lecture 10

lecture 10 - Lecture 10 Lecture 10 Lecture 10 Lecture 10...

Info iconThis preview shows pages 1–4. Sign up to view the full content.

View Full Document Right Arrow Icon

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: Lecture 10 Lecture 10 Lecture 10 Lecture 10 Lecture 10 Analysis of Gene Sequences Analysis of Gene Sequences Analysis of Gene Sequences Analysis of Gene Sequences 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. The Genetic Code Classically, genes are identified by their function. That is the existence of the gene is recognized because of mutations in the gene that give an observable phenotypic change. Historically, many genes have been discovered because of their effects on phenotype. Now, in the era of genomic sequencing, many genes of no known function can be detected by looking for patterns in DNA sequences. The simplest method which works for bacterial and phage genes (but not for most eukaryotic genes as we will see later) is to look for stretches of sequence that lack stop codons. These are known as open reading frames or ORF ORF ORF ORF ORF s. This works because a random sequence should contain an average of one stop codon in every 21 codons. Thus, the probability of a random occurrence of even a short open reading frame of say 100 codons without a stop codon is very small (61/...
View Full Document

Page1 / 8

lecture 10 - Lecture 10 Lecture 10 Lecture 10 Lecture 10...

This preview shows document pages 1 - 4. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online