From DNA to Protein: Understanding Gene Expression
The Central Dogma of Molecular Biology
DNA affects the traits of an organism by providing the instructions for synthesizing proteins. Gene expression involves two main phases: transcription and translation.
Messenger RNA (mRNA) molecules contain information that is used to synthesize polypeptides. To begin transcription, RNA polymerase must bind to a segment of DNA called the promoter. The strand of DNA that is not transcribed is called the coding strand.
Transcription and Translation
During translation, a ribosome assembles a polypeptide whose amino acid sequence is specified by the nucleotide sequence in a molecule of mRNA. Crick and his colleagues proposed that the genetic code consists of a series of blocks of information, called codons.
Gene expression refers to the combined processes of transcription and translation. During splicing of mRNA in eukaryotes, some sequences are cut out of the primary transcript and the remaining sequences are joined together. Most eukaryotic genes contain noncoding sequences called introns.
RNA Processing and the Genetic Code
To remove noncoding sequences in the pre-mRNA of eukaryotes, multiple snRNPs combine with proteins to form a larger complex called the spliceosome. The connection that exists between genes and hereditary traits is based on using the information encoded in genes to synthesize proteins.
Both DNA and RNA are made up of building blocks known as nucleotides. The “one-gene/one-enzyme” hypothesis was proposed by Beadle and Tatum. The polypeptide-making organelles, which consist of protein combined with RNA, are called ribosomes.
Translation and the Role of tRNA
During translation, amino acids are carried to the ribosome by tRNA. During transcription, RNA polymerase synthesizes a molecule of RNA using DNA as a template. Which base in an anticodon will pair with the base adenine in a codon? Uracil.
A codon is composed of three bases. In eukaryotes, translation takes place on ribosomes. Ribosomes are complex aggregates of RNA and proteins. In prokaryotes, the form of RNA polymerase that can accurately initiate synthesis of RNA is called the holoenzyme.
The Mechanics of Translation
During translation, nucleotide sequence information is changed into amino acid sequence information. How many unique mRNA codons can be constructed from the four different RNA nucleotides? 64.
Ribosome movement along the mRNA is called translocation. Specific amino acids are attached to tRNA molecules by aminoacyl-tRNA synthetases. Codons that serve as “stop” signals for translation are recognized by release factors.
When a polypeptide is being assembled, the bond that forms between a newly added amino acid and the previous amino acid in the chain is a peptide bond. During translation in prokaryotes, formation of the initiation complex requires all of the following except RNA polymerase.
Introns, Exons, and mRNA Processing
Eukaryotic mRNA molecules may contain non-coding sequences that must be removed before translation. These are called introns. The location of translation in prokaryotic cells is on ribosomes. In eukaryotes, pre-mRNA processing may involve all of the following except removal of exons from the pre-mRNA.
During translation, uncharged tRNA molecules leave the ribosome from the E site. If the sequence of bases in the template strand of a DNA molecule is 3′ ATCGCTCC 5′, the complementary RNA sequence would be 5′ UAGCGAGG 3′. Which tRNA anticodons would pair with the mRNA that is coded for by this sequence? AUG CGU.
tRNA and the Wobble Hypothesis
Although 61 different codons code for amino acids, cells contain fewer than 61 different tRNAs. Why? Because the 5′ base on the tRNA anticodon has some flexibility (wobble); thus, some tRNA anticodons can pair with more than one mRNA codon.
Prokaryotic vs. Eukaryotic Gene Expression
Eukaryotic and prokaryotic organisms differ in how they process genetic information. Which statements best explain one of these differences? In prokaryotes, translation can begin before transcription is complete. In eukaryotes, transcription and translation are separated in space and time.
What is the first step during transcription initiation in prokaryotes? RNA polymerase binds to the promoter. During translation, translocation refers to moving the ribosome along the mRNA molecule.
RNA Splicing and Genetic Mutations
During protein synthesis in eukaryotes, what happens during RNA splicing? The product of transcription, called the primary transcript, is cut and put back together to produce the mature mRNA.
You are conducting a genetic screen to isolate nutritional mutants in yeast. Specifically, you want to isolate a double mutant that is auxotrophic for methionine and uracil. How would you do this? Grow mutagenized yeast on rich media. Then grow them on minimal media supplemented with methionine and uracil. Finally, replica plate the colonies onto minimal media. Colonies that grow on the supplemented media but not on minimal media are likely double mutants.
Given the sentence “THE FAT CAT ATE THE RED RAT,” which of the following would represent a frameshift mutation? THE FAC ATA TET HER EDR AT
Spliceosomes and RNA Polymerase
During the splicing reaction, the intron-exon junctions are recognized by snRNPs. In prokaryotes, the RNA polymerase holoenzyme consists of the core polymerase plus a sigma subunit. Within the transcription bubble, the 9 most recently added nucleotides in the newly synthesized RNA strand temporarily form a helix with the template DNA strand. The position of the 3′ end of the RNA within the transcription bubble is determined by the position of the RNA polymerase active site.
Alternative Splicing and Protein Diversity
You are working to characterize a novel protein in mice. Analysis shows that high levels of the primary transcript that codes for this protein are found in the pancreas and the liver. However, the protein itself is found only in the pancreas. What is the most likely explanation for this observation? Alternative splicing in the pancreas yields a mature mRNA that can be translated, while alternative splicing in the liver yields a mature mRNA that cannot be translated.