LECTURE 2: Chromosomal Basis of Heredity

• two phases of cell cycle:

  • interphase (nondividing)
  • M phase (dividing)
• interphase: G1 phase, S phase, G2 phase
• G1 phase - longest phase in cell cycle
• G1 phase - cell increase in volume by imbibing water and nutrients. A gap in the reproductive life of the cell.
• S phase - DNA replication or synthesis.
• G2 phase - synthesis of RNA and proteins; mitotic spindles are formed.
• M phase - structural changes of chromosome are visible; may be either mitosis or meiosis.
• Mitosis - undergone by all somatic and germ cells.
• Meiosis - occurs in cells in the sexual cycle.
• Mitosis - one cell division resulting to two daughter cells.
• Meiosis - two cell division resulting to four daughter cells.
• Prophase - chromosomes shorten, thicken, and became visible as thick rods. Nucleolus diminishes in size.
• Metaphase - chromosomes align at the equatorial plane.
• Metaphase - spindle fibers attaches to kinetochore on opposite sides of centromere.
• Anaphase - centromeres doubles and chromatids separates into two opposite poles.
• Telophase - chromosomes regroup into two nuclear structures. Uncoil and lengthens.
• Mitosis - equational division.
• Mitosis produces two identical cells whose chromosomal compositions are identical to each other and their parents.
• Meiosis consists of two consecutive divisions:

  • Meiosis 1 (reductional division)
  • Meiosis 2 (equational division)
• Five stages in Prophase 1: Leptotene, Zygotene, Pachytene, Diplotene, Diakinesis
• Leptotene - chromatin appear long and thin threads with many bead like structures along their lengths.
• Zygotene - synapsis or pairing of homologous chromosomes.
• Zygotene - forms bivalent chromosome.
• Pachytene -chromosome are thicker due to coiling. Bivalent chromosomes breaks and are repaired and further undergoes crossing over and formation of chiasmata.
• Diplotene - longitudinal separation of homologues in a bivalent. synaptonemal complex is no longer functional.
• Diakinesis - terminalization of chiasmata, nucleolus and nuclear membrane disappears, spindle has formed.
• Metaphase 1 - bivalents align at equatorial plane.
• Anaphase 1 - univalents separate from ea other; accounts for reductional phase of meiosis 1
• Telophase 1 - chromosomes regroup and their coiled structure began to relax.
• Meiosis 2 - equational division
• Prophase 2 - chromosomes begun to condense. nuclear membrane disintegrates and nucleus disappears.
• Metaphase 2 - Spindle fibers are formed and the chromosomes align at the equatorial plane.
• Anaphase 2 - sister chromatids of univalent separates and move to opposite poles.
• Telophase 2 - chromosomes uncoil and lengthen. forms four daughter cells.