Lecture 10 Analysis of Gene Sequences Anatomy of a bacterial gene Promoter Coding Sequence(no stop codons 个 个 Transcription Translation Start Translation Stop Transcription Start (AUG qUAG, UAA, or UGA) Terminator S-D Sequence Sequence Element Function Promoter To target RNA polymerase to DNA and to start transcription of a mRNA copy of the gene sequence Transcription terminator To instruct RNA polymerase to stop transcription Shine-Dalgarno sequence S-D sequence in mRNA will load ribosomes to begin transla- and translation start tion. Translation almost always begins at an AUG codon in the mrna (an atG in the dna becomes an aUg in the mRNA copy). Synthesis of the protein thus begins with a methionine Coding Sequence Once translation starts, the coding sequence is translated by the ribosome along with tRNAs which read three bases at a time in linear sequence. Amino acids will be incorporated into the growing polypeptide chain according to the genetic code Translation Stop When one of the three stop codons [UAG(amber), UAA ochre), or UGA] is encountered during translation, the polypeptide will be released from the ribosome Example: A gene coding sequence that is 1, 200 nucleotide base pairs in length(including the Atg but not including the stop codon)will specify the sequence of a protein 1200 /3 400 amino acids long. Since the average molecular weight of an amino acid is 110 da, this gene encodes a protein of about 44 kd- the size of an average proteinLecture 10 Analysis of Gene Sequences Anatomy of a bacterial gene: Promoter Coding Sequence (no stop codons) mRNA: Transcription Translation Start Translation Stop Transcription Start (AUG) (UAG, UAA, or UGA) Terminator S-D Sequence Sequence Element Function Promoter To target RNA polymerase to DNA and to start transcription of a mRNA copy of the gene sequence. Transcription terminator To instruct RNA polymerase to stop transcription. Shine-Dalgarno sequence and translation start S-D sequence in mRNA will load ribosomes to begin transla￾tion. Translation almost always begins at an AUG codon in the mRNA (an ATG in the DNA becomes an AUG in the mRNA copy). Synthesis of the protein thus begins with a methionine. Coding Sequence Once translation starts, the coding sequence is translated by the ribosome along with tRNAs which read three bases at a time in linear sequence. Amino acids will be incorporated into the growing polypeptide chain according to the genetic code. Translation Stop When one of the three stop codons [UAG (amber), UAA (ochre), or UGA] is encountered during translation, the polypeptide will be released from the ribosome. Example: A gene coding sequence that is 1,200 nucleotide base pairs in length (including the ATG but not including the stop codon) will specify the sequence of a protein 1200/3 = 400 amino acids long. Since the average molecular weight of an amino acid is 110 da, this gene encodes a protein of about 44 kd — the size of an average protein