Chromosomes

 

-Basic Structure

-chromosomes are only visible when a cell is dividing so we usually see them in their double-stranded form 

-the halves of a double-stranded chromosome (chromatids) are identical to each other because the DNA (original chromosome) replicated prior to cell division

 

 

-Cells & Their Chromosomes

   -every organism has a specific number of chromosomes in each of their cells
         -gametes / sex cells contain the organism's haploid number
               -human haploid number = 23
         -somatic / body cells contain the organism's diploid number
               -human diploid number = 46 (23 pairs)
                  -"pairs" refer to homologous chromosomes
                      -chromosomes that are the same size, shape, and carry
                        the same trait information
                                

                      -"pairs" exist as a result of fertilization
                         
click here to see an animation of fertilization
                     

-Identification of Chromosomes

   -autosomes vs. sex chromosomes
        -autosomes
            -same for both males & females
            -chromosomes 1 - 22 
        -sex chromosomes 
            -X and Y chromosomes
            -different for males vs. females
            -23rd pair 
                -females: XX
                -males: XY

In males, the 23rd pair is not homologous because the Y chromosome is not identical to the X.  However, the XY pairing is appropriate because X and Y are both sex chromosomes.

   -chromosomes can be distinguished from one another by looking 
     at differences in the
        -size
        -banding pattern
             -dark stripes are caused by the Giemsa stain 
               adhering to the A-T base pairs
                  -stripes (Giemsa bands) vary because the DNA sequence 
                   is different on each chromosome
        -centromere placement
              -metacentric: centromere near the center
                    
              -submetacentric: centromere off-center
                    
              -acrocentric: centromere near one end
                     

 

-Karyotyping 

   -Karyotype
       -a photograph of an individual's chromosomes arranged in a standard format

        -basic steps of karyotyping
             1.  cell is treated with colchicine 
             2.  cell is lysed, stained, and fixed onto a slide
             3.  slide is examined for a clear chromosome spread
(see below)
                 

             4.  slide is photographed 
             5.  each chromosome is cut out of the photograph and placed 
                  in its proper location on the karyotype
(see below) 
                      
                          [karyotype: normal human male]

 

         -karyotypes can reveal problems with
            -chromosome number
               -examples:
                 -Down Syndrome 
                 -Turner Syndrome
            -chromosome structure
                -examples:
                   -Cri du chat Syndrome
                   -CML (chronic myelogenous leukemia)

karyotypes only show chromosomal problems, gene mutations are not visible on a karyotype

            

to more detailed notes
        -
Molecular Biology Notebook Online
        -Colorado State University
         
-Kimball's Biology Pages