Unit 2 Lesson 6: DNA Structure and Function 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. 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 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 Replication happens right before cell division *It only takes a few hours! Replication happens at many places along the strand at once

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 10

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 13

A: RNA! 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 Start at 0:45 or so - https://www.youtube.com/watch?v=gG7uCskUOrA

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” PBS DNA workshop - http://www.pbs.org/wgbh/aso/tryit/dna/#