Lecture 17 Nucleus pp91-95

Nucleus Chinese hamster ovary cells

Nucleus Largest organelle Usually visible with the light microscope Spherical – elliptical shape ~ 5 um in diameter Double membrane surrounding the chromosomes & nucleolus Most cells only have 1 nucleus

Multinuclear – several nuclei (2-50) Exceptions: Anulclear – no nucleus mature red blood cells Multinuclear – several nuclei (2-50) liver cells, skeletal muscle cells, platelet producing cells, bone- dissolving cells

Nucleus Contains nuclear envelope, nucleoli, chromatin, and distinct compartments rich in specific protein sets Gene-containing control center of the cell Contains the genetic library with blueprints for nearly all cellular proteins Dictates the kinds and amounts of proteins to be synthesized

Nucleus Figure 3.28a

4.6 The nucleus is the cell’s genetic control center The nucleus controls the cell’s activities and is responsible for inheritance Inside is a complex of proteins and DNA called chromatin, which makes up the cell’s chromosomes DNA is copied within the nucleus prior to cell division The DNA within the nucleus contains genetic information that must be faithfully replicated and partitioned between two daughter cells to maintain the species. Student Misconceptions and Concerns 1. Students can have trouble relating many cell organelles to their diverse functions. They may not realize that Modules 4.6–4.13 introduce the primary organelles in the order that they function in the production and release of secretory proteins. Products and information generally move from the central nucleus to the rough ER, through the more peripherally located Golgi apparatus and the secretory vesicles, and finally to the outer plasma membrane. Emphasizing the flow from center to periphery in this process will help students to remember the function of individual organelles as they recall the steps of the sequence. 2. Conceptually, some students seem to benefit from the well-developed cell-factory analogy developed in the text. The use of this analogy in lecture might help to anchor these relationships. As mentioned before, challenge students to find exceptions in the analogy, an exercise that promotes critical thinking. Teaching Tips 1. Noting the main flow of genetic information on the board as DNA → RNA → protein will provide a useful reference for students when explaining these processes. As a review, have students note where new molecules of DNA, rRNA, mRNA, ribosomes, and proteins are produced in a cell. 2. If you wish to continue the text’s factory analogy, nuclear pores might be said to function most like the door to the boss’s office. 3. Some of your more knowledgeable students may like to guess the exceptions to the rule of 46 chromosomes per human cell. These exceptions include gametes, some of the cells that produce them, and adult red blood cells in mammals. 4. If you want to challenge your students further, ask them to consider the adaptive advantage of using mRNA to direct the production of proteins instead of using DNA directly. Some biologists suggest that DNA is better protected in the nucleus and that mRNA, exposed to more damaging cross-reactions in the cytosol, is the temporary working copy of the genetic material. In some ways, this is like making a working photocopy of an important document, keeping the original copy safely stored away. Copyright © 2009 Pearson Education, Inc.

4.6 The nucleus is the cell’s genetic control center The nuclear envelope is a double membrane with pores that allow material to flow in and out of the nucleus It is attached to a network of cellular membranes called the endoplasmic reticulum Each of the membranes in the nuclear envelope (membrane) is a lipid bilayer separated by a space of 20–40 nm. Student Misconceptions and Concerns 1. Students can have trouble relating many cell organelles to their diverse functions. They may not realize that Modules 4.6–4.13 introduce the primary organelles in the order that they function in the production and release of secretory proteins. Products and information generally move from the central nucleus to the rough ER, through the more peripherally located Golgi apparatus and the secretory vesicles, and finally to the outer plasma membrane. Emphasizing the flow from center to periphery in this process will help students to remember the function of individual organelles as they recall the steps of the sequence. 2. Conceptually, some students seem to benefit from the well-developed cell-factory analogy developed in the text. The use of this analogy in lecture might help to anchor these relationships. As mentioned before, challenge students to find exceptions in the analogy, an exercise that promotes critical thinking. Teaching Tips 1. Noting the main flow of genetic information on the board as DNA → RNA → protein will provide a useful reference for students when explaining these processes. As a review, have students note where new molecules of DNA, rRNA, mRNA, ribosomes, and proteins are produced in a cell. 2. If you wish to continue the text’s factory analogy, nuclear pores might be said to function most like the door to the boss’s office. 3. Some of your more knowledgeable students may like to guess the exceptions to the rule of 46 chromosomes per human cell. These exceptions include gametes, some of the cells that produce them, and adult red blood cells in mammals. 4. If you want to challenge your students further, ask them to consider the adaptive advantage of using mRNA to direct the production of proteins instead of using DNA directly. Some biologists suggest that DNA is better protected in the nucleus and that mRNA, exposed to more damaging cross-reactions in the cytosol, is the temporary working copy of the genetic material. In some ways, this is like making a working photocopy of an important document, keeping the original copy safely stored away. Copyright © 2009 Pearson Education, Inc.

Two membranes of nuclear envelope Nucleus Nucleolus Chromatin Pore Figure 4.6 TEM (left) and diagram (right) of the nucleus. Endoplasmic reticulum Ribosomes

Nuclear Envelope Selectively permeable double membrane barrier containing pores Encloses jellylike nucleoplasm, which contains essential solutes

Nuclear Envelope Outer membrane is continuous with the rough ER and is studded with ribosomes Inner membrane is lined with the nuclear lamina, which maintains the shape of the nucleus Pore complex regulates transport of large molecules into and out of the nucleus

Nucleoli Dark-staining spherical bodies within nucleus Involved in rRNA synthesis and ribosome subunit assembly Associated with nucleolar organizer regions Contains DNA coding for rRNA Usually one or two per cell © 2013 Pearson Education, Inc.

Surface of nuclear envelope. Figure 3.29b The nucleus. Surface of nuclear envelope. Fracture line of outer membrane Nuclear pores Nucleus Nuclear pore complexes. Each pore is ringed by protein particles. Nuclear lamina. The netlike lamina composed of intermediate filaments formed by lamins lines the inner surface of the nuclear envelope. © 2013 Pearson Education, Inc.

Chromatin Threadlike strands of DNA (30%), histone proteins (60%), and RNA (10%) Arranged in fundamental units called nucleosomes Histones pack long DNA molecules; involved in gene regulation Condense into barlike bodies called chromosomes when cell starts to divide © 2013 Pearson Education, Inc.

Figure 3.30 Chromatin and chromosome structure. 1 DNA double helix (2-nm diameter) Histones 2 Chromatin (“beads on a string”) structure with nucleosomes Linker DNA Nucleosome (10-nm diameter; eight histone proteins wrapped by two winds of the DNA double helix) 3 Tight helical fiber (30-nm diameter) 4 Looped domain structure (300-nm diameter) 5 Chromatid (700-nm diameter) 6 Metaphase chromosome (at midpoint of cell division) consists of two sister chromatids © 2013 Pearson Education, Inc.

Nucleic Acids Composed of carbon, oxygen, hydrogen, nitrogen, and phosphorus Their structural unit, the nucleotide, is composed of N-containing base, a pentose sugar, and a phosphate group

Nucleic Acids Five nitrogen bases contribute to nucleotide structure – adenine (A), guanine (G), cytosine (C), thymine (T), and uracil (U) Two major classes – DNA and RNA

Deoxyribonucleic Acid (DNA) Double-stranded helical molecule found in the nucleus of the cell Replicates itself before the cell divides, ensuring genetic continuity Provides instructions for protein synthesis

Structure of DNA Figure 2.22a

Structure of DNA Figure 2.22b

Ribonucleic Acid (RNA) Single-stranded molecule found in both the nucleus and the cytoplasm of a cell Uses the nitrogenous base uracil instead of thymine Three varieties of RNA: messenger RNA, transfer RNA, and ribosomal RNA

Adenosine Triphosphate (ATP) Source of immediately usable energy for the cell Adenine-containing RNA nucleotide with three phosphate groups

Adenosine Triphosphate (ATP) Figure 2.23

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