Ch 7: From DNA to Protein Flashcards
What is true of eukaryotic mRNAs?
They are translated after they are exported from the nucleus
An RNA chain is synthesized in what direction?
5’-to-3’ only
In eukaryotes, which parts of a gene are transcribed into RNA?
Introns and exons: introns are later spliced out by the activity of the spliceosome, leaving only exons in the mature RNA
How many different aminoacyl-tRNA synthetases do most organisms have?
One for each amino acid, so 20 total.
What is false regarding codons in mRNA molecules?
Some codons code for more than one amino acid
Things that are true:
- some codons do not code of amino acids
- In some cases, several different codons code of the same amino acid
- all codons contain 3 nucleotides
It is thought that, in the early history of life, RNAs not only stored information, but acted as catalysts in the cells. Which reaction is catalyzed by a ribosome in present-day cells?
Peptide bond formation during protein synthesis
Gene expression
The conversion of DNA into RNAs and proteins
What is the flow of genetic information in all living cells?
DNA to RNA to protein
Overview of transcription
Transcription is catalyzed by the enzyme RNA polymerase, which uses nucleotide sequences in the DNA molecule to determine which strand to use as a template, and where to start and stop transcribing.
To begin transcription, RNA polymerase binds to specific DNA sites called promoters that lie immediately upstream of genes. Eukaryotic RNA polymerases require the assembly of a complex of general transcription factors at the promoter, whereas bacterial RNA polymerase requires only one additional subunit called sigma factor.
List the main types of functional RNAs
mRNAs: messenger RNAs, carry the instructions for making proteins
rRNAs: ribosomal RNAs, form the core of ribosomes
tRNAs: transfer RNAs, act as adaptor molecules in protein synthesis
What are the protein-coding genes in eukaryotic DNA comprised of?
Exons: the short, coding regions of the RNA
Introns: non-coding regions.
Both are transcribed into RNA, but the introns are removed from the RNA while it is still in the nucleus, during RNA splicing
Overview of RNA splicing
The process where introns are removed from RNA transcripts, and the exons are joined back together. This is catalyzed by RNAs that are part of small ribonucleoprotein complexes called snRNPs.
The splicing can be done a number of different ways on the same gene, allowing multiple different proteins to be produced from the same gene
What processing steps does mRNA go through before it becomes mature mRNA?
5’ RNA capping
3’ polyadenylation
splicing
Where does translation take place?
The cytoplasm, on large ribonucleoprotein assemblies called ribosomes. As the RNA moves through the ribosome, its message is translated into protein
Codons
sequences of 3 consecutive nucleotides that each correspond to one amino acid.
Most amino acids can be specified by more than one codon.
tRNAs
adaptor molecules in protein synthesis
Each tRNA contains a sequence of three nucleotides, the anticodon, which recognizes a codon in an mRNA through complementary base-pairing, and serves as the source of the amino acid added to the growing polypeptide chain
What starts protein synthesis?
the a ribosome assembles at an initiation codon (AUG) in an mRNA molecule. An RNA in the large ribosomal subunit catalyzes the stepwise linking of amino acids to form the polypeptide chain. A completed chain is released from the ribosome when a stop codon (UAA, UAG, or UGA) is reached
What does the protein concentration in a cell depend on?
The rate at which the mRNA and proteins are synthesized and degraded
Where does protein degradation occur?
in both the cytosol in nucleus, it occurs inside large protein complexes called proteasomes
What is the likely origin of life on earth?
It probably started with self-replicating ribozymes: RNA molecules that can catalyze their own synthesis.
RNA can act both as a catalyst for reactions, and as a carrier of genetic information, so the first cells likely used RNA before DNA replaced it as a more stable structure.
Alternative splicing
the production of different mRNAs (and thus proteins) from the same gene by splicing its RNA transcripts in different ways.
aminoacyl-tRNA synthesis
during protein syntehsis, an enzyme that attaches the correct amino acid to a tRNA molecule to form “charged” aminoacyl-tRNA. This is a high-energy bond, and the energy is used to covalently link the amino acid to the growing polypeptide chain
anticodon
set of 3 consecutive nucleotides in a transfer RNA molecule that recognizes, through base-pairing, the three-nucleotide codon on a messenger RNA molecule. This interaction helps to deliver the correct amino acid to a growing polypeptide chain
general transcription factor
proteins that assemble on the promoters of eukaryotic genes near the start dite of transcription and load the RNA polymerase in the correct position
initiator tRNA
Special tRNA that initiates the translation of an mRNA on a ribosome. It always carries the amino acid methionine (codon AUG)
mRNA
messenger RNA, the RNA molecule that specifies the amino acid sequence of a protein