CHAPTER 1: OVERVIEW OF THE CELL CYCLE THE THREE STAGES OF INTERPHASE: INTERPHASE BEFORE A CELL CAN ENTER CELL DIVISION, IT NEEDS TO PREPARE ITSELF BY REPLICATING ITS GENETIC INFORMATION AND ALL OF THE ORGANELLES. ALL OF THE PREPARATIONS ARE DONE DURING THE INTERPHASE. INTERPHASE PROCEEDS IN THREE STAGES, G1, S, AND G2. CELL DIVISION OPERATES IN A CYCLE. THEREFORE, INTERPHASE IS PRECEDED BY THE PREVIOUS CYCLE OF MITOSIS AND CYTOKENESIS. G1 PHASE: AFTER MITOSIS IS COMPLETE THE NEW DAUGHTER CELL BEGINS TO ACCELERATE ITS BIOCHEMICAL PROCESSES WHICH WERE SLOWED DOWN BY MITOSIS. THE LENGTH OF THE G1 PHASE CREATES THE DIFFERENCE BETWEEN FAST DIVIDING CELLS AND SLOWLY DIVIDING CELLS. THE G1 PHASE CAN BE SLOWED BY REDUCING THE NUTRIENTS AVAILABLE IN A SYSTEM - THUS THE CELL WILL TAKE LONGER TO BUILD UP THE RESOURCES NECESSARY FOR CELL DIVISION. IF THERE IS A SEVERE DEPLETION IN NUTRIENTS THE CELLS CAN VIRTUALLY STOP GROWING. IT IS INTERESTING TO NOTE THAT CELLS THAT AREN'T GROWING ARE ALWAYS STOPPED IN THE G1 PHASE, BEING MITOTICALLY ARRESTED. THIS SUGGESTS THAT ONCE THE CELL ENTERS THE S PHASE, IT IS COMMITTED TO CELL DIVISION, REGARDLESS OF THE EXTERNAL CELL CONDITIONS. S PHASE: THE S PHASE BEGINS WITH THE REPLICATION OF THE CELLULAR DNA. THIS IS DESCRIBED IN FURTHER DETAIL IN DNA REPLICATION. WHEN THE CELLULAR DNA HAS BEEN DUPLICATED, LEAVING THE CELL WITH TWICE AS MANY CHROMOSOMES (EACH CHROMOSOME IS MADE UP OF TWO IDENTICAL CHROMATIDS), THE CELL MOVES ONTO THE G2 PHASE.
G2 PHASE: DURING THIS PHASE PROTEINS, SUCH AS KINASE (WHICH CATALYZES PROTEIN PHOSPHORYLATION), WHICH ARE NECESSARY FOR CELL DIVISION ARE SYNTHESIZED AT THIS TIME. THE CHROMOSOME BEGINS TO CONDENSE AND THE PROTEINS NECESSARY FOR CONSTRUCTION OF THE MITOTIC SPINDLE ALSO ARE SYNTHESIZED. WHEN THE CHROMOSOMES BECOME VISIBLE THE CELL ENTERS THE FIRST STAGE OF MITOSIS, PROPHASE. G0- GAP 0 (G0): THERE ARE TIMES WHEN A CELL WILL LEAVE THE CYCLE AND QUIT DIVIDING. THIS MAY BE A TEMPORARY RESTING PERIOD OR MORE PERMANENT. AN EXAMPLE OF THE LATTER IS A CELL THAT HAS REACHED AN END STAGE OF DEVELOPMENT AND WILL NO LONGER DIVIDE (E.G. NEURON) THE APPROXIMATE TIME A SKIN CELL STAYS IN EACH PHASE OF THE CYCLE IS 48 HOURS. CHAPTER 2: CANCER-UNCONTROLLABLE CELL DIVISION FOR ANY NUMBER OF REASONS, CERTAIN CELLS IN THE BODY CAN DEVELOP ALTERATIONS IN THEIR DNA UPON REPLICATION. OFTEN, THESE CELLS JUST DIE OFF BECAUSE THEY ARE INFERIOR AND DON'T HAVE THE ABILITY TO SURVIVE AS NORMAL CELLS WOULD. HOWEVER, SOME OF THOSE DNA ALTERATIONS CAUSE A CELL TO "GO CRAZY" FOR LACK OF A BETTER EXPRESSION. THE ALTERATIONS IN THE CELL CAN RESULT IN SHORTER CELL CYCLES, MEANING THAT THEY WILL REPLICATE AND DIVIDE MORE QUICKLY. SOMETIMES THE ALTERATIONS WILL AFFECT NORMAL APOPTOSIS OR PROGRAMMED CELLULAR DEATH. THIS MEANS THAT OUR CELLS ARE ONLY DESIGNED TO LIVE FOR A CERTAIN AMOUNT OF TIME BEFORE THEY ARE REPLACED BY NEWLY FORMED CELLS. IF APOPTOSIS IS INTERRUPTED, THE OLD CELLS DON'T DIE OFF THE WAY THEY SHOULD. THEY HANG AROUND WITH THE NEW CELLS AND EVERYONE JUST KEEPS REPLICATING. ULTIMATELY, CANCER IS UNCONTROLLED CELL DIVISION GONE TO THE EXTREME.
CHAPTER 3: MITOSIS MITOSIS IS THE PROCESS BY WHICH A EUKARYOTIC CELL SEPARATES THE CHROMOSOMES IN ITS CELL NUCLEUS INTO TWO IDENTICAL SETS, TWO SEPARATE NUCLEI. IT IS GENERALLY FOLLOWED IMMEDIATELY BY CYTOKINESIS, WHICH DIVIDES THE NUCLEI, CYTOPLASM, ORGANELLES AND CELL MEMBRANE INTO TWO CELLS CONTAINING ROUGHLY EQUAL SHARES OF THESE CELLULAR COMPONENTS. WHEN A PARENT CELL GOES INTO THE PROCESS OF MITOSIS THE ENDING RESULT IS TWO DAUGHTER CELLS EXACTLY THE SAME AS THE PARENT CELL. MITOSIS HAS 5 MAIN PROCESSES. INTERPHASE THE CELL IS ENGAGED IN PERFORMING ITS PREPARE FOR PHASES THAT LEAD UP TO AND CHROMOSOMES ARE NOT NUCLEUS, ALTHOUGH A DARK METABOLIC ACTIVITY AND MITOSIS (THE NEXT FOUR INCLUDE NUCLEAR DIVISION). CLEARLY DISCERNED IN THE SPOT CALLED THE NUCLEOLUS MAY BE VISIBLE. THE CELL MAY CONTAIN A PAIR OF CENTRIOLES (OR MICROTUBULE ORGANIZING CENTERS IN PLANTS) BOTH OF WHICH ARE ORGANIZATIONAL SITES FOR MICROTUBULES. PROPHASE CHROMATIN IN THE AND BECOMES VISIBLE IN THE CHROMOSOMES. THE CENTRIOLES BEGIN MOVING TO AND FIBERS EXTEND FROM THE CROSS THE CELL TO FORM THE MITOTIC SPINDLE. NUCLEUS BEGINS TO CONDENSE LIGHT MICROSCOPE AS NUCLEOLUS DISAPPEARS. OPPOSITE ENDS OF THE CELL CENTROMERES. SOME FIBERS
METAPHASE SPINDLE FIBERS ALIGN MIDDLE OF THE CELL NUCLEUS. THE METAPHASE PLATE. THIS ENSURE THAT IN THE NEXT CHROMOSOMES ARE SEPARATED, THE CHROMOSOMES ALONG THE THIS LINE IS REFERRED TO AS ORGANIZATION HELPS TO PHASE, WHEN THE EACH NEW NUCLEUS WILL RECEIVE ONE COPY OF EACH CHROMOSOME. ANAPHASE THE PAIRED THE KINETOCHORES AND MOVE CELL. MOTION RESULTS FROM A MOVEMENT ALONG THE SPINDLE THE PHYSICAL INTERACTION OF CHROMOSOMES SEPARATE AT TO OPPOSITE SIDES OF THE COMBINATION OF KINETOCHORE MICROTUBULES AND THROUGH POLAR MICROTUBULES. TELOPHASE CHROMATIDS ARRIVE AT NEW MEMBRANES FORM NUCLEI. THE CHROMOSOMES VISIBLE UNDER THE LIGHT FIBERS DISPERSE, AND OPPOSITE POLES OF CELL, AND AROUND THE DAUGHTER DISPERSE AND ARE NO LONGER MICROSCOPE. THE SPINDLE CYTOKINESIS OR THE PARTITIONING OF THE CELL MAY ALSO BEGIN DURING THIS STAGE. CYTOKINESIS IN ANIMAL CELLS, FIBER RING COMPOSED OF A AROUND THE CENTER OF THE CELL INTO TWO DAUGHTER NUCLEUS. IN PLANT CELLS, THE CYTOKINESIS RESULTS WHEN A PROTEIN CALLED ACTIN CELL CONTRACTS PINCHING THE CELLS, EACH WITH ONE RIGID WALL REQUIRES THAT A
CELL PLATE BE SYNTHESIZED BETWEEN THE TWO DAUGHTER CELLS. Chapter 4: Meiosis Overview Meiosis is a specialized process of cell division that produces gametes (eggs and sperm). It is a differentiation pathway, distinct from the mitotic cycle of normally dividing cells. In humans, it is estimated that at least 10% of conceptions have defects that probably occurred during meiosis of either paternal or maternal gametes. Most of these will result in miscarriage. Understanding the process of meiosis is therefore fundamental to understanding human health and development. This page uses simple schematics to compare mitosis to meiosis in a generalized cell. First, consider mitosis (typical cell division) in a normal diploid cell, with two chromosomes. The chromosomes are duplicated during DNA replication, and the duplicates, called sister chromatids, remain attached to one another via sister chromatid cohesion, until chromosome segregation when the spindle pulls the pairs apart. Each daughter cell then receives one of the sisters from each homologue pair. Importantly, this means that the daughter cells are exact copies of the mother cell, with two copies of each chromosome, so they can go through the same process again. Thus, we call this process the cell cycle. This is the usual process of division by which the cells of our bodies renew themselves.
Human somatic cells, with their full set of 46 chromosomes, have what geneticists refer to as a diploid number of chromosomes. Gametes have a haploid number (23). When conception occurs, a human sperm and ovum combine their chromosomes to make a zygote (fertilized egg) with 46 chromosomes. This is the same number that the parents each had in their somatic cells. In doing this, nature is acting conservatively. Each generation inherits the same number of chromosomes. Without reducing their number by half in meiosis first, each new generation would have double the number of chromosomes in their cells as the previous one. Within only 15 generations, humans would have over 1½ million chromosomes per cell and would be a radically different kind of animal. In fact, when a zygote has an extra set of chromosomes, it usually is spontaneously aborted by the mother's reproductive system--it is a lethal condition. Chapter 5: Meiosis I This is a specialized process of cell division that produces gametes (eggs and sperm). It is a differentiation pathway, distinct from the mitotic cycle of normally dividing cells. In humans, it is estimated that at least 10% of conceptions have defects that probably occurred during meiosis of either paternal or maternal gametes. Most of these will result in miscarriage. Understanding the process of meiosis is therefore fundamental to understanding human health and development. This page uses simple schematics to compare mitosis to meiosis in a generalized cell. First, consider mitosis (typical cell division) in a normal diploid cell, with two chromosomes. The chromosomes are duplicated during DNA replication, and the duplicates, called sister chromatids, remain attached to one another via sister chromatid cohesion, until chromosome segregation when the spindle pulls the pairs apart. Each daughter cell then receives one of the sisters from each homologue pair. Importantly, this means that the daughter cells are exact copies of the mother cell, with two copies of each chromosome, so they can go through the same process again. Thus, we call this process the cell cycle. This is the usual process of division by which the cells of our bodies renew themselves.
Chapter 7: comparison of mitosis & meiosis Prior to meiosis or mitosis, DNA replication occurs. In both meiosis and mitosis, the chromosomes condense and become visible under the microscope. In meiosis, homologous pair up or synapse, and crossing over takes place. Homologous chromosomes then align themselves along the metaphase plate and each homologue is drawn to the opposite end of the cell. Meiosis Occurrence of crossing over: yes Occurs in: humans, animals, plants, fungi Number of daughter cells: 4 Creates: sex cells only: female egg cells or male sperm eggs Type of reproduction: sexual
Mitosis Occurrence of crossing over: no Occurs in: all organisms Number of daughter cells: 2 Creates: makes everything but sex cells Type of reproduction: asexual
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