Science, Part 33

Question 1: Name some prenatal tests. Describe them and discuss the positives and negatives of prenatal testing.

Answer 1: Prenatal diagnosis has the ability to detect and predict almost 500 genetic disorders, from very mild to very severe.A major diagnostic tool is amniocentesis, the sampling and testing of the amniotic fluid in which the fetus is suspended. Between the fourteenth and sixteenth week of pregnancy, a syringe is inserted in the amnion and fluid containing embryonic cells is extracted. The cells are cultured and allowed to undergo mitosis. These cells can be analyzed and tested for multiple abnormalities. Biochemical defects such as those that cause sickle-cell anemia may also be discovered. A newer test, chorionic villus sampling, using cells drawn from the chorion (a sac surrounding the amnion) is also available. This test carries a higher risk than amniocentesis, but may be performed much earlier in the pregnancy. Newer and more sophisticated tests are constantly being developed to make earlier and more precise prenatal diagnoses.

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Question 2: Explain and discuss the history and discovery of DNA.

Answer 2: In 1868, Johann Miescher discovered an unknown substance in the cell nucleus that was later known as DNA. The function of this substance remained a mystery for over 75 years. In the early 1950's, experiments with a class of viruses began providing evidence that DNA was the substance that determined heredity. Later in that decade, James Watson and Francis Crick discovered the genetic code and accurately described DNA and its functions. Watson and Crick, armed with evidence from X-ray diffraction, which actually photographed crystallized DNA, began to try to build models of DNA. The breakthrough came when it was recognized that the structure of DNA was arranged in a double helix, with the phosphates and sugars positioned like the outer rungs of a ladder and the nitrogenous bases fitting perfectly between the rails with repeating features. The code was broken, causing a historic breakthrough in scientific research.

Question 3: Discuss the structure and function of DNA.

Answer 3: A DNA molecule is composed of four kinds of nucleotides, the subunits of nucleic acid. A nucleotide consists of a five-carbon sugar (deoxribose), a phosphate group, and one of four nitrogen containing bases: adenine, guanine, thymine, and cytosine. The nucleotides in DNA are structurally similar to each other, but the four types of bases differ in relative amounts from species to species.DNA consists of two strands of nucleotides twisted together into a double helix formation. Hydrogen bonds join the bases of each strand together. Adenine pairs with thymine and cytosine pairs with guanine for the entire length of the DNA molecule. The order and sequence of the bases differ greatly between species, but the base pairing between the two nucleotide stands of DNA is constant for all species. Thus, DNA mirrors both the unity and diversity of life.

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