GENETICS-2-NOTES.pdf

CELL CYCLE Cell Cycle The cycle of growth and asexual reproduction of a cell, consisting of interphase followed by actively dividing cells by prophase, metaphase, anaphase, and telophase. Phases of Cell Cycle 1. Interp...

CELL CYCLE Cell Cycle The cycle of growth and asexual reproduction of a cell, consisting of interphase followed by actively dividing cells by prophase, metaphase, anaphase, and telophase. Phases of Cell Cycle 1. Interphase - the preparatory stage a. Gap 1 Phase ➔ Refers to the gap between the cell division and DNA synthesis ➔ Cell decides to enter the cell cycle ➔ Cells increase in mass ➔ Increase number of organelles ➔ First stage ➔ Preparation stage ➔ Longest phase (about 10 hours out of the total 24 hours) b. S Phase ➔ DNA and RNA synthesis ➔ Cellular contents at this stage becomes double ➔ 5-6 to complete c. Gap 2 Phase ➔ Prepares the cell for entry into mitosis ➔ Spindle fibers are synthesized ➔ RNA and some proteins are synthesized ➔ Cell synthesizes proteins required to assemble machinery for duplication of replicated chromosomes ➔ Major checkpoint at the end of G2 controlling entry to the M Phase ➔ Shortest, 3-4 hours 2. M Phase/Mitotic Phase - the cell and nuclear division stage a. Prophase b. Metaphase c. Anaphase d. Telophase Cell Division 1. Mitosis A type of nuclear division that results in two daughter cell each having a nucleus containing the same number and kind of chromosomes as the mother cell (the same genetic material - DNA) Stages of Mitosis: a. Prophase ➔ Cell prepares for preparation ➔ Chromosomes condense and becomes thicker ➔ The nucleolus becomes less distinct ➔ Nuclear membrane disappear ➔ Spindle fibers travel to the opposite poles b. Metaphase ➔ Chromosomes align at the center or the equatorial plate c. Anaphase ➔ Single chromatid daughter chromosome move away from each other to the opposite poles d. Telophase ➔ Known as “reverse prophase” ➔ Chromosome uncoil ➔ Spindle fiber disappear and nucleolus reappear ➔ New nuclear membrane is synthesized ➔ Cell cytoplasm is divided in the process called “cytokinesis” and finishes cell division 2. Meiosis “Reduction division” Type of nuclear division that give rise to the four reproductive cells (gametes) each with half of the chromosome number of the parent cell There is an exchange of genetic materials Occurs in the reproductive cells Stages of Meiosis a. Meiosis I - undergoes interphase and known as the “reduction division” i. Prophase I (prolonged prophase) ➔ Chromosome appears as long, slender thread with many bead-like structure called “chromomere” ➔ Homologous chromosome, each composed of two sister chromatids come together as pairs in a process called “synapsis” ➔ During the process, chromatids of homologous chromosome exchange segments in a process called “crossing over” ii. Metaphase I ➔ The chromosome tetrads (pair of homologous chromosome) are aligned on the metaphase plate, midway between the two poles of the spindle iii. Anaphase I ➔ Homologue chromosomes separate to the opposite poles iv. Telophase I and Cytokinesis (division of the cytoplasm) ➔ Chromosome uncoil ➔ Nuclear membrane reforms ➔ Cytokinesis divides the cell cytoplasm into two separate cells b. Meiosis II - undergoes interphase except synthesis phase and known as “equational division” The process is exactly the same in mitosis but starts with a haploid cell i. Prophase II ➔ Spindle forms and moves the chromosomes toward the middle of the cell ii. Metaphase II ➔ The chromosomes are aligned on the metaphase plate as they are in mitosis, with the kinetochores of the sister chromatids of each chromosome pointing toward opposite poles iii. Anaphase II ➔ The centromeres of sister chromatids finally separate and the sister chromatids of each pair, now individual daughter chromosomes, move toward the opposite poles of the cells iv. Telophase II ➔ Nuclei form at the cell poles while cytokinesis occurs at the same time ➔ Forms four daughter cells, each with the haploid number of chromosomes Comparison of Mitosis and Meiosis Point of Comparison Mitosis Meiosis Types of Cells Somatic cells Gametes Involved Sex cells Number of Cell 1 2 Division Number of 46 (2n) 46 (2n) Chromosomes in the Parent Cell Number of Diploid (2n) = 46 Haploid (n) = Chromosomes in the 23 Daughter Cells Biological Repair damaged tissues Production of Significance Growth sperm and egg Maintain the number of chromosomes