Translation

Learning Targets Translation is the synthesis of polypeptides on ribosomes The amino acid sequence of polypeptides is determined by mRNA according to the genetic code. Codons of three bases on mRNA correspond to one amino acid in a polypeptide Translation depends on the complementary base paring between codons on mRNA and anticodons on tRNA Use a table of the genetic code to deduce which codon(s) corresponds to which amino acid Use a table of mRNA codons and their corresponding amino acids to deduce the sequence of amino acids coded by a short mRNA strand of known base sequence

Translation is the synthesis of polypeptides on ribosomes. Ribosomes are bundles of RNA and protein that synthesize polypeptides from mRNA and tRNAs. The are mostly found in the cytoplasm of cells.

Codons- a sequence of three nucleotides that code for a specific amino acid

Codons, Anticodons, and Transfer RNA (tRNA) (HHMI Simulation) A codon is three consecutive bases on the strand of mRNA that code for one amino acid (or for the ending of translation). The tRNA (transfer RNA) molecules each have an anti-codon that is complementary to a codon on the mRNA. For example, if the codon is AGU, the anticodon would be UCA. Each tRNA molecule is also carrying an amino acid. The table shows which codons on the mRNA strand code for which amino acids. Notice that some amino acids may be specified by more than one codon. This makes sense when you consider that there are 20 different amino acids, but 64 different possible ways to combine A, C, U, and G. During translation, the tRNA carries its amino acid to the mRNA molecule, which is attached to a ribosome (ribsomes are made of rRNA (ribosomal rRNA) and proteins). As one tRNA at a time carries its amino acid to the mRNA on the ribosome and base-pairs with its complementary codon on the mRNA, the amino acid from the previous tRNA forms a peptide bond with the new amino acid. This condensation reaction is catalyzed by the ribosome. Once the peptide bond is formed, the tRNA can exit the ribosome and be recycled.

What are the consequences of mutations? In Somatic (any cell other than reproductive) cells: Substitution mutations may result in the replacement of one amino acid with a different amino acid Insertion and deletion mutations often result in a frameshift (the entire codon sequence is changed). This could cause an alteration in the protein’s shape or size. Certain types of mutations can lead to cancer. Permanent changes in DNA are called mutations. Different types of mutations can occur. Insertions occur when a new nucleotide is added where there shouldn’t be one. A deletion occurs when a nucleotide is removed. A substitution occurs when a different nucleotide is used instead of what should be there. These mistakes commonly occur during DNA replication. Deletions and Insertions have a much more dramatic effect on gene expression than substitutions, because they cause the entire frame (sequence of nucleotides) to shift when being transcribed and then translated. A substitution, on the other hand, can still cause changes to the resulting protein structure in the end, but it is usually only one amino acid that has been changed. “Somatic” cells include all cells except for germ cells and gametes (remember that germ cells divide to produce gametes). When mutations occur in somatic cells, they cannot be passed on to offspring. These mutations die with the organisms when it dies. Only mutations that occur in germ cells can be passed on to offspring.

What are the consequences of mutations? In germ cells or gametes Mutations may influence or change the development and formation of certain traits in the organism Could lead to evolution The development of the human skull could be the result of a frameshift mutation in the MYH16 gene, found in apes and chimps, which is expressed in their powerful jaw muscles. It is thought that a mutation to the MYH16 gene in our ape-like ancestors may have initiated changes that made evolution of the human brain possible. Chimpanzees still make a functional version of this protein, but our genes have a mutated version of the gene. Because it was a frameshift mutation, human ancestors could not make a special type of strong muscle needed to control the powerful jaw muscle we observe in chimpanzees and apes. Those muscles and their attachment points on the skull, however, take up quite a bit of space. Without the need to use these muscles, the human skull may have evolved to be larger, allowing for a larger brain.

Practice time! Use the codon charts in your textbooks to fill out the worksheets. What you don’t finish is homework.