Exam 4 Study Guide Chapter 13 Section 13 1 Translation of mRNA Depends on Ribosomes and Transfer RNAs Translation of mRNA is the biological polymerization of amino acids into polypeptide chains Translation requires four parts amino acids messenger RNA mRNA ribosomes and transfer RNA tRNA The central question in translation is how triplet ribonucleotides of mRNA direct specific amino acids into their correct position in the polypeptide This was answered by transfer RNA tRNA The tRNA is a class of molecules adapts specific triplet codons in mRNA to their correct amino acids Ribosomes consist of ribosomal proteins and ribosomal RNAs rRNAs o The rRNAs perform important catalytic functions associated with translation They have a large subunit and a small subunit o When the two subunits are associated with each other in a single ribosome the structure is sometimes called a monosome The rRNA aka rDNA genes are clustered at the chromosomal NOR region and they are organized as tandem repeats separated by a non coding spacer DNA sequence The tRNAs are small in size and are very stable They are composed of 75 to 90 nucleotides displaying a nearly identical structure in bacteria and eukaryotes The tRNAs are transcribed and then posttrancriptionally modified which are bases that enhance hydrogen bonding efficiency during translation Robert Holley reported the complete sequence of the tRNA molecule isolated from yeast and proposed that the two dimensional cloverleaf model of tRNA and the anticodon loop from the nitrogenous base of hydrogen bonds Before translation can proceed the tRNA molecules must be chemically linked to their respective amino acids This activation process called charging occurs under the direction of enzymes called aminoacyl tRNA synthetases Originally there were 20 amino acids and 61 triplets that encode amino acids so therefore there must be 20 tRNA molecules and 61 specific tRNAs and enzymes However due to the wobble hypothesis there are only 31 different tRNAs and 20 synthetases The Aminoacyl tRNA synthetases are highly specific enzymes because they recognize only on specific amino acid Section 13 2 Translation of mRNA Can Be Divided into Three Steps The process of translation can be broken down into three parts initiation elongation and termination o Initiation for this to occur it requires the small and large ribosomal subunits mRNA GTP charged initiator tRNA Mg2 and three initiation factors that enhance binding affinity of the various transcriptional components Shine Dalgarno sequence occurs in prokaryotes only and the purine rich sequence upstream of start codon it binds to the small subunit of a ribosome Kozak sequence occurs only in eukaryotes ACCAUGG similar function to Shine sequence but ribosome scans from 5 to 3 on mRNA o Elongation the second stage in translation requires both ribosomal subunits assembled with the mRNA to form the P peptidyl site and the A aminoacyl site The lengthening of the growing polypeptide chain by one amino acid is called elongation The process of elongation is facilitated by elongation factors from each step in the phase o Termination the third stage in translation which is signaled by one or more stop codons UAG UGA and UAA and they are also referred to as termination codons Polyribosomes or polysomes are mRNAs with several ribosomes translating at once and they can form a second initiation complex Section 13 4 Translation is More Complex in Eukaryotes Eukaryotes have larger ribosomes with RNA and protein components that are more complex Transcription occurs in the nucleus the 5 end on mRNA is capped with a 7 MG residue at maturation which is essential for translation Many eukaryotic mRNAs contain a purine A or G three bases upstream from the AUG initiator codon followed by a G and eukaryotic mRNAs require the posttranscriptional addition of a poly A tail on their 3 end Translation occurs in the cytoplasm and transcription occurs in the nucleus The Kozak sequence is considered to increase the efficiency of translation by interacting with the initiator tRNA Eukaryotic ribosomes are larger there is no coupled translation transcription in eukaryotes eukaryotic mRNAs persist longer start codon is identified by the 5 3 scanning and no Fmet in eukaryotes and the membrane is synthesized on the ER first Section 13 5 Study of Inborn Errors in Metabolism Alkaptonuria results from a mutation that leads to a metabolic block which prevents them from metabolizing homogentistic acid The acid accumulates in cells and tissues and is excreted in the urine Oxidation products are black and detected in the diapers of newborns It also accumulates in the cartilaginous areas of the ears and nose leading to darkening Alkaptonuria can also lead to serious cases of arthritis Pedigree analysis indicates that Alkaptonuria has a genetic basis and the afflicted individuals had parents that were first cousins called consanguine Garrod concluded that this inherited condition was the form of an alternate mode of metabolism thus implying hereditary information controls chemical reactions in the body Section 13 6 Studies of Neurospora Led to the One Gene One Enzyme Hypothesis Beadle and Tatum showed that nutritional mutations in the bread mold Neurospora caused the loss of an enzymatic activity that catalyzes an essential reaction in wild type organisms growth If growth occurred on the minimal medium the organisms were able to synthesize all the necessary growth factors and there was no nutritional mutation But if no growth occurred on minimal medium they concluded that the culture contained a nutritional mutation They concluded that each nutritional mutation caused the loss of enzymatic activity that facilitated an essential reaction in wild type organisms called one gene one enzyme hypothesis Section 13 7 Studies of Human Hemoglobin Established that One Gene Encodes One Polypeptide Two factors modified the one gene one enzyme hypothesis 1 nearly all organisms are proteins not all proteins are enzymes and 2 proteins have two or more polypeptide chains Because of this the name has changed from 1 the one gene one protein to the one gene one polypeptide chain which is now the more accurate statement of Beadle and Tatum s basic tenet Sickle cell anemia was the first direct evidence that genes specify proteins other than enzymes came from work on mutant hemoglobin molecules Sickle cell anemia is a recessive genetic disease in which afflicted individuals are homozygous
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