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39 Cards in this Set

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Several tRNA molecules joined together.

Function is filled in after name
Multimeric tRNA precursor
Need to trim off aditional nucleotides from the 3'-5' ends
Monomeric tRNA precursor
a. Cleavage/trimming
b. Nucleoside modifications
c. CCA added to 3' end (nucleotidyl transferase)
Modifications made to monomeric and multimeric precursors.

Need to trim molecules away from initial tRNA molecules.
The point of attachement to the mRNA molecule is through the adenylate molecule
CCA

Added to the 3' end
1. Reductions
2. Deaminations
3. Rearrangements
4. Methylations
Modifications made of tRNA.

Occur at the level of the polynucleotide
In prokaryotes, which RNA type is not extensively processed?
mRNA
In eukaryotes, is tRNA monomeric or multimeric?
monomeric
1. Synthesis of tRnA by RNA pol III

2. Trimming of nucleotides from both the 3' and 5' ends

3. Nucleoside modifications

4. Addition of the sequence CCA to the 3' end
tRNA processing in eukaryotes
1. Synthesis by RNA pol I (nucleolus)

2. Cleavage

3. Trimming

4. Methylation
rRNA processing in eukaryotes

18S, 5.8S and 28S subunits
1. Addition of 5' cap

2. Addition of polyA tail to 3' end (polyadenylation)

3. Methylation of internal nucleoSides

4. RNA splicing
mRNA processing in eukaryotes.

RNA pol II produces heterogenous RNA (hnRNA)
A mixture of precursor mRNA molecules
heterogenous RNA (hnRNA)
a. 7-Methylguanosine is joined to the 5' end of the mRNA in a 5'-triphosphate linkage.

b. Methyl groups are often found at the 2' position of the first and second nucleotides on RNA (DNA does not have a hydroxyl group)
Properties of the 5' cap.

Thought to be crucial to splcing the mRNA

Required for translation (protein synthesis)
The 5'-5' bond is not degraded by ____nucleases
exonucleases
1. Processing (splicing)
2. Stability
3. Transport from nucleus into cytoplasm
4. Translation
Capping promotes mRNA
1. Stability
a. Poly A binding protein binds to poly A tail. This is followed by other subunits binding to tail.
- Prevents degradation of 3' end by exonucleases

2. Enhances translation
Roles of poly(A) tail

According to dogma, histone mRNA's are the only mRNA's that lack a poly(A) tail
Methylation of internal nucleosides is a step in mRNA processing...
however, it's function is unknown
Informational nucleotide sequences
Exons
Non-informational nucleotide sequences
Introns
RNA splicing removes ______ and joins exons together
introns
Introns in a gene can be detected by hybridizing the mRNA with a ss-DNA fragment corresponding to the _______ strand of the gene
nonsense strand (aka template strand/ noncoding strand)
Distribution of introns in genes:

Histone genes and interferon genes have how many introns?
None

Most genes for higher eukaryotes contain 1 to 10 introns
Distribution of introns in genes:

Collagen gene and dystrophin gene have what percentage of their gene coding for introns?
Over 90% of the gene codes for introns
5' splice site
5'-GU

All introns begin with this sequence
3' splice site
AG-3'

All introns end with this sequence
Branch point
An adenylate residue.

Acts as a nucleophile.

In order for splicing to occur, need all three splice points.
The 2' hydroxyl group of the
adenosine residue at the branch point acts as a nucleophile.

•It attacks the phosphodiester bond at the 5' splice site.

•Exon L1 is released.

•”Lariat” structure is formed due to 2',5' phosphodiester bond involving guanosine and adenosine.

•The adenosine residue is involved in three phosphodiester bonds.

•The 3' end of exon L1 acts as a nucleophile attacking the phosphodiester bond at the 3' splice site.

•The intron is released and then degraded.

•Exons L1 and L2 are joined together.
Concept: RNA splicing is a multistep process involving a series of nucleophilic attacks.

A unique structure formed in the process is the 'lariat'
Snurps (SnRNA):

U1
Binds 5' splice site
Snurps (SnRNA):

U2
Binds the branch point
Snurps (SnRNA):

U4
Regulation

When U4 binds to U6, blocks active site (noncompetitive inhibition)
Snurps (SnRNA):

U5
Binds 5' splice site

Displaces U1.
Snurps (SnRNA):

U6
Catalyzes splicing

Contains active site
Snurps (SnRNA):

Enzymatic complex is formed of which SnRNA?
U4, U5 and U6
Alternative splicing is required for producing extra genes
Alternate processing of a primary transcript can generate different proteins from the same gene.
Calcitonin (thyroid)
- Polypeptide (32 aa's)
- participates in calcium and phosphate metabolism
- increases deposition of calcium in bones

Calcitonin gene-related peptide (brain)
- polypeptide (37 aa's)
- potent vasodilator
- modulates acetylcholine receptor function at neuromuscular junction
Alternative splicing can generate numerous isoforms from a single gene.

Each isoform may have different properties
Some functional protein, but not enough

Mutation on exon 1 gives rise to a different splice site
-- Competes with the true splice site

When the wrong splice site is used, loose portion of exon causes frameshift mutation

Leads to termination of translation of exon 2

Will not form full protein
alpha/beta + thalassemia
No functional protein produced from the defective allele
alpha/beta 0 thalassemia
One or more defective alpha genes

Decrease in alpha-globin protein

Decrease in Hb (a2b2)
Free b subunits form a tetramer

b4 is unstable leading to formation of Heinz bodies

Hemolytic anemia
alpha-thalassemia

Two copies of the alpha gene per chromosome (16)
One or more defective beta genes

Decrease in beta-globin protein

Decrease in Hb (a2b2)

Free alpha subunits

Bind to and damage the RBC membrane

Hemolytic anemia
Beta-thalassemia

One copy of the B gene per chromosome (11)
Other mutations that can lead to thalassemia:
1. Promoter site mutation

2. Mutations in signal for addition of poly(A) tail

3. Other mutations