Gene Expression: From DNA to Protein Synthesis
Gene Expression
The process by which DNA directs the synthesis of proteins.
What Situation Did Archibald Garrod Suggest Caused Inborn Errors of Metabolism?
Garrod suggested that genes dictate phenotypes through enzymes that catalyze specific chemical reactions in the cell.
Describe One Example Garrod Used to Illustrate His Hypothesis
Alkaptonuria: Urine is black because it contains the chemical alkapton, which darkens upon exposure to air.
State the Hypothesis Formulated by George Beadle While Studying Eye Color Mutations in Drosophila
Each of the various mutations affecting eye color blocks pigment synthesis at a specific step by preventing the production of the enzyme that catalyzes that step.
What Strategy Did Beadle and Tatum Adopt to Test This Hypothesis?
They bombarded Neurospora with X-rays and then looked among the survivors for mutants that differed in their nutritional needs from the wild-type mold.
Which Organism Did Beadle and Tatum Use in Their Research? How Did This Organism’s Nutritional Requirements Facilitate This Research?
Wild-type Neurospora. It can survive in the lab on a moist support medium called agar, mixed with only inorganic salts, glucose, and the vitamin biotin.
How Were Neurospora Spores Treated to Increase the Mutation Rate?
They allowed them to grow on a complete growth medium, which consisted of a minimal medium supplemented with all 20 amino acids and a few other nutrients.
Cite Two Significant Findings That Resulted from the Research of Beadle and Tatum
- The function of a gene is to dictate the production of a specific enzyme.
- Not all proteins are enzymes.
What Revision of Detail Did This Hypothesis Undergo as More Information Was Gained?
Not all proteins are enzymes. Many proteins are constructed from two or more different polypeptide chains, and each polypeptide is specified by its own gene.
Find Three Ways in Which RNA Differs from DNA
- RNA contains ribose instead of deoxyribose.
- RNA has the nitrogenous base Uracil instead of Thymine.
- RNA consists of a single strand.
What Are the Monomers of DNA and RNA? Of Proteins?
In DNA or RNA, the monomers are the four types of nucleotides. The monomers of proteins are amino acids.
Transcription
The synthesis of RNA under the direction of DNA.
Translation
The synthesis of a polypeptide, which occurs under the direction of mRNA.
In Eukaryotes, What Is Pre-mRNA Called?
Primary Transcript
Central Dogma of Molecular Genetics
Cells are governed by a molecular chain of command with a directional flow of genetic information. DNA → RNA → Protein
What Is the Coding Strand Called?
The template strand.
Describe Nirenberg’s Experiment in Which He Identified the First Codon
Nirenberg synthesized an artificial mRNA by linking identical RNA nucleotides containing Uracil as their base. No matter where this message started or stopped, it could contain only one codon in repetition: UUU.
What Was the First Codon-Amino Acid Pair to Be Identified?
UUU
Why Is the Genetic Code Said to Be Redundant but Not Ambiguous?
Because there are codons that specify the same amino acids, but none of them ever specify any other amino acid.
Reading Frame
The triplet grouping of ribonucleotides used by the translation machinery during polypeptide synthesis.
Name the Enzyme That Uses the DNA Template Strand to Transcribe a New mRNA Strand
RNA Polymerase
Which Enzyme, DNA Polymerase III or RNA Polymerase, Does Not Require a Primer to Begin Synthesis?
RNA Polymerase
Transcription Unit
The stretch of DNA that is transcribed into an RNA molecule.
What Is the TATA Box?
A DNA sequence in eukaryotic promoters crucial in forming the transcription initiation complex.
What Comprises a Transcription Initiation Complex?
Transcription factors and RNA polymerase bound to a promoter.
What Happens at the 5′ End?
Receives a 5′ cap.
What Happens at the 3′ End?
An enzyme adds 50 to 205 more adenine nucleotides, forming a poly-A tail.
What Are Three Important Functions of the 5′ Cap and the Poly-A Tail?
- They facilitate the export of the mature mRNA from the nucleus.
- They help protect the mRNA from degradation by hydrolytic enzymes.
- They help ribosomes attach to the 5′ end of the mRNA once the mRNA reaches the cytoplasm.
Distinguish Between Introns and Exons
The noncoding segments of nucleic acid that lie between coding regions are called intervening sequences, or introns. The other regions are exons because they are eventually expressed.
What Are snRNPs?
Small nuclear ribonucleoproteins that recognize splice sites. Made up of RNA and protein molecules.
What Type of Small RNA Is the RNA in a snRNP?
Small Nuclear RNA
How Do Spliceosomes Work?
They interact with certain sites along an intron, releasing the intron and joining together the two exons that flanked the intron.
Explain How Splice Sites Are Recognized
snRNPs form a spliceosome on a pre-mRNA molecule. snRNA base-pairs with nucleotides at specific sites along the intron.
Ribozyme
RNA molecules that function as enzymes.
What Commonly Held Idea Was Rendered Obsolete by the Discovery of Ribozymes?
All biological catalysts are proteins.
What Are Three Properties of RNA That Allow It to Function as an Enzyme?
- Because RNA is single-stranded, a region of an RNA molecule may base pair with a complementary region elsewhere in the same molecule.
- Some of the bases in RNA contain functional groups that may participate in catalysis.
- The ability of RNA to hydrogen bond with other nucleic acid molecules adds specificity to its catalytic activity.
What Is the Consequence of Alternative Splicing of Identical mRNA Transcripts?
A single gene can encode more than one kind of polypeptide.
Anticodon
At the end of tRNA, a nucleotide triplet which base-pairs with a complementary codon on mRNA.
How Many Different Aminoacyl-tRNA Synthetases Are There?
20
Scientists Expected to Find One Aminoacyl-tRNA Synthetase per Codon, but Far Fewer Have Been Discovered. How Does Wobble Explain This?
Wobble explains why the synonymous codons for a given amino acid can differ in their third base, but usually not in their other bases.
Describe the Structure of a Eukaryotic Ribosome
A ribosome is made up of two large and small subunits. The ribosomal subunits are constructed of proteins and RNA molecules named ribosomal RNAs or rRNAs. In eukaryotes, they are made in the nucleolus.
How Does a Prokaryotic Ribosome Differ from a Eukaryotic Ribosome? What Is the Medical Significance of This Difference?
Those of eukaryotes are slightly larger and differ somewhat from bacterial ribosomes in their molecular composition. Certain antibiotic drugs can inactivate bacterial ribosomes without inhibiting the ability of eukaryotic ribosomes to make proteins.
What Is a Release Factor? By What Mechanism Is Termination Accomplished?
A protein that binds directly to the stop codon in the A site. The hydrolysis of two more GTP molecules.
Polyribosome
A string of ribosomes trailing along the mRNA.
What Are Some of the Things That Will Result in a Final-Form Functional Protein?
A polypeptide chain begins to coil and fold spontaneously as a consequence of its amino acid sequence, forming a protein with a specific shape.
Describe Three Types of Post-Translational Modifications
- The attachment of sugars, lipids, phosphate groups, or other additions.
- Removal of one or more amino acids from the leading end of the polypeptide chain.
- A polypeptide chain may be enzymatically cleaved into two or more pieces.
Mutation
Changes to the genetic information of a cell or virus.
Point Mutations
Chemical changes in a single base pair of a gene.
Frameshift Mutations
Insertion or deletion of nucleotides that alter the frame of the genetic message, the triplet grouping of bases on the mRNA that is read during translation.
Identify Two Mechanisms by Which Frameshifts May Occur
Base pair substitutions and base pair insertions or deletions.
Nonsense Mutation
A point mutation changes a codon for an amino acid into a stop codon and causes translation to be stopped prematurely.
Missense Mutation
Substitutions that change one amino acid to another one.
How Can a Base Pair Substitution Result in a Silent Mutation?
A change in base pair may transform one codon into another that is translated into the same amino acid.
What Are Two Categories of Mutagens?
Physical and chemical.
Describe the Action of Different Types of Chemical Mutagens
Base analogs are chemicals that are similar to normal DNA bases but pair incorrectly during DNA replication. Some other chemical mutagens interfere with correct DNA replication by inserting themselves into the DNA and distorting the double helix.
Describe Two Important Ways in Which Bacterial and Eukaryotic Gene Expression Differ
A bacterial cell ensures a streamlined operation. In the absence of a nucleus, it can simultaneously transcribe and translate the same gene. The eukaryotic cell’s nuclear envelope segregates transcription from translation and provides a compartment for extensive RNA processing.
Gene
A DNA sequence that codes for a specific polypeptide chain.