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Elementary Science Education

Reproduction on a cellular level

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      Bernard Nebel

      The life cycle of innumerable organisms (animal, plant, fungi) have been examined on in great detail including down to the cellular level. The crucial finding is that that the life cycle of every organism includes a stage in which meiosis (the separation of chromosome pairs to form gametes, eggs or sperm, each with a single set of chromosomes) occurs. Then there is fertilization, the uniting of egg and sperm to restore the two sets of analogous chromosomes. The fertilized egg is the first cell of the baby of the next generation. You cannot overemphasize that this separation and recombination of chromosomes, hence genes, is a basic feature of all living things.  [Have students reflect on: Why should such a complex process be a basic feature of all living things (plants, animals, fungi)?]

      In most members of the animal kingdom, eggs and sperm are produced by separate animals, females and males. This creates a problem of bringing eggs and sperm together for fertilization. The following videos show that nature has various means of assuring fertilization ranging from very inefficient to very efficient. It starts with organisms such as sea urchins that simply release eggs and sperm into the water. See following video:


      Salmon go a step further; the female makes a “nest” into which she deposits eggs.The male then spreads sperm over the eggs. See video:

      A further step in assuring that eggs are fertilized is seen in frogs and toads. The male sits atop the female and  showers eggs with sperm as the female extrudes them. 


      The final stage of increasing efficiency is seen in mammals, birds and insects. Anatomical differences between males and females enable the male to insert sperm into the female’s body and fertilization occurs within her body. In birds and insects fertilized eggs are laid; in mammals, babies develop within the female’s body. 

      2. The male inserting sperm into the female’s body requires the female being receptive to the act, i.e., mating. To bring this about, animals, especially birds exhibit remarkable courtship behaviors. See the following and related videos. Note that there are significant anatomical features supporting the courtship behavior.

      In many insects, the female secretes a chemical substance known as a pheromone with a highly specific odor. The male, which may be as much as a mile away, detects the odor and follows it upwind to find the female as shown in the following video.

      Traps bated with artificially produced pheromone are used in controlling certain insect pests. This is an area of active ongoing research. 

      By the way, a male moth responding to just 3 or 4 molecules of a pheromone present in the air represents the most sensitive biological signaling mechanism known.  


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