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