12 One Gene, One Polypeptide In the 1940s, Beadle and Tatum showed that when an altered gene resulted in an altered phenotype, that altered phenotype always showed up as an altered enzyme. This lead to the one-gene, one-enzyme hypothesis.

Figure 12.1 One Gene, One Enzyme (Part 1)

Figure 12.1 One Gene, One Enzyme (Part 2)

12 One Gene, One Polypeptide The gene–enzyme connection has undergone several modifications. Some enzymes are composed of different subunits coded for by separate genes. This suggests, instead of the one-gene, one enzyme hypothesis, a one-gene, one- polypeptide relationship.

12 DNA, RNA, and the Flow of Information The expression of a gene takes place in two steps:  Transcription makes a single-stranded RNA copy of a segment of the DNA.  Translation uses information encoded in the RNA to make a polypeptide.

Figure 12.2 The Central Dogma

12 DNA, RNA, and the Flow of Information Messenger RNA, or mRNA moves from the nucleus of eukaryotic cells into the cytoplasm, where it serves as a template for protein synthesis. Transfer RNA, or tRNA, is the link between the code of the mRNA and the amino acids of the polypeptide, specifying the correct amino acid sequence in a protein.

12 DNA, RNA, and the Flow of Information Certain viruses use RNA rather than DNA as their information molecule during transmission. HIV and certain tumor viruses (called retroviruses) have RNA as their infectious information molecule; they convert it to a DNA copy inside the host cell and then use it to make more RNA.

12 Transcription: DNA-Directed RNA Synthesis In normal prokaryotic and eukaryotic cells, transcription requires the following:  A DNA template for complementary base pairing  The appropriate ribonucleoside triphosphates (ATP, GTP, CTP, and UTP)  The enzyme RNA polymerase

12 Transcription: DNA-Directed RNA Synthesis Just one DNA strand (the template strand) is used to make the RNA. For different genes in the same DNA molecule, the roles of these strands may be reversed.

12 Transcription: DNA-Directed RNA Synthesis Initiation  Begins at promoter. Elongation  Transcript is antiparellel  More mistakes than replication Termination.  Fall off or helped off.

Figure 12.4 (Part 1) DNA is Transcribed in RNA

Figure 12.4 (Part 2) DNA is Transcribed in RNA

Figure 12.4 (Part 3) DNA is Transcribed in RNA

12 The Genetic Code Genetic code relates genes (DNA) to mRNA and mRNA to the amino acids of proteins. mRNA is read in three-base segments called codons. The 64 possible codons code for only 20 amino acids and the start and stop signals.  Redundancy  Wobble

Figure 12.5 The Universal Genetic Code

12 Preparation for Translation: Linking RNAs, Amino Acids, and Ribosomes The codon in mRNA and the amino acid in a protein are related by way of an adapter—a specific tRNA molecule. tRNA has three functions:  It carries an amino acid.  It associates with mRNA molecules.  It interacts with ribosomes.

Figure 12.7 Transfer RNA

12 Preparation for Translation: Linking RNAs, Amino Acids, and Ribosomes Midpoint in the sequence are three bases called the anticodon.  contact point between the tRNA and the mRNA.  complementary (and antiparallel) to the mRNA codon.  codon and anticodon unite by complementary base pairing.

12 Preparation for Translation: Linking RNAs, Amino Acids, and Ribosomes Each ribosome has two subunits: a large one and a small one.  Each made of rRNA and protein. When they are not translating, the two subunits are separate.

Figure 12.9 Ribosome Structure

12 Translation: RNA-Directed Polypeptide Synthesis Translation begins with an initiation complex: a charged tRNA with its amino acid and a small subunit, both bound to the mRNA. The start codon (AUG) designates the first amino acid in all proteins. The large subunit then joins the complex.

Figure The Initiation of Translation

12 Translation: RNA-Directed Polypeptide Synthesis Ribosomes move in the 5-to-3 direction on the mRNA. The peptide forms in the N–to–C direction.

Figure Translation: The Elongation Stage

Figure The Termination of Translation

12 Regulation of Translation Many antibiotics are considered magic bullets because they will affect the ribosomes of bacteria and have no effect on our ribosomes.

12 Post translational Events Two post translational events can occur after the polypeptide has been synthesized:  The polypeptide may be moved to another location in the cell, or secreted.  The polypeptide may be modified by the addition of chemical groups, folding, or trimming.

12 Post translational Events As the polypeptide chain forms, it folds into its 3-D shape. The amino acid sequence also contains an “address label” indicating where in the cell the polypeptide belongs. It gives one of two sets of instructions:  Finish translation and be released to the cytoplasm.  Stall translation, go to the ER, and finish synthesis at the ER surface.

Figure A Signal Sequence Moves a Polypeptide into the ER (Part 1)

Figure A Signal Sequence Moves a Polypeptide into the ER (Part 2)

12 Post translational Events Most proteins are modified after translation. Three types of modifications:  Proteolysis (cleaving)  Glycosylation (adding sugars)  Phosphorylation (adding phosphate groups)

12 Mutations: Heritable Changes in Genes Mutations are heritable changes in DNA— changes that are passed on to daughter cells. Multicellular organisms have two types of mutations:  Somatic mutations are passed on during mitosis, but not to subsequent generations.  Germ-line mutations are mutations that occur in cells that give rise to gametes.

12 Mutations: Heritable Changes in Genes All mutations are alterations of the DNA nucleotide sequence and are of two types:  Point mutations are mutations of single genes.  Chromosomal mutations are changes in the arrangements of chromosomal DNA segments.

12 Mutations: Heritable Changes in Genes Point mutations result from the addition or subtraction of a base or the substitution of one base for another.  Can happen spontaneously or due to mutagens.  Many are silent

12 Mutations: Heritable Changes in Genes Some mutations, called missense mutations, cause an amino acid substitution. An example in humans is sickle-cell anemia, a defect in the  -globin subunits of hemoglobin. The  -globin in sickle-cell differs from the normal by only one amino acid.

Figure Sickled and Normal Red Blood Cells

12 Mutations: Heritable Changes in Genes Nonsense mutations are base substitutions that substitute a stop codon. The shortened proteins are usually not functional.

12 Mutations: Heritable Changes in Genes A frame-shift mutation consists of the insertion or deletion of a single base in a gene. This type of mutation shifts the code, changing many of the codons to different codons. These shifts almost always lead to the production of nonfunctional proteins.

12 Mutations: Heritable Changes in Genes DNA molecules can break and re-form, causing four different types of mutations:  Deletions are a loss of a chromosomal segment.  Duplications are a repeat of a segment.  Inversions result from breaking and rejoining when segments get reattached in the opposite orientation.  Translocations result when a portion of one chromosome attaches to another.

12 Mutations: Heritable Changes in Genes Mutations have both benefits and costs. Germ line mutations provide genetic diversity for evolution, but usually produce an organism that does poorly in its environment. Somatic mutations do not affect offspring, but can cause cancer.