From Genes to Proteins Flashcards
What is Transcription?
The synthesis of messenger RNA (mRNA) using DNA as a template. RNA polymerase uses DNA as a template to produce pre-mRNA
What is translation?
The production of proteins from mature mRNA, builds the protein encoded by the original gene
What are promoter elements in eukaryotes?
TATA BOX
How is transcription started?
The short run of T & A bases can vary slightly from gene to gene and is probably the best characterised promoter element (TATA box)
Why are T & As used?
they form the lowest energy base pairs so are easiest to unwind
What is the first step in transcription?
INITIATION
Where does RNA polymerase bind?
Binds to DNA upstream (5’) of the gene at a promoter sequence
What Co factors does RNA polymerase use and why?
cofactors= general transcription factors
TFIID recognises the TATA box and ensures that the correct start site is used
What does alteration of promoter strength cause?
Can have deleterious effects upon a cell, often resulting in disease
What is the second step in transcription?
ELONGATION
What happens during elongation?
The DNA double helix unwinds, RNA polymerase reads template strand and adds complementary nucleotides
What is the final step in transcription?
TERMINATION
What stops transcription?
Terminator sequences are found close to the ends of noncoding sequences
What stops transcription in Pol I genes
- termination factor
- through a mechanism similar to rho-dependent termination in bacteria (rRNA)
What stops transcription in pol II genes?
can continue for hundreds of or thousands of nucelotides beyond the end of the noncoding sequence (mRNA)
What stops transcription of Pol III genes
ends after transcribing a termination sequence that includes a polyuracil stretch (tRNA)
What is splicing?
removal of intronic sequences from the pre-mRNA. Coding regions (Exonic sequences) are spliced together. Num of exons varies. forms mature mRNA
How does splicing occur?
2 transesterification reactions
What are the steps in splicing?
- 1st reaction is the free 2’- hydroxy group
- the 3’ hydroxy group of the 5’ exon attacks and cleaves the phosphodiester linkage at the 3’ splice site
- The products are spliced mRNA product and excised intron
What is the excised intron called?
lariat product
What is self splicing?
The intron itself folds into a specific conformation without the involvement of other RNA or protein
What is alternative splicing?
the process by which a given gene is spliced into more than one type of mRNA molecule
Why is splicing important
- Fundamental role in cellular metabolism
- Protein diversity
- Regulation of gene and protein content
- Evolution of new and improved proteins
- Key to cancer pathology
How is protein diversity brought about?
Changes in the number & sequence of exons and introns present in the RNA sequence
What is a frameshift mutation?
The addition (insertion) or loss (deletion) of a base would result in a change of reading frame and proteins with altered function
What can frameshift mutation cause?
Array of diseases
Where does translation occur?
RIBOSOME
What is a ribosome comprised of?
2 subunits, large and small, each subunit exists separated in the cytoplasm, but join together on the mRNA molecule
What are tRNA molecules
Adaptor molecules, one end can read the triplet code on mRNA via complementary base pairing & the other attaches to a specific amino acid
What does rRNA do?
catalyses the attachment of each new amino acid to the growing chain
forms the ribosome
What is the first step in translation?
Initiation
- Translation begins with formation of a complex on the mRNA
Three initiation factor proteins bind to small subunit of the ribosome
The preinitiation complex and a methionine carrying tRNA then bind to mRNA forming an initiation complex
What are the 3 initiation factor proteins?
IF1, IF2, IF3
What is the second step in translation?
ELONGATION
- Ribosome moves along mRNA in a 5-3 direction, elongation factor G is required for TRANSLOCATION
- Methionine- carrying tRNA starts out in the middle slot of the ribosome
What acts as the energy source for translation?
Guanosine triphosphate acts as energy source
On binding of the tRNA- amino acid complex
GTP= guanosine diphosphate
How are peptide bonds formed between amino acids?
peptidyl transferase activity
What is the 3rd step in translation?
TERMINATION
- Three stop codons UAA, UAG, UGA
How are stop codons recognised?
By release factors which fit into the P site
When a ribosome reaches the stop codon it dissociates and protein is released
What is the genetic code?
64 triplets called codons
Each codon except 3 codes for one of the 20 amino acids
Some aa encoded for by more than one codon
What is a triplet?
Sequence of three nucleotides found on a strand of DNA or RNA to code for a specific amino acid
What are the 3 stop codons?
UAA (U ARE ANNOYING)
UGA (U GO AWAY)
UAG (U ARE GONE)
What is the Methionine codon (Met)
AUG (A universal go)
What is the wobble hypthesis?
Suggests that specificity for the base at the 3’ end of the condon is not always observed
explains why multiple codons can code for one amino acid
