DNA

Chapter 17 : Chapter 17 From Gene to Protein

Slide 2 : Overview: The Flow of Genetic Information The information content of DNA Is in the form of specific sequences of nucleotides along the DNA strands

Slide 3 : The DNA inherited by an organism Leads to specific traits by dictating the synthesis of proteins The process by which DNA directs protein synthesis, gene expression Includes two stages, called transcription and translation

Slide 4 : The ribosome Is part of the cellular machinery for translation, polypeptide synthesis Figure 17.1

Slide 5 : Concept 17.1: Genes specify proteins via transcription and translation

Evidence from the Study of Metabolic Defects : Evidence from the Study of Metabolic Defects In 1909, British physician Archibald Garrod Was the first to suggest that genes dictate phenotypes through enzymes that catalyze specific chemical reactions in the cell

Nutritional Mutants in Neurospora: Scientific Inquiry : Nutritional Mutants in Neurospora: Scientific Inquiry Beadle and Tatum causes bread mold to mutate with X-rays Creating mutants that could not survive on minimal medium

Slide 8 : Using genetic crosses They determined that their mutants fell into three classes, each mutated in a different gene Figure 17.2 Working with the mold Neurospora crassa, George Beadle and Edward Tatum had isolated mutants requiring arginine in their growth medium and had shown genetically that these mutants fell into three classes, each defective in a different gene. From other considerations, they suspected that the metabolic pathway of arginine biosynthesis included the precursors ornithine and citrulline. Their most famous experiment, shown here, tested both their one gene–one enzyme hypothesis and their postulated arginine pathway. In this experiment, they grew their three classes of mutants under the four different conditions shown in the Results section below. The wild-type strain required only the minimal medium for growth. The three classes of mutants had different growth requirements

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Slide 10 : Beadle and Tatum developed the “one gene–one enzyme hypothesis” Which states that the function of a gene is to dictate the production of a specific enzyme

The Products of Gene Expression: A Developing Story : The Products of Gene Expression: A Developing Story As researchers learned more about proteins They made minor revision to the one gene–one enzyme hypothesis Genes code for polypeptide chains or for RNA molecules

Basic Principles of Transcription and Translation : Basic Principles of Transcription and Translation Transcription Is the synthesis of RNA under the direction of DNA Produces messenger RNA (mRNA) Translation Is the actual synthesis of a polypeptide, which occurs under the direction of mRNA Occurs on ribosomes

Slide 13 : In prokaryotes Transcription and translation occur together Figure 17.3a

Slide 14 : In eukaryotes RNA transcripts are modified before becoming true mRNA

Slide 15 : Cells are governed by a cellular chain of command DNA ??RNA ??protein

The Genetic Code : The Genetic Code How many bases correspond to an amino acid?

Codons: Triplets of Bases : Codons: Triplets of Bases Genetic information Is encoded as a sequence of nonoverlapping base triplets, or codons

Slide 18 : During transcription The gene determines the sequence of bases along the length of an mRNA molecule

Cracking the Code : Cracking the Code A codon in messenger RNA Is either translated into an amino acid or serves as a translational stop signal

Cracking the Code : Cracking the Code A codon in messenger RNA Is either translated into an amino acid or serves as a translational stop signal This diagram assists in knowing which amino acid the mRNA codes for:

Slide 21 : Codons must be read in the correct reading frame For the specified polypeptide to be produced

Evolution of the Genetic Code : Evolution of the Genetic Code The genetic code is nearly universal Shared by organisms from the simplest bacteria to the most complex animals

Slide 23 : In laboratory experiments Genes can be transcribed and translated after being transplanted from one species to another Figure 17.6

Slide 24 : Concept 17.2: Transcription is the DNA-directed synthesis of RNA: a closer look

Molecular Components of Transcription : Molecular Components of Transcription RNA synthesis Is catalyzed by RNA polymerase, which pries the DNA strands apart and hooks together the RNA nucleotides Follows the same base-pairing rules as DNA, except that in RNA, uracil substitutes for thymine

Synthesis of an RNA Transcript : Synthesis of an RNA Transcript The stages of transcription are Initiation Elongation Termination

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RNA Polymerase Binding and Initiation of Transcription : RNA Polymerase Binding and Initiation of Transcription Promoters signal the initiation of RNA synthesis Transcription factors Help eukaryotic RNA polymerase recognize promoter sequences Figure 17.8

Elongation of the RNA Strand : Elongation of the RNA Strand As RNA polymerase moves along the DNA It continues to untwist the double helix, exposing about 10 to 20 DNA bases at a time for pairing with RNA nucleotides

Termination of Transcription : Termination of Transcription The mechanisms of termination Are different in prokaryotes and eukaryotes

Slide 31 : Concept 17.3: Eukaryotic cells modify RNA after transcription Enzymes in the eukaryotic nucleus Modify pre-mRNA in specific ways before the genetic messages are dispatched to the cytoplasm

Alteration of mRNA Ends : Alteration of mRNA Ends Each end of a pre-mRNA molecule is modified in a particular way The 5? end receives a modified nucleotide cap The 3? end gets a poly-A tail Figure 17.9

Split Genes and RNA Splicing : Split Genes and RNA Splicing RNA splicing Removes introns and joins exons Figure 17.10

Slide 34 : Is carried out by spliceosomes in some cases Figure 17.11

Ribozymes : Ribozymes Ribozymes Are catalytic RNA molecules that function as enzymes and can splice RNA

The Functional and Evolutionary Importance of Introns : The Functional and Evolutionary Importance of Introns The presence of introns Allows for alternative RNA splicing

Slide 37 : Proteins often have a modular architecture Consisting of discrete structural and functional regions called domains In many cases Different exons code for the different domains in a protein Figure 17.12

Slide 38 : Concept 17.4: Translation is the RNA-directed synthesis of a polypeptide: a closer look

Molecular Components of Translation : Molecular Components of Translation A cell translates an mRNA message into protein With the help of transfer RNA (tRNA)

Slide 40 : Translation: the basic concept Figure 17.13

Slide 41 : Molecules of tRNA are not all identical Each carries a specific amino acid on one end Each has an anticodon on the other end

The Structure and Function of Transfer RNA : The Structure and Function of Transfer RNA A C C A tRNA molecule Consists of a single RNA strand that is only about 80 nucleotides long Is roughly L-shaped

Slide 43 : Figure 17.14b

Slide 44 : A specific enzyme called an aminoacyl-tRNA synthetase Joins each amino acid to the correct tRNA Figure 17.15 Amino acid ATP Adenosine Pyrophosphate Adenosine Adenosine Phosphates tRNA P P P P P Pi Pi Pi P AMP Aminoacyl tRNA (an “activated amino acid”) Aminoacyl-tRNA synthetase (enzyme) Active site binds the amino acid and ATP. 1

Ribosomes : Ribosomes Ribosomes Facilitate the specific coupling of tRNA anticodons with mRNA codons during protein synthesis

Slide 46 : The ribosomal subunits Are constructed of proteins and RNA molecules named ribosomal RNA or rRNA Figure 17.16a

Slide 47 : The ribosome has three binding sites for tRNA The P site The A site The E site Figure 17.16b E P A

Slide 48 : Figure 17.16c

Building a Polypeptide : Building a Polypeptide We can divide translation into three stages Initiation Elongation Termination

Ribosome Association and Initiation of Translation : Ribosome Association and Initiation of Translation The initiation stage of translation Brings together mRNA, tRNA bearing the first amino acid of the polypeptide, and two subunits of a ribosome

Elongation of the Polypeptide Chain : Elongation of the Polypeptide Chain In the elongation stage of translation Amino acids are added one by one to the preceding amino acid

Termination of Translation : Termination of Translation The final stage of translation is termination When the ribosome reaches a stop codon in the mRNA

Polyribosomes : Polyribosomes A number of ribosomes can translate a single mRNA molecule simultaneously Forming a polyribosome

Completing and Targeting the Functional Protein : Completing and Targeting the Functional Protein Polypeptide chains Undergo modifications after the translation process

Protein Folding and Post-Translational Modifications : Protein Folding and Post-Translational Modifications After translation Proteins may be modified in ways that affect their three-dimensional shape

Targeting Polypeptides to Specific Locations : Targeting Polypeptides to Specific Locations Two populations of ribosomes are evident in cells Free and bound Free ribosomes in the cytosol Initiate the synthesis of all proteins

Slide 57 : Proteins destined for the endomembrane system or for secretion Must be transported into the ER Have signal peptides to which a signal-recognition particle (SRP) binds, enabling the translation ribosome to bind to the ER

Slide 58 : The signal mechanism for targeting proteins to the ER

Slide 59 : Concept 17.5: RNA plays multiple roles in the cell: a review RNA Can hydrogen-bond to other nucleic acid molecules Can assume a specific three-dimensional shape Has functional groups that allow it to act as a catalyst

Slide 60 : Types of RNA in a Eukaryotic Cell Table 17.1

Slide 61 : Concept 17.6: Comparing gene expression in prokaryotes and eukaryotes reveals key differences Prokaryotic cells lack a nuclear envelope Allowing translation to begin while transcription is still in progress Figure 17.22

Slide 62 : In a eukaryotic cell The nuclear envelope separates transcription from translation Extensive RNA processing occurs in the nucleus

Slide 63 : Concept 17.7: Point mutations can affect protein structure and function Mutations Are changes in the genetic material of a cell Point mutations Are changes in just one base pair of a gene

Slide 64 : The change of a single nucleotide in the DNA’s template strand Leads to the production of an abnormal protein Figure 17.23

Types of Point Mutations : Types of Point Mutations Point mutations within a gene can be divided into two general categories Base-pair substitutions Base-pair insertions or deletions

Substitutions : Substitutions A base-pair substitution Is the replacement of one nucleotide and its partner with another pair of nucleotides Can cause missense or nonsense Figure 17.24

Insertions and Deletions : Insertions and Deletions Insertions and deletions Are additions or losses of nucleotide pairs in a gene May produce frameshift mutations Figure 17.25

Mutagens : Mutagens Spontaneous mutations Can occur during DNA replication, recombination, or repair

Slide 69 : Mutagens Are physical or chemical agents that can cause mutations

What is a gene? revisiting the question : What is a gene? revisiting the question A gene Is a region of DNA whose final product is either a polypeptide or an RNA molecule

Slide 71 : A summary of transcription and translation in a eukaryotic cell Figure 17.26