MBG PART TWO: Lecture 6 Flashcards
What does a continuous sequence of nucleotides in the DNA encode?
A continuous sequence of amino acids in the protein
What is the number of nucleotides proportional to?
The number of amino acids (3:1)
What is the 5’ untranslated region (5’UTR)?
Does not code for amino acids. Binds to the ribosomal complex (not part of coding region)
What is the Protein Coding Region?
Comprises the codons that specify the amino acids. Begins with a start codon and ends with a stop codon.
What is 3’UTR?
Does not code for amino acids. Affects the stability of the mRNA and regulates its translation.
How can transcription and translation occur in prokaryotes?
Simultaneously (no nucleus and the mRNA sequence direction corresponds to the DNA)
What happens to the mRNA in Eukaryotes?
mRNA is extensively modified to translation and must be transported out of the nucleus.
In Eukaryotes, what happens to the template strand?
The entire template strand of the gene produces a pre-mRNA which is then processes to make mature messenger mRNA that can be translated into protein.
What are exons?
Coding sequences
What are introns?
Non-Coding Sequences
How do you remove introns?
RNA splicing along with additional RNA processing
Why is the removal of introns required?
Too form the mRNA that will be translated into a polypeptide.
What can exons vary in?
Length and relative position on the gene.
What type of introns and exons are the best studied?
Nuclear pre-mRNA introns and exons
What does splicing of the introns require?
A spliceosomal complex that contains a small nuclear RNAs (snRNAs) and proteins to remove the intron.
What can introns influence?
How genes are expressed
What do some introns have?
Self-splicing capabilities
What are introns removed from?
pre-mRNA by the action of the splicesome.
What does addition to 5’ cap do?
Facilitates binding of ribosome to 5’ end of mRNA, increases mRNA.
What does the 3’ cleavage and addition of poly(A) tail do?
Increases stability of mRNA, facilitates binding of ribosome to mRNA.
What does RNA splicing do?
Removes noncoding introns from pre-mRNA, facilitates export of mRNA to cytoplasm, allows fo multiple proteins to be produced through alternative
What does RNA editing do?
Alters nucleotide sequence of mRNA.
How is RNA transcription initiated?
When core RNA polymerase binds to the promoter with the help of sigma.
How does RNA processing work with the 5’ cap?
One of the three phosphate groups at the 5’ end of the mRNA is removed and a guanine nucleotide is added. Methyl groups are added to position 7 of the base of the terminal guanine nucleotide
Where can methyl groups be added to?
- 7’ position of the terminal Guanine
- 2’ OH group of the sugar to the first and sometimes second nucleotide.
- To adenine if it is the second nucleotide, sometimes.
What is added to the first and second transcribed nucleotide?
CH3 group
What is not transcribed?
Methyl Guanosine
What might be added adenine if present?
One extra CH3 group
Where is Pre-mRNA cleaved?
At a position from 11 to 30 nucleotides down-stream of the consensus sequence, in the 3’ untranslated region
Where does the addition of adenine nucleotides (polyadenylation) take place and what does it generate?
At the 3’ end of the pre-mRNA, generating the poly(A)tail.
How many Adenine’s are added to the 3’end?
5 - 250 A’s are added.
How are A’s added?
Without a template.
What is the Poly A consensus sequence?
AAUAAA is the polyadenylation signal located 11 to 30 nucleotides upstream of the cleavage site
What does the Poly A Consensus sequence affect?
The stability of mRNA, facilitates attachment of the ribosome and export out of the nucleus.
What is RNA splicing?
Removal of introns from pre-mRNA
What must removal of introns be?
Precise in order to properly fiuse the 3’ end of one exon to the 5’ end of the next exon.
What does EVERY intron have?
Three conserved sequences that are required for its precise removal
What are the three conserved sequences?
1.) 5’ splice sequences containing the junction sequences GU
2.) 3’ splice sequence containing the junction sequence AG
3.) Intron branch point: A conserved “A” residue located 18 - 40 nt upstream of 3’ splice site.
What does a spliceosome contain?
Five small nuclear RNAs (snRNAs)
What do the snRNAs associate with?
About 300 small proteins to form small nuclear ribonucleoproteins (snRNPs)
What does Each snRNP contain?
One snRNA molecule and multiple proteins.
How many snRNPs does the spliceosome contain?
Five (U1, 2,4,5,6)
What do snRNAs of the splice some form?
Complementary base pairing with the pre-mRNA
Where does U1 attach?
To the 5’ splice site
Where does U2 attach?
To the branch point
What does the addition of U4, U5, U6 join?
The spliceosome
What is Step 1 of RNA splicing?
Pre/mRNA is cut at the 5’ splice site and the 5’ end of the intron attaches to the branch point. Called a Lariat.
5’ phosphate of the G nucleotide binds with the 2’ OH group of the A incueltodie at the branch point.
What is Step 2 of RNA splicing?
Cut is made at the 3’ splice site and simultaneously the 3’ end of exon 1 becomes covalently attached to the 5’ end of exon 2.
Intron is released as lariat and degraded
Why is RNA less stable than DNA and Why use it?
The presence of the unique 2’OH group in ribose causes it to react intramolecularly with the 3’OH site resulting in phosphate bond breakage (reactions due to electronegativity)
What does the addition of U4,U5, and U6 cause?
A conformational change bringing the 5’ splice site close to the branch
What happens to U1 and U4?
They are released
Where does base pairing occur?
Between U6 and U2 and between U6 and the 5’ splice site
What is transesterification?
One phosphodiester linkage is replaced by another
What do the two transesterification reactions by U6 cause?
Joining of two exons together and releasing the Lariat
