Lecture 18 Flashcards
What was the nature of the defect that caused hemophilia in the family of Tsar Nicholas?
Point mutation in F9, a gene on the X chromosome that encodes blood coagulation factor IX; this A to G mutation occurs just upstream of exon 4 in the F9 gene and is predicted to create a new splice acceptor site that could lead to production of a truncated factor IX protein
What is the concept of collinearity and noncollinearity?
Collinearity: Suggests that a continuous sequence of nucleotides in DNA encodes a continuous sequence of AA in a protein; number of nucleotides in a gene should be proportional to the number of AA in the encoded protein
Noncollinearity: DNA much longer than mRNA; was discovered in eukaryotic genes through hybridization
What are introns and exons?
Introns: Noncoding sequences
Exons: Coding sequences
What are the components of a gene?
- DNA sequences that code for all exons and introns
- Sequences at the beginning and end of the RNA that are not translated into a protein, including the entire transcription unit; promoter, RNA coding sequence, and terminator
What are the primary post transcriptional modifications to mRNA?
- Addition of 5’ cap, which facilitates binding of ribosome to 5’ end of mRNA
- 3’ cleavage and addition of poly(A) tail, increases stability of mRNA, facilitates binding of ribosome to mRNA
- RNA splicing via spliceosome, removes introns and splices together exons and exports mRNA exons to cytoplasm
- RNA editing, which alters nucleotide sequence of mRNA
What is the basic concept associated with mRNA splicing?
Critical consensus sequences are present at the 5’ splice site and the 3’ splice site; a weak consensus sequence exists at the branch point (in the middle of the intron)
What are guide RNAs?
Has sequences that are partly complementary to those of the preedited mRNA and pairs it with it; after pairing, the mRNA undergoes cleavage and new nucleotides are added, with sequences in the gRNA serving as a template; ends of the mRNA are then joined together
Basically: Guide RNA adds nucleotides to the mRNA that were not encoded by the DNA
What are siRNAs and microRNAs? How do they function?
siRNAs: Class of double-stranded RNA molecules, 20-25 bp in length, similar to miRNA, and operating within the RNA interference (RNAi) pathway; interfere with the expression of specific genes with complementary nucleotide sequences by degrading mRNA after transcription, preventing translation
miRNAs: Small, non-coding RNA molecule about 22 nucleotides long, that functions in RNA silencing and post-transcriptional regulation of gene expression
What is the significance of the codon/anticodon interaction between tRNA and mRNA?
The binding between the codon and anticodon leads to the translation of the mRNA and formation of a protein; codon is in the 5’ to 3’ direction while the anticodon is in the 3’ to 5’ direction (they are antiparallel to each other)
Explain the process of mRNA splicing
- mRNA is cut at the 5’ splice set
- 5’ end of the intron attaches to the branch point
- Cut is made at the 3’ splice site
- Intron is released as a lariat, and the 2 exons are spliced together
- Bond holding the lariat is broken, and the linear intron is degraded
- Spliced mRNA is exported to the cytoplasm and translated
If a splice site were mutated so that splicing did not take place, what would the effect be on the protein encoded by the mRNA?
It would be shorter than normal
In RNA silencing, siRNAs and miRNAs usually bind to which part of the mRNA molecules that they control?
3’ UTR
How are rare bases incorporated into tRNAs?
By chemical changes in one of the standard bases
