Cell Cycle PDF

www.trinityuniversity.edu.ng CELL CYCLE Nwaogu Victor Cell cycle is the series of events that take place in the cell that cause it to divide in two daughter cell and also when it is temporarily suspended in nondividing resting stage (Go phase) Two major periods comprises the cell cycle (1) In...

www.trinityuniversity.edu.ng CELL CYCLE Nwaogu Victor Cell cycle is the series of events that take place in the cell that cause it to divide in two daughter cell and also when it is temporarily suspended in nondividing resting stage (Go phase) Two major periods comprises the cell cycle (1) Interphase, ( interval between cell division (2) M phase ( mitosis, the period of cell division  INTERPHASE Interphase is series of changes that take place in a newly formed cell and it's nucleus before it becomes capable of division again. It is also called preparatory phase or inter mitosis. Interphase is divided Into 3 separate phases,  G1 Phase. The G1 phase is the gap of time between mitosis (M phase) and DNA synthesis (S-phase). The G1 phase is the phase where RNA, protein, and organelle synthesis occurs. The G1phase lasts about 5 hours in a typical mammalian cell with a 16-hour cell cycle. G1 Checkpoint. Cdk2-cyclin D and Cdk2- cyclin E mediate the G1 S S phase transition at the G1checkpoint. S (Synthesis) Phase. The S phase is the phase where DNA synthesis occurs. The S phase lasts about 7 hours in a typical mammalian cell with a 16-hour cell cycle. G2 Phase. The G2 phase is the gap of time between DNA synthesis (S phase) and mitosis (M phase). The G2 phase is the phase where high levels of ATP synthesis occur. The G2 phase lasts about 3 hours in a typical mammalian cell with a 16-hour cell cycle. G2 Checkpoint. Cdk1-cyclin A and Cdk1-cyclin B mediate the G2 S M phase transition at the G2 checkpoint. M (Mitosis) Phase. The M phase is the phase where cell division occurs. The M phase is divided into six stages called prophase, prometaphase, metaphase, anaphase, telophase, and cytokinesis. The M phase lasts about 1 hour in a typical mammalian cell with a 16- hour cell cycle. Mitosis is the process by which a cell with the diploid number of chromosomes, which inhumans is 46, passes on the diploid number of chromosomes to daughter cells. The term diploid is classically used to refer to a cell containing 46 chromosomes.  The term “haploid” is classically used to refer to a cell containing 23 chromosomes. Mitosis ensures that the diploid number of 46 chromosomes is maintained in cells. 1. Prophase. The chromatin condenses to form well-defined chromosomes. Each chromosome has been duplicated during the S phase and has a specific DNA sequence called the centromere that is required for proper segregation The centrosome complex, which is the microtubule organizing center (MTOC), splits into two and each half begins to move to opposite poles of the cell. The mitotic spindle (microtubules) forms between the centrosomes. 2. Prometaphase. The nuclear envelope is disrupted, which allows the microtubules access to the chromosomes. The nucleolus disappears. The kinetochores (protein complexes) assemble at each centromere on the chromosomes. Certain microtubules of the mitotic spindle bind to the kinetochores and are called kinetochore microtubules. Other microtubules of the mitotic spindle are now called polar microtubules and astral microtubules. 3. Metaphase. The chromosomes align at the metaphase plate. The cells can be arrested in this stage by microtubule inhibitors (e.g., colchicine). The cells arrested in this stage can be used for karyotype analysis. 4. Anaphase. The centromeres split, the kinetochores separate, and the chromosomes move to opposite poles. The kinetochore microtubules shorten. The polar microtubules lengthen. 5. Telophase. The chromosomes begin to decondense to form chromatin. The nuclear envelope re- forms. The nucleolus reappears. The kinetochore microtubules disappear. The polar microtubules continue to lengthen. 6. Cytokinesis. The cytoplasm divides by a process called cleavage. A cleavage furrow forms around the middle of the cell. A contractile ring consisting of actin and myosin filaments is found at the cleavage furrow. Meiosis is a specialized process of cell division that occurs only during the production of gametes within the female ovary or male testes. Meiosis consists of two divisions (meiosis I and II), which result in the formation of four gametes, each containing half the number of chromosomes (23 single chromosomes) and half the amount of DNA (1N) found in normal somatic cells (46 single chromosomes, 2N). In general, meiosis consists of two cell divisions (meiosis I and meiosis II) but only one round of DNA replication that results in the formation of four gametes, each containing half the number of chromosomes (23 chromosomes) and half the amount of DNA (1N) found in normal somatic cells (46 chromosomes, 2N). Meiosis I. Events that occur during meiosis I include: 1. DNA replication 2. Synapsis. Synapsis refers to the pairing of each duplicated chromosome with its homologue, which occurs only in meiosis I (not meiosis II or mitosis). 3. Crossover. Crossover refers to the equal exchange of large segments of DNA between the maternal chromatid and paternal chromatid (i.e., nonsister chromatids) at the chiasma, which occurs during prophase (pachytene stage) of meiosis I a. Crossover introduces one level of genetic variability among the gametes. b. During crossover, two other events (i.e., unequal crossover and unequal sister chromatid exchange) may occur, which introduces variable number tandem repeat (VNTR) polymorphisms, duplications, or deletions into the human nuclear genome 4. Alignment. Alignment refers to the process whereby homologous duplicated chromosomes align at the metaphase plate. 5. Disjunction. Disjunction refers to the separation of the 46 maternal and paternal duplicated chromosomes in the 23 homologous pairs from each other into separate secondary gametocytes (Note: the centromeres do not split).  This random distribution of maternal and paternal homologous duplicated chromosomes introduces another level of genetic variability among the gametes. 6. Cell division. Meiosis I is often called the reduction division, because the number of chromosomes is reduced by half, to the haploid (23 duplicated chromosomes, 2N DNA content) number in the two secondary gametocytes that are formed. Meiosis II. Events that occur during meiosis II include: 1. Synapsis: absent. 2. Crossover: absent. 3. Alignment: 23 duplicated chromosomes align at the metaphase plate. 4. Disjunction: 23 duplicated chromosomes separate to form 23 single chromosomes when the centromeres split. 5. Cell division: gametes (23 single chromosomes, 1N) are formed. COMPARISOM OF MEIOESIS AND MITOSIS Which of the following is a major characteristic of meiosis I? (A) Splitting of the centromere (B) Pairing of homologous chromosomes (C) Reducing the amount of DNA to 1N (D) Achieving the diploid number of chromosomes (E) Producing primordial germ cells During meiosis, pairing of homologous chromosomes occurs, which permits large segments of DNA to be exchanged. What is this process called? (A) Synapsis (B) Nondisjunction (C) Alignment (D) Crossing over (E) Disjunction In the process of meiosis, DNA replication of each chromosome occurs, forming a structure consisting of two sister chromatids attached to a single centromere. What is this structure? (A) A duplicated chromosome (B) Two chromosomes (C) A synapsed chromosome (D) A crossover chromosome (E) A homologous pair 4. Which of the following chromosome compositions in a sperm normally results in the production of a genetic female if fertilization occurs? (A) 23 homologous pairs of chromosomes (B) 22 homologous pairs of chromosomes (C) 23 autosomes plus an X chromosome (D) 22 autosomes plus a Y chromosome (E) 22 autosomes plus an X chromosome Which of the following describes the number of chromosomes and amount of DNA in a gamete? (A) 46 chromosomes, 1N (B) 46 chromosomes, 2N (C) 23 chromosomes, 1N (D) 23 chromosomes, 2N (E) 23 chromosomes, 4N A normal somatic cell contains a total of 46 chromosomes. What is the normal complement of chromosomes found in a sperm? (A) 22 autosomes plus a sex chromosome (B) 23 autosomes plus a sex chromosome (C) 22 autosomes (D) 23 autosomes (E) 23 paired autosomes Which of the following describes the main difference between meiosis and mitosis? (A) homologous chromosomes pair during meiosis (B) the number of chromosomes is reduced by half during mitosis (C) after meiosis is complete, there are 46 chromosomes in each cell (D) after mitosis is complete there are 23 chromosomes in each cell. Crossing over and random segregation produce much of the genetic variation in human populations. These events occur dur ing which of the following? (A) mitosis (B) meiosis (C) fertilization (D) transcription Tetraploid cells are the result of the failure of which one of the following processes? (A) anaphase of mitosis (B) S (synthesis) phase of the cell cycle (C) cytokinesis of mitosis (D) G1 phase of the cell cycle The “reduction division” in which the number of chromosomes in a germ cell is reduced from 46 to 23 chromosomes occurs during which of the following? (A) mitosis (B) meiosis I (C) meiosis II (D) synapsis One of the two places in the cell cycle where a response to DNA damage occurs is which one of the following? (A) G0 phase (B) metaphase (C) m (synthesis) phase (D) G2 checkpoint_