Science

• Cell


  In biology, the smallest unit that can live on its own and that makes up all living organisms and the tissues of the body
• Contributors to the cell theory


  • Robert Hooke
  • Anton van Leeuwenhoek
  • Matthias Schleiden
  • Theodor Schwann
  • Rudolf Virchow
• Prokaryotic cell reproduction


  Eukaryotic cell reproduction
• Reproduction


  Organisms must reproduce for species to persist
• Types of reproduction


  • Asexual reproduction
  • Sexual reproduction
• Reasons cells divide and make more of themselves

  • Growth
  • Repair
  • Asexual reproduction of single-celled organisms (binary fission)
• Stages of cellular reproduction

  • Cellular growth and maturation
  • Cell division
• Chromosomes


  Rod-like structures made of DNA and protein

  
• Proteins in chromosomes


  • Histones - proteins that DNA wraps around, helps hold shape of chromosome and allows DNA to pack tightly
  • Nonhistones - do not aid in packing of DNA, instead they control specific regions of the DNA
• Parts of the chromosome

  • Chromatids - two identical halves of a replicated chromosome after the Synthesis phase or the S phase of the cell cycle
  • Centromere - the attachment points of the two chromatids of a chromosome
  • Short arm (p arm) - upper arms of the chromosome which is usually shorter
  • Q arm - lower arms of the chromosome which is usually longer

  
• Organelles involved in cell division


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• Cell cycle


  A series of events that takes place in a cell as it grows and divides
• Phases of the cell cycle

  • Interphase (3 stages - Gap 1, Synthesis, Gap 2)
  • Mitosis (4 stages + cytokinesis - Prophase, Metaphase, Anaphase, Telophase)

  
• Interphase

  Non-dividing state with 3 sub-stages: Gap 1 - cell grows in size and organelles replicated, Synthesis - replication of DNA and synthesis of proteins associated with DNA, Gap 2 - synthesis of proteins associated with mitosis
• Mitosis


  Division of somatic cells (non-reproductive cells) in eukaryotic organisms. A single cell divides into two identical daughter cells, each receiving a complete set of chromosomes from the parent cell.

  
• Stages of mitosis


  • Prophase
  • Metaphase
  • Anaphase
  • Telophase
  • Cytokinesis
• Prophase


  1. DNA coils very tightly, shortening and thickening the chromosomes (condense)

  2. Nuclear membrane breaks into pieces

  3. Nucleolus disappears

  4. The centrosomes (an organelle that makes microtubules) appears and migrate to opposite sides

  5. Spindle fibers start to form and are free to attach to the chromosomes
• Metaphase


  1. Spindle lines up the chromosomes on the metaphase plate

  2. Centromeres are attached to spindle fibers

  
• Anaphase


  1. Centromeres split and some spindle fibers shorten as they pull the sister chromatids toward opposite poles of the cells

  2. Other microtubules lengthen to move the poles apart
• Telophase


  1. The spindle disassembles

  2. Chromosomes begin to unwind

  3. Nucleolus and nuclear envelope form

  4. As this phase ends, the division of the genetic material is complete and the cell contains two nuclei

  
• Cytokinesis


  1. After mitosis (division of nucleus), the cytoplasm splits into two

  2. In animal cells, cleavage furrow is developed

  3. In plant cells, cell plate develops

  
• What mitosis actually looks like
• What happens after mitosis
• Products of mitosis


  The product of mitosis is 2 daughter cells that are identical to each other & to the mother cell

  
• The daughter cells in humans will have 46 chromosomes (23 pairs), each chromosome having the same gene sequence
• Importance of mitosis


  • Growth and development
  • Replacement and regeneration of cells (repair)
  • Asexual reproduction for unicellular organisms
  • Maintenance of an organism
• Cellular activities corresponding to mitotic phases

  • DNA replicates
  • Nuclear membrane reforms
  • Cytoplasm completely divides
  • Protein and RNA synthesis occur
  • Cell prepares for the actual cell division
  • Chromosomes align at the equatorial plane
  • Chromosomes condense and become visible
  • Sister chromatids separate and migrate to the opposite pole

  
• Meiosis is preceded by interphase which includes chromosome replication

  
• Meiosis


  Two meiotic divisions - Meiosis I and Meiosis II. Called reduction-division. Original cell is diploid (2n), four daughter cells produced that are haploid (1n).

  
• Meiosis produces gametes (eggs & sperm). Occurs in the testes in males (spermatogenesis) and in the ovaries in females (oogenesis).
• Meiosis I (reduction division)


  Cell division that reduces the chromosome number by one-half.
• Interphase I


  Similar to interphase in mitosis. Chromosomes (DNA) replicate in the S phase. Each duplicated chromosome consist of two identical sister chromatids attached at their centromeres. Centriole pairs also replicate.

  
• Prophase I


  Nucleus & nucleolus disappear. Spindle forms. Chromosomes condense & synapsis (pairing) occurs. Tetrads form & crossing over occurs.
• Homologous chromosomes


  Paired chromosomes - one from each parent. Both chromosomes of a pair carry genes that control the same inherited characters.
• Crossing over


  Segments of non-sister chromatids break and reattach to the other chromatid. Causes genetic recombination.
• Metaphase I

  Tetrads align on the equator. Independent assortment occurs - chromosomes separate randomly causing genetic recombination.
• Anaphase I

  Homologous chromosomes separate and move towards the poles. Sister chromatids remain attached at their centromeres.
• Telophase I


  Each pole now has haploid (1n) set of chromosomes. Cytokinesis occurs and two haploid daughter cells are formed.
• Meiosis II


  No interphase II or very short. No DNA replication. Similar to mitosis - sister chromatids separate.

  
• Prophase II


  Same as prophase in mitosis. Nucleus & nucleolus disappear. Chromosomes condense. Spindle forms.