Meiosis

 11.1 - Reproduction
Asexual reproduction only requires one parent.  The offspring have exactly the same DNA of the parent.  Some examples of asexual reproduction are...
  1. Binary Fission - The means by which bacteria and other
      prokaryotic organisms reproduce.
  2. Fragmentation - The organism's body breaks into pieces,
      which then grow into new organisms.  (starfish)
  3. Budding - A small copy of the parent organisms grows out of
      the parent until it is large enough to break off and live
      independantly.  (sea anemones)
  4. Parthenogenesis - All of the organisms are female.  The
      produce eggs that do not need to be fertilized, but which
      develop into offsping on their own.  (Daphnia)

Sexual reproduction requires two parents.  The offspring receive a combination of their parents' DNA, so they are not genetic copies.  Sexual reproduction allows for more variety of characteristics among a species, which makes the species more adaptable to changes in the environment.

Cells in a sexually-reproducing organism can be divided into two groups:
  1. Germ Cells produce gametes (egg cells and sperm cells),
      which participate in sexual reproduction.
  2. Somatic Cells are all the rest of the cells in the body, and do
      not participate in sexual reproduction.

Humans have 46 chromosomes, which are arranged in 23 pairs of homologous chromosomes.  Homologous chromosomes are chromosomes that have the same genes and the same structure.  They also pair up during meiosis.
  - Diploid refers to a cell/organism that has two sets of
     chromosomes.  Our somatic cells are diploid. (2n)
  - Haploid refers to a cell/organism that has one set of
     chromosomes.  Our gametes are haploid. (1n)
The n-number is the number of chromosomes that an organism has.  In humans, the n = 23.  Since our cells have two sets of chromosomes, we are considered 2n, which is another way of saying that we are diploid.  For us, 2n = 46 chromosomes.

We are 2n, but our gametes are 1n.  When a 1n sperm fertilizes a 1n egg cell, the resulting zygote is 2n.  Having 1n gametes allows us to maintain the same number of chromosomes in each generation.

Chromosomes can be divided into two groups...
  1. Autosomes, which do not determine gender.
  2. Sex Chromosomes, which do determine gender.
      XX results in a female.
      XY results in a male.

The purpose of meiosis is to reduce the n-number (diploid to haploid) in order to make gametes.

 11.2 - Meiosis
See the attached PowerPoint presentations on Meiosis and Genetic Variation.


Subpages (1): Files for Meiosis