MCQ1 Prep Flashcards
What is the Shine Dalgarno sequence?
Purine-rich (AG) region of prokarytoic mRNA upstream of start codon, helps to position ribosome to ensure correct translation.
AUG codon
The start codon, signals where translation should start, coding for methionine (formylmethionine in prokaryotes) establishing a reading frame.
mRNA
Messenger RNA is a transcribed copy of the genomic DNA that is taken to the ribosome to specify the amino acid sequence.
tRNA
Adapter molecule that brings specific amino acids to the ribosome. Each tRNA contains an anticodon region complementary to mRNA codon.
Reading frame
The way nucleotides of mRNA are divided into consecutive, non-overlapping triplets (codons). A frameshift mutation can result in a mismatched amino acid sequence.
Peptide bond formation
The carboxyl group and the amino group of two amino acids join to form a peptide bond, facilitated by peptidyl transferase.
Steps of translation of prokaryotes.
- Binding of small ribosomal subunit
- Positioning of start codon
- Anticodon alignment
- Joining of large ribosomal subunit
- Ready for elongation
Codon-anticodon recognition
Specific between mRNA codons and tRNA anticodons. Ribosomes proofread the tRNA before it is attached.
Role of RNA polymerase?
Responsible for synthesizing mRNA from the DNA template in transcription, catalyzing ribonucleotide addition in 5’ to 3’ direction.
Promoter sequence
Specific regions of DNA that signal for the start site of transcription. Easily recognized by RNA polymerase.
Differences in eukaryotic and prokaryotic transcription
In prokaryotes, transcription occurs in the cytoplasm, and involves simple transcription factors, whereas, in eukaryotes it has a complex array of transcription factors as well as regulatory mechanisms and takes place in the nucleus.
What is the significance of ATG codon?
DNA sequence TAC codes for ATG on RNA and signals methionine synthesis.
What machinery is involved in eukaryotic transcription?
3 RNA polymerases (I,II,III), and numerous transcription factors (TFIIA and TFIIB), as well as a pre-initiation complex.
Initiation of transcription
Transcription factors bind to the promoter region, followed by the assembly of the pre-initiation complex. RNA polymerase II joins the complex, and upon additional factor addition, transcription is initiated.
mRNA synthesis conclusion (Pro and eukaryotes)
In prokaryotes, a terminator sequence inhibits RNA polymerase, stopping transcription. In eukaryotes, a polyadenylation sequence is transcribed which leads to cleavage of nascent RNA and termination of transcription.
Post transcriptional modifications
5’ capping, 3’ polyadenylation and splicing (removing introns and joining exons) all occur in eukaryotes post transcription.
