Chapter 9:   Chromosomes, Mitosis, and Meiosis

 

Eukaryotic chromosomes contain DNA and protein

 

              The chromosomes carry the genetic information in eukaryotes

 

              The chromosomes are so named as they may be stained by certain dyes

 

                          Chromosomes are composed of chromatin, which is composed of protein and DNA

 

                                      When cells are not dividing, the genetic material is decondensed

 

                                      Chromosomes become visible as distinct structures when the cell divides

 

              DNA is organized into informational units called genes

 

                          Chromosomes contain hundreds to thousands of genes

 

                          Humans are thought to have about 35,000 - 45,000 genes

 

              Chromosomes of different species differ in number and information content

 

                          Humans and several other species of organisms have 46 chromosomes

 

                                      The average number of chromosomes is between 10 and 50

 

                          The number of chromosomes is not indicative of complexity

 

The cell cycle is a sequence of cell growth and division

 

              Cells divide when they reach a certain size

 

              Cell division involves mitosis and cytokinesis

 

                          Mitosis involves division of the chromosomes

 

                          Cytokinesis involves division of the cytoplasm

 

                                      Mitosis without cytokinesis results in multinucleate cells

 

              Chromosomes become duplicated during interphase

 

                          Cells are very active during interphase, synthesizing biological molecules and growing– the G₁ (gap) phase

 

                          The S (synthesis) phase is marked by DNA replication

 

                          The G₂ (gap) phase occurs between the S phase and mitosis

 

              Mitosis ensures orderly distribution of chromosomes

 

                          Mitosis may be divided into 4 stages        

 

                          During prophase, duplicated chromosomes become visible with the light microscope

 

                                      Chromatin condenses into chromosomes

 

                                      Each chromosome has two duplicated units, termed chromatids

 

                                      Sister chromatids are bound at the centromere

 

                                      Centromeres have kinetochores to which microtubules will bind

 

                                      The mitotic spindle, composed of microtubules, forms between the poles

 

                                      The MTOC (microtubule organizing center) is found in plant and animal cells

 

                                                  A pair of centrioles is in the middle of each MTOC in animal cells and some plant cells (but not in the cells of flowering plants)

 

                                                  The centrioles are surrounded by pericentriolar material

 

                                      Asters extend from the MTOCs at the poles (in cells that have centrioles)

 

                                      The nucleolus disappears

 

                          At metaphase duplicated chromosomes line up on the midplane

 

                                      Chromatids are highly condensed during metaphase, and may be photographed for a karyotype

 

                                      Polar microtubules extend from the pole to the equator, and typically overlap

 

                                      Kinetochore microtubules extend from the pole to the kinetochores

 

                          During anaphase, chromosomes move toward the poles

 

                                      Chromatids separate at the centromeres and are now referred to as chromosomes

 

                                      The chromosomes are pulled by the kinetochore microtubules to the poles and form a "V” shape

 

                                      The mechanism by which the microtubules and other mitotic spindle components move the chromosomes is largely unknown

 

                          During telophase, two separate nuclei are formed

 

                                      The cell returns to the conditions similar to interphase

 

                                      The nuclear envelope reforms; the nucleoli reappear

 

              Cytokinesis is the formation of two separate daughter cells

 

                          Cytokinesis begins during telophase

 

                          In animal cells, the cells develop a furrow, caused by contractile actin filaments that encircle the equatorial region

 

                          In plant cells, a cell plate forms, originating from the Golgi complex

 

              Mitosis typically produces two cells genetically identical to the parent cell

 

              Most cytoplasmic organelles are distributed randomly to the daughter cells

 

                          Mitochondria and chloroplasts divide independently during interphase

 

              The cell cycle is controlled by an internal genetic program interacting with external signals

 

                          Eukaryotic cells typically divide less frequently than prokaryotes

 

                          Protein kinases are involved in control of mitosis

 

                                      Protein kinases are active when complexed with cyclins, which are regulatory proteins

 

                                      When Cdk complexes with a certain cyclin, it activates specific enzymes, and inactivates other enzymes

 

                          Colchicine is one of a number of drugs that can block cell division in eukaryotes

 

                                      Colchicine acts by interfering with spindle formation

 

                          Cytokinins are plant hormones that stimulate mitosis

 

                          Various protein growth factors stimulate mitosis

    

      

Sexual life cycles require a mechanism to reduce the chromosome number

 

              Asexual reproduction involves splitting, budding, or fragmentation of the parent

 

                          The offspring formed by asexual reproduction are clones of the parent

 

                          Asexual reproduction is typically rapid

 

              Sexual reproduction involves the union of gametes to form a zygote

 

                          The offspring of sexual reproduction are not identical to the parents

 

              Somatic cells contain homologous chromosomes

 

                          The genetic material of homologous chromosomes is not necessarily identical

 

                          Diploid cells contain two sets of chromosomes (2n)

 

                          Haploid cells have only one set of chromosomes (n)

 

                          In humans, the diploid number is 46

 

                          Polyploid cells contain more than two sets of chromosomes, e.g. 3n

 

                                      Polyploidy is important in plant evolution

 

Diploid cells undergo meiosis to form haploid cells

 

              Meiosis potentially produces four haploid cells

 

              Meiosis produces haploid cells with unique gene combinations

 

                          Meiosis involves two separate divisions

 

              The position of meiosis in the life cycle varies among groups

 

                          Germ line cells undergo gametogenesis

 

                                      Spermatogenesis produces sperm

 

                                      Oogenesis typically produces eggs, or a single ovum and two or more polar bodies 

 

                          Meiosis does not always immediately precede gamete formation

 

                                      Some organisms are haploid for most of their lives

 

                                      Plants and some algae exhibit alternation of generation

 

                                                  The diploid stage is the sporophyte generation

 

                                                  The haploid stage is the gametophyte generation

 

                                                  In higher plants, the dominant stage is the sporophyte generation

 

              Meiosis produces haploid cells with unique gene combinations

 

                          The gametes produced by meiosis differ genetically

 

              In meiosis, homologous chromosomes are separated into different daughter cells

 

                          Meiosis I and meiosis II each include prophase, metaphase,     anaphase, and telophase

 

                          Prophase I includes synapsis and crossing-over

 

                                      Homologous chromosomes pair and undergo synapsis

 

                                      One member of a pair is the maternal homologue, the other is the paternal homologue

 

                                      Synapsis is the association of four chromatids (two from each homologue)

 

                                      The resulting complex is called a bivalent or tetrad

 

                                                  In humans, there are 23 tetrads and 92 chromatids in this phase

 

                                      The synaptonemal complex forms between the members of the tetrad and genetic material is exchanged by crossing over

 

                                      Crossing-over results in great genetic variation

 

                                      Prophase I in the formation of egg cells is often lengthy

 

                                      In the oocytes of some amphibians, the chromosomes take on unusual shapes, called lampbrush chromosomes

 

                                      Homologous chromosomes are held together at chiasmata, which are the sites of crossing over

 

                                      Other typical events of prophase occur

 

                          During meiosis I, homologous chromosomes separate

 

                                      Tetrads align at the equator in metaphase I

 

                                      The sister kinetochores of each homologue are attached to spindle fibers attached to only one of the poles

 

                                      During anaphase I, the homologous chromosomes separate and move to the poles

 

                                      Each pole receives a mixture of maternal and paternal chromosomes

 

                                      In telophase I, chromosomes decondense, the nuclear membrane may reform and cytokinesis usually occurs

 

                                      Interkinesis separates meiosis I and II; no DNA synthesis occurs

 

                                                  It is a very short period and may be absent in some organisms

 

                          Chromatids separate in meiosis II

 

                                      Prophase II is brief, involves recondensation of the chromosomes, and events are very similar to those of prophase in mitosis (as are most stages of meiosis II)

 

                                      Chromosomes line up at the equator in metaphase II

 

                                      The chromatids separate in anaphase II (they are now called chromosomes)

 

                                      In telophase II, there is one copy of each homologous chromosome at each pole

 

                                      The end product is typically 4 haploid cells           

 

The events of mitosis and meiosis lead to contrasting outcomes

 

              Mitosis is a single division and results in two genetically identical daughter cells

 

                          Homologous chromosomes do not experience crossing over

 

              Meiosis is two sets of divisional processes, and results in four genetically different cells

 

                          Due to synapsis and independent separation of sister chromatids, a great deal of genetic diversity results