that renature at very different rates  About 40% of the calf DNA renatures more rapidly than bacterial DNA © 2016 Pearson Education, Inc. Repeated DNA Sequences  The more rapidly annealing sequences contain repeated DNA sequences that are present in multiple copies. The rest is nonrepeated DNA  Categories of repeated DNA: tandemly repeated DNA and interspersed repeated DNA © 2016 Pearson Education, Inc. Tandemly Repeated DNA  One major category of DNA repeats is called tandemly repeated DNA  The multiple copies are arranged next to each other in a row  It accounts for 10–15% of a typical mammalian genome; a repeat unit can measure anywhere from 1 to 2000 bp, most of the time less than 10 bases © 2016 Pearson Education, Inc. Simple-Sequence Repeats  The tandem repeats that are less than 10 bases per repeat comprise a subcategory called simplesequence repeated DNA  There can be as many as several hundred thousand copies at selected sites in the genome  It was originally called satellite DNA © 2016 Pearson Education, Inc. Types of Repeat Sequences  The amount of satellite DNA at any given site can vary enormously; typically it ranges from 105 to 107 bp in overall length  Variable number tandem repeats (VNTRs) refer to short repeats  Minisatellites are short, 102 to 105 bp in length  Microsatellites (or short tandem repeats, STRs) are even shorter, 10–100 bp in length, but with numerous sites in the genome © 2016 Pearson Education, Inc. Interspersed Repeated DNA  Interspersed repeated DNAs are scattered around the genome  Single repeats are hundreds or thousands of bases in length, and the dispersed copies, numbering in hundreds of thousands of copies, are similar but not identical to one another  They account for 25–50% of mammalian genomes © 2016 Pearson Education, Inc. Types of Interspersed Repeated DNA  Most interspersed repeated DNA consists of families of transposable elements (transposons), which can move around the genome and leave copies of themselves behind  Roughly half of the human genome consists of these mobile elements  The most abundant are called LINEs (long interspersed nuclear elements) © 2016 Pearson Education, Inc. © 2016 Pearson Education, Inc. LINEs and SINEs  LINEs are 6000–8000 bp long and contain genes required for their own mobilization  SINEs are short interspersed nuclear elements and are less than 500 bp  These rely on enzymes from other elements for their movement; the most common SINEs in humans are Alu sequences, which account for 10% of the human genome © 2016 Pearson Education, Inc. Eukaryotes Package Some of Their DNA in Mitochondria and Chloroplasts  Mitochondria and chloroplasts have their own chromosomes, which lack histones and is circular  Both organelles can encode some of their own polypeptides but depend on the nuclear genome to encode the rest of them © 2016 Pearson Education, Inc. The Human Mitochondrion  The genome of the human mitochondrion has been sequenced  It is 16,569 base pairs long and encodes 37 genes, about 5% of all the RNAs and proteins needed by the mitochondrion © 2016 Pearson Education, Inc. Chloroplast Genomes  Chloroplasts usually possess circular DNA molecules of about 120,000 bp in length, containing around 120 genes  Subunits of some multimeric protein complexes are encoded by the nuclear genome; this is true for both chloroplasts and mitochondria © 2016 Pearson Education, Inc. 16.4 The Nucleus  The nucleus is the site within the eukaryotic cell where the chromosomes are localized and replicated and the DNA they contain is transcribed © 2016 Pearson Education, Inc. © 2016 Pearson Education, Inc. A Double-Membrane Nuclear Envelope Surrounds the Nucleus  The nucleus is bounded by a nuclear envelope with an inner and an outer membrane separated by a perinuclear space  The outer membrane is continuous with the ER and contains proteins that bind actin and intermediate filaments (IFs) of the cytoskeleton  Tubular invaginations of the envelope project into the nucleus © 2016 Pearson Education, Inc. © 2016 Pearson Education, Inc. Nuclear Pores  Nuclear pores are specialized channels where inner and outer membranes are fused  They provide direct contact between the cytosol and the nucleoplasm © 2016 Pearson Education, Inc. Molecules Enter and Exit the Nucleus Through Nuclear Pores  Enzymes and proteins needed in the nucleus must be imported from the cytoplasm  RNAs that need to be translated and components of ribosomes must be exported from the nucleus  In addition to all the traffic through the pores, they also mediate passage of small particles, molecules, and ions © 2016 Pearson Education, Inc. © 2016 Pearson Education, Inc. Simple Diffusion of Small Molecules Through Nuclear Pores  Small particles, less than 10 nm in diameter, pass through pores at a rate proportional to the size of the particle  The NPC contains tiny aqueous diffusion channels through which small particles freely move © 2016 Pearson Education, Inc. Active Transport of Large Proteins and RNA Through Nuclear Pores  Some proteins needed in the nucleus are too large to easily diffuse through the nuclear pores  These large particles are actively transported across the membrane  Nuclear localization signals (NLS) enable the protein to be recognized and transported by the nuclear pore complex © 2016 Pearson Education, Inc. Nuclear Localization Signals  An NLS is usually 8–30 amino acids in length and often contains proline and the basic amino acids lysine and arginine © 2016 Pearson Education, Inc. The Nucleus Is Mechanically Integrated with the Rest of the Cell  The nuclear matrix (nucleoskeleton) is an insoluble fibrous network that helps maintain the shape of the nucleus  The nuclear lamina is a thin dense meshwork of fibers lining the inner surface of the inner nuclear membrane  It is made of intermediate filaments made from lamins © 2016 Pearson Education, Inc. The Nucleolus Is Involved in Ribosome Formation  The nucleolus is the place in the nucleus where ribosomal subunits are assembled  Fibrils in the nucleolus contain DNA that is being transcribed into ribosomal RNA (rRNA)  Granules in the nucleolus are rRNA molecules being packaged with proteins © 2016 Pearson Education, Inc. © 2016 Pearson Education, Inc.