Lesson Overview - East Tennessee State University

14.2

Lesson OverviewRibosomes and Protein SynthesisThe Genetic CodeThe first step in decoding genetic messages is to transcribe a nucleotidebase sequence from DNA to mRNA.This transcribed information contains a code for making proteins.

Lesson OverviewRibosomes and Protein SynthesisThe Genetic CodeProteins are made by joining amino acids together into long chains, calledpolypeptides.As many as 20 different amino acids are commonly found in polypeptides.

Lesson OverviewRibosomes and Protein SynthesisThe Genetic CodeThe specific amino acids in a polypeptide, and the order in which they arejoined, determine the properties of different proteins.The sequence of amino acids influences the shape of the protein, which inturn determines its function.

Lesson OverviewRibosomes and Protein SynthesisThe Genetic CodeRNA contains four different bases: adenine, cytosine, guanine, and uracil.These bases form a “language,” or genetic code, with just four “letters”: A,C, G, and U.

Lesson OverviewRibosomes and Protein SynthesisThe Genetic CodeEach three-letter “word” in mRNA is known as a codon.A codon consists of three consecutive bases that specify a single aminoacid to be added to the polypeptide chain.

Lesson OverviewRibosomes and Protein SynthesisHow to Read CodonsBecause there are fourdifferent bases in RNA, thereare 64 possible three-basecodons (4 4 4 64) inthe genetic code.This circular table shows theamino acid to which each ofthe 64 codons corresponds.To read a codon, start at themiddle of the circle and moveoutward.

Lesson OverviewRibosomes and Protein SynthesisHow to Read CodonsMost amino acids can bespecified by more than onecodon.For example, six differentcodons—UUA, UUG, CUU, CUC,CUA, and CUG—specify leucine.But only one codon—UGG—specifies the amino acidtryptophan.

Lesson OverviewRibosomes and Protein SynthesisStart and Stop CodonsThe genetic code has punctuationmarks.The methionine codon AUGserves as the initiation, or “start,”codon for protein synthesis.Following the start codon, mRNAis read, three bases at a time, untilit reaches one of three different“stop” codons, which endtranslation.

Lesson OverviewRibosomes and Protein SynthesisTranslationThe sequence of nucleotide bases in an mRNA molecule is a set ofinstructions that gives the order in which amino acids should be joined toproduce a polypeptide.The forming of a protein requires the folding of one or more polypeptidechains.Ribosomes use the sequence of codons in mRNA to assemble aminoacids into polypeptide chains.The decoding of an mRNA message into a protein is a process known astranslation.

Lesson OverviewRibosomes and Protein SynthesisSteps in TranslationMessenger RNA is transcribed in the nucleus and then enters thecytoplasm for translation.

Lesson OverviewRibosomes and Protein SynthesisSteps in TranslationTranslation begins when aribosome attaches to anmRNA molecule in thecytoplasm.As the ribosome reads eachcodon of mRNA, it directstRNA to bring the specifiedamino acid into the ribosome.One at a time, the ribosomethen attaches each amino acidto the growing chain.

Lesson OverviewRibosomes and Protein SynthesisSteps in TranslationEach tRNA molecule carries justone kind of amino acid.In addition, each tRNA moleculehas three unpaired bases,collectively called theanticodon—which iscomplementary to one mRNAcodon.The tRNA molecule formethionine has the anticodonUAC, which pairs with themethionine codon, AUG.

Lesson OverviewRibosomes and Protein SynthesisSteps in TranslationThe ribosome has a secondbinding site for a tRNA moleculefor the next codon.If that next codon is UUC, atRNA molecule with an AAGanticodon brings the amino acidphenylalanine into theribosome.

Lesson OverviewRibosomes and Protein SynthesisSteps in TranslationThe ribosome helps form apeptide bond between the firstand second amino acids—methionine and phenylalanine.At the same time, the bondholding the first tRNA moleculeto its amino acid is broken.

Lesson OverviewRibosomes and Protein SynthesisSteps in TranslationThat tRNA then moves into athird binding site, from which itexits the ribosome.The ribosome then moves tothe third codon, where tRNAbrings it the amino acidspecified by the third codon.

Lesson OverviewRibosomes and Protein SynthesisSteps in TranslationThe polypeptide chaincontinues to grow until theribosome reaches a “stop”codon on the mRNAmolecule.When the ribosome reachesa stop codon, it releases boththe newly formed polypeptideand the mRNA molecule,completing the process oftranslation.

Lesson OverviewRibosomes and Protein SynthesisThe Roles of tRNA and rRNA inTranslationRibosomes are composed of roughly 80 proteins and three or fourdifferent rRNA molecules.These rRNA molecules help hold ribosomal proteins in place and helplocate the beginning of the mRNA message.They may even carry out the chemical reaction that joins amino acidstogether.

Lesson OverviewRibosomes and Protein SynthesisThe Molecular Basis of HeredityMost genes contain instructions for assembling proteins.

Lesson OverviewRibosomes and Protein SynthesisThe Molecular Basis of HeredityMany proteins are enzymes, which catalyze and regulate chemicalreactions.A gene that codes for an enzyme to produce pigment can control the colorof a flower. Another gene produces proteins that regulate patterns of tissuegrowth in a leaf. Yet another may trigger the female or male pattern ofdevelopment in an embryo.Proteins are microscopic tools, each specifically designed to build oroperate a component of a living cell.

Lesson OverviewRibosomes and Protein SynthesisThe Molecular Basis of HeredityMolecular biology seeks to explain living organisms by studying them at themolecular level, using molecules like DNA and RNA.The central dogma of molecular biology is that information istransferred from DNA to RNA to protein.There are many exceptions to this “dogma,” but it serves as a usefulgeneralization that helps explain how genes work.

Lesson OverviewRibosomes and Protein SynthesisThe Molecular Basis of HeredityGene expression is the way in which DNA, RNA, and proteins areinvolved in putting genetic information into action in living cells.DNA carries information for specifying the traits of an organism.The cell uses the sequence of bases in DNA as a template for makingmRNA.

Lesson OverviewRibosomes and Protein SynthesisThe Molecular Basis of HeredityThe codons of mRNA specify the sequence of amino acids in a protein.Proteins, in turn, play a key role in producing an organism’s traits.

Lesson OverviewRibosomes and Protein SynthesisThe Molecular Basis of HeredityOne of the most interesting discoveries of molecular biology is the nearuniversal nature of the genetic code.Although some organisms show slight variations in the amino acidsassigned to particular codons, the code is always read three bases at atime and in the same direction.Despite their enormous diversity in form and function, living organismsdisplay remarkable unity at life’s most basic level, the molecular biology ofthe gene.

Ribosomes and Protein Synthesis. Steps in Translation . Each tRNA molecule carries just one kind of amino acid. In addition, each tRNA molecule has three unpaired bases, collectively called the . anticodon —which is complementary to one mRNA codon. The tRNA molecule for methionine has the anticodon UAC, which pairs with the methionine codon, AUG.