Lecture 9 Gene Expression.pptx

Lecture 9: Gene Expression 8I01101 8iology Department College of Science IMSIU CONCEPT 17.1: Genes specify proteins via transcription and translation • The DNA inherited by an organism leads to specific traits by dictating the synthesis of proteins • Proteins are the links between genotype and phe...

Lecture 9: Gene Expression 8I01101 8iology Department College of Science IMSIU CONCEPT 17.1: Genes specify proteins via transcription and translation • The DNA inherited by an organism leads to specific traits by dictating the synthesis of proteins • Proteins are the links between genotype and phenotype • Gene expression, the process by which DNA directs protein synthesis, includes two stages: transcription and translation • How was the relationship between proteins and DNA discovered? © 2021 Pearson Education Ltd. Basic Principles of Transcription and Translation • RNA is the bridge between genes and protein synthesis • Transcription is the synthesis of RNA using information in DNA • Transcription produces messenger RNA (mRNA) • Translation is the synthesis of a polypeptide, using information in the mRNA • Ribosomes are the sites of translation © 2021 Pearson Education Ltd. Codons: Triplets of Nucleotides • The flow of information from gene to protein is based on a triplet code: a series of nonoverlapping, three-nucleotide words • The words of a gene are transcribed into complementary nonoverlapping three-nucleotide words of mRNA • These words are then translated into a chain of amino acids, forming a polypeptide © 2021 Pearson Education Ltd. Figure 17.5 © 2021 Pearson Education Ltd. Cracking the Code • All 64 codons were deciphered by the mid-1960s • 0 f the 64 triplets, 61 code for amino acids; 3 triplets are "stop" signals to end translation • The genetic code is redundant (more than one codon may specify a particular amino acid) but not ambiguous; no codon specifies more than one amino acid • Codons must be read in the correct reading frame (correct groupings) in order for the specified polypeptide to be produced © 2021 Pearson Education Ltd. Figure 17.6 © 2021 Pearson Education Ltd. Synthesis of an RNA Transcript • The three stages of transcription: - Initiation - Elongation - Termination © 2021 Pearson Education Ltd. RNA Polymerase Binding and Initiation of Transcription • Promoters signal the transcription start point and usually extend several dozen nucleotide pairs upstream of the start point • Transcription factors help guide the binding of RNA polymerase and the initiation of transcription • The completed assembly of transcription factors and RNA polymerase II bound to a promoter is called a transcription initiation complex • A promoter called a TATA box is crucial in forming the initiation complex in eukaryotes © 2021 Pearson Education Ltd. Elongation of the RNA Strand • As RNA polymerase moves along the DNA, it untwists the double helix, 10-20 nucleotides at a time • Nucleotides are added to the 3' end of the growing RNA molecule • Transcription progresses at a rate of 40 nucleotides per second in eukaryotes • A gene can be transcribed simultaneously by several RNA polymerases © 2021 Pearson Education Ltd. Termination of Transcription • The mechanisms of termination are different in bacteria and eukaryotes • In bacteria, the polymerase stops transcription at the end of the terminator and the mRNA can be translated without further modification • In eukaryotes, RNA polymerase II transcribes the polyadenylation signal sequence; the RNA transcript is released 10-35 nucleotides past this polyadenylation sequence © 2021 Pearson Education Ltd. CONCEPT 17.4: Translation is the RNA-directed synthesis of a polypeptide: a closer look • Genetic information flows from mRNA to protein through the process of translation © 2021 Pearson Education Ltd. Molecular Components of Translation • A cell translates an mRNA message into protein with the help of transfer RNA (tRNA) • tRNAs transfer amino acids to the growing polypeptide in a ribosome • Translation is a complex process in terms of its biochemistry and mechanics © 2021 Pearson Education Ltd. The Structure and Function of Ribosomes • Ribosomes facilitate specific coupling of tRNA anticodons with mRNA codons in protein synthesis • Eukaryotic ribosomes are somewhat larger than bacterial ribosomes and differ in their molecular composition • Some antibiotic drugs specifically inactivate bacterial ribosomes without harming eukaryotic ribosomes • The two ribosomal subunits (large and small) are made of proteins and ribosomal RNAs (rRNAs) © 2021 Pearson Education Ltd. • A ribosome has three binding sites for tRNA - The P site holds the tRNA that carries the growing polypeptide chain - The A site holds the tRNA that carries the next amino acid to be added to the chain - The E site is the exit site, where discharged tRNAs leave the ribosome © 2021 Pearson Education Ltd. Building a Polypeptide • The three stages of translation: - Initiation - Elongation - Termination • All three stages require protein "factors" that aid in the translation process • Energy is required for some steps, too © 2021 Pearson Education Ltd. Ribosome Association and Initiation of Translation • The initiation of translation starts when the small ribosomal subunit binds with mRNA and a special initiator tRNA • The initiator tRNA carries the amino acid methionine • Then the small subunit moves along the mRNA until it reaches the start codon (AUG) • Proteins called initiation factors bring in the large subunit that completes the translation initiation complex © 2021 Pearson Education Ltd. Termination of Translation • Elongation continues until a stop codon in the mRNA reaches the A site • The A site accepts a protein called a release factor • The release factor causes the addition of a water molecule instead of an amino acid • This reaction releases the polypeptide, and the translation assembly comes apart © 2021 Pearson Education Ltd. Substitutions • A nucleotide-pair substitution replaces one nucleotide and its partner with another pair of nucleotides • Silent mutations have no effect on the amino acid produced by a codon because of redundancy in the genetic code • Missense mutations still code for an amino acid, but not the correct amino acid • Nonsense mutations change an amino acid codon into a stop codon; most lead to a nonfunctional protein © 2021 Pearson Education Ltd. Insertions and Deletions • Insertions and deletions are additions or losses of nucleotide pairs in a gene • These mutations have a disastrous effect on the resulting protein more often than substitutions do • Insertion or deletion of nucleotides may alter the reading frame, producing a frameshift mutation • Insertions or deletions outside the coding part of a gene could affect how the gene is expressed © 2021 Pearson Education Ltd.