Chapter 17 Flashcards
What are the 3 major classes of RNA?
mRNA, rRNA, and tRNA
What is the purpose of mRNA?
carries genetic information from the DNA to the ribosomes (m for messenger)
What is the purpose of rRNA?
composes ribosomes, site of protein synthesis (r for ribosome)
What is the purpose of tRNA?
brings amino acids to the ribosomes (t for transfer)
In eukaryotic cells, transcription cannot begin until…
several transcription factors have bound to the promoter.
The anticodon of a particular tRNA molecule is
complementary to the corresponding mRNA codon.
true of a codon
- consists of three nucleotides
- is the basic unit of the genetic code.
- may code for the same amino acid as another codon
- never codes for more than one amino acid.
initiation
binding of transcription factors and RNA polymerase II to a promoter- transcription initiation complex
elongation
RNA polymerase untwists the double helix and transcription progresses at a rate of 40 nucleotides per second in eukaryotes. Assembles mRNA in 5’-3’ direction. No primer needed, start the chain from scratch.
termination
transcribes polyadenylation signal (AAUAAA), then enzymes cut the pre-mRNA free from polymerase and release it.
base-pair substitution
replaces one nucleotide and its partner with another pair of nucleotides
Silent mutations have no effect on the amino acid produced by a codon because of redundancy in the genetic code
Missense mutations
still code for an amino acid, but not necessarily the right amino acid
Nonsense mutations
change an amino acid codon into a stop codon, nearly always leading to a nonfunctional protein
Insertions and deletions
are additions or losses of nucleotide pairs in a gene
These mutations have a disastrous effect on the resulting protein more often than substitutions do
Insertion or deletion of nucleotides may alter the reading frame, producing a frameshift mutation
Mutagens
are physical or chemical agents that can cause mutations
a gene can be defined as
a region of DNA that can be expressed to produce a final functional product, either a polypeptide or an RNA molecule
polyribosome (or polysome)
A number of ribosomes can translate a single mRNA simultaneously,enable a cell to make many copies of a polypeptide very quickly
The termination of translation
occurs when a stop codon in the mRNA reaches the A site of the ribosome
The A site accepts a protein called a release factor
The release factor causes the addition of a water molecule instead of an amino acid
This reaction releases the polypeptide, and the translation assembly then comes apart
The elongation cycle of translation
Amino acids are added one at a time
Each new tRNA-amino acid complex at the second binding site receives a peptide from a tRNA at the first binding site, a peptide
bond forms, the first tRNA breaks away, and the ribosome moves forward one codon.
The initiation of translation
Small ribosomal subunit attaches to mRNA at start codon
Codons read in the 5’ to 3’ direction along m RNA. Anticodon of the initiator tRNA-methionine complex binds to start codon
Large ribosomal subunit joins to the small subunit
The initiation of translation
Chain initiation
Small ribosomal subunit attaches to mRNA at start codon
Codons read in the 5’ to 3’ direction along m RNA. Anticodon of the initiator tRNA-methionine complex binds to start codon
Large ribosomal subunit joins to the small subunit
introns
The noncoding segments of nucleic acid that lie between coding regions are called intervening sequences
exon
a sequence within the primary transcript that remains in the RNA, after RNA processing; also refers to the region of DNA from which this sequence was transcribed
RNA splicing
After synthesis of a eukaryotic primary RNA transcript, the removal of portions of transcript (introns) that will not be included in the mRNA and the joining together of the remaining portions (exons)
codon
A sequence of three nucleotides on an mRNA molecule
During translation, nucleotide base triplets (codons) in mRNA are read in sequence in which direction?
5’ → 3’ direction along the mRNA.
RNA polymerase
untwists a portion of the DNA double helix
What is a key difference between eukaryotic and prokaryotic gene expression?
In prokaryotic cells, the RNA transcript is immediately available as mRNA without processing.
Two different proteins with mostly different structures are translated from two different mRNAs. These mRNAs, however, were transcribed from the same gene. Which mechanism could best account for this?
Exons from the same gene could be spliced in different ways to make different mRNAs.
Which statement correctly describes mRNA processing?
Introns are cut out of the primary transcript, and the resulting exons are spliced together.
Which accurately describes the usual process of eukaryotic transcription?
Both introns and exons are transcribed, but the RNA transcribed from introns does not leave the nucleus.
summary of protein synthesis
Messenger RNA is made on a DNA template, and then amino-acid-bearing transfer RNAs bind to the mRNA through codon-anticodon pairing.
During translation in a eukaryotic cell, __________.
polypeptides are synthesized at ribosomes, according to instructions carried by mRNA
During translation, amino acid chain elongation occurs until __________
the ribosome encounters a “stop” codon
What is their proper sequence for these steps?
- translation
- RNA processing
- transcription
- modification of protein
3, 2, 1, 4
