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7th Grade Cells and Heredity (Mod A) Unit 2 Lesson 6: DNA Structure and Function DNA • Deoxyribonucleic Acid • Genetic material of a cell Many scientists from all over the world contributed to our understanding of DNA. Write these names & dates in your notebook. Leave 3 lines between each one. • 1857 • 1869 • 1951 • 1953 – Gregor Mendel – – Johann Fredrich Miechner - Rosalind Franklin & Maurice Wilkins– – James Watson & Francis Crick - • 1857 – Gregor Mendel – did experiments with pea plants; observed offspring had same traits as parents. Hypothsized that parents pass down traits to offspring. • 1869 – Johann Fredrich Miechner – isolated “nuclein” from white blood cells - DNA • 1951 - Rosalind Franklin & Maurice Wilkins–made images of DNA with x-rays • 1953 – James Watson & Francis Crick - used Franklin & Wilkins images to make 3D model of DNA • <repeated info – skip if you did the previous slide> • Many scientists from all over the world contributed to our understanding of DNA. • Some scientists discovered the chemicals that make up DNA, others learned how these chemicals fit together. • Still others determined the three-dimensional structure of the DNA molecule. • 1951 - Rosalind Franklin and Maurice Wilkins made images of DNA with x-rays • 1953 - James Watson and Francis Crick credited with building first model of DNA DNA Structure • Shape is double helix • Sides (a.k.a. backbone) made of sugars and phosphate groups • “Rungs” made of pairs of bases • • • • Adenine Thymine Cytosine Guanine • Base + sugar + phosphate = nucleotide (“building block” of DNA) • Bases always pair in specific ways – complementary bases • adenine (A) pairs with thymine (T) • cytosine (C) pairs with guanine (G) • How can you remember this? 4 nucleotides • The ORDER of the nucleotides matters – it is the code that tells cells what proteins to build • Segments of DNA that code for a certain trait are called genes, which determine your traits • Each gene codes for a specific protein DNA Replication: making copies • 1. The double helix unwinds (“unzips” and the two strands separate • Each strand is used as a pattern for the new strand • 2. bases on each side are exposed, and complementary nucleotides are added • For example: an nucleotide containing thymine attaches to an exposed adenine • 3. Now you have two identical DNA molecules, each containing one old strand and one new strand! Replication happens right before cell division *It only takes a few hours! Replication happens at many places along the strand at once DNA does not always copy correctly! Mutations: changes in the number, type or order of bases on a piece of DNA • In a deletion mutation, a base is left out. • In an insertion mutation, an extra base is added. • The most common mutation, substitution, happens when one base replaces another. Unit 2 Lesson 6 DNA Structure and Function • Which type of mutation is shown in each row? (The first row is the original sequence.) Copyright © Houghton Mifflin Harcourt Publishing Company Mutations can be positive or negative, but most are neutral. How do mutations happen? • Random error • Damage to the DNA molecule by mutagens • Ex. UV light and chemicals in cigarette smoke •Cells make proteins that can fix errors in DNA, but sometimes the mistake is not corrected & mistake becomes part of the genetic code. • A genetic disorder results from mutations that harm the normal function of the cell. • Some genetic disorders are inherited, or passed on from parent to offspring. • Other disorders result from mutations during a person’s lifetime. Most cancers fall in this category. Copyright © Houghton Mifflin Harcourt Publishing Company • Q: What cell organelle makes proteins? • A: Ribosomes • Q: Where are ribosomes found? • A: In the cytoplasm and on rough ER • Q: Where is the code for making the proteins? • A: On the DNA • Q: Where is the DNA? • A: In the nucleus • Q: How does the info from the DNA inside the nucleus get outside the nucleus to the ribosomes? •A: RNA! RNA = ribonucleic acid • Like DNA, RNA has a sugar-phosphate backbone and the bases adenine (A), guanine (G), and cytosine (C) • Instead of thymine (T), RNA contains uracil (U). • Unlike DNA, it is only one strand, not two • Three types of RNA have special roles in making proteins. • mRNA – messenger RNA • tRNA – transfer RNA • rRNA – ribosomal RNA Transcription: copying DNA to an mRNA strand • (mRNA = messenger RNA) • 1. DNA strand unwinds (just like in replication) • 2. mRNA fills in the complementary nucleotides (just like in replication) • Only one gene at a time is transcribed, not the whole strand • 3. When transcription is complete, DNA strand winds up again Translation: proteins are made from the mRNA code • 1. mRNA travels outside the nucleus to a ribosome made of rRNA (ribosomal RNA) • 2. As mRNA passes through the ribosome tRNA (transfer RNA) molecules deliver amino acids to ribosome • Each group of three bases on the mRNA strand code for one amino acid • The order of bases tells what amino acids to move into the ribosome • 3. Amino acids join together to make proteins Together, transcription and translation are often called “protein synthesis”