BIOL 160 - Chapter 9 - Cellular Reproduction PDF

Chapter 9 BIOL 60 Cellular reproduction 1 Chapter 9 At a Glance  9.1 What are the functions of cell division?  9.2 What Occurs During the Prokaryotic Cell Cycle?  9.3 How Is the DNA in Eukaryotic Chromosomes Organized?  9.4 What Occurs During the Eukaryotic Cell C...

Chapter 9 BIOL 60 Cellular reproduction 1 Chapter 9 At a Glance  9.1 What are the functions of cell division?  9.2 What Occurs During the Prokaryotic Cell Cycle?  9.3 How Is the DNA in Eukaryotic Chromosomes Organized?  9.4 What Occurs During the Eukaryotic Cell Cycle?  9.5 How Does Mitotic Cell Division Produce Genetically Identical Daughter Cells? 2 9.1 What are the functions of cell division?  Cells reproduce by cell division, in which a parent cell normally gives rise to two daughter cells  Each daughter cell receives a complete set of hereditary information from the parent cell and about half its cytoplasm  The hereditary information is usually identical with that of the parent cell 3 9.1 What are the functions of cell division?  Cell division transmits hereditary information to each daughter cell  The hereditary information in all cells is deoxyribonucleic acid (DNA)  Each DNA molecule consists of a long chain composed of smaller subunits called nucleotides  Each nucleotide consists of a phosphate, a sugar (deoxyribose), and one of four bases – adenine (A), thymine (T), guanine (G), or cytosine (C) 4 9.1 What are the functions of cell division?  The DNA in a chromosome consists of two long strands of nucleotides wound around each other. This structure is called a double helix  The units of inheritance called genes, are segments of DNA  The specific sequence of nucleotides in genes spell out the instructions for making the proteins of a cell  When a cell divides, it replicates its DNA to make two identical copies, and gives each daughter cell one of the two copies 5 9.1 What are the functions of cell division?  Cell division is required for growth, development and repair of multicellular organisms  The cell division of eukaryotic cells by which organisms grow or increase in number is called mitotic cell division  After cell division, the daughter cells may differentiate, becoming specialized for specific functions  The repeating pattern of divide, grow, and differentiate, then divide again is called the cell cycle 6 9.1 What are the functions of cell division?  Most multicellular organisms have three categories of cells  Stem cells  Other cells capable of dividing  Permanently differentiated cells 7 9.1 What are the functions of cell division?  Stem cells have two important characteristics: self-renewal, and potency.  Stem cells self-renew because they retain the ability to divide, perhaps for the entire life of the organism  Stem cells include most of the daughter cells formed by the first few cell divisions of a fertilized egg, as well as a few adult cells  When a stem cell divides, usually one daughter remains a stem cell, thus continuing the line; the other daughter eventually differentiates  Some stem cells in early embryos can produce any of the specialized cell types of the entire body.  Potency means that the dividing stem cells produce daughter cells that can differentiate into a variety of specialized cell types. 8 9.1 What are the functions of cell division?  Some cells other than stem cells are capable of continuing to divide, but typically differentiate into only one or two different cell types  Dividing liver cells, for example, can only become more liver cells  Permanently differentiated cells differentiate and never divide again  For example, most heart and brain cells cannot divide 9 9.1 What are the functions of cell division?  Cell division is required for sexual and asexual reproduction  Sexual reproduction in eukaryotic organisms occurs when offspring are produced by the fusion of gametes (sperm and eggs) from two adults  Cells in the adult’s reproductive system undergo a specialized type of cell division called meiotic cell division  Products of meiotic cell division, such as gametes, have exactly half the genetic information of their parent cells and reestablish the full genetic complement when they fuse  Reproduction in which offspring are formed from a single parent, without having a sperm fertilize an egg, is called asexual reproduction 10 9.2 What Occurs During the Prokaryotic Cell Cycle?  The DNA of a prokaryotic cell is contained in a single, circular chromosome about a millimeter or two in circumference  Unlike eukaryotic chromosomes, prokaryotic chromosomes are not contained in a membrane- bound nucleus  The prokaryotic cell cycle consists of a relatively long period of growth followed by binary fission (or “splitting in two”) 11 9.2 What Occurs During the Prokaryotic Cell Cycle?  The prokaryotic cell cycle has five stages: 1. At the start of the growth phase, the single prokaryotic chromosome is usually attached at one point to the plasma membrane of the cell 2. During the growth phase, the circular DNA chromosome replicates, producing two identical chromosomes that become attached to the plasma membrane at nearby, but separate, sites 3. As the cell increases in size, new plasma membrane is added between the attachment points, pushing the duplicated chromosomes apart 4. The plasma membrane grows inward between the two chromosome copies 5. Fusion of membrane along the cell equator completes separation (binary fission) of the cells, producing two daughter cells, each containing one of the chromosomes  The daughter cells are genetically identical 12 The Prokaryotic Cell Cycle 13 9.3 How Is the DNA in Eukaryotic Chromosomes Organized?  Eukaryotic chromosomes differ from prokaryotic chromosomes in important ways  Eukaryotic chromosomes are separated from the cytoplasm by a membrane-bound nucleus  Eukaryotic cells always have multiple chromosomes  Eurkaryotic chromosomes are longer and have more DNA than prokaryotic chromosomes (human chromosomes are 10 to 80 times longer and have 10 to 50 times more DNA)  These differences account for the complexity of eukaryotic cell division 14 9.3 How Is the DNA in Eukaryotic Chromosomes Organized?  The eukaryotic chromosome consists of a linear DNA double helix bound to proteins  Each human chromosome contains a single DNA double helix, about 50 million to 250 million nucleotides long  Most of the time, the DNA in each chromosome is wound around proteins called histones  These DNA-histone spools are further folded into coils  Another layer of folding occurs as the coiled strand folds into loops, which are then attached to protein scaffolding, so that the chromsome is 1,000 times shorter than the extended DNA molecule  During cell division, more proteins fold up the DNA and histones, until it is 10 times shorter than during its resting state 15 9.3 How Is the DNA in Eukaryotic Chromosomes Organized? 16 9.3 How Is the DNA in Eukaryotic Chromosomes Organized?  Genes are segments of the DNA of a chromosome  Genes are sequences of DNA from hundreds to thousands of nucleotides long  Each gene occupies a specific place, or locus (plural, loci) on the chromosome  In addition to genes, every chromosome has specialized regions that are crucial to its structure and function  Two telomeres  One centromere  The two ends of a chromosome consist of repeated nucleotide sequences called telomeres, which are essential for chromosome stability  The second specialized region of the chromosome is the centromere, which has two principal functions 1. It temporarily holds two daughter DNA double helices together after DNA replication 2. It is the attachment site for microtubules that move the chromosomes during cell division 17 9.3 How Is the DNA in Eukaryotic Chromosomes Organized? 18 9.3 How Is the DNA in Eukaryotic Chromosomes Organized?  Duplicated chromosomes separate during cell division  Prior to cell division, the DNA is replicated  At the end of DNA replication, a duplicated chromosome consists of two identical DNA double helices, called sister chromatids, which are attached to each other at the centromere  During mitotic cell division, the two sister chromatids separate, each becoming an independent chromosome that is delivered to one of the two daughter cells 19 9.4 What Occurs During the Eukaryotic Cell Cycle?  The eukaryotic cell cycle consists of interphase and cell division  Interphase is a time for acquisition of nutrients, growth, and chromosome duplication  During cell division, one copy of every chromosome and half of the cytoplasm and organelles are parceled out into the two daughter cells 20 9.4 What Occurs During the Eukaryotic Cell Cycle?  During interphase, a cell grows in size, replicates its DNA, and often differentiates  Most eukaryotic cells spend the majority of their time in interphase  Interphase is divided into three phases  G1 (growth phase 1) is a time for acquisition of nutrients and growth to proper size  S (synthesis phase) is characterized by DNA synthesis, during which every chromosome is replicated  G2 (growth phase 2) includes completion of cell growth and preparation for division of the cell into daughter cells 21 9.4 What Occurs During the Eukaryotic Cell Cycle? 22 9.5 How Does Mitotic Cell Division Produce Genetically Identical Daughter Cells?  Mitosis consists of four phases followed by cytokinesis  Prophase  Metaphase  Anaphase  Telophase  Cytokinesis 23 9.5 How Does Mitotic Cell Division Produce Genetically Identical Daughter Cells?  Three major events occur in prophase 1. Duplicated chromosomes condense and the nucleolus begins to disappear  Sister chromatids (formed during S phase) in each duplicated chromosome coil up, forming small compact bodies 2. Spindle microtubules form  Pairs of centrioles, which serve as loci from which spindle microtubules form, begin to migrate to opposite sides of the cell, to regions called spindle poles  In resting cells, centrioles exist in pairs, but during S phase, each duplicates itself, creating two pairs (which migrate to opposite poles in prophase)  Two types of spindle microtubules form: polar microtubules and kinetochore microtubules  The spindle microtubules radiate from the poles, both toward the nucleus, forming a basket around it and outward toward the plasma membrane  The nuclear envelop disintegrates, releasing the duplicated chromosomes  Plants, fungi, and some algae lack centrioles but still form spindle poles 24 9.5 How Does Mitotic Cell Division Produce Genetically Identical Daughter Cells?  Three major events occur in prophase (continued) 3. Chromosomes are captured by the spindle  Each sister chromatid has a kinetochore structure at its centromere to which kinetochore microtubules attach  Each chromatid of a given chromosome is attached to a microtubule emanating from a spindle pole opposite that of its sister  Because sister chomatids are attached to microtubules from opposite poles, they will be drawn to opposite poles later in mitosis and, ultimately, enter different daughter cells 25 9.5 How Does Mitotic Cell Division Produce Genetically Identical Daughter Cells?  During mitotic metaphase, chromosomes are drawn to the center of the spindle  A kinetochore microtubule from one pole that is attached to a chromatid’s kinetochore lengthens or shortens, as necessary, to draw the chromosome to the cell’s equator, in a line perpendicular to the spindle - The kinetochore microtubule on the other sister chromatid does the opposite  The two kinetochores of each duplicated chromosome face opposite spindle poles so that each chromosome is attached by its chromatids to opposite poles 26 9.5 How Does Mitotic Cell Division Produce Genetically Identical Daughter Cells? 27 9.5 How Does Mitotic Cell Division Produce Genetically Identical Daughter Cells?  During mitotic anaphase, daughter chromosomes (formerly sister chromatids) are drawn to opposite poles  Sister chromatids separate during anaphase into daughter chromosomes  Motor proteins in kinetochores pull chromatids apart along the kinetochore microtubules and toward opposite poles  At about the same time, polar microtubules from opposite poles attach to one another where they overlap at the equator  These polar microtubules then simultaneously lengthen and push on one another, which forces the poles of the cell apart, so that the cell assumes an oval shape  Clusters of chromosomes that gather at each pole contain one copy of every chromosome  Because the copies of a given chromosome were derived from identical sister chromatids, the daughter chromosomes at each pole are also identical - This is how mitotic cell division produces genetically identical cells 28 9.5 How Does Mitotic Cell Division Produce Genetically Identical Daughter Cells?  Telophase is the end stage of mitotic cell division  The spindle microtubules disintegrate  A nuclear membrane forms around each group of chromosomes at the pole  Chromosomes unwind (decondense) and revert to their extended state  The nucleoli (which disappeared in prophase) reappear 29 9.5 How Does Mitotic Cell Division Produce Genetically Identical Daughter Cells?  Cytokinesis in animal cells  Microfilaments attached to the plasma membrane form a ring around the equator of a cell  The ring contracts and constricts the cell’s equator  Eventually, contraction of the ring pinches off the membrane, forming two daughter cells, each with a nucleus identical with the other  Following cytokinesis, animal cells enter G1 of interphase, thus completing the cell cycle 30 9.5 How Does Mitotic Cell Division Produce Genetically Identical Daughter Cells? 31 9.5 How Does Mitotic Cell Division Produce Genetically Identical Daughter Cells?  Cytokinesis in plant cells  Stiff plant cell walls prevent the “pinching off” of cytokinesis seen in animal cells, which only have a plasma membrane  Instead, carbohydrate-filled vesicles assemble along the cell’s equator, between the daughter nuclei  The vesicles fuse into a continuous flattened sac, surrounded by plasma membrane and filled with sticky carbohydrates  This is called a cell plate  The plasma membranes of the plate fuse with the plasma membrane of the cell, forming two cells, with the carbohydrate in between becoming part of the cell wall  As in animals, plant cells enter G1 of interphase following cytokinesis, thus completing the cell cycle 32 9.5 How Does Mitotic Cell Division Produce Genetically Identical Daughter Cells? 33 9.5 How Does Mitotic Cell Division Produce Genetically Identical Daughter Cells? 34