Module 7 Molecular Genetics
Lesson 3.7.1
3.7.1 page 2
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The discovery of the molecule responsible for heredity took many years and the great efforts of many scientists. During the same time, other scientists were trying to determine the structure of DNA.
DNA, then known as “nucleic acid” was first isolated from the nuclei of white blood cells by Friedrich Miescher in 1869. At this time, there was no connection made between this molecule and heredity. It took experiments by Phoebus Levene, Frederick Griffith, Alfred Hershey and Martha Chase to finally prove that DNA was the molecule responsible for heredity and not protein.
While the scientists mentioned above were looking for the agent of heredity, other scientists were studying the DNA molecule more closely. Phoebus Levene first discovered that DNA was made up of chains of four different nucleotides (adenine (A), guanine (G), cytosine (C) and thymine (T). He also noted that RNA was also made up of four nucleotides but had the base uracil (U) instead of thymine (T). Levene, unfortunately, made an incorrect discovery and proposed that all nucleotides were found in the same concentration and in the same repeating order. This led scientists to believe that DNA couldn’t be the molecule of heredity. Later, when Erwin Chargraff found that adenine (A) and thymine (T) are found in equal amounts in any sample of DNA, and cytosine and guanine were also found in equal amounts (but different to A and T), Levene’s incorrect discovery was thrown out the window and Chargaff’s rule of constant relationships between A and T, and C and G were accepted. Later, the combination of Rosalind Franklin’s use of x-ray photography, and the work of James Watson and Francis Crick eventually produced the double-helix structure model of the DNA molecule.
DNA: deoxyribonucleic acid (DNA), a double-stranded nucleic acid molecule that governs the processes of heredity in the cells of all organisms: composed of nucleotides containing a phosphate group, a nitrogenous base and deoxyribose
Chargaff’s Rule: in any sample of DNA, a constant relationship in which the amount of adenine is always approximately equal to the amount of thymine, and the amount of cytosine is always approximately equal to the amount of guanine
Double-helix: spiral shape most commonly associated with DNA, made up of two long strands of nucleotides bound together and twisted
Learn more about the contributions of the above-mentioned scientists by reading pages 624 to 627. Summarize your readings by creating a timeline that includes scientists, experiments and major discoveries that lead to the identification of the hereditary agent and the structure of the DNA molecule. Place this timeline in your course folder for further reference when studying.
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The following video titled Introduction to Molecular Genetics: The Search for the Genetic Code goes over the same scientists and experiments that lead to the discovery of DNA purpose and structure as those in your textbook readings. You may choose to watch this video in place of doing the textbook readings above, OR you may use this video to review areas that are confusing for you. To view this video you may need to get a password from your instructor in order to access the LearnAlberta website.
The Structure of DNA
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Now that you have learned the history of how DNA was discovered, let's look in detail at the structure of the DNA molecule. Since DNA is a 3-D molecule, you may find it easier to visualize the structure by watching a video. The following video shows the structure of the DNA molecule. Pay attention to the double-helix shape and the arrangement of the deoxyribose sugars, phosphates and nitrogen bases in the molecule. The relationship between DNA, chromosomes and cells is also reviewed in the video. As an extra, you can also watch students perform a lab where they extract and isolate DNA from calf thymus cells—you may skip this section if your time is limited. Stop watching the video when you get to the “Replication of DNA,” this will be covered in Lesson 2. You may need to contact your instructor for a password in order to access the LearnAlberta website so that you may view this video.
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Complementary base pairs: refers to the hydrogen bonded, nitrogenous base pairs of adenosine and thymine, and of cytosine and guanine in the DNA double helix
Antiparallel: describes the property by which the 5’ to 3’ phosphate bridges run in opposite directions on each strand of nucleotides in a double-stranded DNA molecule
If you prefer, you may read about the structure of DNA in your textbook on pages 628 and 629. In this case, pay attention to the structure shown in Figure 18.6 and note the arrangement of the deoxyribose sugars, phosphates and nitrogen bases in the molecule. You should also note the definition of complementary base pairs, and the idea that the two strands of nucleotides in the double-helix run antiparallel to each other. Adding a diagram that illustrates these features to your course folder would be an excellent idea.
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