This is a model of the portion of the DNA molecule. Notice the labeled components of the molecule with the area of hydrogen bonding (steel rod).





DNA is a double helix. The center or interior of the molecule consists of paired bases. It is always A - T (adenine - thymine) and G - C (guanine - cytosine). The outisde "backbone" of the molecule is composed of repetitive units of deoxyribose sugar [D] and phosphate bonds [rubber band]. Actually DNA is composed of a double strand of units called nucleotides. The nucleotide is labeled in the next picture.

A Nucleotide

Thoracic-Abdominal Cavity

The Lower Respiratory Tract

Position of the Lungs in Relation To Cardiovascular & Digestive Organs

Various organisms have specialized tissue which undergo meiosis. It could be the zygotes of algae or fungi; the microspore and megaspore mother cells of plants; or spermatogonial and oogonial cells of human beings and other animals.




During the process of meisosis, you begin with a diploid or 2N cell, like a zygote, with two sets of chromosomes in it (2 pair yellow, 2 pair green).





2N (Diploid)


The chromosomes become shortened, thickened, and will double themselves.



















2N (Diploid)




The chromosomes of these 2N cells will align themselves in the equatorial position of the cell.



















2N Diploid





The chromosome pairs are about to divide. The next phase, the chromosomes are pulling apart and cytoplasm is beginning to split between them.






























Once this first division is completed you will see two yellow and two green chromosomes within 1N (haploid) cells on opposite sides of each other.


































Let's look at the new 1N (haploid) cell produced. Each cell has its chromatids or chromosome halves pulled apart. But first they will align themselves in the equatorial position of the cell.













Then the chromosome halves will begin to pull apart with each chromosome half going in opposite directions with the cytoplasm splitting the two cells again.















Each cell produces two new 1N (haploid) cells for a total of 4 (tetrad) from the 2 cells.




Models provided by the Biology Department. Thanks to Dr. Billy Williams (BWilliams@dscc.edu)

for the information provided.