Nuclear and Chromosomal Structure - Molecular Biology PDF

Nuclear and Chromosomal Structure Alberts MBotC Chapter 4 (and a little bit of Chapter 12)  The Big Picture Eukaryotic cells are defined by the presence of the nucleus, which contains the genetic material The nucleus is bounded by a d...

Nuclear and Chromosomal Structure Alberts MBotC Chapter 4 (and a little bit of Chapter 12)  The Big Picture Eukaryotic cells are defined by the presence of the nucleus, which contains the genetic material The nucleus is bounded by a double membrane (i.e. two lipid bilayers) that communicate with the cytoplasm via large nuclear pores The DNA in eukaryotic cells is in the form of chromatin, a complex with histones and other proteins Chromatin is a dynamic structure, which can change depending on the needs of the cell. Modification of histones is an important mechanism for regulating chromatin structure, and as a result, gene expression  The Nucleus The most obvious internal structure of eukaryotic cells, and their defining characteristic, is the nucleus The nucleus contains the genetic material of DNA within a double membrane structure that is continuous with the endoplasmic reticulum The nuclear membrane is supported by a fibrous nuclear lamina, which provides structural support The nucleus is compartmentalized into subdomains, but these are dynamic – they are not separated by membranes, and can be assembled/disassembled as needed The best-understood subdomain of the nucleus is the nucleolus, which is the site of ribosome assembly  Prokaryotes don’t have nuclei. What does that tell us about their DNA? ? A. It floats around freely in the cytoplasm ? B. There might be some organized nuclear-type region, but it isn’t surrounded by a membrane C. Who cares about prokaryotes? That is for a microbiology class! ? ? Figure 4-9a Molecular Biology of the Cell (© Garland Science 2008) Figure 12-8 Molecular Biology of the Cell (© Garland Science 2008)  Nuclear lamina: meshwork of proteins called lamins Figure 12-19 Molecular Biology of the Cell (© Garland Science 2008)  Progeria – A Disease of Nuclear Structure Progeria (aka Hutchinson-Gilford progeria syndrome) is a genetic disorder that causes premature aging Disease is caused by a point mutation in the protein lamin A, which is normally a component of the nuclear lamina Mutation prevents lamin A from incorporating in the lamina, reducing structural support for the nuclear membrane Also prevents the normal organization of chromatin, affecting the ability of cells to divide properly  Nuclear Pores The double bilayer of the nuclear membrane presents a major barrier to the movement of material between the cytoplasm and the nucleus Transport between the nucleus and cytoplasm is regulated via large nuclear pores The nuclear pore complex consists of about 30 different proteins (nucleoporins) that span the double membrane Small ions and molecules (< 5 kilodaltons) can diffuse freely through the pores. Proteins smaller than about 60 kilodaltons can still transit the pores via passive diffusion, but more slowly Larger macromolecules, and even large protein complexes (such as ribosomes), can traverse the nuclear pores by active transport (more on this in a later lecture) Figure 12-9b Molecular Biology of the Cell (© Garland Science 2008) Figure 12-9a Molecular Biology of the Cell (© Garland Science 2008) Figure 12-9c Molecular Biology of the Cell (© Garland Science 2008)  Chromatin and Chromosomal Structure The DNA in the nucleus of a eukaryotic cell is divided into a set of chromosomes Each chromosome is a single VERY long strand of DNA Bacterial (E. coli) chromosome: ~ 1 mm (circular) E. coli cell: ~ 2 µm in length Human genome: ~ 2 m (46 linear chromosomes) Human cell nucleus: ~ 5 µm in diameter How does the DNA fit?  If you put all of the DNA in your body end to end, about how long do you think it would be? ? A. About 2 times the circumference of the earth ? B. About 5 round-trips from the earth to the moon C. About 10 round-trips from the earth to the sun D. About the distance from earth to Neptune ? ?  Chromatin and Chromosomal Structure Each chromosome is a single VERY long strand of DNA in a complex with a variety of proteins and RNA molecules: chromatin The structure of chromatin serves to condense the linear DNA molecules to fit within the nucleus Chromatin is a highly dynamic structure that changes to regulate gene expression, DNA replication, and cell division  Chromatin and Chromosomal Structure The basic unit of chromatin is the nucleosome: ~150 nucleotides of DNA wound around a core of histone proteins – The histone core is composed of two copies each of 4 different histones (H2A, H2B, H3, H4) If stretched out, the nucleosomes would look like “beads on a string” Between the “beads” is a short stretch of linker DNA that ranges from a few nucleotides to ~80 nucleotides, such that the nucleosome structure repeats on average every 200 nucleotides or so Figure 4-22 Molecular Biology of the Cell (© Garland Science 2008) Figure 4-24 Molecular Biology of the Cell (© Garland Science 2008)  Chromatin and Chromosomal Structure During interphase (i.e. when cells are not actively dividing), the chromatin is condensed into a fiber that is ~30 nm in diameter The actual packing structure of the 30 nm fiber is not known, and several different models have been suggested, based on different imaging techniques Interactions between the N-terminal tails of the histones in neighboring nucleosomes are important in forming the 30 nm fiber The linker histone H1 binds to the outside of each nucleosome and also helps to condense the chromatin Figure 4-31 Molecular Biology of the Cell (© Garland Science 2008) Figure 4-33a Molecular Biology of the Cell (© Garland Science 2008) Figure 4-34 Molecular Biology of the Cell (© Garland Science 2008)  Heterochromatin and Euchromatin Interphase chromatin is not homogeneous, but consists of at least two types Heterochromatin is highly condensed, and represents DNA that is resistant to gene expression (“silenced”). It also includes some specialized structures (centromeres, telomeres) Euchromatin is less condensed and remains accessible to the RNA transcription machinery Chromatin structure can be regulated by covalent modifications to the histone tails Figure 4-39a Molecular Biology of the Cell (© Garland Science 2008) Figure 4-33b Molecular Biology of the Cell (© Garland Science 2008) Figure 4-38 Molecular Biology of the Cell (© Garland Science 2008) Figure 4-39b Molecular Biology of the Cell (© Garland Science 2008)  Chromatin and Chromosomal Structure 30 nm fibers are still much too long to fit into a nucleus, so further condensation is needed – Structure is not well understood, but likely involves extensive looping and interactions with many non-histone proteins, including the nuclear lamina Mitotic chromosomes represent the most highly condensed form of chromatin As with the lower levels of packing, the fine details are unknown, but can be postulated Figure 4-57 Molecular Biology of the Cell (© Garland Science 2008) Figure 4-72 Molecular Biology of the Cell (© Garland Science 2008)  A Bit More On Chromosomal Structure Within each chromosome are several specialized sequences that are required for proper duplication The replication origin is a site where DNA duplication is initiated. Eukaryotic chromosomes generally have many replication origins, to allow for more rapid duplication The centromere is the site of attachment to the mitotic spindle, which allows one copy of each duplicated chromosome to be pulled into each daughter cell during mitosis The telomere prevents the ends of chromosomes from being mistaken for broken DNA, and allows for proper duplication of the chromosome ends  Which structure(s) would you expect to find in a prokaryotic chromosome? ? A. Telomere ? B. Centromere C. Origin of replication D. All of the above ? ? Figure 4-21 Molecular Biology of the Cell (© Garland Science 2008)  The Big Picture - Review Eukaryotic cells are defined by the presence of the nucleus, which contains the genetic material The nucleus is bounded by a double membrane (i.e. two lipid bilayers) that communicate with the cytoplasm via large nuclear pores The DNA in eukaryotic cells is in the form of chromatin, a complex with histones and other proteins Chromatin is a dynamic structure, which can change depending on the needs of the cell. Modification of histones is an important mechanism for regulating chromatin structure, and as a result, gene expression