2.2 - Replicating Cells

2.2 - Replicating Cells

1. Making More DNA

   • Single & multicellular organisms must copy DNA in order to reproduce
     • Each cell contains exact copies of chromosomes in the zygote. Zygote = egg fertilised by sperm

2. Replication

   • 1 step of DNA replication is )-- the strands of double helix split from each other & base is exposed
     • 2 step nucleotides pair up with exposed bases following complementary basing rule, sugar & phosphate molecules bond with nucleotides & new strand of DNA is formed
       • result - step 3, 2 identical DNA molecules (each double helix) identical to parent DNA
   • Replication occurs on both sides of DNA strands. Result is Chromatids

3. Cell Division

   • Cells divide when organisms grow.
     • Mitosis
       • Produces two daughter cells (identical to parent cell)
         • Cell begins to divide, DNA coils up & separates chromosomes become visible. Each Chromosome has 2 chromatids
           • Membrane breaks down, Chromosomes line up across equator of cell & network of fibres appear, extending from the poles.
             • Chromatids separate and become 2 independent chromosomes . Network of fibres contracts, pulling chromosome to opposite poles.
               • New nuclear membrane encloses the chromosomes at each pole. Uncoils & no longer visible as individual strands
                 • Division of nucleus is complete- cytoplasm divides and result is two identical daughter cells.
       • Reasons for cell replication
         • 1) growth , 2) wound & tissue repair, 3) asexual reproduction, 4) research, 5) tissue regeneration

4. Diploid & Haploid

   • Number of chromosomes in your body - Diploid number, described as 2n means two sets
     • One copy of chromosome - Haploid number, known as n

5. Asexual reproduction

   • Unicellular organisms with a nucleus reproduce by mitosis. Ex-- yeast and protists (Amoeba & Euglena)
     • Plants & Animals sometimes reproduce asexually )-- Means offsprings are produced without union of gametes. But produced by mitosis of a particular cell. grasses is an example.
       • Grasses reproduce by budding when conditions are favourable. Cells on the side of the body multiply by mitosis and new hydra forms

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