Genetics - Translation

UNIT: GENETICS LEVEL: STANDARD TRANSLATION Content Statements: D1.2.5 Transla-on as the synthesis of polypep-des from mRNA D1.2.6 Roles of mRNA, ribosomes and tRNA in transla-on D1.2.7 Complementary base pairing between tRNA and mRNA D1.2.8 Features of the gene-c code D1.2.9 Using the gene-c code expressed as a table of mRNA codons D1.2.10 Stepwise movement of the ribosome along mRNA and linkage of amino acids by pep-de bonds to the growing polypep-de chain D1.2.11 Muta-ons that change protein structure TRANSLATION Transla-on is the process of protein synthesis, in which gene-c informa-on encoded in mRNA is translated into a sequence of amino acids (polypep-de). • Messenger RNA (mRNA) is transported to the ribosomes (in the cytosol) • Ribosomes read the mRNA sequence in triplets of bases called codons • Codons code for specific amino acids according to a genetic code • Amino acids are brought to the ribosome by transfer RNA (tRNA) • Transfer RNA binds to specific codons via complementary anticodons • Anticodons cause tRNA molecules to line up according to codon order • Peptide bonds form between amino acids (catalysed by the ribosome) • Polypeptides are produced as the ribosome moves along the mRNA Hint: MR CAT APP RIBOSOMES The ribosome is made up of two dis-nct subunits – each composed of protein and ribosomal RNA (rRNA): • The small subunit binds to the messenger RNA (mRNA), which is read in triplets of bases called codons • The large subunit contains binding sites for two tRNA molecules, which each carry a specific amino acid The role of the ribosome is to transfer the amino acid from one tRNA molecule to the next, sequen-ally growing a polypep-de chain as it moves along the mRNA. Once a given tRNA molecule has transferred its amino acid to a chain, it is released in order to collect another amino acid for future use. Ribosomes ini-ate transla-on when they reach a start codon (AUG) and terminate transla-on when they reach a stop codon. The start codon acts to establish the appropriate reading frame in which the triplets of mRNA bases are read (translated) by the par-cular ribosome. Mul-ple ribosomes can translate an mRNA sequence at once. amino acid tRNA mRNA UAC AAC an-codon AUG UUG codon ribosome Components of TranslaCon GENETIC CODE The gene-c code is the set of rules that iden-fies which amino acid corresponds to each mRNA codon. It is typically represented as a table. The gene-c code possesses two key characteris-cs: • Universality: Almost every living organism uses the same code (there are a few viral exceptions) • Degeneracy: Some codons code for the same amino acid (there are 64 codons and only 20 amino acids) The GeneCc Code UUU UUC UUA UUG Phe Leu CUU CUC CUA CUG Leu AUU AUC AUA AUG Ile Met GUU GUC GUA GUG Val UCU UCC UCA UCG Ser CCU CCC CCA CCG Pro ACU ACC ACA ACG Thr GCU GCC GCA GCG Ala UAU UAC UAA UAG CAU CAC CAA CAG AAU AAC AAA AAG GAU GAC GAA GAG TranslaCon Tyr STOP His Gln Asn Lys Asp Glu UGU UGC UGA UGG CGU CGC CGA CGG AGU AGC AGA AGG GGU GGC GGA GGG tRNA Cys polypep-de amino acid STOP Trp Arg an-codon Ser Arg codon mRNA Gly ribosome The codons are recognised by an-codons on specific tRNA molecules. The codons and the an-codons are complementary in base sequence, ensuring correct pairing. Guanine and cytosine undergo complementary pairing, while adenine partners with uracil. For instance, a codon with a sequence of AUG will correspond to an-codons with a sequence of UAC. Complementary pairing ensures sequences are correctly translated. GENETIC DISEASES As proteins are encoded by DNA sequences (genes), a change in nucleo-de sequence can alter the protein structure. If a change in protein sequence leads to the abroga-on of its func-on, a disease condi-on may arise. An example of a gene-c disease caused by a muta-on that changes protein structure is sickle cell anaemia. Sickle cell anaemia results from a muta-on to the gene coding for haemoglobin. The mRNA sequence is changed from GAG → GUG, which in turn changes the amino acid sequence from glutamic acid to valine (Glu → Val). The haemoglobin now forms insoluble fibrous strands that cannot carry oxygen as effec-vely (resul-ng in anaemia). The fibrous strands also change the shape of the red blood cell to a sickle shape, which are destroyed more rapidly than normal cells, leading to a much lower red blood cell count. mRNA mRNA G A G G U G Glu Val Protein Normal Soluble Haemoglobin in Normal Red Blood Cells Insoluble Mutant d Haemoglobin in ‘Sickle’ Red Blood Cells Protein