Exam Results and Gene Expression: Transcription and Translation - Prof. S. Crousillac, Study notes of Biology

Download Exam Results and Gene Expression: Transcription and Translation - Prof. S. Crousillac and more Study notes Biology in PDF only on Docsity! Exam #3 Results As Bs Cs Ds Fs 73 153 103 68 65 (16%) (33%) (22%) (15%) (14%) 462 students took the exam Class average: ~78.5% Highest grade: 101% (2 of these) Lowest grade: not a 101% (460 of these) Chapter 12-Gene Expression and Regulation ornithine enzyme 1 gene B gene A enzyme 2 arginine amino acid needed in protein synthesis citrulline The biochemical pathway for synthesis of the amino acid arginine involves two steps, each catalyzed by a different enzyme. A blockage in a pathway not only stops the production of something we need but it also may lead to the accumulation of something we don’t want. Fig. 12.1 George Beadle and Edward Tatum conducted experiment with bread mold, Neurospora crassa, and demonstrated that different mutants (generated by X-rays) had the pathway of arginine (an amino acid) synthesis blocked at different steps. The suggestions of Garrod were confirmed in the 1930's From these results they formulated the one gene-one enzyme hypothesis Beadle and Tatum deduced that the 3 different mutant types each lacked a different enzyme in the pathway which synthesizes arginine. -The RNA is complementary to the DNA -RNA that is synthesized from a gene coding for a protein is called messenger RNA (mRNA), or a transcript Differences between DNA and RNA: 1. sugar - deoxyribose (DNA) verses ribose (RNA) 2. base - thymine (DNA) is replaced by uracil (RNA) Step two - RNA to protein This process is called translation. ---Translation is the synthesis of a polypeptide (protein) directed by mRNA Why is this process referred to as translation?? Converting the language of nucleic acids into the language of proteins: • 4 bases in nucleic acids • 20 amino acids in proteins During translation, proteins are synthesized according to the genetic message of sequential codons in the mRNA. Translation occurs on the ribosomes. ---Codon is a three-nucleotide sequence in mRNA that specifies which amino acid will be added to a growing polypeptide or signals the termination of translation. In the genetic code, a triplet of nucleotides specifies an amino acid There are 4 nucleotides in RNA and 20 amino acids. -4 different nucleotides, combining as codons, which are 3-nucleotide sequences. Math would dictate that there could potentially be as many as 64 amino acids (43) But there are only 20, since each amino acid is usually encoded by more than one codon. 60 of the 64 possible codons code for amino acids. The remaining four codons signal the translation to start (start codon) and translation to stop (stop codon). Since there are only 20 amino acids, more than one codon can code for the same amino acid. This relationship makes the code redundant, or degenerate. The codons usually differ only at the third position. There is no ambiguity in the triplet code since a given triplet codes for one and only one amino acid. RNA polymerase Initiation gene 1 gene 2 gene 3 DNA RNA polymerase binds to the promoter region of DNA near the beginning of a gene, separating the double helix near the promoter. DNA Fig. 12.3 Elongation RNA polymerase travels along the DNA template strand, catalyzing the addition of ribose nucleotides into an RNA molecule. The nucleotides in the RNA are complementary to the template strand of the DNA. RNA DNA template strand Fig. 12.3 Termination At the end of a gene, RNA polymerase encounters a sequence of DNA called a termination signal. RNA polymerase detaches from the DNA and releases the RNA molecule. Fig. 12.3 What is the order of steps in the transcription process? • 1. Initiation, elongation, termination • 2. Initiation, termination, elongation • 3. Elongation, initiation, termination • 4. Elongation, termination, initiation How does initiation begin? • 1. DNA polymerase finds a promoter region and binds, causing the DNA double helix to unwind. • 2. RNA polymerase finds a promoter region and binds, causing the DNA double helix to unwind. • 3. DNA synthase finds a promoter region and binds, causing the DNA double helix to unwind. • 4. RNA synthase finds a promoter region and binds, causing the DNA double helix to unwind. If the mRNA transcript AUGCGCUGCAAU were to leave the nucleus and undergo translation at a ribosome, what would be the sequence of anticodons translating this nucleotide into protein? • 1. TACGCGACGTTA • 2. ATGCGCTGCAAT • 3. AUGCGCUGCAAU • 4. UACGCGACGUUA The ribosome coordinates the pairing of tRNA anticodons with mRNA codons The ribosome is where proteins are built. Fig. 12.1 -Composed of ribosomal RNA (rRNA) and protein -The subunits combine as a ribosome only when they are translating a protein large subunit small subunit Fig. 12.1 The P site -holds the tRNA with the polypeptide chain attached. The A site -hold the tRNA with the next amino acid to be added. large subunit small subunit P A tRNA/amino acid binding sites Fig. 12.1 Transfer RNA (tRNA) -tRNA acts as the "interpreter" between the nucleotide “language” of mRNA and the amino acid “language” of proteins. Fig. 12.1 Transfer RNA (tRNA) anticodon In part of this role as “interpreter,” the tRNA must “read” the mRNA. This is accomplished by the anticodon portion of the tRNA Fig. 12.1 Transfer RNA (tRNA) anticodon The other portion the tRNA’s role as “interpreter” is to transfer the correct amino acid from the cytoplasmic pool of amino acids to the ribosome for protein synthesis. attached amino acid Fig. 12.1 mRNA The initiation complex binds to an mRNA molecule. The methionine (met) tRNA anticodon (UAC) base- pairs with the start codon (AUG) of the mRNA. Initiation: methionine tRNA small ribosomal subunit first tRNA at the P site large ribosomal subunit catalytic site A site Initiation: Fig. 12.7 catalytic site Elongation:Elongation: The next tRNA occupies the A site and the anticodon hydrogen bonds to the codon of the mRNA. Fig. 12.7 peptide bond Elongation: this leaves the tRNA at the P site with no amino acid, and the tRNA at the A site with a dipeptide attached. Fig. 12.7 catalytic site Translocation: the tRNA vacates the P site Fig. 12.7 catalytic site Translocation: the ribosome moves one codon down the mRNA. This translocates the tRNA (with the growing peptide) from the A site to the P site Fig. 12.7 completed peptide stop codon Termination: The mRNA reaches a stop codon: UAA, UAG, or UGA ---Stop codon is a triplet codon that signals the end of translation. Fig. 12.7 ---Mutations are permanent changes in the DNA that can involve large chromosomal regions or a single nucleotide pair. ---Point mutations are mutations limited to one or two nucleotides in a single gene, and can affect the function of a protein. el) es 2 2 DNA (Template Properties Strand) mRNA Amino Acid — of Amino Acid Functional Effect on Protein Disease Original codon 6 CTC GAG Glutamic acid — Hydrophilic Normal protein function None Mutation 1 CTT GAA Glutamic acid = Hydrophilic Neutral; normal protein function None Mutation 2 GTC CAG Glutamine Hydrophilic Neutral; normal protein function None Mutation 3 CAC GUG Valine Hydrophobic Loses water solubility; compromises Sickle-cell anemia protein function Original codon 17) TTC AAG Lysine Hydrophilic Normal protein function None Mutation 4 ATC UAG Stop codon Ends translation after Synthesizes only part of the protein; Beta-thalassemia amino acid 16 eliminates protein function Table 12-4 ---Deletion is the deletion of one or more nucleotide pairs from a gene Fig. 11.8c Ionizing Radiation Radiation can cause a process called ionization in our DNA. Causes DNA molecules to lose or gain electrons, resulting in the breaking apart of our DNA, and re- arrangement of bases (mutations). X-rays can cause radiation damage. But the radiation from x-rays cannot pass through lead. This is why a lead apron is used during x-rays. Ultraviolet radiation from the sun can have the same effect, and this radiation is blocked by sunscreen.