INITIAL STUDENT EMAIL:

Dr. K:  My name is Steven and my professor just finished lecturing on meiosis.  We are going to be tested on this stuff in a couple of weeks and  I just cannot seem to get it.  My biggest problem is the material about homologous chromosomes and crossing-over.

INITIAL DR. K REPLY:

Hi Steven!  The first thing to remember is that  MEIOSIS is only found in diploid cells.  You see . . . meiosis is sometimes called "The Reduction Division" - it is a special process of cell division that begins with a DIPLOID CELL (chromosomes in pairs = two sets of chromosomes) and results in four HAPLOID CELLS (one set of chromosomes in each cell).

1.  Let's start with HOMOLOGOUS CHROMOSOMES.  Both you and I are diploid animals - every one of our cells contains two sets of chromosomes.  Where did we get these two sets of chromosomes?  One set came from one parent, and the other came from the second parent.  A human being has a total of 23 different types of chromosomes, BUT . . . we each have a grand total of 46 chromosomes, because we have two of each.  

Here is a link to a photograph of a set of human chromosomes - you will note that numbers 1-22 are represented by two chromosomes each . . . pair #23 are the sex chromosomes and they are identified as X and Y in the photograph.

http://homepages.uel.ac.uk/V.K.Sieber/solidktp.jpg

 

Now . . . each pair of chromosomes is officially called a HOMOLOGOUS PAIR.  "Homo" means same . . . so a homologous pair is two chromosomes that are basically the same.  For example:  Chromosome #1 inherited from your mom contains an entire set of genes, which might include a gene for eye color.  Chromosome #1 inherited from your dad contains the same set of genes!  The only difference is that the genes from your mom might be slightly different from the genes from your dad . . . you might inherit a gene for blue eyes from your mom and one for brown eyes from your dad.  Each chromosome in a HOMOLOGOUS PAIR is called a HOMOLOG.

During MEIOSIS, the HOMOLOGS separate (specifically, during MEIOSIS I).  

Here are links to three  illustrations of meiosis . . . you will note that the HOMOLOGS are paired during Prophase I, they line up at Metaphase I, and they separate at Anaphase I

http://www.uic.edu/classes/bios/bios100/lecturesf04am/prophase1m.jpg

http://www.uic.edu/classes/bios/bios100/lecturesf04am/metaphase1m.jpg

http://www.uic.edu/classes/bios/bios100/lecturesf04am/anaphase1m.jpg

 SO . . . HOMOLOGOUS CHROMOSOMES are found in DIPLOID cells - these are paired chromosomes that will become separated through the process of meiosis.

 2.  Let's continue with crossing-over.  You have noted that the homologous chromosomes seem to be paired during PROPHASE I of MEIOSIS [ A homologous pair is often called a TETRAD - owing to the presence of four chromatids.]  Anyway . . . the chromosomes get very sticky and are somewhat fragile.  At this stage of the game, chromatids might lay across one another (each cross-over point is called a CHIASMA (CHIASMATA plural).  A CHIASMA is a location where breakage and reattachment will take place  -  pieces of chromatids will actually switch places!  

Here is a link to an illustration of crossing over that might help you to visualize the breakage, the reattachment, and the exchange of chromatid parts.

http://www.synapses.co.uk/genetics/crosso1.gif

Here is another link to an illustration of the process of meiosis - you will see the process of crossing-over during prophase I and the recombinant chromatids moving into the final products (you will note that the blue and red chromatids have exchanged parts).

http://www.mun.ca/biology/desmid/brian/BIOL2060/BIOL2060-20/2017.jpg

You might note that the illustration labels the final chromatids as "recombinant chromatids" - this means that these new chromatids are "recombined" . . . they have new unique gene combinations that they did not have before!  You might think of this as a "shuffling" of genes between the homologs.

SO . . . CROSSING-OVER is a process that takes place between HOMOLOGOUS CHROMOSOMES during PROPHASE I of meiosis.  It is a process characterized by breakage and reattachment of chromatids resulting in the exchange of chromatid parts.  This results in a new unique combination of genes on each HOMOLOG!