Topic 8 From Gene …to Protein Biology 1001 October 17, 2005

IIIA. Details of Transcription 5’- GCCAGGCTAAACTA -3’DNA coding strand 3’- CGGTCCGATTTGAT -5’DNA template strand 5’- GCCAGG …-3’RNA synthesis 3’- CGGTCCGATTTGAT -5’ 5’- GCCAGGCUAAACUA -3’ messenger RNA (mRNA)  RNA is similar to DNA except that it is always single-stranded, the sugar has a 2’-OH group, and the nitrogenous bases are A, C, G, & Uracil  Messenger RNA is transcribed or “copied” from a DNA template according to the base pairing rules, except that U pairs with A  The mRNA sequence is the same as the DNA coding strand sequence, except that there are Us instead of As

Figure Transcription has three stages initiation, elongation and termination

IIIB. Translation – the Basic Concept  Translation is the synthesis of a polypeptide under the direction of the mRNA  The nucleotide sequence of the mRNA is converted to the amino acid sequence of a polypeptide  The site at which this occurs is in the cell is the ribosome  Amino acids are carried to the growing polypeptide chain by tRNA molecules Figure 17.13

Figure – The structure of transfer RNA Figure – The anatomy of a ribosome

IIIC. Features of Gene Expression in Eukaryotes  Transcription occurs in the nucleus, translation in the cytoplasm  Transcription initially results in a pre-mRNA or primary transcript which undergoes RNA processing before being translated  The ribosomal subunits (rRNA & proteins) are assembled in the nucleolus Figure 17.26

IV. The Genetic Code  Four nucleotides need to specify 20 amino acids, so the genetic code is a triplet code  4 1 = 4, 4 2 = 16, 4 3 = 64  A codon is a sequence of three nucleotides that specifies a particular amino acid; 3 of the 64 possible codons function as stop signals and one (AUG) codes for the amino acid methionine & functions as a start signal  The code is read from the mRNA which is transcribed from the template strand of DNA  The code is degenerate – it is redundant but not ambiguous  The sequence needs to be read in non-overlapping tandem groups of three and in the correct reading frame  The code is nearly universal

IV. The Genetic Code Figure 17.4 Figure 17.5

V. The Effect of Mutation on Protein Function Mutation – a change in the genetic material Point mutation – a change in just one nucleotide (base) pair A substitution of one nucleotide pair for another produces silent, missense, or nonsense mutations An indel (insertion or deletion of a base pair) produces a frameshift mutation Mutations are either spontaneous errors during DNA replication or caused by mutagens – physical or chemical agents that interact with and damage DNA  Egs. Physical – UV, X-rays  Egs. Chemical – base analogs, intercalating agentsbase analogsintercalating agents

Figure Base-pair substitution Silent

Review Videos of Transcription and Translation (These animations are intended to aid your conceptual understanding. I won’t test you on the content unless I mentioned it elsewhere in Topic 8)