CHAPTER 17 FROM GENE TO PROTEIN Section C: The
CHAPTER 17FROM GENE TO PROTEINSection C: The Synthesis of Protein1. Translation is the RNA-directed synthesis of a polypeptide: a closer look2. Signal peptides target some eukaryotic polypeptides to specific destinationsin the cell3. RNA plays multiple roles in the cell: a review4. Comparing protein synthesis in prokaryotes and eukaryotes: a review5. Point mutations can affect protein structure and function6. What is a gene? revisiting the questionCopyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
1. Translations is the RNA-directedsynthesis of a polypeptide: a closer look In the process of translation, acell interprets a series of codonsalong a mRNA molecule. Transfer RNA (tRNA)transfers amino acids from thecytoplasm’s pool to a ribosome. The ribosome adds eachamino acid carried by tRNAto the growing end of thepolypeptide chain.Fig. 17.12Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
During translation, each type of tRNA links amRNA codon with the appropriate amino acid. Each tRNA arriving at the ribosome carries aspecific amino acid at one end and has a specificnucleotide triplet, an anticodon, at the other. The anticodon base-pairs with a complementarycodon on mRNA. If the codon on mRNA is UUU, a tRNA with an AAAanticodon and carrying phenyalanine will bind to it. Codon by codon, tRNAs deposit amino acids in theprescribed order and the ribosome joins them into apolypeptide chain.Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Like other types of RNA, tRNA molecules aretranscribed from DNA templates in the nucleus. Once it reaches the cytoplasm, each tRNA is usedrepeatedly to pick up its designated amino acid in the cytosol, to deposit the amino acid at the ribosome, and to return to the cytosol to pick up another copy of thatamino acid.Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
A tRNA molecule consists of a strand of about 80nucleotides that folds back on itself to form athree-dimensional structure. It includes a loop containing the anticodon and anattachment site at the 3’ end for an amino acid.Copyright 2002 Pearson Education, Inc., publishing as Benjamin CummingsFig. 17.13
If each anticodon had to be a perfect match to eachcodon, we would expect to find 61 types of tRNA,but the actual number is about 45. The anticodons of some tRNAs recognize morethan one codon. This is possible because the rules for base pairingbetween the third base of the codon and anticodonare relaxed (called wobble). At the wobble position, U on the anticodon can bindwith A or G in the third position of a codon. Some tRNA anticodons include a modified form ofadenine, inosine, which can hydrogen bond with U, C,or A on the codon.Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Each amino acid is joinedto the correct tRNA byaminoacyl-tRNAsynthetase. The 20 differentsynthetases match the 20different amino acids. Each has active sites foronly a specific tRNA andamino acid combination. The synthetase catalyzes acovalent bond between them,forming aminoacyl-tRNAor activated amino acid.Fig. 17.14Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Ribosomes facilitate the specific coupling of thetRNA anticodons with mRNA codons. Each ribosome has a large and a small subunit. These are composed of proteins and ribosomal RNA(rRNA), the most abundant RNA in the cell.Fig. 17.15aCopyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
After rRNA genes are transcribed to rRNA in thenucleus, the rRNA and proteins form the subunits inthe nucleolus. The subunits exit the nucleus via nuclear pores. The large and small subunits join to form afunctional ribosome only when they attach to anmRNA molecule. While very similar in structure and function,prokaryotic and eukaryotic ribosomes have enoughdifferences that certain antibiotic drugs (liketetracycline) can paralyze prokaryotic ribosomeswithout inhibiting eukaryotic ribosomes.Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Each ribosome has a binding site for mRNA andthree binding sites for tRNA molecules. The P site holds the tRNA carrying the growingpolypeptide chain. The A site carries the tRNA with the next amino acid. Discharged tRNAs leave the ribosome at the E site.Fig. 17.15b &cCopyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Recent advances in our understanding of thestructure of the ribosome strongly supports thehypothesis that rRNA, not protein, carries out theribosome’s functions. RNA is the main constituent at the interphase betweenthe two subunits and of the A and P sites. It is the catalyst forpeptide bond formationFig. 17.16Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Translation can be divided into three stages:initiationelongationtermination All three phase require protein “factors” that aid inthe translation process. Both initiation and chain elongation require energyprovided by the hydrolysis of GTP.Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Initiation brings together mRNA, a tRNA with thefirst amino acid, and the two ribosomal subunits. First, a small ribosomal subunit binds with mRNA and aspecial initiator tRNA, which carries methionine andattaches to the start codon. Initiation factors bring in the large subunit such that theinitiator tRNA occupies the P site.Copyright 2002 Pearson Education, Inc., publishing as Benjamin CummingsFig. 17.17
Elongation consists of a series of three stepcycles as each amino acid is added to theproceeding one. During codon recognition, an elongation factorassists hydrogen bonding between the mRNAcodon under the A site with the corresondinganticodon of tRNA carrying the appropriateamino acid. This step requires the hydrolysis of two GTP.Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
During peptide bond formation, an rRNAmolecule catalyzes the formation of a peptide bondbetween the polypeptide in the P site with the newamino acid in the A site. This step separates the tRNA at the P site from thegrowing polypeptide chain and transfers the chain,now one amino acid longer, to the tRNA at the Asite.Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
During translocation, the ribosome moves thetRNA with the attached polypeptide from the Asite to the P site. Because the anticodon remains bonded to the mRNAcodon, the mRNA moves along with it. The next codon is now available at the A site. The tRNA that had been in the P site is moved to the Esite and then leaves the ribosome. Translocation is fueled by the hydrolysis of GTP. Effectively, translocation ensures that the mRNA is“read” 5’ - 3’ codon by codon.Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
The three steps of elongation continue codon bycodon to add amino acids until the polypeptidechain is completed.Fig. 17.18Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Termination occurs when one of the three stopcodons reaches the A site. A release factor binds to the stop codon andhydrolyzes the bond between the polypeptide andits tRNA in the P site. This frees the polypeptide and the translationcomplex disassembles.Fig. 17.19Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Typically a single mRNA is used to make many copies of apolypeptide simultaneously. Multiple ribosomes, polyribosomes, may trail along thesame mRNA. A ribosome requires less than a minute to translate anaverage-sized mRNA into a polypeptide.Fig. 17.20Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
During and after synthesis, a polypeptide coils andfolds to its three-dimensional shape spontaneously. The primary structure, the order of amino acids,determines the secondary and tertiary structure. Chaperone proteins may aid correct folding. In addition, proteins may require posttranslationalmodifications before doing their particular job. This may require additions like sugars, lipids, orphosphate groups to amino acids. Enzymes may remove some amino acids or cleavewhole polypeptide chains. Two or more polypeptides may join to form a protein.Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
2. Signal peptides target some eukaryoticpolypeptides to specific destinations in thecell Two populations of ribosomes, free and bound, areactive participants in protein synthesis. Free ribosomes are suspended in the cytosol andsynthesize proteins that reside in the cytosol. Bound ribosomes are attached to the cytosolic side ofthe endoplasmic reticulum. They synthesize proteins of the endomembrane system aswell as proteins secreted from the cell.Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
While bound and free ribosomes are identical instructure, their location depends on the type ofprotein that they are synthesizing. Translation in all ribosomes begins in the cytosol, buta polypeptide destined for the endomembrane systemor for export has a specific signal peptide region ator near the leading end. This consists of a sequence of about 20 amino acids. A signal recognition particle (SRP) binds to thesignal peptide and attaches it and its ribosome to areceptor protein in the ER membrane. The SRP consists of a protein-RNA complex.Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 17.21Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
After binding, the SRP leaves and protein synthesisresumes with the growing polypeptide snakingacross the membrane into the cisternal space via aprotein pore. An enzyme usually cleaves the signal polypeptide. Secretory proteins are released entirely into thecisternal space, but membrane proteins remainpartially embedded in the ER membrane.Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Other kinds of signal peptides are used to targetpolypeptides to mitochondria, chloroplasts, thenucleus, and other organelles that are not part ofthe endomembrane system. In these cases, translation is completed in the cytosolbefore the polypeptide is imported into the organelle. While the mechanisms of translocation vary, each ofthese polypeptides has a “postal” code that ensures itsdelivery to the correct cellular location.Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
3. RNA plays multiple roles in the cell: areview The cellular machinery of protein synthesis and ERtargeting is dominated by various kinds of RNA. The diverse functions of RNA are based, in part, on itsability to form hydrogen bonds with other nucleic acidmolecules (DNA or RNA). It can also assume a specific three-dimensional shape byforming hydrogen bonds between bases in different partsof its polynucleotide chain. DNA may be the genetic material of all living cellstoday, but RNA is much more versatile.Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
The diversefunctions ofRNA range fromstructural toinformational tocatalytic.Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
4. Comparing protein synthesis inprokaryotes and eukaryotes: a review Although bacteria and eukaryotes carry outtranscription and translation in very similar ways,they do have differences in cellular machinery andin details of the processes. Eukaryotic RNA polymerases differ from those ofprokaryotes and require transcription factors. They differ in how transcription is terminated. Their ribosomes are also different.Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
In one bigdifferences,prokaryotes cantranscribe andtranslate the samegenesimultaneously. The new proteinquickly diffusesto its operatingsite.Fig. 17.22Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
In eukaryotes, the nuclear envelope segregatestranscription from translation. In addition, extensive RNA processing is insertedbetween these processes. This provides additional steps whose regulation helpscoordinate the elaborate activities of a eukaryotic cell. In addition, eukaryotic cells have complicatedmechanisms for targeting proteins to theappropriate organelle.Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
5. Point mutations can affect proteinstructure and function Mutations are changes in the genetic material of acell (or virus). These include large-scale mutations in which longsegments of DNA are affected (for example,translocations, duplications, and inversions). A chemical change in just one base pair of a genecauses a point mutation. If these occur in gametes or cells producing gametes,they may be transmitted to future generations.Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
For example, sickle-cell disease is caused by amutation of a single base pair in the gene thatcodes for one of the polypeptides of hemoglobin. A change in a single nucleotide from T to A in the DNAtemplate leads to an abnormal protein.Fig. 17.23Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
A point mutation that results in replacement of apair of complimentary nucleotides with anothernucleotide pair is called a base-pair substitution. Some base-pair substitutions have little or noimpact on protein function. In silent mutations, alterations of nucleotides stillindicate the same amino acids because of redundancy inthe genetic code. Other changes lead to switches from one amino acid toanother with similar properties. Still other mutations may occur in a region where theexact amino acid sequence is not essential for function.Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Other base-pair substitutions cause a readilydetectable change in a protein. These are usually detrimental but can occasionally leadto an improved protein or one with novel capabilities. Changes in amino acids at crucial sites, especially activesites, are likely to impact function. Missense mutations are those that still code for anamino acid but change the indicated amino acid. Nonsense mutations change an amino acid codoninto a stop codon, nearly always leading to anonfunctional protein.Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 17.24Copyright Pearson Education, Inc., publishing as Benjamin Cummings
Insertions and deletions are additions or losses ofnucleotide pairs in a gene. These have a disastrous effect on the resulting proteinmore often than substitutions do. Unless these mutations occur in multiples of three,they cause a frameshift mutation. All the nucleotides downstream of the deletion orinsertion will be improperly grouped into codons. The result will be extensive missense, ending sooner orlater in nonsense - premature termination.Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Fig. 17.24Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Mutations can occur in a number of ways. Errors can occur during DNA replication, DNA repair,or DNA recombination. These can lead to base-pair substitutions, insertions, ordeletions, as well as mutations affecting longer stretchesof DNA. These are called spontaneous mutations.Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Mutagens are chemical or physical agents thatinteract with DNA to cause mutations. Physical agents include high-energy radiation likeX-rays and ultraviolet light. Chemical mutagens may operate in several ways. Some chemicals are base analogues that may besubstituted into DNA, but that pair incorrectly duringDNA replication. Other mutagens interfere with DNA replication byinserting into DNA and distorting the double helix. Still others cause chemical changes in bases that changetheir pairing properties.Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Researchers have developed various methods totest the mutagenic activity of different chemicals. These tests are often used as a preliminary screen ofchemicals to identify those that may cause cancer. This make sense because most carcinogens aremutagenic and most mutagens are carcinogenic.Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
6. What is a gene? revisiting the question The Mendelian concept of a gene views it as adiscrete unit of inheritance that affects phenotype. Morgan and his colleagues assigned genes to specificloci on chromosomes. We can also view a gene as a specific nucleotidesequence along a region of a DNA molecule. We can define a gene functionally as a DNAsequence that codes for a specific polypeptide chain.Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Transcription,RNA processing,and translation arethe processes thatlink DNAsequences to thesynthesis of aspecificpolypeptide chain.Fig. 17.25Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Even the one gene-one polypeptide definition mustbe refined and applied selectively. Most eukaryotic genes contain large introns that haveno corresponding segments in polypeptides. Promotors and other regulatory regions of DNA are nottranscribed either, but they must be present fortranscription to occur. Our definition must also include the various types ofRNA that are not translated into polypeptides. A gene is a region of DNA whose final product iseither a polypeptide or an RNA molecule.Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Section C: The Synthesis of Protein 1.Translation is the RNA-directed synthesis of a polypeptide: a closer look 2. Signal peptides target some eukaryotic polypeptides to specific destinations in the cell 3. RNA plays multiple roles in the cell: a review 4. Comparing protein synthesis in prokaryotes and eukaryotes: a review 5.
Part One: Heir of Ash Chapter 1 Chapter 2 Chapter 3 Chapter 4 Chapter 5 Chapter 6 Chapter 7 Chapter 8 Chapter 9 Chapter 10 Chapter 11 Chapter 12 Chapter 13 Chapter 14 Chapter 15 Chapter 16 Chapter 17 Chapter 18 Chapter 19 Chapter 20 Chapter 21 Chapter 22 Chapter 23 Chapter 24 Chapter 25 Chapter 26 Chapter 27 Chapter 28 Chapter 29 Chapter 30 .
TO KILL A MOCKINGBIRD. Contents Dedication Epigraph Part One Chapter 1 Chapter 2 Chapter 3 Chapter 4 Chapter 5 Chapter 6 Chapter 7 Chapter 8 Chapter 9 Chapter 10 Chapter 11 Part Two Chapter 12 Chapter 13 Chapter 14 Chapter 15 Chapter 16 Chapter 17 Chapter 18. Chapter 19 Chapter 20 Chapter 21 Chapter 22 Chapter 23 Chapter 24 Chapter 25 Chapter 26
One Gene-One Enzyme Hypothesis (Beadle & Tatum) The function of a gene is to dictate the production of a specific enzyme One Gene—One Enzyme but not all proteins are enzymes those proteins are coded by genes too One Gene—One Protein but many proteins are composed of several polypeptides, each of which has its own gene One Gene—One Polypeptide
DEDICATION PART ONE Chapter 1 Chapter 2 Chapter 3 Chapter 4 Chapter 5 Chapter 6 Chapter 7 Chapter 8 Chapter 9 Chapter 10 Chapter 11 PART TWO Chapter 12 Chapter 13 Chapter 14 Chapter 15 Chapter 16 Chapter 17 Chapter 18 Chapter 19 Chapter 20 Chapter 21 Chapter 22 Chapter 23 .
AQA GCE Biology A2 Award 2411 Unit 5 DNA & Gene Expression Unit 5 Control in Cells & Organisms DNA & Gene Expression Practice Exam Questions . AQA GCE Biology A2 Award 2411 Unit 5 DNA & Gene Expression Syllabus reference . AQA GCE Biology A2 Award 2411 Unit 5 DNA & Gene Expression 1 Total 5 marks . AQA GCE Biology A2 Award 2411 Unit 5 DNA & Gene Expression 2 . AQA GCE Biology A2 Award 2411 .
this genotype is caused by more than one gene because there are 4 phenotypes not 3 in F2 (9:3:3:1) Ð1 gene F2 would have 3 phenotypes 1:2:1 ratio Complementary Gene Action : one good copy of each gene is needed for expression of the final phenotype Ð9:7 ratio Epistasis : one gene can mask the effect of another gene
About the husband’s secret. Dedication Epigraph Pandora Monday Chapter One Chapter Two Chapter Three Chapter Four Chapter Five Tuesday Chapter Six Chapter Seven. Chapter Eight Chapter Nine Chapter Ten Chapter Eleven Chapter Twelve Chapter Thirteen Chapter Fourteen Chapter Fifteen Chapter Sixteen Chapter Seventeen Chapter Eighteen
18.4 35 18.5 35 I Solutions to Applying the Concepts Questions II Answers to End-of-chapter Conceptual Questions Chapter 1 37 Chapter 2 38 Chapter 3 39 Chapter 4 40 Chapter 5 43 Chapter 6 45 Chapter 7 46 Chapter 8 47 Chapter 9 50 Chapter 10 52 Chapter 11 55 Chapter 12 56 Chapter 13 57 Chapter 14 61 Chapter 15 62 Chapter 16 63 Chapter 17 65 .
