BIOL FOR SCI MAJ I
BIOL FOR SCI MAJ I BIOL 1201
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This 29 page Class Notes was uploaded by Cassie Koepp on Tuesday October 13, 2015. The Class Notes belongs to BIOL 1201 at Louisiana State University taught by Steven Pomarico in Fall. Since its upload, it has received 22 views. For similar materials see /class/222835/biol-1201-louisiana-state-university in Biological Sciences at Louisiana State University.
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Date Created: 10/13/15
Biology 1201 Dr Pomarico 1 Exam 2 Review Chapter 5 1 Which of the following are not polysaccharides a glycogen b starch c chitin d hydrocarbon 2 Which of the following are not true of pentoses a they are found in nucleic acids b they can occur in ring structure c they have the formula C5H1205 d they may be an aldose or a ketose 3 Polymerization is a process that a creates bonds between amino acids in the formation of a peptide chain b involves the removal of a water molecule c links the phosphate of one nucleotide with the sugar of the next d all of the above 4 Disaccharides can differ from each other in all of the follwing ways except a in the number of their monosaccharides b as Enantiomers c in the location of the glycosidic linkage d in their structural formulas 5 Which of the follwing is not true of cellulose a It differs from starch because of the con guration of glucose and the geometry of the glycosidic linkage b it maybe hydrogen bonded to neighboring cellulose molecules to form micro brils c few organisms have enzymes that hydrolyze its glycosidic linkages d its monomers are glucose with nitrogen containing appendages 6 Plants store most of their energy as a glucose b glycogen c starch d cellulose 7 What happens when a protein denatures a it loses its primary structure b it loses its secondary and tertiary structure c it hydrolyzes into component amino acids d it becomes irreversibly insoluble and precipitates 8 The alpha helix of protein is a part of the tertiary structure and is stabilized by disulfide bridges b a double helix c stabilized by hydrogen bonds and commonly found in fibrous proteins d found in some regions of globular proteins and stabilized by hydrophobic interactions 9 Pleated sheets are characterized by a disul de bridges between amino acids b parallel regions of the polypeptide chain held together by hydrophobic interactions c folds stabilized by hydrogen bonds between segments of polypeptide chains membrane sheets oomposed of phospholipids 10 How are the nucleotide monomers connected to form a polynucleotide a oovalent bonds between the sugar ofone nucleotide and the phosphate of the next b ester linkages between the carboxyl group ofone nucleotide and the hydroxyl group on the ribose of the next c hydrogen bonds between complementary nitrogenous base pairs d ionic attractions between phosphate groups HZN CNH 11 What is the best description of this molecule a chitin b amino acid c polypeptide d nucleotide 12 If the nucleotide sequenoe ofone strand ofa DNA helix is 339 GCCTAA 539 what would be the sequence on the complementary strand a 39 GC 39 d 339 CGGA39I39I39 539 13 A molecule with the chemical formula C15H32015 is probably a a carbo ydrate b protein c nucleic acid d hydrocarbon 14 Which of the following is are me for the class of large biological molecules known as lipids a they are insoluble in water b they are an important constituent ofoell membranes c they oontain twice as much energy as an equivalent weight of polysaocharide d all of the above 15 Saturated fatty acids a are the predominant fatty acid in oom oil b have double bonds between carbon aton39s of the fatty acids c have a higher ratio of hydrogen to carbon than do unsaturated fatty acids d are usually liquid at room temperature Biology 1201 Dr Pomarico 3 16 What is the term used for a change in a protein s three dimensional shape or conformation due to disruption of hydrogen bonds disul de bridges or ionic bonds a hydrolysis b destabilization c renaturation d denaturation 17 Which of the following are nitrogenous bases of the purine type a cytosine and guanine b guanine and adenine c adenine and thymine d thymine and uracil 18 The difference between the sugar in DNA and the sugar in RNA is that the sugar in DNA a is a six carbon sugar and the sugar in RNA is a five carbon sugar b can form a double stranded molecule c can attach to a phosphate d contains one less oxygen 19 In a hydrolysis reaction and water is a monomers are assembled to produce a polymer produced b a polymer is broken up into its constituents monomers produced c a polymer is broken up into its constituents monomers consumed d monomers are assembled to produce a polymer consumed Chapter 6 20 All of the following are part of a prokaryotic ce except a an endoplasmic reticulum b a cell wall c a plasma membrane d ribosomes 21 The pores in the nuclear envelope provide for the movement of a proteins into the nucleus b ribosomal components out of the nucleus c mRNA out of the nucleus d all of the above 22 Which of the following is incorrectly paired with its function a peroxisome contains enzymes that break down H20 b ECM extracellular matrix supports and anchors ces communicates between inside and outside of the cell c Lysosome contains pigments in plant cells d Golgi apparatus processes tags and ships ceuar products 23 The innermost portion of a mature plant cell wall is the a primary cell wall b secondary cell wall c middle amea d pasmodesmata 24 Of the following which is probably the most common route fir membrane flow in the endomembrane system a rough ER9Golgi9lysosomes9nuclear membrane9plasma membrane b rough ER9transport vesicles9Golgi9vesicles9plasma membrane c nuclear envelope9 rough ER9Golgi9smooth ER9lysosomes d rough ER9 vesicles9Golgi9smooth ER9pasma membrane 25 The volume enclosed by the plasma membrane of plant cells is often much larger than the corresponding volume in animal cells The most reasonable explanation for this observation is th t a a plant cells are capable of having a much higher surface to volume ratio than animal cells b plants have a much more highly convoluted folded plasma membrane than animal cells c plant cells contain a large vacuole that reduces the volume of the cytoplasm d animal cells are more spherical while plant cells are elongated 26 Large numbers of ribosomes are present in cells that specialize in producing which of the following molecules a lipids b starches c proteins d steroids 27 Of the following which is probably the most common route for membrane ow in the endomembrane system a Golgi 9 Lysosome 9 ER 9 plasma membrane b ER 9 chloroplasts 9 mitochondrion 9 cell membrane c Nuclear envelope 9 Lysosome 9 Golgi 9 plasma membrane d rough ER 9 vesicles 9 Golgi 9 plasma membrane 28 Which of the following are capable of converting light energy into chemical energy a chloroplasts b mitochondria c Golgi bodies d lysosomes 29 Which of the following pairs is mis matched a nucleolous ribosomal RNA b nucleus DNA replication c Lysosome protein synthesis d cell membrane lipid bilayer 30 is composed of DNA and protein a mitochondrion b ribosome c centriole d chromosome Chapter 7 31 All of the following are part of the cell membrane except a lipids b nucleic acids c phosphate groups d proteins Biology 1201 Dr Pomarico 5 32 The presence of cholesterol in the plasma membrane of some animals a enables the membrane to stay uid more easily when cell temperature drops b makes the membrane less exible allowing it to sustain greater pressure from within the cell c enables the animal to remove hydrogen atoms from saturated phospholipids d enables the animal to add hydrogen atoms from unsaturated phospholipids 33 According to the fluid mosaic model of cell membranes which of the following is a true statement about membrane phospholipids a they can move laterally along the plane of the membrane b they bond with ions to transport them through the membrane c They have hydroplilic tails in the interior of the membrane d They occur in an uninterrupted bilayer with membrane proteins restricted to the surface of the membrane 34 The surface of an integral membrane protein would be best described as a hydrophilic b hydrophobic c amphipathetic d completely covered with phospholipids 35 Which of the following is a reasonable explanation for why unsaturated fatty acids help keep any membrane more fluid at lower temperatures a Unsaturated fatty acids permit more water in the interior of the membrane b The double bonds form a kink in the fatty acid tail forcing adjacent lipids to be further apart c The double bonds block interaction among the hydrophilic head groups of the lipids d Unsaturated fatty acids permit more water in the interior of the membrane 36 Which of the following would likely move through the lipid bilayer of a plasma membrane most rapidly a glucose b K c an amino acid d C02 37 The movement of a substance across a biological membrane against its concentration gradient with the help of energy input is a diffusion b active transport c osmosIs d facilitated diffusion 38 A single layer of phospholipid molecules coats the water in a beaker Which part of the molecules will face the air a the phosphate groups b the hydrocarbon tails c the phospholipids would dissolve in the water and not form a membrane coat d the gylocolipid regions 39 The fluidity of membranes in a plant in cold weather may be maintained by a increasing the number of phosphoipids with saturated hydrocarbon tails b increasing the concentration of cholesterol in the membrane c increasing the proportion of peripheral proteins d increasing the number of phospholipids with unsaturated hydrocarbon tails 40 A plant cell placed in a hypotonic environment will a plasmolyze b shrivel c become turgid d become flaccid 41 Ions diffuse across membranes down their a chemical gradients b concentration gradients c electrical gradients d electrochemical gradients 42 Which of the following is not true of the carrier molecules in facilitated diffusion a they increase the speed of transport across a membrane b they can concentrate solute molecules on one side of the membrane c they may have specific binding sites for the molecules they transport d they may undergo a conformational change upon binding of solute 43 Cotransport may involve a active transport of two solutes through a transport protein b passive transport of two solutes through a transport protein c transport of one solute against its concentration gradient in tandem with another that is diffusing down its concentration gradient d ion diffusion against the electrochemical gradient 44 The proton pump in plant cells is the functional equivalent of an animal cell s a cotransport mechanism b sodium potassium pump c contractile vacuole for osmoregulation d ATP pump 45 All of the following processes take material into cells except a endocytosis b exocytosis c active transport d carrier facilited diffusion 46 Pinocytosis involves a the fusion of a newly formed food vacuole with a Lysosome b receptor mediated endocytosis and the formation of vesicles c the pinching in of the plasma membrane around small droplets of external fluid d the accumulation of specific large molecules in a cell 47 Watering a houseplant with too concentrated a solution of fertilizer can result in wilting because a the uptake of ions into plant cells makes the cells hypertonic b the soil solution becomes hypertonic causing the cells to lose water Biology 1201 Dr Pomarico 7 c diffusion down the electrochemical gradient will cause a disruption of membrane potential and accompanying loss of water d the plant will grow faster than it can transport water and maintain proper water balance 48 Which of the following is the most probable description of an integral transmembrane protein a amphipatheitc with a hydrophilic head and a hydrophobic tail region b a globular protein with hydrophobic amino acids in the interior and hydrophilic amino acids arranged around the outside c a glycoprotein with oligosaccharides attached to the portion of the protein facing the exterior of the cell and cytoskeletal elements facing inside the cell d a middle region composed of a helical stretches of hydrophobic amino acids with the hydrophilic regions at both ends of the protein Chapter 8 49 Which term most precisely describes the cellular process of breaking down large molecules into smaller ones a metabolism b anabolism c dehydration d catabolism 50 Which of the following is are true for anabolic pathways a They do not depend on enzymes b they are highly regulated sequences of chemical reactions c they release energy as they degrade polymers to monomers d they release energy as they build up polymers from monomers 51 According to the first law of thermodynamics a for every action there is an equal and opposite reaction b the total amount of energy in the universe is conserved or constant c potential energy is converted to kinetic energy and kinetic energy is converted to heat d energy can be transferred or transformed but disorder always increases Chapter 1 Themes in the Study of Life 8252010 74200 PM 1 Evolution is a central theme in biology o A change in the population of organisms over time o The kind of change a change in the genetic make up in a population how often certain genes occur in a population of organisms in that species 0 How much time generational time which changes from each organism when organism is on planet until it can reproduce o Variability is the norm with respect to populations 0 Some are better able to survive and reproduce The ones that are more suited are more likely to be passing on their genes to the next generation and so those genes are more likely to pass on which starts to change the group of organisms in that generation as a whole o Very dependent on environmental pressures o Not about religion but about the sequence of events and change that occur 2 Hierarchy of Biological Organization o Structure of biological organization picture in outline 0 Based on the way the smaller pieces ineract o Gives a building mechanism 3 Emergent Properties o Each level of biological organization property that emerges as a result of interaction between components 0 Examples are in outline o Interactions of components bring forth new components that aren t in the primary components 4 Energy capacity to do work o life has the ability to acquire material and energy material and energy are often transformed we are open systems goes in and out of us thermodynamics all organisms are apart of it 5 Structure and Function related to one another 0 O O O o Can guess function of something based on the structure like the bones of the hand 6 Cellular basis of life o prokaryotic cells one room houseare cells which lark internal cellular membranes 0 no nucleus or organelles o in terms of regulation and how the events occur is actually more complicated than in a eukaryotic cell c eukaryotic cell cells are cells which have a membrane enclosed nucleus and membraneenclosed organelles 0 may be a bit more simpler 7 DNA the bases of heritable information o how the cell runs its busy o large majority of how cell functions Is determined o If its alive It uses DNA o Inheritance acquistion of trait by transmission of DNA parent to offspring o Transmitting DNA to new generations is built into functions of reproduction 8 Growth and Development o instructions provided by DNA o multicellular organism 9 ability to maintain their structure and regulate o Homeostasisability to make and regulate an internal environment 0 Function with incredibly high efficiency Body temperature 0 Uses negative and positive feedback How is the Diversity of Life Arranged o Arrange so we are able to study it better o Give us as much of a power of inference that we can get o The Three Domain System 0 Bacteriaprokaryotic o Archaeaprokaryotic o Eukaryaeukaryotic Protistia Plantae Fungi Animalia Divison and Phlyum same size set of the hierarchy o Phylumjust animals o Divisoneverything else Naming of an organism Genus and species taxonomy Science is an activity that often involves one of 2 methods discovery science and hypothesis based science o Natural causality o Obey natural laws o Natural Laws don t change with time or distance DATA o Quantitative data o Qualitative datadescriptive The scientific Methodlogical thought process o Hypotheticodeductive o Formal set of steps 0 Hypotheis possible explanation Theorystrongly verified explanation for a phenomenon accounts for many facts and explains a verity of phenomena lawabsolutely proven O 8252010 74200 PM 8252010 74200 PM Crossing Over Gametes Bva X bbvv Expected F1 12 Bva amp 12 bbvv Actual F1 out of 2300 975normalBva 944 black vestigal bbvv 206other recombinants 185other recombinants Why aren t all the offspring parental types Meiosis crossing over More space in betweenmore space of crossing over Recombination frequency3 91 total recombinant 17 9 17 map units apart 2300 total Nondisjuction Meiosis 1 abnormal n1 n1 n1n1 gametes Meiosis 2 abnormal n1 n1 n n gametes Complete dominance normal heterozygous amp homozygous dominant show dominant phenotype Incomplete dominance heterozygous not complete eX White ower red ower pink ower Epistasis one gene alters the phenotype of another gene eX mouse fur Bbblack or brown amp Ccpigment or no pigment ifno pigment NO color shows BBcc Pleiotropy one gene is responsible for multiple phenotypes Polygenic one phenotype is given by multiple alleles eX skin color Codominence both alleles show up fully eX bloodAB DNA replication Helicaseunwinds DNA Binding protein holds strands apart Primase joins RNA nucleotides to make a short stretch of RNA starting point for making new DNA DNA polymerase adds DNA nucleotides to make DNA strands along the old ones Can ONLY move 3 5 making 5 3 DNA leading strandthe replication fork DNA polymerase 3 lagging strandaway from the replication fork DNA polymerase 2 DNA polymerase 1 replaces RNA nucleotides Ch 17 1 transcription DNA pre mRNA 2 RNA processing pre mRNA altered and mRNA created Translation mRNA leaves nucleus ribosome reads amp links amino acids together Thus making a polypeptide 539quot Transcription RNA polymerase attaches to promoter does segment at a time in the 3 5 direction creating a 5 3 RNA strand it releases the segment of RNA as it is made and moves on down the template strand RNA processing 5 and 3 untranslated regions are modified 5 9modified by guanine 3 9modified by 30200 adenine the coding segment is from the start codon to the stop codon removal of noncoding segments within by splicosomes snRNP protein Translation initiation small ribosomal subunit binds to leader 5 Methianine binds to AUG start codon Psite Initiation complex small and large ribosomal subunits and mRNA elongation sites on the large ribosomal subunit E where the anticodons eXit P9 where the amino acids bind together A where the amino acids bind to the codons termination UAA UGA UAG stop codons release factorreleases the polypeptide chain when it binds to the stop codon Chapter 17 FROM THE GENE TO THE PROTEIN DNA is the instructions program that tell the cell what to do Proteins are the results of those instructions Molecular Genetics The Protein vs DNA Debate 1611 gtgtgtgtgtgtThe study of metabolic defects provided evidence that genes lead to proteins Archibald Garrod first proposed this relationship in 1909 suggested that genes dictated phenotypes via enzymes inherited diseases were the result of the lack of an enzyme gtgtgtHow genes control metabolism Continuing to Link Genes to Chemicals Muller Beable and Tatum 1612 The suggestion of Garrod were con rmed in the 193039s George Beadle and Edward Tatum conducted experiment with yeast Neurosporacrassa and demonstrated that different mutants generated by Xrays had the pathway of arginine an amino acid synthesis blocked at different steps see fig 172 medium ornithine citrulline arginine MM no Wildtype yeast could survive onminimal medium The mutants were auxotrophs that required added nutrients Auxotrophs Beadle and Tatum deduced that the 3 different mutant types each lacked a different enzyme in the pathway that synthesizes arginine From these results they formulated the one geneone enzyme hypothesis This has since been modified to the one geneone polypeptide hypothesis because Not all proteins are enzymes Many enzymes are comprised of2 polypeptide subunits gtgtgtgtgtgtThe steps from DNA to protein see fig 173 Transcription and Translation An Overview 1711 DNA gt RNA gt Proteins gtgtgtStep one DNA to RNA This process is called transcription Transcription The RNA is complementary to the DNA RNA that is synthesized from a gene coding for a protein is called messenger RNA mRNA DNA verses RNA Similar because they are both polymers of nucleotides Structural differences sugar deoxyribose DNA verses ribose RNA base thymine DNA is replaced by uracil RNA gtgtgtStep two RNA to protein This process is called translation Translation Why translation Different language of nucleic acids and proteins 4 bases in nucleic acids 20 amino acids in proteins Translation occurs on the ribosomes ln prokaryotes since there is no nucleus transcription and translation occur in rapid succession ln eukaryotes the two processes are separated in time and space Transcription occurs in the nucleus The mRNA must be modified before moving into the cytoplasm This modification process is called RNA processing and occurs only in eukaryotes Translation occurs in the cytoplasm gtgtgtgtgtgtTranscription of DNA to RNA see fig 177 Transcription RNA Formation from the DNA Template 1712 Like DNA replication transcription of a DNA sequence to mRNA for a protein occurs by building the new molecule in a 539 gt 339 direction This means reading the template in the 339 gt 539 direction During transcription ofa gene only one strand template strand of the DNA39s two strands is read Different genes use different strands as the template strand The enzyme RNA polymerase catalyzes transcription This enzyme 1 Separates the DNA helix at a speci c sequence see fig 178 The initiation sequence aka startpoint 2 Enzymatically synthesizes the RNA chain that is complementary to the 339 gt 539 DNA strand 3 Stops the synthesis at the terminator The initiation site gene terminator make up the transcription unit see fig 177 Transcription unit gtgtgtlnitiation of transcription see fig 178 RNA polymerase binds to the DNA at the promoter region of the gene Promoter ln eukaryotes RNA polymerase cannot recognize the promoter without the help of transcription factors Transcription factors The transcription of eukaryote mRNA by RNA polymerase usually requires a specific transcription factor that binds to a DNA region known as a TATA box TATA box The RNA polymerase recognizes the TATA transcription factorDNA complex and binds the DNA Other transcription factors or initiation factors may bind before transcription begins The active RNA polymerase separates the two DNA strands at the initiation site and transcription begins gtgtgtElongation ofthe RNA strand Once transcription begins RNA polymerase performs two functions 1 Untwisting the DNA double helix for about ten nucleotides to expose the DNA template 2 Catalyze the linkage of new RNA nucleotide to the 339 end of the RNA polymer Elongation of mRNA occurs at about 30 60 nucleotides per second As elongation proceeds the RNADNA base pairs separates the DNADNA double helix reforms Several sequential RNA transcripts can be generated from a single gene ie as one transcript is being initiated others may be at various stages ofelongation gtgtgtTermination of transcription Transcription Termination and RNA Protection 1713 At the end ofthe DNA transcription unit for a gene there is a terminator Terminator May involve the interaction oftermination factors with the DNA gtgtgtgtgtgtEukaryotic cells modify RNA after transcription Before eukaryotic mRNA is exported from the nucleus it is processed in two ways 1 Both ends are covalently altered see g 179 2 Intervening sequences introns are removed and the remaining sequence is spliced together gtgtgtModi cation of the mRNA ends During mRNA processing both the 539 and 339 ends are modi ed The 539 end has a 539 cap added 539 cap protects the mRNA from degradation acts with the leader sequence to bind the mRNA to ribosomes The 339 end of the mRNA has a polyA tail added PoyA tail inhibits degradation aids in export of mRNA from the nucleus attached to a trailer sequence at the end ofthe mRNA gtgtgtRNA splicing Posttranscriptional Modification RNA Splicing 1714 ln eukaryote the original RNA transcript is the complement ofthe DNA sequence for the gene however the functional mRNA in the cytoplasm is much shorter Between transcription and translation the RNA is processed to remove parts ofthe sequence see fig 1710 The original transcript is a precursor mRNA or premRNA The DNA and the complementary premRNA consists of the coding sequence interrupted by noncoding segments called intervening sequences or introns ntrons The coding sequences of the DNA and premRNA are known as exons because they are the sequence that is expressed as proteins Exons The exon regions often code for different domains ofa protein see g 1712 domains Because these domains are on separate exons the opportunity for a coded function to be swapped from one gene to another is greater RNA splicing as part of the RNA processing that occurs before the mRNA leaves the nucleus removes the introns from the hnRNA RNA splicing the boundary ofthe exons and introns is marked by sequences known as splicing sites RNA splicing also occurs during posttranscriptional modification oftRNA andrRNA RNA splicing involves enzymes and other protein factors In addition small nuclear ribonucleoproteins snRNPs play a key role Several snRNPs along with other proteins assemble to make a spliceosomesee g 1711 Spliceosome The spliceosome brings together the exons and excises the intron The exons are joined and the excised intron is released as a lariatshaped loop Differential splicing removal ofall versus some introns can result in to different mRNA products from a single premRNA transcript gtgtgtgtgtgtTranslation is the RNAdirected synthesis of polypeptides Translation Ribosomal and Transfer RNA 1721 During translation proteins are synthesized according to the genetic message of sequential codons in the mRNA Each threenucleotide triplet forms a codon Codon transfer RNA tRNA acts as the quotinterpreterquot between the nucleotide language of mRNA of the amino acid language of proteins In part of this role as interpreter the tRNA must read the mRNA This is accomplished by the anticodon portion of the tRNA see fig 1714 Anticodon The Role of Transfer RNA Charging a tRNA Molecule 1722 The other portion the tRNA s role as interpreter is to transfer the correct amino acid from the cytoplasmic pool of amino acids to the ribosome for protein synthesis This is possible because each tRNA is specific for a single amino acid The specificity ofthis correct pairing is accomplished by a group of enzymes known as aminoacyItRNAsynthetases see g 1715 AminoacyltRNAsynthetases This is a twostep process 1 Activation of the amino acid 2 Transfer of the activated amino acid to the tRNA This process occurs before the anticodon pairs up with the codon on the mRNA Using this one codon gt one anticodon gt one amino acid method the gene is decoded to protein see fig 1713 gtgtgtThe ribosome is where proteins are built The ribosome coordinates the pairing oftRNA anticodons with mRNA codons Eukaryotic ribosome structure see fig 1716 Two subunits large and small Composed of 60 ribosomal RNA rRNA and 40 protein The ribosomal subunits are made in the nucleolus The subunits combine as a ribosome only when they are translating a protein In addition to the mRNA binding site the groove between subunits the ribosome also has 3 tRNA binding sites E P and A The tRNA discharge or exit E site releases the tRNA once the peptide chain has been transferred to the next amino acid The peptidyltRNA binding P site holds the tRNA with the polypeptide chain attached The aminoacyltRNA binding A site hold the aminoacyltRNA with the next amino acid to be added The ribosome holds all the components together as the next amino acid is transferred to the growing polypeptide chain gtgtgtgtgtgtln the genetic code a triplet of nucleotides specifies an amino acid Translation Initiation Events 1723 Dictated by the math since there are 4 nucleotides and 20 amino acids if it was a 11 relationship then only 4 amino acids would be needed a 21 would result in 16 possible amino acids 42 as a 31 there could be as many as 64 amino acids 43 So a triplet of nucleotides is the smallest size that could code for all the amino acids gtgtgtThe genetic quotcodequot se fig 175 61 of the 64 possible triplet code for amino acids The remaining three triplets signal the translation to stop Since there are only 20 amino acids more than one triplet can code for the same amino acid This relationship is known as redundancy and usually the codons differ only at the third position There is no ambiguity in the triplet code since a given triplet codes for one and only one amino acid The correct ordering and grouping of nucleotide is an important aspect ofthe translation ofthe triplet codons The correct ordering is the reading frame Reading frame The genetic code is nearly universal see fig 176 There are few eukaryotic exceptions The genes in the mitochondria and the chloroplast can vary The rst two positions of the anticodon compliment the codon exactly the third position allows wobble in the basepairing Wobbe Because ofwobble only 45 different tRNAs are needed to complement the 64 possible codons gtgtgtBuilding a polypeptide Protein synthesis occurs in three stages 1 Initiation 2 Elongation 3 Termination Initiation of translation see g 1717 1 Assembly omeNA initiatortRNA with methionine and small ribosome subunit lnitiator tRNA binds to the mRNA start codonAUG Requires protein initiation factors 2 Large ribosome subunitjoins complex and the initiatortRNA is in the P site The A site and E site are empty The elongation cycle oftranslation see fig 1718 Translation Elongation The Initiation of Elongation 1724 1 The next tRNA occupies the A site and the anticodon hydrogen bonds to the codon ofthe mRNA energy for this step is provided by the hydrolysis of GTPgt GDP P require a protein elongation factor I Peptide bond formation the methionine from the initiator tRNA forms a peptide bond with the amino acid on the tRNA at the A site this reaction as catalyzed by a ribozyme this leaves the tRNA at the P site with no amino acid and the tRNA at the A site with a dipeptide attached A Translocation the empty tRNA moves to the E site and leaves the mRNA with the attached tRNA moves through ribosome in 539 gt 339 direction This translocates the tRNA with the growing peptide from the A site to the P site energy for this step is provided by the hydrolysis of GTPgt GDP P eongation proceeds at 1 million amino acids per minute The termination oftranslation see fig 1719 Elongation Continued and Termination 1725 1 The mRNA reaches a stop codon UAA UAG or UGA 2 A release factor binds to the A site there is no amino acid associated with the release factor 3 The peptidyltransferase adds H20 instead ofan amino acid to the end the the polypeptide chain this frees polypeptide from tRNA in P site gtgtgtPoyribosomes can quickly make many copies of a protein from a single mRNA see fig 1720 Polypeptide Destinations Signal Peptides and ER Ribosomes 1731 Poyribosomes Once a ribosome passes the initiation codon a new ribosome can bind and begin translation Several ribosomes may translate an mRNA at once making many copies of a polypeptide gtgtgtProteins are targeted for specific destinations by signal peptides see g 1721 lfa protein is to be inserted into a membrane integral proteins orto be shipped to a speci c compartment or out of the cell then it is directed to the correct location by a signal peptide The signal peptide combines with a signal recognition particle SRP to direct the protein to the proper location signal peptide signal recognition particle SRP gtgtgtCoupled transcriptiontranslation in prokaryotes see fig 1722 In prokaryotes the mRNA may be translated as soon as the initiation codon is transcribed No nucleusto separate the new mRNA from the ribosomes No mRNA processing splicing or modi cation needed in prokaryotes gtgtgtgtgtgtPoint mutations can affect the function of a protein see g 1723 Genetic Mutation 1541 Genetic Mutation Different Forms of Point Mutations 1542 Point mutations gtgtgtTypes of point mutations A Substitutions Substitutions May have little or no effect Change may not change amino acids wobble and redundancy Different amino acid may not cause a change Change may be drastic Basepair substitution mutations are usually missense mutations or nonsense mutations Missense mutations Nonsense mutations B Insertion or Deletion Genetic Mutation Insertion and Deletion 1543 lnsertion Deetion Both insertions and deletions can result in a frameshift mutation Frameshift mutations gtgtgtMutagenesis Mutagenesis may be spontaneous or more often is the result of mutagens Mutagens Regardless of the cause of mutations the most common phenotypic effect is cancerous cell growth
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