Chapter 17 Flashcards
Faulty gene
Gene that contains incorrect information.
The DNA inherited by an organism leads to specific traits by dictating the synthesis of proteins and of RNA molecules involved in protein synthesis. In other words, proteins are the link between genotype and phenotype.
True
One gene-one enzyme hypothesis
Initially proposed by Archibald Garrod and dubbed by Beadly and Tatum, states that the function of a gene is to dictate the production of a specific enzyme.
One gene-one polypeptide hypothesis
States that each gene dictates the production of a single polypeptide chain.
Getting from DNA to protein requires two major stages:
Transcription and translation
Transcription
Is the synthesis of RNA using information in the DNA.
Messenger RNA (mRNA)
A type of RNA, synthesized using a DNA template, that attaches to ribosomes in the cytoplasm and specifies the primary structure of a protein.
Translation
The synthesis of a polypeptide using the information in the mRNA.
The sites of translation:
Ribosomes
In eukaryotic cells, transcription occurs in the…
Nucleus
The transcription of a protein-coding eukaryotic gene results in pre-mRNA, and further processing yields the finished mRNA.
True
Primary transcript
An initial RNA transcript from any gene; also called pre-mRNA when transcribed from a protein-coding gene.
Cells are governed by a molecular chain of command with a directional flow of genetic information, called the central dogma; shown here:
DNA —> RNA —> Protein
Triplets of nucleotide bases are the smallest units of uniform length that can code for all the amino acids.
True
Triplet code
The genetic instructions for a polypeptide chain are written in the DNA as a series of nonoverlapping, three nucleotide words.
For each gene, only one of the two DNA strands is transcribed; this strand is called the…
Template strand because it provides the pattern, or template, for the sequence of nucleotides in an RNA transcript.
Like a new strand of DNA, the RNA molecule is synthesized in an antiparallel direction to the template strand of DNA.
True
Codons
mRNA nucleotide triplets. Customarily written in the 5’ —> 3’ direction.
The term codon is also used for the DNA nucleotide triplets along the non template strand.
True
Coding strand
The non template DNA strand.
Because codons are nucleotide triplets, the number of nucleotides making up a genetic message must be three times the number of amino acids in the protein product.
True
61 of the 64 triplets code for amino acids. The three codons that do not designate amino acids are “stop” signals, or termination codons, marking the end of translation.
True
Codon AUG
Has a dual function: it codes for the amino acid methionine (Met) and also functions as a “start” signal, or initiation codon.
Reading frame
On an mRNA, the triplet grouping of ribonucleotides used by the translation machinery during polypeptide synthesis.
RNA polymerase
An enzyme that pries the two strands of DNA apart and joins together RNA nucleotides complementary to the DNA template strand, thus elongating the RNA polynucleotide.
RNA polymerases can assemble a polynucleotide only in its 5’ —> 3’ direction.
True
RNA polymerases are able to start a chain from scratch; they don’t need a primer.
True
Promoter
The DNA sequence where RNA polymerase attaches and initiates transcription.
In bacteria, the sequence that signals the end of transcription is called the ____
Terminator
Molecular biologists refer to the direction of transcription as “downstream” and the other direction as “upstream”.
True
Transcription unit
The stretch of DNA downstream from the promoter that is transcribed into an RNA molecule.
RNA polymerase II
The RNA polymerase used for pre-mRNA synthesis.
Three stages of transcription
Initiation, elongation, and termination of the RNA chain.
Start point
The nucleotide where RNA synthesis actually begins.
-Is part of the promoter.
Transcription factors
Mediate the binding of RNA polymerase and the initiation of transcription.
-Present only in eukaryotes.
Only after transcription factors are attached to the promoter does RNA polymerase II bind to it.
True
Transcription initiation complex
The whole complex of transcription factors and RNA polymerase II bound to the promoter.
TATA box
A DNA sequence in eukaryotic promoters crucial in forming the transcription initiation complex.
Transcription progresses at a rate of about ________ nucleotides per second in eukaryotes.
40
Which sequence marks the end of mRNA transcription in eukaryotes?
The polyadenylation sequence (AAUAAA)
RNA processing produces…
An mRNA molecule ready for translation.
5’ cap
A modified form of guanine (G) nucleotide added onto the 5’ end after transcription of the first 20-40 nucleotides.
The 3’ end of the pre-mRNA molecule is also modified before the mRNA exits the nucleus.
True
poly-A tail
An additional 50-250 adenine nucleotides added to the 3’ end of the mRNA in RNA processing.
The 5’ cap and the poly-A tail share several important functions. First, they seem to facilitate the export of the mature mRNA from the nucleus. Second, they help protect the mRNA from degradation by hydrolytic enzymes. And third, they help ribosomes attach to the 5’ end of the mRNA once the mRNA reaches the cytoplasm.
True
UTR
Untranslated region
-Function in ribosome binding
RNA splicing
After synthesis of a eukaryotic primary RNA transcript, the removal of portions of the transcript (introns) that will not be included in the mRNA and the joining together of the remains portions (exons).
Most eukaryotic genes and their RNA transcripts have long noncoding stretches of nucleotides, regions that are not translated.
True
Intervening sequences or introns
The noncoding segments of nucleic acid that lie between coding regions.
Exons are so named because…
They are eventually expressed, usually by being translated into amino acid sequences.
-Or because they exit the nucleus
The terms intron and exon are used for both RNA sequences and the DNA sequences that encode them.
True
Spliceosome
A large complex made of proteins and small RNAs that accomplishes the removal of introns.
Ribozymes
RNA molecules that function as enzymes.
Self-splicing
When the intron RNA functions as a ribozyme and catalyzes it’s own excision.
Three properties of RNA enable some RNA molecules to function as enzymes. First, because RNA is single-stranded, a region of an RNA molecule may base-pair, in an antiparallel arrangement, with a complementary region elsewhere in the same molecule; this gives the molecule a particular three-dimensional structure. Second, like certain amino acids in an enzymatic protein, some of the bases in RNA contain functional groups that can participate in catalysis. Third, the ability of RNA to hydrogen-bond with other nucleic acid molecules (either RNA or DNA) adds a specificity to its catalytic activity.
Alternative RNA splicing
A type of eukaryotic gene regulation at the RNA-processing level in which different mRNA molecules are produced from the same primary transcript, depending on which RNA segments are treated as exons and which as introns.
Because of alternative splicing, the number of different protein products an organism produces can be much greater than its number of genes.
True
Proteins often have a modular architecture consisting of discrete structural and functional regions called ________
Domains
In quite a few cases, different exons code for the different domains of a protein.
True
Exon shuffling
Introns crossing over with exons potentially creating new proteins.
Transfer RNA (tRNA)
-The translator
Functions to transfer amino acids to a growing polypeptide in a ribosome.
A tRNA molecule consists of a single RNA strand that is only about 80 nucleotides long (whereas most mRNA molecules have hundreds of nucleotides).
True
Anticodon
The particular nucleotide triplets that base-pairs to a specific mRNA codon.
The loop extending from one end of the L-shaped tRNA includes the anticodon. From the other end of the L protrudes its 3’ end, which is the attachment site for an amino acid.
True
The tRNA molecule is a translator in the sense that it can read a nucleic acid word (the mRNA codon) and interpret it as a protein word (the amino acid).
True
Aminoacyl-tRNA synthetase
An enzyme that joins each amino acid to the appropriate tRNA.
Linkage if a tRNA to its amino acid is an endergonic process that occurs at the expense of ATP, which loses two phosphate groups, becoming Amp (adenosine monophophate).
True
Aminoacyl tRNA or charged tRNA
tRNA with its corresponding amino acid covalently bonded to it.
Wobble
The flexible base pairing at the third codon position that allows for some tRNAs to bind to more than one codon.
In ribosomes, a large and a small subunit join to from a functional ribosome only when attached to an mRNA molecule.
True
P site
Peptidyl-tRNA binding site holds the tRNA carrying the growing polypeptide chain.
A site
Aminoacyl-tRNA binding site) holds the tRNA carrying the next amino acid to be added to the chain.
E site
Discharged tRNAs leave the ribosome from the exit site.
As the polypeptide becomes longer, it passes through an exit tunnel in the ribosome’s large subunit.
True
Translation initiation complex
The union of mRNA , initiator tRNA, and a small ribosomal subunit followed by the attachment of a large ribosomal subunit.
Initiation factors
Are required to form the translation initiation complex.
In the elongation stage of translation, amino acids are added one by one to the previous amino acid at the C-terminus of the growing chain.
True
Elongation factors
Are required in the elongation stage of translation.
The mRNA is moved through the ribosome in one direction only, 5’ end first.
True
Release factor
Causes the addition of a water molecule instead of an amino acid to the polypeptide chain.
-Shaped like a tRNA, instead of an aminoacyl tRNA
Breakdown of the translation assembly requires the hydrolysis of two more GTP molecules.
True
Polypeptide synthesis always begins in the cytosol as a free ribosome starts to translate an mRNA molecule. There the process continues to completion— unless the growing polypeptide itself cues the ribosome to attach to the ER.
True
Signal peptide
Targets the protein to the ER
Signal recognition particle (SRP)
A protein-RNA complex that recognizes signal peptides and functions as an escort that brings the ribosome to a receptor protein built into the ER membrane.
Polyribosomes (polysomes)
Enable a cell to make many copies of a polypeptide very quickly.
Point mutations
Changes in a single nucleotide pair of a gene.
Point mutations within a gene can be divided into two general categories:
-Single nucleotide-pair substitutions
- nucleotide-pair insertions or deletions
Nucleotide-pair substitution
The replacement of one nucleotide and its partner with another pair of nucleotides.
Silent mutation
Has no observable effect on the phenotype.
Missense mutations
Substitutions that change one amino acid to another.
Nonsense mutation
A point mutation that changes a codon for an amino acid into a stop codon.
Insertions and deletions
Are additions or losses of nucleotide pairs in a gene.
Frameshift mutation
Occurs whenever the number of nucleotides inserted or deleted is not a multiple of three.
Mutagens
Physical and chemical agents that interact with DNA in ways that cause mutations.
