SCIENCE -Cellular Reproduction- 8

• Chromosome


  Self-replicating genetic material that directs the activities and functions of the cells

  
• DNA (Deoxyribonucleic acid)


  Genetic material located inside a chromosome in the nucleus of the cell

  
• DNA


  • Helical structure consisting of two strands
  • Organized and packaged by proteins or histones to form different levels of chromosome packaging

  
• Chromosome packaging


  1. DNA helix forms nucleosomes

  2. Nucleosomes form 30-nanometer chromatin fiber (solenoid)

  3. Solenoid forms chromatin loops

  4. Chromatin loops condense to form mitotic chromosome (1,400 nm)

  
• Chromatids


  Two identical strands of a replicated chromosome

  
• Centromere


  Constricted region that holds the two chromatids together
• Telomeres


  Protective caps at the ends of chromosomes
• Chromatin


  DNA strands attached and coiled, looking like beads on a string, measuring up to 11 nm
• Chromatin fiber


  Series of chromatin molecules forming a 30-nanometer coiled nucleosomes called solenoid
• Chromatin loops


  Supercoiled chromatin fibers forming loops, leading to condensation of chromosomes up to 700 nm
• Mitotic/Metaphase chromosome


  Replicated and packaged DNA forming a chromosome measuring up to 1,400 nm

  
• Chromatids


  • Two identical halves of a replicated chromosome after the Synthesis phase or the S phase of the cell cycle

  
• Centromere


  • Attachment points of the two chromatids of a chromosome, dividing the chromosome into two "arms"

  
• Short arm (p arm)


  • Upper, usually shorter arms of the chromosome
• Long arm (q arm)


  • Lower, usually longer arms of the chromosome
• Chromosome number of selected organisms

  • Drosophila melanogaster (fruit fly): 8
  • Canis familiaris (dog): 78
  • Homo sapiens (man): 46
  • Oryza sativa (rice): 24
  • Zea mays (corn): 20

  
• Interphase


  Interval between two cell divisions, where the cell grows and DNA replicates

  
• Interphase


  1. G1 phase: cell grows, protein and RNA synthesis, mitochondria increase

  2. S phase: DNA synthesis, chromosome replication

  3. G2 phase: rapid cell growth, preparation for cell division

  
• Mitosis


  Cellular process where two nuclei and two cells are produced from the division of the original nucleus, with the same chromosome number

  
• Stages of Mitosis


  1. Prophase: chromosome condensation, nuclear membrane breakdown

  2. Metaphase: chromosomes align at equatorial plane, spindle fibers attach to kinetochores

  3. Anaphase: sister chromatids separate and move to opposite poles

  4. Telophase: chromosomes reach poles, nuclear membranes reform, cytokinesis occurs

  
• Cell plate


  Structure formed in plant cells during telophase, where membrane-enclosed vesicles fuse to create a new cell wall between daughter cells
• Cleavage furrow


  Grooves that form in animal cells during telophase, deepening to separate the two daughter cells
• Somatic cells


  Body cells with two complete sets of chromosomes (diploid)

  
• Gametes


  Sex cells with one set of chromosomes (haploid)
• Meiosis


  Cell division process that produces gametes, involving two cell divisions to halve the chromosome number

  
• Meiosis I


  Reduction division, producing two haploid daughter cells from one diploid parent cell
• Substages of Prophase I

  1. Leptotene: chromosomes visible as threads

  2. Zygotene: homologous chromosomes pair up

  3. Pachytene: chromosomes condense, crossing over occurs

  4. Diplotene: homologous chromosomes separate but remain connected at chiasmata

  5. Diakinesis: chromosomes continue to condense

  
• Leptotene


  Each chromosome is made up of sister chromatids. These are long threadlike structures which result from the replication of DNA during the Synthesis or S phase of the cell cycle.

  
• Zygotene


  The homologous chromosomes start to pair off through the process known as synapsis. Pairs of chromosomes that are similar in size and shape are called homologous chromosomes or tetrads.

  
• Pachytene


  The repeated coiling of chromosomes occurs resulting to its contraction and thickening making the homologous pair of chromosomes to be very close to each other. At this stage, the process called crossing over happens. Here, the exchange of segments between the sister chromatids of the homologous chromosomes occurs. The exchanging process form a cross-linkage called a chiasma. After crossing over, the sister chromatids of each chromosome may not be identical with each other based on the genetic material they contain. Crossing-over is a complicated process that results to genetic variability.

  
• Diplotene


  The two homologous chromosomes forming a tetrad begin to repel one another and move apart. They are held only by the chiasma.

  
• Diakinesis


  This is the last stage of meiosis prophase 1. Diakinesis stage is characterized by chiasmata terminalization. After diakinesis, the dividing cell enters metaphase. At this stage, bivalents or homologous pair chromosomes distribute them evenly in the nucleus. The nuclear membrane breaks down and the nucleolus disappears. Chiasma moves towards the end, which is called terminalization. Chromatids remain attached only at the terminal chiasmata and enter the metaphase stage.

  
• Metaphase I


  Spindle fibers from the centrosomes of each pole connect to bivalents or tetrads through the kinetochores. Homologous chromosomes line up at the equatorial plane. There is double alignment of the chromosomes.

  
• Anaphase I


  Spindle fibers begin to contract and separate the bivalent or tetrads. Homologous chromosomes separate and migrate to each pole of the cell.

  
• Telophase I


  Chromosomes decondense. Nuclear membranes reform. Cytokinesis or the cytoplasmic division occurs and two haploid (N) daughter cells with chromosomes with two sister chromatids are formed.

  
• Prophase II


  Chromosomes (chromatids) condense to form metaphase chromosomes. Nuclear membrane dissolves and nucleolus disappears. Centrosomes move towards each pole of the cell.

  
• Metaphase II


  Spindle fibers attach to chromatids at the kinetochores. Chromosomes line up at the equatorial plane. (Single alignment of chromosomes).

  
• Anaphase II


  Spindle fibers shortened and separated the sister chromatids. Chromatids that are now called chromosomes move towards each pole of the cell.

  
• Telophase II


  Single-stranded chromosomes decondense. Nuclear membrane and nucleolus reforms. Cytoplasm divides (cytokinesis). Four haploid (n) daughter cells are formed.

  
• Differences in Characteristics between Mitosis and Meiosis


  • Location: Somatic cells (Mitosis) vs Reproductive cells (Meiosis)
  • Chromosome number of parent cell: Diploid (2n) or haploid (n) (Mitosis) vs Diploid (2n) (Meiosis)
  • Chromosome number of daughter cell: Diploid (2n) or haploid (n) (Mitosis) vs Haploid (n) (Meiosis)
  • Number of daughter cells produced: Two (Mitosis) vs Four (Meiosis)
  • Number of nuclear divisions: One (Mitosis) vs Two (Meiosis)
  • Steps required to complete the cell division phase: Prophase, Metaphase, Anaphase, and Telophase (Mitosis) vs Prophase I, Metaphase I, Anaphase I, Telophase I, Prophase II, Metaphase II, Anaphase II, and Telophase II (Meiosis)
  • Occurrence of synapsis: None (Mitosis) vs Yes (Meiosis)
  • Occurrence of crossing over: None (Mitosis) vs Yes (Meiosis)
  • Presence of chiasma: Absent (Mitosis) vs Present (Meiosis)
  • Kind of reproduction associated with: Asexual (Mitosis) vs Sexual (Meiosis)