BRIDGES 2014. DNA ➔ RNA ➔ PROTEIN ➔ TRAIT Genotype Phenotype.
From DNA to Protein: Genotype to Phenotype
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Transcript of From DNA to Protein: Genotype to Phenotype
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12From DNA to Protein: Genotype to Phenotype
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12 One Gene, One Polypeptide
• A gene is defined as a DNA sequence that encodes information.
• 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.
• Their results suggested that mutations cause a defect in only one enzyme in a metabolic pathway.
• This lead to the one-gene, one-enzyme hypothesis.
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Figure 12.1 One Gene, One Enzyme (Part 2)
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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.
Today, we know some genes encode functional RNA molecules, such as ribozymes.
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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.
For functional RNAs, this is the final step.
Translation uses information encoded in the RNA to make a polypeptide.
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12 DNA, RNA, and the Flow of Information
• RNA (ribonucleic acid) differs from DNA in three ways:
Single stranded.
The sugar in RNA is ribose, not deoxyribose.
RNA has uracil instead of thymine.
• RNA can base-pair with single-stranded DNA (adenine pairs with uracil instead of thymine) and also can fold over and base-pair with itself.
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Figure 12.2 The Central Dogma
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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.
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Figure 12.3 From Gene to Protein
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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 (rATP, rGTP, rCTP, and rUTP) to act as substrates
The enzyme RNA polymerase
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12 Transcription: DNA-Directed RNA Synthesis
• The first step of transcription, initiation, begins at a promoter, a special sequence of DNA.
• There is at least one promoter for each gene to be transcribed.
• The RNA polymerase (synthesizes RNA during transcription) binds to the promoter region when that protein is needed by the cell.
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Figure 12.4 (Part 1) DNA is Transcribed in RNA
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12 Transcription: DNA-Directed RNA Synthesis
• After binding, RNA polymerase unwinds the DNA and reads the template in the 3-to-5 direction (elongation).
• The new RNA elongates from its 5 end to its 3 end; thus the RNA transcript is antiparallel to the DNA template strand.
• RNA polymerization is always 5’ to 3’ (needs free 3’ –OH to add nucleotide).
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Figure 12.4 (Part 2) DNA is Transcribed in RNA
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Figure 12.4 (Part 3) DNA is Transcribed in RNA
Particular base sequences in the DNA specify termination – the signal that the end of the gene has been reached and
transcription can terminate.
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12 The Genetic Code
• A 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.
• Each codon is assigned only one amino acid.
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Figure 12.5 The Universal Genetic Code
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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.
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12 Preparation for Translation: Linking RNAs, Amino Acids, and Ribosomes
• A tRNA molecule has 75 to 80 nucleotides and a three-dimensional shape.
• The shape is maintained by complementary base pairing and hydrogen bonding.
• The three-dimensional shape of the tRNAs allows them to combine with the binding sites of the ribosome.
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12 Preparation for Translation: Linking RNAs, Amino Acids, and Ribosomes
• At the 3 end of every tRNA molecule is a site to which its specific amino acid binds covalently.
• Midpoint in the sequence are three bases called the anticodon.
• The anticodon is the contact point between the tRNA and the mRNA.
• The anticodon is complementary (and antiparallel) to the mRNA codon.
• The codon and anticodon unite by complementary base pairing.
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Figure 12.7 Transfer RNA
Anticodon
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12 Preparation for Translation: Linking RNAs, Amino Acids, and Ribosomes
• The ribosome is a complex protein assembly where protein synthesis takes place.
• The ribosome binds to the mRNA, and then the correct transfer RNA comes in and binds to bring in the correct amino acid – thus building the protein chain.
• Each ribosome has two subunits: a large and a small one.
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12 Preparation for Translation: Linking RNAs, Amino Acids, and Ribosomes
• The ribosome validates the three-base-pair match between the mRNA and the tRNA.
• If hydrogen bonds have not formed between all three base pairs, the tRNA is ejected from the ribosome.
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Figure 12.10 The Initiation of Translation
The ribosome attaches to the
mRNA at a special
ribosome recognition
sequence upstream of the start sequence
AUG.
The start codon (AUG)
designates the first amino acid in all proteins =
methionine.
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Figure 12.11 Translation: The Elongation Stage
The ribosome helps form a peptide bond between the last amino acid of the growing protein and the amino acid attached to the incoming tRNA.
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Figure 12.12 The Termination of Translation
When a stop codon—UAA, UAG, or UGA—enters the ribosome, it signals the ribosome to release the formed protein.
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Figure 12.13 A Polysome (Part 1)
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Figure 12.13 A Polysome (Part 2)
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12 Posttranslational Events
• Two posttranslational 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.
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Figure 12.14 Destinations for Newly Translated Polypeptides in a Eukaryotic Cell
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12 Posttranslational 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.
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12 Posttranslational Events
• Most proteins are modified after translation.
• These modifications are often essential to the functioning of the protein.
• Three types of modifications:
Proteolysis (cleaving)
Glycosylation (adding sugars)
Phosphorylation (adding phosphate groups)
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Figure 12.16 Posttranslational Modifications to Proteins
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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.
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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.
• Point mutations can occur as a result of mistakes during DNA replication or can be caused by environmental mutagens.
• Because of degeneracy (redundancy) in the genetic code, some point mutations result in no change in the amino acids in the protein.
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Silent Mutation
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12 Mutations: Heritable Changes in Genes
• Some 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.
• These mutations may reduce the functioning of a protein or disable it completely.
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Missense mutation
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Figure 12.17 Sickled and Normal Red Blood Cells
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12 Mutations: Heritable Changes in Genes
• Some mutations are base substitutions that substitute a stop codon.
• The shortened proteins are usually not functional.
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Nonsense mutation
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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.
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Frame-shift mutation
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12 Mutations: Heritable Changes in Genes
• Spontaneous mutations are permanent changes, caused by any of several mechanisms.
• Induced mutations are changes caused by some outside agent (mutagen).