1_43 Flashcards
M) PTMs, oxidative/substrate phosp., P) GC, elution, cross flow vs dead end (43 cards)
What are post-transcriptional modifications (PTMs)?
Post-transcriptional modifications are chemical and structural changes made to newly synthesized pre-mRNA (hnRNA) in eukaryotic cells after transcription but before translation.
These modifications convert the immature pre-mRNA into mature mRNA that is stable, export-ready, and can be translated into protein by ribosomes.
What are histones and what are common histone PTMs?
Histones are proteins around which DNA is wrapped in the nucleus to form nucleosomes, the basic unit of chromatin structure.
Common PTMs: Acetylation, methylation, phosphorylation, ubiquini
Histones: What is acetylation as a PTM?
Acetylation: Addition of an acetyl group (–COCH₃) to lysine residues. Carried out by histone acetyltransferases (HATs). Neutralizes positive charge, loosening DNA-histone interaction → promotes transcription. Removed by histone deacetylases (HDACs).
Histones: What is Methylation as a PTM?
Methylation: Addition of 1/2/3 methyl groups to lysine or arginine.
Effect: Can either activate or repress transcription, depending on the residue and methylation state which act as signals to turn genes on/off
Enzymes: Histone methyltransferases (HMTs) and demethylases.
Histones: What is phosphorylation as a PTM?
Addition of phosphate groups to serine, threonine, or tyrosine residues. Important in cell cycle regulation and DNA damage response.
Histones: What is ubiquitination as a PTM?
Addition of the small protein ubiquitin. Involved in transcriptional regulation and proteasomal degradation.
What is the function of histone PTMs?
- PTMs act as molecular signals, forming a “histone code” that recruits specific reader proteins to modulate chromatin structure.
- They determine whether DNA is in euchromatin (open, active) or heterochromatin (closed, repressed) state.
What is hnRNA?
Heterogeneous nuclear RNA (hnRNA) or pre-mRNA is the initial transcript produced from DNA during transcription.
It undergoes several post-transcriptional modifications before becoming mature mRNA that can be translated into protein.
What are main steps of PTMs involving hnRNA?
1) 5’ Capping
2) 3’ Polyadenylation
3) Splicing
What are exons and introns?
- Exons: Segments of pre-mRNA that code for proteins and are retained in the mature mRNA.
- Introns: Non-coding sequences that are removed during splicing.
What is alternative splicing?
- Alternative splicing is a process by which different combinations of exons are joined together to produce multiple mRNA variants from a single pre-mRNA transcript.
- This allows a single gene to code for multiple protein isoforms.
What are the types of alternative splicing?
- Exon skipping: An exon is skipped entirely.
- Mutually exclusive exons: Only one of two exons is included.
- Alternative 5’ splice site: A different start point of an exon is selected.
- Alternative 3’ splice site: A different end point of an exon is used.
- Intron retention: An intron remains in the mRNA (usually repressed or degraded).
What is the biological importance of alternative splicing?
- Increases proteomic diversity without increasing genome size.
- Regulates tissue-specific expression and developmental timing.
- Misregulation can lead to disease (e.g., cancer, spinal muscular atrophy).
PTMs: What is the 5’ capping?
o A 7-methylguanosine cap is added to the 5’ end of the pre-mRNA.
o Functions:
1) Protects mRNA from degradation
2) Facilitates ribosome binding during translation
3) Helps in nuclear export of mRNA.
PTMs: What is the 3’ Polyadenylation?
o Addition of a poly(A) tail (a stretch of ~200 adenine nucleotides) to the 3’ end.
o Poly(A) polymerase cleaves at the 3’ end pre-mRNA and then adds the poly(A) tail
o Functions: Enhances mRNA stability, Assists in nuclear export, Promotes translation efficiency.
PTMs: What is the Splicing?
o Removal of introns (non-coding regions) and joining of exons (coding sequences).
o Carried out by the spliceosome, a complex of small nuclear RNAs and proteins (snRNPs).
o Enables the production of mature mRNA that contains only coding sequences.
What is substrate-level phosphorylation?
It is the direct transfer of a phosphate group from a phosphorylated intermediate to ADP to form ATP, using an enzyme.
In which cellular processes does substrate-level phosphorylation occur?
It occurs in glycolysis and the citric acid (Krebs) cycle.
Does substrate-level phosphorylation require oxygen?
No, it does not require oxygen.
How much ATP is produced via substrate-level phosphorylation per glucose molecule?
It produces a small amount, typically 2–4 ATP per glucose molecule.
What is oxidative phosphorylation?
It is the process of ATP synthesis powered by electrons passing through the electron transport chain and driven by a proton gradient across the inner mitochondrial membrane.
Where does oxidative phosphorylation take place?
In the inner mitochondrial membrane.
Does oxidative phosphorylation require oxygen?
Yes, oxygen is the final electron acceptor in the electron transport chain.
What role does the electron transport chain play in oxidative phosphorylation?
It transfers electrons from NADH and FADH₂, pumps protons to create a gradient, and drives ATP synthesis via ATP synthase.
