Unit 2 Lesson 6 DNA Structure and Function Copyright © Houghton Mifflin Harcourt Publishing Company

Cracking the Code What is DNA? Unit 2 Lesson 6 DNA Structure and Function Cracking the Code What is DNA? The genetic material in cells is contained in a molecule called deoxyribonucleic acid, or DNA. Scientists describe DNA as containing a code. A code is a set of rules and symbols used to carry information. To understand how DNA functions, you first need to learn about the structure of the DNA molecule. Copyright © Houghton Mifflin Harcourt Publishing Company 2

Unit 2 Lesson 6 DNA Structure and Function How was DNA discovered? Many scientists from all over the world contributed to our understanding of DNA. Some scientists discovered the chemicals that make up DNA, and others learned how these chemicals fit together. Still others determined the three-dimensional structure of the DNA molecule. Copyright © Houghton Mifflin Harcourt Publishing Company 3

Unraveling DNA What does DNA look like? Unit 2 Lesson 6 DNA Structure and Function Unraveling DNA What does DNA look like? Experiments and imaging techniques have helped scientists to infer the shape of DNA. The structure of DNA is a twisted ladder shape called a double helix. Copyright © Houghton Mifflin Harcourt Publishing Company 4

Unit 2 Lesson 6 DNA Structure and Function What does DNA look like? The two sides of the ladder are made of sugars and phosphate groups. The rungs of the ladder are made of pairs of bases. Copyright © Houghton Mifflin Harcourt Publishing Company 5

Unit 2 Lesson 6 DNA Structure and Function What does DNA look like? A base, a sugar, and a phosphate group make a building block of DNA called a nucleotide. There are four different nucleotides in DNA. Copyright © Houghton Mifflin Harcourt Publishing Company 6

Unit 2 Lesson 6 DNA Structure and Function What does DNA look like? The bases in nucleotides are paired, or complementary. Adenine always pairs with thymine (A-T). Cytosine always pairs with guanine (C-G). The order of the nucleotides in DNA is a code that carries information. Copyright © Houghton Mifflin Harcourt Publishing Company 7

Unit 2 Lesson 6 DNA Structure and Function What does DNA look like? Genes are segments of DNA that relate to a certain trait. The code in the nucleotide order has information about which proteins the cells should build. The types of proteins that your body makes help determine your traits. Copyright © Houghton Mifflin Harcourt Publishing Company 8

Replication and Mutation Unit 2 Lesson 6 DNA Structure and Function Replication and Mutation How are copies of DNA made? The cell makes copies of DNA molecules through a process known as replication. During replication, the two strands of DNA separate. The bases on each side of the molecule are used as a pattern for a new strand. Copyright © Houghton Mifflin Harcourt Publishing Company 9

How are copies of DNA made? Unit 2 Lesson 6 DNA Structure and Function How are copies of DNA made? As bases on the original molecule are exposed, complementary nucleotides are added. When replication is complete, there are two identical DNA molecules. Copyright © Houghton Mifflin Harcourt Publishing Company 10

How are copies of DNA made? Unit 2 Lesson 6 DNA Structure and Function How are copies of DNA made? Describe what is happening in the diagram. Copyright © Houghton Mifflin Harcourt Publishing Company 11

When are copies of DNA made? Unit 2 Lesson 6 DNA Structure and Function When are copies of DNA made? Before a cell divides, it copies its DNA. Our cells can replicate DNA in just a few hours, because replication begins in many places along a DNA strand. Many groups of proteins are working to replicate your DNA at the same time. Copyright © Houghton Mifflin Harcourt Publishing Company 12

Unit 2 Lesson 6 DNA Structure and Function What are mutations? Mutations are changes in the number, type, or order of bases on a piece of DNA. There are three main kinds of mutations: deletions, insertions, and substitutions. Copyright © Houghton Mifflin Harcourt Publishing Company 13

What are mutations? In a deletion mutation, a base is left out. Unit 2 Lesson 6 DNA Structure and Function What are mutations? 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. Copyright © Houghton Mifflin Harcourt Publishing Company 14

Unit 2 Lesson 6 DNA Structure and Function What are mutations? Which type of mutation is shown in each row? (The first row is the original sequence.) Copyright © Houghton Mifflin Harcourt Publishing Company 15

Unit 2 Lesson 6 DNA Structure and Function What are mutations? Mutations can happen by random error, and also by damage to the DNA molecule by physical or chemical agents called mutagens. Cells make proteins that can fix errors in DNA, but sometimes the mistake is not corrected. The mistake then becomes part of the genetic code. Copyright © Houghton Mifflin Harcourt Publishing Company 16

Unit 2 Lesson 6 DNA Structure and Function What are mutations? 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 17

Protein Factory What is the role of DNA and RNA in building proteins? Unit 2 Lesson 6 DNA Structure and Function Protein Factory What is the role of DNA and RNA in building proteins? Some of the information in the DNA is copied to a separate molecule called RNA, or ribonucleic acid. RNA is used to build proteins. Copyright © Houghton Mifflin Harcourt Publishing Company 18

What is the role of DNA and RNA in building proteins? Unit 2 Lesson 6 DNA Structure and Function What is the role of DNA and RNA in building proteins? 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). Three types of RNA have special roles in making proteins. Copyright © Houghton Mifflin Harcourt Publishing Company 19

What is the role of DNA and RNA in building proteins? Unit 2 Lesson 6 DNA Structure and Function What is the role of DNA and RNA in building proteins? When a cell needs to make a protein, it makes an RNA copy of a section of the DNA. This is called transcription. In transcription, DNA is used as a template to make a complementary strand of messenger RNA (mRNA). The information in the mRNA is then used to build proteins. This is called translation. Copyright © Houghton Mifflin Harcourt Publishing Company 20

What is the role of DNA and RNA in building proteins? Unit 2 Lesson 6 DNA Structure and Function What is the role of DNA and RNA in building proteins? In translation, the mRNA passes through a protein assembly line within a ribosome. Copyright © Houghton Mifflin Harcourt Publishing Company 21

What is the role of DNA and RNA in building proteins? Unit 2 Lesson 6 DNA Structure and Function What is the role of DNA and RNA in building proteins? A ribosome is a cell organelle made of ribosomal RNA (rRNA) and protein. As mRNA passes through, transfer RNA (tRNA) delivers amino acids to the ribosomes. The order of the bases codes for which amino acid is attached. The amino acids are joined together to form a protein. Copyright © Houghton Mifflin Harcourt Publishing Company 22