GEN 3000 Notes Set 2 01 11 2016 Dr Tsai Clemson University Chapter 2 Continued Mitosis Meiosis Mitosis the duplication of cells Produces Diploid Cells Mitosis occurs regularly in the body such as in bone marrow and blood production or inside the lining of the gut New cells are created through mitosis There are some exceptions such as mature neurons which don t usually undergo a full cell cycle Before Mitosis occurs a cell is in Interphase which is composed of G1 S DNA replication and G2 During interphase you cannot see chromosomes with light microscope It s a jumbled up mess inside the nucleus During the M phase or mitosis we re focusing on replication and the chromosomes condense to the point that they can be seen with a light microscope DNA is ALREADY REPLICATED AT THIS POINT Note Mitosis is dynamic and fluid sometimes making it difficult to determine exact beginning and ending points of phases Other sources may list them differently For this class study according to the information listed below Stages of Mitosis 1 Prophase The already replicated chromosomes condense and the mitotic spindle forms from the centrosomes These centrosomes are only present in animal cells and will not be found in plants During this phase we re looking at diploid cells each chromosome consists of 2 sister chromatids We ll see the centrosomes start to move to opposite ends of the cell and they organize the spindle fibers which are also called microtubules or microtubular filaments The nuclear membrane also called the nuclear envelope begins to break down 2 Prometaphase The nuclear envelope disappears completely and microtubules contact the chromatids They will attach at the centromere of each chromosome and then will be able to move them Once contact is established they will either lengthen or shorten to move the chromosomes to the metaphase plate 3 Metaphase Trigger word Middle single plane called the metaphase plate Unlike in Meiosis which we ll look at later the chromosomes don t care what the other chromosomes are doing Spindle fibers arrange the chromosomes in a 4 Anaphase Trigger word Apart move toward the opposite poles creating separate chromosomes This separation is called disjunction If they do not separate this is called nondisjunction Sister chromatids are separated and they 5 Telophase Trigger word Twins nuclear membrane reforms to form 2 nuclei The chromosomes begin to disappear from view If everything has gone properly they will be exact copies Chromosomes arrive at spindle poles and the Cytokinesis The division of the plasma membrane and organelles considered to happen at the same time as telophase and some consider it part of telophase while others say it is a separate phase itself Most important factors of each phase Interphase nondistinct mushy chromosomes and growth DNA replication Prophase Condesnation of chromosomes Prometaphase Spindles form Metaphase Middle line metaphase plate Anaphase Separation of Sister chromatids Early Telophase Separation into two identical cells Late Telophase Formation of new nuclei Spindle fibers are going to connect to the centromere to form the kinetochore Spindle fibers are made up of tubulin subunits that can be added or removed according to which direction they need to move There are molecular motors that help with this They ll lengthen by adding subunits and shorten the tubules by removing subunits This process is ESSENTIAL If the spindle fibers cannot change length the process will halt and kill the cell Some drugs specifically target spindle fibers to freeze them or prevent the molecular motors from working which forces the cell to shut down This is used commonly in cancer treatments If something happens and a single fiber breaks the process of disjunction won t work properly You can end up with something like Turner syndrome lacking a Y chromosome or a second half to the X chromosome Has consequences on the cell and is usually fatal How do chromosomes move When we classify chromosome types by centromere location we re looking at metaphase chromosomes that have been through the S phase and are arranged with their homologous pair Both sister chromatids are present or should be Counting Chromosomes The cell is diploid during almost the entire process of Mitosis Say a cell starts out in G1 with 4 chromosomes During the S phase all of those chromosomes are going to be replicated to result in sister chromatids but each pair is still considered one chromosome This continues through G2 Prophase Prometaphase and Metaphase Only once we get to anaphase where the sister chromatids separate does the number temporarily change to 8 In telophase the separation of cells returns chromosomes numbers to 4 Controlling the Cell Cycle The cell cycle is virtually the same in all eukaryotic diploid organisms The cycle requires proteins with specific patters which has helped us understand the checkpoints of the cell cycle The two proteins we focus on are Cyclin B and Cyclin Dependent Kinase CDK Cyclin B gradually builds from interphase to the end of mitosis where it suddenly drops to start over again in the next interphase CDK maintains a steady level throughout the process The cooperation of the two cycles results in a curved pattern If the proteins do not follow their patterns the cell cycle cannot continue properly If a protein stays present when it should decrease in amount it may lead to a particular type of cancer However this also allows us to use targeted treatment for the illness Meiosis the production of gametes Produces haploid gametes Has two round of Prophase Metaphase Anaphase and Telophase The first round of meiosis is considered the reduction division where the cells are reduced from diploid to haploid and stay as such through the second round The second round of meiosis is more like mitosis with sister chromatid separation Stages of Meiosis 1 Prophase 1 Starts a lot like mitosis where the chromatin condenses and the nuclear envelope begins to break down Unlike in mitosis homologous chromosomes from mom and dad line up together to create a synapsis very close association These two sets of chromosomes are also called a tetrad or bivalent There are 4 chromatids total in this junction two sets of sister chromatids CROSSING OVER TAKES PLACE IN PROPHASE 1 where at a chiasma direct contact of chromatids genetic material is physically broken and swapped as the homologous non sister chromatids entangle The chromatids will then have pieces from both parents