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Introduction to Nucleic Acids and Application to Infectious Disease Detection (cont.)

Chromosomes

Slide 1

We begin by looking at chromosomes.  In slide 1 a karyotype of human chromosomes is seen.  Every normal, non-reproductive (somatic) human cell contains 23 pairs of chromosomes.  Typically chromosomes are analyzed or a karyotype is prepared from cells that are actively dividing or undergoing mitotic division.  Cells most frequently used are peripheral white blood cells (WBC’s), but bone marrow cells, skins cells, tumor cells or amniotic fluid cells can also be used, depending upon what the physician is looking for.

Each chromosome seen here looks like an “X”, because these are actually duplicate pairs of chromosomes during mitotic division.  A single chromosome is known as a “chromatid.”  You can see this illustrated in the lower left corner of slide 1.  So in each cell there are 23 pairs of chromatids (except ova and sperm which are haploid).  When this cell with duplicate chromosome material divides, ½ of one pair goes to one cell & the other ½ or sister chromatid goes to the other cell.  This way, all 23 chromosome pairs are passed on to each new cell.

Slide 2

Chromosomes contain DNA which contains hereditary information.  In slide 2, the image at the bottom is a chromosome prior to cell division as we discussed earlier – notice the x-shape.  The top image is DNA – the double stranded helix.  This slide illustrates how DNA is wrapped around proteins (represented as yellow circles), known as “histones.”  The histone wrapped DNA is further packed together as “nucleosomes.”  These nucleosomes are further compacted by coiling – this is known as a “solenoid.”  These are further condensed by supercoiling & ultimately what can be seen microscopically are “furry or fuzzy” looking chromosomes.  There is a little over 6 feet of DNA condensed in the 23 pairs of chromosome in a 2um nucleus!  An incredible feat!  The compacted DNA is 50,000 times shorter than its extended length.

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