Cellular Reproduction

Cloning, anyone?

What is the purpose of reproduction?

A. Modes of reproduction

1. Asexual reproduction
   • Single-celled organisms: fission, budding
   • Fungi: spores
   • Plants: cuttings
   • Animals: fragmentation, regeneration
2. Sexual reproduction
   1. Syngamy: identical gametes
      • Many protozoa, algae, fungi
   2. Fertilization: egg & sperm
      • Plants, animals

B. Reproduction of a cell

• Note: Although Prokaryotes have a simpler method for growth and reproduction, they also must precisely duplicate and pass on their chromosomes to each daughter cell.

What are genes? Where are they found?

Chromosomes

1. DNA wrapped around histone proteins (eukaryotes): chromatin
   • Nucleus: 2 m of DNA packed into 5 um space
2. Chromosomes condense only during cell division
   • DNA is already doubled ("sister chromatids")
   • Centromere joins chromatids

The cell cycle

1. G0: non-dividing cells
2. Interphase: Preparation for division
   • G1 phase: growth & preparation
   • S phase: DNA synthesis (replication)
   • G2 phase: growth & preparation 
3. M phase: Mitosis (nucleus divides)
   • Prophase: chromosomes condense, nuclear envelope disappears, spindle forms
   • Metaphase: chromosomes lined up in center of spindle
   • Anaphase: chromatids separate and move to opposite poles
   • Telophase: nuclear envelope forms, spindle dissolves, chromosomes uncoil
4. Cytokinesis: cell divides
   • Animal cells: cleavage furrow (actin)
   • Plant cells: cell plate forms in cytoplasm (vesicles from Golgi)
5. Cell cycle checkpoints & signals
   • G1 checkpoint (p53): is DNA damaged?
   • G2 checkpoint:: did DNA replicate correctly?
   • M checkpoint: did chromosomes get distributed evenly?
   • Cyclins: cellular proteins that regulate the cell cycle
   • Growth factors: external signals that stimulate cell division
   • Contact inhibition: external signal that inhibits cell division
   • Apoptosis: programmed/stimulated cell death
6. Cancer: loss of cell cycle control
   • cancer cells are undifferentiated
   • cancer cells have abnormal DNA & fail to undergo apoptosis
   • cancer cells lack contact inhibition & form tumors
   • cancer cells promote blood vessel growth: angiogenesis
   • cancer cells may invade tissues and spread: metastasis

C. Sexual reproduction: Meiosis

How many chromosomes do our cells have?

Life cycle of sexual organisms

1. Fertilization: zygote (2N)
2. Gametogenesis: sperm & egg (N)
3. Alternation of generations

Meiosis

1. Meiosis consists of 2 cell divisions
2. Meiosis I reduces the number of chromosomes to 1/2
   • Prophase I: Homologous chromosomes pair up (synapsis) = forms tetrads
     • Crossing over occurs
     • The human karyotype
   • Metaphase I: Tetrads line up at center of spindle
   • Anaphase I: Homologues are separated (dyads; centromeres unbroken)
   • Telophase I: followed by cytokinesis
3. Interkinesis
4. Meiosis II separates sister chromatids
   • Similar to meiosis
   • Starts with haploid number of chromosomes (23 in humans)
5. Meiosis produces 4 distinct gametes

Origins of genetic diversity

1. Random assortment of chromosomes
2. Crossing over
3. Random fertilization

Applications

1. In-vitro fertilization
2. Cloning - human or otherwise
3. Genetic testing - heritable traits
4. Chromosomal abnormalities: nondisjunction
   • Trisomy 21 (Down syndrome)
   • XXY (Klinefelter syndrome)
   • XO (Turner syndrome)
5. Cancer