1 Lectures 1/25, 1/30/2012 Topics: DNA as genes Mutation, genetic drift, gene flow Reading: Chapter 2, pp. 32-38; Chapter 3, pp. 44-47; Chapter 4, pp. 73-76; Web Links: http://learn.genetics.utah.edu/ This website has many useful resources http://www.youtube.com/watch?v=4jtmOZaIvS0&feature=related Cool video of DNA replication in real time http://www.nature.com/scitable/topicpage/discovery-of-dna-structure-and-function-watson-397 This is a short paper describing the discovery and function of DNA structure ------------------------------------------------- DNA and Darwin 1. The Modern Synthesis essentially is a unification of Darwin’s Theory of Natural Selection and genetics (Mendel and understanding DNA’s properties). a When Darwin proposed his Theory of Natural Selection, scientists still didn’t understand how traits are passed from parent to offspring. b. The prevalent view at the time was called blending inheritance. As an example of blending inheritance, a red flower and a white flower would produce pink flower offspring. Another example: a tall parent and a short parent would always produce medium height children. c. Gregor Mendel’s work on pea plants (we discuss this in detail in Lectures 10/5 and 10/10 ) showed that blending inheritance is not accurate. d. The discovery of DNA (Watson, Crick, Franklin) taught us about the mechanistic basis for inheritance. DNA Structure and Function 1. Take home points: a. DNA is the universal genetic material of all life. All living things on Earth have DNA. In many instances, scientists have even been able to extract and sequence DNA from fossils (extinct) organisms. b. DNA has two major functions 1. It stores heritable genetic information. 2. A portion of the DNA sequence also has embedded information; once that code is ‘solved’ or ‘read’ using the Genetic Code, proteins can be produced. Proteins are the ‘workers’ of the cell. 2. DNA Structure2 a. The human genome (i.e. our complete DNA sequence) is 3.0 billion bases long! b. Every cell of our body carries one genome in the nucleus. But DNA is not ‘floating’ around in the nucleus, it is packaged tightly into structures called chromosomes. Humans have 23 pairs of chromosomes (one set of 23 from each parent). The number of chromosomes varies in different animals (see Figure 3.5 of your text). c. The basic building block of DNA, called a nucleotide, is composed of three parts: The Base, the Sugar, and a Phosphate group (see Figure 3.7). 1. There are four (4) Bases of DNA called A (Adenine), C (Cytosine), G (Guanine), and T (Thymidine). 2. One strand of DNA is basically a chain of nucleotides joined by the very strong phosphodiester bond (the bond formed between phosphate and sugar of adjacent nucleotides (links in a growing chain). 3. DNA structure, however, is a double-stranded helix. Two DNA strands form this helix structure by pairing of complementary bases via hydrogen bonds. The base pairs that are formed are universally conserved. a. A base pairs with T via two hydrogen bonds. b. G base pairs with C via three hydrogen bonds. 4. Thus the double stranded DNA helix is formed by two antiparallel DNA strands, each consisting of chains of nucleotides, that are then joined together by hydrogen bonding between complementary bases. You will see a movie of how Watson, Crick and Franklin solved this structure of DNA. By solving this structure, these scientists allowed us to understand how DNA is copied and inherited and how mistakes in DNA (i.e. mutations) are formed. 3. DNA Replication, i.e. How DNA is copied a. The entire genome is copied in the nucleus prior to cell division (mitosis or meiosis). b. DNA repair systems are found in every organism, they make sure copying the genome occurs accurately. Still, however, mistakes in the copying process occur. These are called mutations. Mutations create variation from one individual to the next in a population. Some mutations are harmful and some can be beneficial. Most are neutral (have no effect). See below for more about mutation. c. DNA replication is semi-conservative. Prior to DNA replication, the DNA unwinds from it’s tightly wound conformation in chromosomes and the hydrogen bonds between complementary bases on each of the two DNA strands break, causing the two DNA strands to separate. Each parent or original DNA strand then serves as the template for a new growing DNA strand, based on complementary base pairing.3 Evolutionary Forces a. Darwin’s original Theory of Natural Selection is an evolutionary force that acts on existing variation. b. How does variation in populations arise? Mutation is the primary source of new genetic material. Mutation is in DNA, arising as a result of errors in replication. Mutation is a second evolutionary force. Most mutations have no effect, or they are neutral. Some mutations are very harmful. Some are very beneficial or adaptive. c. Gene flow is a third evolutionary force. Gene flow is the spread of new genetic material from one population to another of the same species. If there is a lot of gene flow between populations, the populations are not that genetically distinct from one another. If populations are isolated from one another with barriers to gene flow, the populations become genetically distinct from one another. This can lead to speciation. d. Genetic drift is a fourth evolutionary force. It is a random change in the frequency of alleles; genetic drift has a stronger impact on small populations vs. large populations. See Figure 2.19 in your text for an example of