Cellular Reproduction

 Describe the debate surrounding spontaneous generation and how Redi’s and Pasteur’s experiments ended that debate.  Sequence the events of the cell cycle in which new body cells are produced.  Analyze the ways in which events of the cell cycle are controlled.

 Debating spontaneous generation.  Some believed nonliving things could give rise to living things.  Francesco Redi performed an experiment in 1668 to test this belief.  Maggots were thought to be spontaneously generated from decaying meat.  Redi observed maggots turned into flies.  He suspected they came from flies themselves.

 In 1675, microorganisms were discovered.  This reopened the debate on spontaneous generation.  In 1864, Louis Pasteur finally developed an experiment to end the debate.  At this time, air was considered to be an essential ingredient necessary for spontaneous generation.  He conducted experiment in which air was allowed to enter a flask of nutrient broth.

 Was the precursor for today’s pasteurization process.  His experiment led to a major biological theory: Theory of Biogenesis  At the present time and under present conditions on Earth, all organisms are produced from other organisms.  Fits in well with the cell theory discussed earlier.

 Cell reproduction occurs when parent cells divide.  Two daughter cells are the result.  Contributes to overall growth of an organism.  Also helps repair damaged tissue, replace cells that are lost from outer surfaces (skin), and helps us to resist disease.  Smaller is better: SA to volume ratio we studied in lab.

 Most cells are in a non-reproducing phase called interphase.  Cell spends most of its life in this stage.  Stage length varies with type of cell.  Interphase begins when cell reproduction is completed.  Cell carries out normal cell activities during this phase.

 Interphase  G1 – cell growth  S – DNA replicated  G2 – preparation for cell division  Mitosis  Prophase  Metaphase  Anaphase  Telophase

 Chromatin begins to coil up into structures known as chromatids. Two identical chromatids are attached at the center in a location called the centromere.  A chromosome is this double stranded structure attached at the centromere.  Spindle also forms during prophase. It is a football-shaped band of fibers that originate from two centrioles.  Centrioles have migrated to opposite ends of the cell.

 Nuclear membrane disintegrates as well:

 Chromosomes attach to the spindle fibers.  They then meet in the middle of the cell.

 The chromosomes separate into sister chromatids again by being pulled apart at the centromere by the spindles.  Chromatids move away from the middle.

 Plasma membranes in animal cells begin to pinch together.  Basically propase in reverse: nuclear membrane reforms, chromatids relax back into chromatin form, two sets of identical chromatin now located at each end of the cell.  Result of Mitosis: One 2n cell becomes two 2n daughter cells.

 Cell plate

 Cells grow at different rates depending on needs.  Red blood cells and skin cells multiply rapidly.  Some muscle and nerve cells remain in interphase their whole lives.  Liver cells only divide when repairs need to be made.

 If cells begin to grow rapidly and growth is not stopped, cancerous areas can occur.  Cancer is an example of uncontrollable cell growth.  Over time, this growth can outcompete vital cells and tissues for nutrients and space.

 One way cells regulate growth is by coming into contact with other cells.  When this occurs, cells stop reproducing.  Controlled by proteins in the cell.  As scientists figure out what triggers cell growth, information could be used to cure cancers, replace damaged tissues, etc.

 Have single chromosome (circular).  Don’t really have a cell cycle.  Reproduce by binary fission.  Chromosome attaches to cell membrane  Cell elongates.  Chromosome is replicated.  Cell divides into 2 with 1 chromosome in each.

 Objectives:  Sequence the series of events by which reproductive cells are produced in complex plants and animals.  Analyze the significance of meiosis with respect to adaptation and evolution.

 Number of chromosomes varies from species to species.  Humans46  Fruit flies8  Camel70  Porpoise44  Bat44  King Crab208  Soybean40  Pea14

 How many chromosomes do your parents have in their cells? Grandparents?  How does this number remain the same from one generation to the next?  The answer lies in the process of meiosis.

 In the cells of animals, chromosomes usually come in pairs.  Humans with 46 chromosomes have 23 pairs.  Cells with 2 of each chromosome are said to be diploid (2n) n=number of different pairs.

 The two members of each pair are referred to as homologous chromosomes, or homologues.  The DNA for each homologue carries the information for the same traits, although the exact information may differ.  Example: Eye color: One may carry info for brown eyes and one may carry info for blue eyes.

 When two parent organisms mate to produce single cell, the single fertilized egg is called a zygote.  A zygote results from the union of two different kinds of gametes, which are the sex cells (eggs and sperm).  The fusion of the egg and sperm is called fertilization.  Eggs and sperm are haploid (n) cells. They have one set of the chromosome pairs, so they have 23 chromosomes in humans.

 Fusion of sex cells is sexual reproduction.

 Haploid cells cannot be produced by mitosis.  Meiosis – the process by which haploid cells are formed from diploid cells.  Not limited to animal cells. Happens in plant cells also. Haploid cells are called spores instead of gametes.

 Prophase I  Metaphase I  Anaphase I  Telophase I  Prophase II  Metaphase II  Anaphase II  Telophase II

 Interesting differences:  Meiosis I begins before birth, then process stops.  At sexual maturity, several cells continue on with the process.  Usually result is only one egg instead of 4.  Why is this an advantage?

 Crossing over provides for genetic variation.  Provides for a better chance at survival for the species as a whole.  The reshuffling of chromosomes and the genetic information they carry is one of the mechanisms for what is called genetic recombination.  This allows for inheritable variation.