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O7. Sexual life cycles involve meiosis and fertilisation.


Go here: Meiosis, Fertilisation and Chromosomal Disorders for an interactive page on these ideas.


Student Outcome: O7.1

Understand that diploid cells contain pairs of homologous chromosomes.


Homologous Chromosomes


If a dividing cell is stained with a special fluorescent dye and examined under a microscope during cell division, the individual chromosomes can be distinguished. They can then be photographed and studied. This is a difficult and skilled procedure, and it often helps if the chromosomes are cut out and arranged in order of size.



This display is called a karyotype, and it shows several features:


  • Different species have different number of chromosomes, but all members of the same species have the same number. Humans have 46, chickens have 78, goldfish have 94, fruit flies have 8, potatoes have 48, and so on. The number of chromosomes does not appear to be related to the number of genes or amount of DNA.
  • The chromosomes are numbered from largest to smallest.
  • Chromosomes come in pairs, called homologous pairs ("same shaped"). So there are two chromosome number 1s, two chromosome number 2s, etc, and humans really have 23 pairs of chromosomes. Homologous chromosomes are a result of sexual reproduction, and the homologous pairs are the maternal (inherited from the mother) and paternal (inherited from the father) versions of the same chromosome, so they have the same sequence of genes
  • One pair of chromosomes is different in males and females. These are called the sex chromosomes, and are non-homologous in one of the sexes. In humans the sex chromosomes are homologous in females (XX) and non-homologous in males (XY). (In birds it is the other way round!) The non-sex chromosomes are sometimes called autosomes, so humans have 22 pairs of autosomes, and 1 pair of sex chromosomes.


Source: http://www.biologymad.com/master.html?http://www.biologymad.com/CellDivision/CellDivision.htm


Student Outcome: O7.2

Explain why the products of meiosis are haploid cells and contain a single set of chromosomes.


In large, multicellular organisms like ourselves, our somatic cells (body cells) are all diploid. Meiosis, therefore, takes place in specialized reproductive organs, the gonads, to produce haploid cells that will eventually fuse in pairs to produce new diploid zygotes again. Meiosis in such organisms is said to be gametic.


The haploid products produced at the end of Meiosis II go on to differentiate in quite complex ways to produce the recognizable sexual gametes; the sperm and eggs. Since each of these gametes has a very different role, and method of accomplishing that role, the two gametes are often very, very different in physical form, and the path of differentiation they take after Meiosis II.


In other organisms, such as the brewer's yeast, the haploid products produced at the end of Meiosis II are packaged and then released as independent, free living forms in their own right. This is said to be a sporic outcome of meiosis. The haploid form of a plant or fungus may live for a long time in this genetic state before recombining its biological information with that of another individual and re-entering the diploid state for a time.


Many fungi, however, only exist as diploids for a tiny, brief part of their life cycle. In these species fertilization, the fusion of two haploid cells to produce a zygote, is followed almost immediately by a meiotic division of this very same cell to produce haploids once more.


This zygotic type of meiosis is only used, therefore, to mix up and randomize the biological information contributed by two parents into the huge possible alternate forms in the various offspring. It is pure variation, which, after all, is the main purpose of meiosis and sexual reproduction.


Source: http://www.brooklyn.cuny.edu/bc/ahp/LAD/C9/C9_m_meio2.html


Here is an introductory video on meiosis



This picture compares meiosis with mitosis:


Student Outcome: O7.3

Explain the importance of crossing over and independent assortment in meiosis.

  • Crossing Over


Each red or blue butterfly-shaped structure in the animation (see: http://www.execulink.com/~ekimmel/crossing_over.htm) represents one chromosome that has replicated so that it's sister chromatids remain joined at their centromeres. When the chromosomes move together and pair up, they are called a homologous pair. The four chromatids together are called a tetrad and the process of pairing up is referred to as synapsis.


While paired, non-sister chromatids from opposite chromosomes touch and "cross over" each other. Each point of cross over is called a chiasma. While they are touching, the non-sister chromatids exchange genes. When the chromatids pull away later in meiosis, they are genetically different from when they started due to crossing over.


And because the chromatids are now genetically different, when they separate as chromosomes into haploid gametes (ie. sperm and egg), all the gametes will be genetically unique!


Source: http://www.execulink.com/~ekimmel/crossing_over.htm


Crossing over made simple!


  • Random Assortment (Note: also called independent assortment)


In meiosis I, the orientation of paternal and maternal homologues at the metaphase plate is random. Therefore, although each cell produced by meiosis contains only one of each homologue, the number of possible combinations of maternal and paternal homologues is 2n, where n = the haploid number of chromosomes. In this diagram, the haploid number is 3, and 8 (23) different combinations are produced.


Random assortment of homologues in humans produces 223 (8,388,608) different combinations of chromosomes.


Furthermore, because of crossing over, none of these chromosomes is "pure" maternal or paternal. The distribution of recombinant and non-recombinant sister chromatids View into the daughter cells at anaphase II is also random.


So I think it is safe to conclude that of all the billions of sperm produced by a man during his lifetime (and the hundreds of eggs that mature over the life of a woman), no two have exactly the same gene content.


Source: http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/M/Meiosis.html#crossing_over


Yeah! Found an animation that show this! Pretty poor but you get the idea. Go here and be stunned!


Student Outcome: O7.4

Know that fertilisation restores the diploid number.


  • Fertilization


By reducing the number of chromosomes from 2n (diploid) to n (haploid),the stage is set for the union of two genomes. If the parents differ genetically, new combinations of genes can occur in their offspring.


Taking these three mechanisms together, I think that it is safe to conclude that no two human beings have ever shared an identical genome unless they had an identical sibling; that is a sibling produced from the same fertilized egg.


Source: http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/M/Meiosis.html#crossing_over


Short video of fertilisation. Nice graphics



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