MB PRELIM LEC 3: RNA TRANSCRIPTION Flashcards
Copying of 1 strand of DNA into RNA
mRNA carries the information in DNA to the ribosomes, where it is translated into protein
TRANSCRIPTION
DNA can only store information. How will this information be used?
____________
- It is the production of RNA and protein using a DNA template
GENE EXPRESSION
Setting the stage for transcription to begin:
DNA must be released locally from histones & the helix unwound, involve the participation of:
DNA BINDING PROTEINS
TRANSCRIPTION FACTORS
HISTONE MODIFICATION ENZYMES
RNA POLYMERASE
RNA polymerase & its supporting accessory proteins assemble on DNA at a specific site (promoter)
TRANSCRIPTION INITIATION
PROKARYOTE:
_____________: assembly of large & small subunits of RNA polymerase & additional sigma factors at the promoter
BASAL TRANSCRIPTION COMPLEX
EUKARYOTES:
_________: assembly of RNA polymerase & up to 20 additional factors for accurate initiation
TRANSCRIPTION COMPLEX
how many bases/sec can RNA polymerase build?
50 - 100 bases/sec
how many bases/sec can DNA polymerase build?
1,000 bases/sec
Some are responsive to protein products; HIGH LEVEL OF GENE PRODUCT induce termination of its own synthesis.
TERMINATION IN?
PROKARYOTES
LARGEST COMPONENT of cellular RNA (80%-90% of the total cellular RNA)
- Various types are named for their sedimentation coefficient (S) in density-gradient centrifugation
- Important in structural & functional part of the ribosomes
RIBOSOMAL RNA
Initial connection between the information stored in DNA & the translation apparatus that will ultimately produce the protein products responsible for the phenotype
MESSENGER RNA
MESSENGER RNA IN __________ are:
- Synthesized & simultaneously translated into protein
- Sometimes POLYCISTRONIC (1 mRNA codes for more than 1 protein)
PROKARYOTES
MESSENGER RNA IN __________ are:
- MONOCISTRONIC (having only 1 protein per mRNA)
- Can produce different proteins from the same DNA sequences by:
- starting the RNA synthesis in different places or
- by processing the mRNA differently
EUKARYOTES
EUKARYOTES mRNA
messages are transcribed CONSTANTLY & are relatively abundant in the cell
CONSTITUTIVE TRANSCRIPTION
EUKARYOTES mRNA
messages are transcribed only a CERTAIN TIMES during the cell cycle under particular conditions
INDUCIBLE/REGULATORY TRANSCRIPTION
Addition of adenosines to the 3’ end of mRNA
POLYADENYLATION
is the enzyme responsible for adding the adenines to the end of the subscript
POLYADENYLATE POLYMERASE
- A structure (5’-5’ pyrophosphate linkage of 7-methyl guanosine to either 2’ O-methyl adenine of the mRNA) that BLOCKS THE EUKARYOTIC mRNA at the 5’ terminus
- Confers a protective function & serves as a recognition signal for the translational apparatus
CAP
Removal of intron sequences from mRNA
SPLICING
noncoding (intervening) sequences, does not code for amino acids
INTRONS
Remaining sequences that code for the protein product
EXONS
newly transcribed mRNA, much larger than mature RNA because it contains introns
HETERONUCLEAR RNA (hnRNA)
Splicing Mechanism:
Special sequences of RNA that are able to splice themselves
SELF - SPLICING
Splicing Mechanism:
Nuclear macromolecule complex within which splicing reactions occur to remove introns from pre-mRNAs
SPLICEOSOME
- Removal of introns from RNA USING DIFFERENT BREAKPOINTS
- Exons from the same gene are joined in different combinations, leading to different, but related, mRNA transcripts
- Modifies products of gene by ALTERNATE INSERTION OF DIFFERENT EXONS
- Has been found in over 40 different genes
ALTERNATIVE SPLICING
Some of the diseases resulting from abnormalities in splicing process:
- mutations in the splice recognition sequences of the ẞ-globin genes
ẞ-THALASSEMIAS
Some of the diseases resulting from abnormalities in splicing process:
- production of antibodies to RNA- protein complexes
CERTAIN AUTOIMMUNE CONDITIONS
- FUNCTIONS IN SPLICING IN EUKARYOTES, allowing for precise alignment and correct excision of introns
- Stays in the nucleus after its transcription by RNA polymerase I or III
SMALL NUCLEAR RNA (snRNA)
- ADAPTOR MOLECULES during the translation process
- Relatively short, single-stranded polynucleotides of 73-93 bases in length, MW 24,000-31,000
- At least 1 tRNA for each amino acid
CCA at the 3’ end: amino acid will be covalently attached to the tRNA
TRANSFER RNA (tRNA)
the end of tRNA to which an amino acid becomes bound
ACCEPTOR ARM
tRNA loops:
TψC loop
ANTICODON LOOP
VARIABLE LOOP
D LOOP
TψC loop ( stands for the modified pseudouridine), seven-base loop, contains the sequence _________-
5’ - TTCG - 3’
SPECIAL RECOGNITION SITE FOR THE RIBOSOME to allow a tRNA-ribosome complex to form during the process of protein synthesis
TψC loop
Larger in longer tRNAs, HELPS IN RECOGNITION OF THE tRNA molecule
VARIABLE LOOP
- 8- to 12-base loop, relatively rich in dihydrouridine (modified nucleotide), plays an important role in STABILIZING RNA STRUCTURE
D LOOP
solved the 1st tRNA sequence (alanine tRNA of yeast = 76 bases long & 10 of these are modified)
ROBERT HOLLEY & COLLEAGUES AT CORNELL UNIV. 1964
OTHER RNA’S FUNCTIONS:
- RNA synthesis & processing
- Influence numerous cellular processes such as?
- PLASMID REPLICATION
- BACTERIOPHAGE DEVELOPMENT
- CHROMOSOME STRUCTURE AND DEVELOPMENT
- It refers to any events in the life cycle of RNA molecules:
● Transcription
● Folding/unfolding
● Modification
● Processing
● Degradation
RNA METABOLISM
OTHER RNA-METABOLIZING ENZYMES
- Ubiquitous, stable enzymes that degrade all types of RNA
RIBONUCLEASES
OTHER RNA-METABOLIZING ENZYMES
- Required in RNA synthesis & processing to catalyze the UNWINDING of dsRNA
- Have been characterized in prokaryotes & eukaryotes
- Some work exclusively on RNA, others on DNA (RNA heteroduplexes & DNA substrates
- Involved in removal of proteins from RNA-protein complexes
RNA HELICASES
- Production of RNA and protein using a DNA template
- Key determinant of phenotype
- Some genes (products are in continual use by the cell), gene expression is constant (constitutive)
- Other genes, gene expression is tightly regulated throughout the life of the cell
GENE EXPRESSION
Most immediate & well-studied level of control of gene expression:
TRANSCRIPTION INITIATION
Regulation of mRNA Synthesis at Initiation
2 FACTORS RESPONSIBLE:
CIS FACTORS & TRANS FACTORS
DNA sequences that MARK PLACES ON DNA involved in the initiation & control of RNA synthesis
CIS FACTORS
Proteins that BIND TO THE CIS SEQUENCES & direct the assembly of transcription complexes at the proper gene
TRANS FACTORS
SERIES OF STRUCTURAL GENES transcribed together on 1 mRNA & subsequently separated into individual proteins
OPERON
LOCATION OF CIS REGULATORY ELEMENTS:
- closer to the gene (proximal elements only)
PROKARYOTES OR EUKARYOTES?
PROKARYOTES
LOCATION OF CIS REGULATORY ELEMENTS:
- thousands of base pairs away from the genes they control (distal elements)
- in or around the genes they control (proximal elements)
PROKARYOTES OR EUKARYOTES?
EUKARYOTES
REPRESSOR PROTEIN BINDS TO THE OPERATOR SEQUENCE & prevents transcription of the operon
LAC OPERON IN THE ABSENCE OF LACTOSE
LACTOSE BINDS TO REPRESSOR PROTEIN & changes its conformation & lowers its affinity to bind the operator sequence, resulting in the expression of the operon
LAC OPERON IN THE PRESENCE OF LACTOSE
MODE OF REGULATION IN PROKARYOTES
- ENZYME INDUCTION
- ENZYME REPRESSION
- ACTIVATION
MODE OF REGULATION IN PROKARYOTES
- inducer prevents the repressor from binding the operator to turn on expression
EX: found in lac operon
ENZYME INDUCTION
MODE OF REGULATION IN PROKARYOTES
- corepressors must bind to a repressor to turn off expression
EX: found in arg operon
ENZYME REPRESSION
MODE OF REGULATION IN PROKARYOTES
- activator binds with RNA polymerase to turn on transcription
EX: found in mal operon
ACTIVATION
- Another mechanism of control in bacteria
- FORMATION OF STEMS & LOOPS IN THE RNA TRANSCRIPT by intrastrand
- H bonding of complementary bases (allow/prevent transcription)
ATTENUATION
Several factors affecting the stability of the RNA transcript:
- RNA STRUCTURE
- PRESENCE OF EXONUCLEASE & ENDONUCLEASE THAT DIGESTS RNA
POST - TRANSCRIPTIONAL REGULATION
- enzymatic & structural alterations in RNA interfere with its processing
- alternate splicing in combination with protein factors is responsible for many tissue- & development-specific gene expression patterns (hematopoiesis)
EUKARYOTES OR PROKARYOTES?
EUKARYOTES
POST - TRANSCRIPTIONAL REGULATION
- RNA transcription & translation are CONCURRENT, protecting the RNA from the processing & exogenous factors
- RNA stability is affected by secondary structure (folding of the RNA molecule) & polyadenylation of 3’ end of the transcripts
- codon usage & cofactor availability may alter translation speed
EUKARYOTES OR PROKARYOTES?
PROKARYOTES
