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BrooklynMessersmith Ch 17 - Coggle Diagram
BrooklynMessersmith Ch 17
Translation
Elongation
Translation proceeds along the mRNA in a
5′ → 3′ direction
The ribosome and mRNA move relative to each other, codon by codon
Elongation occurs in three steps: codon recognition, peptide bond formation, and translocation
The ribosome and mRNA move relative to each other, codon by codon
Each addition involves proteins called elongation factors
During elongation, amino acids are added one
by one to the C-terminus of the growing chain
Termination
The release factor causes the addition of a water molecule instead of an amino acid
This reaction releases the polypeptide, and the translation assembly comes apart
The A site accepts a protein called a release factor
Initiation
Then the small subunit moves along the mRNA until it reaches the start codon
Proteins called initiation factors bring in the large subunit that completes the translation initiation complex
First, a small ribosomal subunit binds with mRNA and a special initiator tRNA
The start codon (AUG) signals the start of translation
tRNA
tRNAs transfer amino acids to the growing polypeptide in a ribosome
Structure
A tRNA molecule consists of a single RNA strand that is only about 80 nucleotides long
Because of hydrogen bonds, tRNA actually twists and folds into a three-dimensional molecule
Each has an anticodon on the other end; the anticodon base-pairs with a complementary codon on mRNA
tRNA is roughly L-shaped with the 5' and 3' ends both located near one end of the structure
Each tRNA molecule enables translation of a given mRNA codon into a certain amino acid
The protruding 3' end acts as an attachment site for an amino acid
A cell translates an mRNA message into protein with the help of transfer RNA (tRNA)
Transcription
Elongation
Transcription progresses at a rate of 40 nucleotides per second in eukaryotes
A gene can be transcribed simultaneously by several RNA polymerases
As RNA polymerase moves along the DNA, it untwists the double helix, 10 to 20 bases at a time
Nucleotides are added to the 3′ end of the
growing RNA molecule
Termination
The mechanisms of termination are different in bacteria and eukaryotes
In bacteria, the polymerase stops transcription at the end of the terminator and the mRNA can be translated without further modification
In eukaryotes, RNA polymerase II transcribes the polyadenylation signal sequence; the RNA transcript is released 10–35 nucleotides past this polyadenylation sequence
Initiation
A promoter called a TATA box is crucial in forming the initiation complex in eukaryotes
The completed assembly of transcription factors and RNA polymerase II bound to a promoter is called a transcription initiation complex
RNA
RNA polymerase does not need any primer
RNA synthesis follows the same base-pairing rules as DNA, except that uracil substitutes for thymine
The RNA is complementary to the DNA template strand
The DNA sequence where RNA polymerase attaches is called the promoter
RNA synthesis is catalyzed by RNA polymerase, which pries the DNA strands apart and joins together the RNA nucleotides
Mutations
Insertion & Deletion
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
Insertions and deletions are additions or losses of nucleotide pairs in a gene
Substitution
Silent mutations have no effect on the amino acid produced by a codon because of redundancy in the genetic code
A nucleotide-pair substitution replaces one nucleotide and its partner with another pair of nucleotides
Missense mutations still code for an amino acid, but not the correct amino acid
Nonsense mutations change an amino acid codon into a stop codon; most lead to a nonfunctional protein
Mutagens are physical or chemical agents that can cause mutations
Important terms
Signal recognition peptide
A signal-recognition particle (SRP) binds to the signal peptide
The SRP escorts the ribosome to a receptor protein built into the ER membrane
Polypeptides destined for the ER or for secretion are marked by a signal peptide
Polyribosome
Polyribosomes enable a cell to make many copies of a polypeptide very quickly
Multiple ribosomes can translate a single mRNA simultaneously, forming a polyribosome (or polysome)
RNA splicing
RNA splicing removes introns and joins exons, creating an mRNA molecule with a continuous coding sequence
In some cases, RNA splicing is carried out by spliceosomes
