Themes > Science > Life Sciences > Physical Anthropology > Heredity and Beyond > DNA Testbook > DNA 101.

..The Cell
..Chromosomes
..Genes
..Mitosis
 
The Cell
A eukaryotic cell.
 Note the nucleus in the middle.
DNA is found in cells, the smallest living units in your body or the smallest separate living organisms.  Every living organism is made of cells.  Microscopic organisms such as algae, bacteria, and protozoas are made of only one cell.  A person on the other hand is made of billions of specialized cells.  These cells such as skin, brain and bone cells work together to form organs which make the systems of the body.  Inside every cell is the information that determines what function the cell performs.  This information is in the form of DNA which is found in chromosomes that exist in the nucleus of your cells.
There are two types of cells, eukaryotic and prokaryotic cells.  Eukaryotic cells found in a variety of organisms such as yeast, plants, and animals have a nucleus while procaryotic cells like bacteria do not.  Eukaryotic cells have a nucleus while prokaryotic cells only have a nuclear area.
The most important part of the cell is the nucleus.  The nucleus has a nuclear envelope or a coating to keep its material together.  There are pores on this envelope that allow certain materials like RNA into the cytoplasm.  Inside this nucleus or nuclear area is a material called a chromosome.
 

Chromosomes

 
Prokaryotic cell with a circular chromosome in the middle.  There is no nucleus.
 
Click on the picture to see a chromosome.
A chromosome is a long strand of DNA that contains certain genes.  These genes are portions of the DNA strand and carry the genetic information of the cell.  In a eukaryotic cell there are multiple chromosomes, and each of these is part of a pair.  Both of the chromosomes in a pair contain the genes for the same trait.  However, a procaryotic cell has a long circular chromosome that contains all of the genes of the cell.
 
 
  
Organism Number of Chromosome Pairs
Tomato 12 Pairs
Mouse 20 Pairs
Human  23 Pairs
Dog 39 Pairs
 
 
 

Cells in the human body are eukaryotic and contain twenty-three pairs of chromosomes.  Other organisms have a different chromosome count.

In the disorganized group of 46 chromosomes in your cells’ nucleus, are twenty three pairs.  They can be aligned with their pairs into a karyotype.  Scientists pair these chromosomes by their size, shape, and bands, or centromere.  These bands are present after the chromosomes have been dyed.



A Point to Ponder 
Each chromosome of a pair stands for the same traits as its counterpart. 

 
Genes

Genes are portions of the DNA strand and tell the body what proteins to make.  These proteins help carry out the functions of the cell.  They form the structure of your skin and hair, act as enzymes to break down your food, and regulate the different processes of the cell.
 
A Point to Ponder    
Every cell in your body has the same genes, and DNA.  Your cells have all of the genetic information for your whole body.  Your skin cells have the genetic make up of your skin and your muscles.  Your skin does not use this extra information.

Mitosis

Mitosis is the name given to basic cell division.  Cells divide when hormones instruct them to; usually to replace dying cells or as part of the healing process.  There are five stages to this division, the first of which is interphase.   During interphase, the cell rests, and the chromosomes make copies of themselves.  After interphase comes prophase, the second stage of mitosis.  During prophase, the chromosomes change shape and the walls of the nucleus begin to break down.  The third phase, metaphase,  is when the chromosomes line up at their centers.  During anaphase, the fourth stage, each forming cell is given its own identical set of chromosomes.  The fifth stage, telophase, occurs when the two newly formed daughter cells pull away from each other and begin forming new nuclei.  When these new nuclei are formed, the cells re-enter interphase and await their next division.


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