Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Overview Section 2 The Structure of DNA DNA

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu A Winding Watson and Crick determined that a DNA molecule is a —two strands twisted around each other, like a winding staircase. are the subunits that make up DNA. Each nucleotide is made of three parts: a phosphate group, a five-carbon sugar molecule, and a nitrogen-containing base. The five-carbon sugar in DNA nucleotides is called Section 1 The Structure of DNA DNA

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu DNA Double Section 1 The Structure of DNA DNA

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu DNA STRUCTURE tmhttp:// tm A gene is a segment of DNA that directs the formation ofA gene is a segment of DNA that directs the formation of DNA

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu A Winding Staircase, continued The nitrogen base in a nucleotide can be either a bulky, double-ring or a smaller, single-ring Section 1 The Structure of DNA DNA

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Structure of a – know the picture, page 345 Section 1 The Structure of DNA DNA bonds hold nucleotides together.

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Discovering DNA’s Structure Chargaff’s Observations In 1949, Erwin Chargaff observed that for each organism he studied, the amount of always equaled the amount of thymine Likewise, the amount of always equaled the amount of cytosine (G=C). Complimentary However, the amount of adenine and thymine and of guanine and cytosine varied between Section 1 The Structure of DNA DNA

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Wilkins and Photographs By analyzing the complex patterns on diffraction photo, scientists can determine the structure of the molecule. In 1952, Maurice Wilkins and Rosalind Franklin developed high-quality X-ray photographs of strands of DNA. These photographs suggested that the DNA molecule resembled a tightly coiled helix and was composed of two or three chains of.(repeating subunits of DNA/RNA) Section 1 The Structure of DNA DNA

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Discovering DNA’s Structure, continued Watson and Crick’s DNA Model In 1953, Watson and Crick built a model of DNA with the configuration of a double helix, a “spiral staircase” of two strands of nucleotides twisting around a The double-helical model of DNA takes into account observations and the patterns on Franklin’s X-ray diffraction photographs. Section 1 The Structure of DNA DNA

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Pairing Between Bases An adenine on one strand always pairs with a on the opposite strand, and a guanine on one strand always pairs with a on the opposite strand. These rules are supported by Chargaff’s observations. The strictness of base-pairing results in two strands that contain base pairs. Section 1 The Structure of DNA DNA

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Discovering DNA’s Structure, continued The diagram of DNA below the helix makes it easier to visualize the base-pairing that occurs between Section 1 The Structure of DNA DNA

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Roles of Enzymes in DNA Replication The complementary structure of DNA is used as a basis to make exact copies of the DNA each time a cell divided. The process of making a copy of DNA is called D DNA replication occurs during the of the cell cycle, before a cell divides. Section 1 The Replication of DNA DNA

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Roles of Enzymes in DNA Replication, continued DNA replication occurs in three steps: Step 1 DNA open the double helix by breaking the hydrogen bonds that link the complementary nitrogen bases between the two strands. The areas where the double helix separates are called Section 1 The Replication of DNA DNA

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Roles of Enzymes in DNA Replication, continued Step 2 At the replication fork, enzymes known as DNA move along each of the DNA strands. DNA polymerases add nucleotides to the exposed nitrogen bases, according to the base-pairing rules. Step 3 Two DNA molecules form that are identical to the original Section 1 The Replication of DNA DNA

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu DNA Section 1 The Replication of DNA DNA

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 1 The Replication of DNA DNA

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Roles of Enzymes in DNA Replication Checking for In the course of DNA replication, errors sometimes occur and the wrong is added to the new strand. An important feature of DNA replication is that DNA polymerases have a role. This proofreading reduces errors in DNA replication to about one error per 1 billion Section 1 The Replication of DNA DNA

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu The Rate of Replication Replication does not begin at one end of the DNA molecule and end at the other. The circular DNA molecules found in usually have two replication forks that begin at a single point. The replication forks move away from each other until they meet on the opposite side of the Section 1 The Replication of DNA DNA

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu The Rate of Replication, continued In cells, each chromosome contains a single, long strand of DNA. Each human chromosome is replicated in about sections that are 100,000 nucleotides long, each section with its own starting point. With multiple replication forks working in concert, an entire human chromosome can be replicated in about hours. Section 1 The Replication of DNA DNA

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 1 The Replication of DNA DNA

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Decoding the Information in DNA, such as eye color, are determined by proteins that are built according to instructions coded in DNA. Proteins, however, are not built directly from DNA. acid is also involved. Like DNA, ribonucleic acid (RNA) is a nucleic acid—a molecule made of linked together. Section 1 From Genes to Proteins DNA

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Ribonucleic Acid Section 1 From Genes to Proteins DNA

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Decoding the Information in DNA, continued RNA differs from DNA in three ways: 1. RNA consists of a of nucleotides instead of the two strands found in DNA. 2. RNA nucleotides contain the five-carbon sugar ribose rather than the sugar deoxyribose, which is found in 3. In addition to the A, G, and C nitrogen bases found in DNA, RNA nucleotides can have a nitrogen base called Section 1 From Genes to Proteins DNA

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Decoding the Information in DNA, continued The instructions for making a protein are transferred from a gene to an RNA molecule in a process called Cells then use two different types of RNA to read the instructions on the RNA molecule and put together the amino acids that make up the protein in a process called Section 1 From Genes to Proteins DNA

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Decoding the Information in DNA, continued The entire process by which proteins are made based on the information encoded in DNA is called gene or protein synthesis. Section 1 From Genes to Proteins DNA

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Transfer of Information from DNA to RNA The first step in the making of a protein, transcription, takes the information found in a gene in the DNA and to a molecule of RNA. RNA, an enzyme that adds and links complementary RNA nucleotides during transcription, is required. Section 2 From Genes to Proteins DNA

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Transfer of Information from DNA to RNA, continued The three steps of transcription are: Step 1 RNA polymerase binds to the gene’s Step 2 The two DNA strands unwind and Step 3 Complementary RNA nucleotides are Section 2 From Genes to Proteins DNA

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Section 2 From Genes to Proteins DNA

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Types of RNA mRNA – chain tRNA – rRNA - Section 2 From Genes to Proteins DNA

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu The Genetic Code: Three-Nucleotide “Words” Different types of RNA are made during depending on the gene being expressed. When a cell needs a particular protein, it is that is made. Messenger RNA is a form of RNA that carries the instructions for making a protein from a gene and delivers it to the site of Section 2 From Genes to Proteins DNA

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu The Genetic Code: Three-Nucleotide “Words”, The information is translated from the language of RNA—nucleotides—to the language of proteins— The RNA instructions are written as a series of three-nucleotide sequences on the mRNA called Stop codon stop translation The genetic code of mRNA is the amino acids and “start” and “stop” signals that are coded for by each of the possible mRNA codons. Section 2 From Genes to Proteins DNA

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu in mRNA Section 2 From Genes to Proteins DNA

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu RNA’s Roles in Translation RNA molecules and help in the synthesis of proteins. Transfer RNA (tRNA -Translation takes place in the. Here molecules are single strands of RNA that temporarily carry a specific amino acid on one end. An is a three-nucleotide sequence on a tRNA that is complementary to an mRNA codon. Section 2 From Genes to Proteins DNA

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu RNA’s Roles in Translation, continued Ribosomes are composed of both proteins and Ribosomal RNA (rRNA) molecules are RNA molecules that are part of the structure of ribosomes. Each ribosome temporarily holds one mRNA and tRNA molecules. signal stops transcription Section 2 From Genes to Proteins DNA

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Translation: Forming the First Peptide Bond Links amino acids together = Section 3 From Genes to Proteins DNA

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Translation: Assembling Section 3 From Genes to Proteins DNA nacode.htm

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Intervening DNA in Eukaryotic Genes In eukaryotes, many genes are interrupted by —long segments of nucleotides that have no coding information. are the portions of a gene that are translated (expressed) into proteins. After a eukaryotic gene is transcribed, the introns in the resulting mRNA are cut out by complex assemblies of RNA and protein called Section 3 Gene Regulation and Structure DNA

Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Mutations Because the genetic message is read as a series of triplet nucleotides, and of one or two nucleotides can upset the triplet groupings. Section 3 Gene Regulation and Structure DNA