RNA and the Genetic Code Flashcards
Central Dogma of Biology
- DNA
a. Transcription (in 5’ –> 3’ direction) - RNA
b. Translation (N-terminus –> C-terminus direction) - Protein
OR
- RNA
a. Reverse Transcriptase - DNA
b. Replication - DNA
And then begin the top sequence….
Messenger RNA (mRNA)
Carries info. specifying amino acid sequence from DNA in the nucleus to the cytoplasm to be Transcribed by RNA polymerase and then translated into proteins at the ribosome through codons
RNA Polymerase
Transcribes mRNA which can then be translated into proteins through codons
Monocistronic
Eukaryotes:
Each mRNA translates into one protein product
Polycistronic
Prokaryotes:
Beginning translation at different parts of the mRNA resulting in new proteins from the same original genes
Transfer RNA (tRNA)
- Converts the language of nucleic acids into amino acids and peptides
- The codon of mRNA recognized by anticodon of tRNA
- The 3’ end of tRNA connects an amino acid becoming charged or activated in the cytoplasm
- Each amino acid is activated by Aminoacyl-tRNA-Synthetase and ATP
Ribosomal RNA (rRNA)
Synthesized in the Nucleolus and used during protein assembly in the cytoplasm at the ribosome; Enzymatically active
Ribozymes
Catalyze the formation of peptide bonds
Characteristics of the Genetic Code
- Unambiguous
- Degenerate
- Wobble Position
Unambiguous
Each codon is specific for an amino acid
Degenerate
More than one codon can speficy an amino acid
Wobble Position
3rd base of a codon can change without coding for a new amino acid (silent or degenerate position) to protect against changes in DNA or RNA
Point Mutation
Affects one of the nucleotides in a codon
Missense Mutation
Expressed:
One amino acid is substituted for another (produces a codon that changes the amino acid)
Nonsense Mutation
Expressed:
Truncation - codes for a premature stop codon
Frameshift Mutation
Nucleotides are added or deleted from the mRNA that changes reading frame of subsequent codons
Stop Codons
UAA
UGA
UAG
Start Codons
AUG - Codes for Methionine
Transcription
Creation of mRNA from DNA Template
- RNA is synthesized by DNA dependent RNA Polymerase
RNA Polymerase II
Binds to the TATA box (-25) and is the main player in transcribing mRNA
Transcription Factors
Protein that controls the rate of transcription of genetic information from DNA to messenger RNA, by binding to the TATA promoter and aiding RNA Polymerase
RNA Polymerase I
In the Nucleolus to synthesize rRNA
RNA Polymerase II
In the Nucleus - Synthesizes the hnRNA (pre-mRNA) and small nuclear RNA (snRNA)
RNA Polymerase III
In the Nucleus - synthesizes tRNA and some rRNA
Post-Transcriptional Processing
Maturation of hnRNA into mRNA
Splicing
Involved in Transcription: Removing introns (non-conding DNA) and ligating exons (coding DNA) by the spliceosome.
snRNA and snRNP complex recognizes splice sites and forms a lariat and then degrades what is not needed
5’ Cap
Involved in Transcription:
7-methylguanylatetriphosphate Cap
- Recognized by the ribosome as the binding site
- Protects the mRNA from degradation in the cytoplasm
Poly (A) Tail
Involved in Transcription:
Added to 3’ end to protect again rapid degradation; Also assists with export from the nucleus
Translation
In the cytoplasm - Converting the mRNA transcript into a functional protein
Ribosome
Involved in Translation:
Composed of rRNA and proteins that contain subunits which bind together during protein synthesis. Overall the mRNA and aminoacyl-tRNA complex are brought together to generate the protein
Initiation
Involved in Translation:
Small ribosomal subunits binds to mRNA and then the large subunits binds the small subunit assested by initiation factors (IFs)
Steps of Translation
- Initiation
- Elongation
- Termination
Initiation in Prokaryotes
30S subunit binds the Shine-Dalgamo Sequence at the Fmet start site
Initiation in Eukaryotes
40S subunit binds the 5’cap at the AUG (methionine) start codon
Elongation
Involved in Translation: Occurs for each amino acid and consists of: - A site - P site - E site - Elongation factors - Signal sequences
A Site
Part of Elongation - holds the incoming aminoacyl tRNA complex
P Site
Part of Elongation - holds the tRNA that carries the growing peptide chain and forms the peptide bond with peptidyltransferase and GTP
E Site
Part of Elongation - inactivated (uncharged) tRNA pauses and then exits the ribosome
Elongation Factors
Part of Elongation - locate and recruit charged tRNA and GTP and removed GDP
Signal Sequences
Part of Elongation - tell a peptide where to go so that they can be secreted
Termination
Involved in Translation:
Stop codon moves into A site and releases factor binds adding a water to the peptide. Termination factors hydrolyze the completed polypeptide from the final tRNA and ribosomal units dissociate
Post Translational Processing
- Proper protein folding by chaperones
- Formation of the quaternary structure
- Cleavage of proteins or signal sequences
- Addition of other biomolecules like phosphorylation, carboxylation, glycosylation, etc…
Operon
Inducible or repressible clusters of genes transcribed as a single mRNA
What’s Involved in Gene Expression?
Starting Upstream…
Regulator - Codes for the repressor
Promotor - binds RNA Polymerase
Operator - non-transcribable region binds repressor
Structural - codes for the protein of interest
Inducible System
Repressor bound tightly to the operator system and acts as a road block. Inducer must bind the repressor protein so that RNA Polymerase can move down the gene
Repressible System
Allows constant production of a protein product. The repressor is inactive until it binds to corepressor which then binds the operator to prevent more transcription
Negative Control
Product can be co-repressed to end transcription
Transcription Factors in Eukaryotes
- DNA Binding Domain that binds nucleotide in promotor region
- Activation Domain that binds TFs and regulatory proteins to remodel chromatin
Gene Amplification
Ekuaryotes: Signal molecules (steroids and secondary messengers) which bind the TFs that attach to a sequence DNA (Response Element). Once bound the TFs promote increased expression of the gene
Enhancer
Response element group that controls gene expression by multiple signals
Duplication
Eukayotes:
Genes that can be duplicated in series on the same chromosome OR in parallel by opening the gene with helicases and permitting DNA replication only of that gene
Histone Acetylase
Chromatin Remodeling:
Acetylate lys residue decreases the positive charge to open chromatin
Histone Deacetylase
Chromatin Remodeling:
Removes acetyl from histones to close the chromatin
DNA Methylases
Chromatin Remodeling:
Add methyls to nucleotides to silence gene expression
Silent Mutation
Have no effect on protein synthesis
Transcription steps in Eukaryotes
- Helicase and Topoisomerase unwind DNA double helix
- RNA Polymerase II bind to TATA Box within promoter region of gene
- hnRNA synthesized from DNA template (Antisense strand)
- 5’ Cap added
- Poly (A) Tail added to 3’ end
- Splicing done by splicesome; Introns removed and exons ligated together
Alternative Splicing
Combines different exons to acquire different gene products
Promoters
Transcription Factor:
DNA that initiates transcription of a particular gene. Found within 25 base pairs of the transcription start site
Enhancers
Transcription Factor:
Short region of DNA that can be bound by proteins (activators) to increase the likelihood that transcription of a particular gene will occur. Found more than 25 base pairs away from the transcription start site
