DNA & GENE EXPRESSION Section 1 DNA & RNA Structure Setting the Scene Nucleic Acid Structure - - DNA and RNA are nucleic acids that help store and express genetic information Nucleic acids are made of a sugar-phosphate backbone and nitrogenous bases (A, T, G, C, U) DNA Base Pairs In DNA, A-T G-C In RNA, U replaces T. A-U G-C Way to remember: Apples go with Trees Cars go with Garages Base Pairs There are two types of bases: - Purines: made of two rings - Pyrimidines: made of one ring Purines pair with pyrimidines! Coding Ability - The sequence of nitrogenous bases (A, T, G, C, U) codes for the structures in an organism DNA stores genetic information, and RNA helps express genetic information (details on this later) DNA vs. RNA DNA - - Double stranded Bases: Adenine, Thymine, Guanine, Cytosine Deoxyribose sugar in sugar-phosphate backbone RNA - - Single stranded Bases: Adenine, Uracil, Guanine, Cytosine Ribose sugar in sugarphosphate backbone DNA Replication DNA Replication - DNA is replicated during mitosis, in order to produce two identical sets of DNA - DNA replication is semiconservative, meaning one strand is conserved (kept from the original), and another is made from scratch DNA Replication AAATTTGGGCCC What is the matching sequence? AAATTTGGGCCC TTTAAACCCGGG What is the matching sequence? TTTAAACCCGGG DNA Replication AAATTTGGGCCC TTTAAACCCGGG Original DNA AAATTTGGGCCC TTTAAACCCGGG Two identical copies AAATTTGGGCCC TTTAAACCCGGG Replication Enzymes - There are many enzymes involved in the steps of DNA replication Some replication enzymes: ● ● ● ● Helicase “unzips” the DNA Polymerase adds new bases Primase makes starting points for polymerase Ligase “glues” DNA fragments together Section 2 Transcription & Translation DNA Expression - Proteins are largely responsible for the traits and structures in organisms - Proteins are made by ribosomes in the cytoplasm - The instructions for protein making are found in DNA in the nucleus - DNA cannot leave the nucleus, so a messenger is needed to transfer info Transcription - A strand of messenger RNA (mRNA) is made based on the DNA section that needs to be expressed in the nucleus - Making this mRNA copy of DNA is called transcription - The mRNA contains Us instead of Ts, which signals to the cell that it can travel out of the nucleus and deliver the information to the ribosome Practice What would the mRNA transcript sequence be for this DNA segment? DNA: CGTACT Practice What would the mRNA transcript sequence be for this DNA segment? DNA: CGTACT mRNA: GCAUGA Translation - The mRNA leaves the nucleus and goes to the ribosome (the protein maker of the cell) - The ribosome translates the mRNA into an amino acid chain, which will then be folded into a protein Translation - The ribosome is made out of ribosomal RNA (rRNA) - The ribosome reads the mRNA bases in threes - A group of three bases is called a codon - Each codon codes for a specific amino acid - Transfer RNAs (tRNAs) bring amino acids to the ribosome that match the codon Codons - We can use a codon chart to see which codons code for which amino acids Practice: DNA: CGTACT mRNA: GCAUGA Amino acids: Codons - We can use a codon chart to see which codons code for which amino acids Practice: DNA: CGTACT mRNA: GCAUGA Amino acids: Ala, Stop Stop Codons - Stop codons tell the ribosome that the amino acid chain is finished - If a stop codon occurs too early, the protein will almost always be nonfunctional DNA: CGTACT mRNA: GCAUGA Amino acids: Ala, Stop Codons - Multiple different codons code for the same amino acid - Why is this beneficial? Codons - Multiple different codons code for the same amino acid - This increases the chances that a protein will still turn out normal even if a mutation occurs! Overview Section 3 Mutations Reminder: Chromosomes are tightly packed DNA Three Sources of Mutations - Errors during DNA replication: during mitosis, DNA may be incorrectly copied - Errors during meiosis: during production of sex cells (meiosis), chromosomes may not separate correctly - Environmental factors: damage can be done to DNA from lifestyle or environment (smoking, sun exposure, toxic chemicals, etc.) Two Categories of Mutations 1st Category: Point mutations: change one base in one area on the DNA. - Substitution: one base is replaced with a different one Insertion: an additional base is inserted into the base sequence Deletion: a base is deleted from the base sequence Results of Point Mutations - Silent mutation: still produce the same amino acid sequence → normal protein - Missense mutation: produces a different amino acid sequence - Nonsense mutation: produces a stop codon → amino acid sequence gets cut short Results of Point Mutations - Remember that the ribosome reads in threes! - Insertions and deletions shift the reading frame of the ribosome, so they are called frameshift mutations - Substitution mutations usually only affect one codon, but frameshift mutations affect every codon after the mistake Two Categories of Mutations 2nd Category: Chromosomal mutations: change entire chromosomes. - Chromosomal mutations may result in an organism having too many, too few, or reversed sections of chromosomes Sources of Chromosomal Mutations - Most chromosomal mutations occur from mistakes during meiosis (production of sex cells) - Chromosomes may not be replicated or separate properly Section 4 Genetic Modification & Biotechnology Problem: Restaurants and grocery stores want to be able to sell apple slices, but once you slice the apple, it begins to brown. Is it possible to create an apple that doesn’t turn brown? How would you do this? Can we make an apple that doesn’t turn brown? Step One: Understand why an apple turns brown in the first place. Mini experiment: Some companies slow the browning process by adding lemon juice to the slices. Design an experiment to determine if this method really does work. Are there other materials that also might work to slow browning? Historically, we have found ways to change organisms. Selective Breeding Choosing the “best” traits for breeding Most domestic animals are products of this process, also known as artificial selection. Sometimes new breeds are created because a random mutation changes some aspect of the organism. Wild mustard plants are the ancestors of many of these grocery store staples. Scottish Fold cat - a mutation results in the ears turning downward. Most domestic animals are the result of artificial selection Artificial selection is when animals with desirable traits are purposefully bred Could selective breeding be used to create a nonbrowning apple? What would you need? Organisms can also be changed by crossbreeding them with other organisms to try to blend the traits. Poodles don’t shed, if you want a non-shedding type of golden retriever, try a “golden-doodle.” You could also get a “labradoodle!” These are also referred to as hybrids. Some of the fruit you eat are actually hybrids. How could you use hybridization to create a non-browning apple? Tangerine + grapefruit = tangelo Granny smith apples do not turn brown as quickly as other apples. Granny smith apples are a cross between the european wild apple and the domestic apple What if we could insert genes into a species without having to go through the breeding process? This is GENETIC MODIFICATION (genetic engineering) Organisms that contain genes from more than one species (or inserted genes) are called: TRANSGENIC organisms Gene causes these mice to glow in the dark. Normally, the gene is found in jellyfish. Transgenic Organisms - Organisms that contain genes from other species 1.Bacteria that contain human insulin genes 2.Animals that can produce more milk 3.Plants that are resistant to pests GMO = genetically modified organism ● organism whose genome (DNA) has been altered by the techniques of genetic engineering ● Genes are usually added ● Sometimes genes are removed How could we use genetic modification to create a nonbrowning apple? What would we need to know? Is there a gene that causes apples to turn brown? DNA extraction – process where DNA is removed from cells The sequence of DNA can be used for: - paternity tests - solving crimes - establishing lineages (evolution) - diagnosing disorders How is DNA analyzed? Gel Electrophoresis - DNA is cut into fragments, and separated by size. This image shows a DNA fingerprint where DNA from a bloodstain at a crime scene is compared to suspect DNA. Which suspect is a match to the bloodstain? This is what a DNA fingerprint would look like for a paternity test. Which male is the father? Electrophoresis can diagnosis sickle cell anemia. Cloning Clones are organisms with the same genetic code. The first clones were made with this process, called artificial twinning. Somatic Cell Nuclear Transfer We have the technology to clone humans, but should we? Can you think of any good reasons for cloning an organism? What about a human? Clones do exist naturally. These are identical twins. 1. An animal that contains the genes of another species, like the glow-in-the-dark mice, is called a __________ organism. 2. A _______ is a genetically identical organism. 3. In humans, a naturally occurring clone is a ________ 4. The first mammal that was cloned from a skin cell was a _______ 5. For cloning, it is necessary to have which of the following? Check all that apply. ___Cell from the adult to be cloned ___Egg with nucleus intact ___Embryo ___Surrogate mother ___ Sperm ___ Stem cells ___ Egg with nucleus removed Can we make an apple that doesn’t turn brown? Step One: Understand why an apple turns brown in the first place. Apples turn brown due to the action of an enzyme called polyphenol oxidase (PPO) This enzyme is controlled by the PPO gene. Did lemon juice work on your apples? Construct an explanation for why lemon juice would slow browning. What would happen if we silenced the PPO Gene? This “arctic apple” has been modified to remove the gene that causes browning. Why is this apple not considered to be transgenic? The PPO gene (polyphenol oxidase) has been silenced these apples. They will not turn brown when sliced. Would you eat them? Food for Thought: Some states and some countries required food companies to label products that have been genetically modified. Some companies also make clear when their products are non GMO. 1. What are some reasons for NOT wanting to label foods as GMO? 2. What are reasons FOR labeling GMO foods? 3. If the law required GMO labeling, would the arctic apple need a label? Why or why not?
