• RNA is single-stranded; substitutes the sugar ribose for deoxyribose and the base uracil for thymine • Messenger RNA or mRNA, conveys the DNA recipe for protein synthesis to the cell cytoplasm . • mRNA binds to ribosome , each three-base codon of the mRNA links to a specific form of transfer RNA (tRNA) containing the complementary three-base sequence. • This tRNA, in turn, transfers a single amino acid to a growing protein chain. • Each codon directs the addition of one amino acid to the protein. Note: the same amino acid can be added by different codons; in this illustration, the mRNA sequences GCA and GCC are both specifying the addition of the amino acid alanine (Ala).
Important • Both DNA and RNA have a direction: one end is the 3’ the other is the 5’ end. • Thus, _codons_ are read in one direction only. • Also, note there is redundancy in the genetic code: the different sequences can specify for the same amino acid. Example: UUA and UUG = Leucine
When things go wrong… • Mutations : changes in the DNA sequence, that may be passed along to future generations. • Point mutations : a single base substitution THE CAT SAW THE R AT THE CAT SAW THE H AT • Deletion : a small DNA segment is lost THE C AT SAW THE RAT THE AT SAW THE RAT • Insertion : a segment of DNA is added THE CAT SAW THE RAT THE C H AT SAW THE RAT
Mutations • Frame- _shift_ mutation : modification of the reading frame after a deletion or insertion, resulting in all codons downstream being different. For example: THE RAT SAW THE CAT AND RAN If you take out the “R” in “RAT” and shift the frames, you get: THE ATS AWT HEC ATA NDR AN The resulting sentence (or mRNA message) is meaningless! THE OLD MAN AND THE SEA THE LDM ANA NDT HES EA
Somatic Mutations • Somatic mutations: occur in _body_ cells, or cells that do not lead to gametes. • Somatic mutations that occur in leaves, roots or stems are usually not passed on to future generations… • UNLESS the plant reproduced _asexually_ .