Chapter 9 The Continuity of Life Cellular Reproduction 9 1 Why Do Cells Divide Cells reproduce by cell division parent cell normally gives rise to 2 daughter cells Each daughter cell receives a complete set of DNA information o Includes chromosomes varies by species o Cytoplasm is parceled out evenly between two cells o The units of inheritance called genes specific segments of DNA exons o Non coding regions also copied and passed on introns o Specific sequences of nucleotides are instructions The Structure of DNA Cell division is required for growth and development o Process by which organism s cells grow larger or increase in number is called mitotic cell o Daughter cells may differentiate becoming specialized for specific functions o The repeating pattern of divide grow differentiate and then divide again is called the cell division cycle o Most multicellular organisms have three categories of cells 1 Stem cells 2 Permanently differentiated cells 3 Other cells capable of dividing 1 Stem cells have 2 important characteristics self renewal and the ability to differentiate into a variety of cell types wildcards o Can self renew because they retain the ability to divide perhaps for the entire life of the organism well as a few adult cells o Include most of the daughter cells formed by the first few cell divisions of a fertilized egg as o When a stem cell divides usually one daughter remains a stem cell thus continuing the line the other daughter eventually differentiates o Some stem cells in early embryos can produce any of the specialized cell types of the entire body 2 Permanently differentiated cells differentiate and never divide again o Ex Most heart and brain cells cannot divide o Once they die they are gone forever 3 Some cells other than stem cells are capable of continuing to divide but typically differentiate into only one or two different cell types o Ex Dividing liver cells can only become more liver cells Cell division is required for sexual and asexual reproduction o Sexual reproduction in eukaryotic organisms occurs when offspring are produced by the fusion of gametes sperm and eggs from 2 adults Gametes have exactly half the genetic information of their parent cells and reestablish Reproductive cells undergo a specialized type of cell division called MEIOSIS the full genetic complement when they fuse o Asexual reproduction forms offspring from a single parent without having a sperm fertilize an egg Sexual and asexual reproduction is seen in many different organisms o Bacteria and single celled eukaryotic Paramecium organisms reproduce asexually o Some multicellular organisms reproduce asexually o Many plant and fungi reproduce both asexually and sexually Groves of aspen may reproduce asexually by shoots growing up from the root system of a single parent tree Aspen may also reproduce sexually from seeds Mode of reproduction depends on many factors Cell Division Enables Asexual Reproduction 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 in circumference 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 The prokaryotic cell cycle has 5 distinct stages 1 The single prokaryotic chromosome is usually attached to the plasma membrane of the cell 2 Chromosome replicates producing two identical copies that attach to plasma membrane at nearby but separate sites 3 As the cell increases in size new plasma membrane is added pushing chromosomes apart 4 The plasma membrane grows inward between the two chromosome copies 5 The parent cell divides into two daughter cells with each one genetically identical to the parent cell and containing a single circular chromosome The Prokaryotic Cell Cycle 9 3 DNA Organization in Eukaryotic Chromosomes Eukaryotic chromosomes differ from prokaryotic chromosomes in important ways o Eukaryotic chromosomes are separated from the cytoplasm by a membrane bound nucleus o Eukaryotic cells always have multiple chromosomes o Eukaryotic chromosomes are longer and have more DNA than prokaryotic DNA o These differences account for the complexity of eukaryotic cell division The eukaryotic chromosome consists of a linear DNA double helix bound to proteins It is all about compacting the DNA o DNA exists as a DOUBLE HELIX o Helix wrapped around HISTONES cluster of protein particles o One NUCLEOSOME represents the helix wrapped 2x around the histone particle o CHROMATIN represents a collection of multiple nucleosomes looks like beads on a string o CHROMSOMES are compacted chromatin o It is almost 50 000x shorter than when stretched Compacting DNA Chromosome Structure Genes are segments of the DNA of a chromosome o Genes are sequences of DNA from hundreds to thousands of nucleotides long o Each gene occupies a specific place or locus plural loci on the chromosome o In addition to genes every chromosome has specialized regions that are crucial to its structure and function One centromere location can vary Two telomeres one on each end Telomeres o The 2 ends of a chromosome consist of repeated nucleotide sequences called telomeres which are essential for chromosome stability Centromeres o It temporarily holds 2 daughter DNA double helices together after DNA replication o It is the attachment site for microtubules that move the chromosomes during cell division The Principal Features of a Eukaryotic Chromosome during Cell Division Duplicated chromosomes separate during cell division o Prior to cell division the DNA is replicated o At the end of DNA replication a duplicated chromosome consists of 2 identical DNA double helices Called sister chromatids Attached to each other at the centromere o During mitotic cell division the 2 sister chromatids separate Each one becomes an independent chromosome Delivered to one of the 2 daughter cells Eukaryotic chromosomes usually occur in pairs with similar genetic information o When an entire set of stained chromosomes from a single cell are examined its karyotype we see that most cells contain pairs of chromosomes o Both members of each pair are the same length and shape and have the same staining pattern o Each chromosome in a pair carries the same genes arranged in the same order o Chromosomes that contain the same genes are called homologous chromosomes or homologues Ex pair 16 o Cells with
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