Transcript 2009 WH Freeman and Company

LECTURE
CONNECTIONS
14 | RNA Molecules and RNA Processing
© 2009 W. H. Freeman and Company
In eukaryotic cells, RNA molecules are often extensively
modified after transcription.
Different proteins are produced from the same DNA
sequence.
RNA undergoes alternative processing.
14.1 Many Genes Have Complex Structures
Genes encode proteins.
Proteins are made of amino acids, so a gene
contains the nucleotides that specify the amino
acids of a protein.
Amino acids have a variety of roles in metabolism.
The chemical properties of the amino acids of
proteins determine the biological activity of the
protein.
Gene Organization
• The concept of
- colinearity: there is a direct correspondence between the
nucleotide sequence of DNA and the amino acid sequence of
a protein. Bacteria and many viruses.
- noncolinearity
Eukaryotic cells contain far more DNA than is required to
encode proteins.
Nuclear RNAs undergo some type of change before they are
exported to the cytoplasm.
Regions of DNA might not be transcribed.
Gene Organization
• Exons: coding regions
• Introns: noncoding regions
• All the introns and the exons are initially transcribed
into RNA but, after transcription, the introns are
remove by splicing.
The coding sequences of many eukaryotic genes
are disrupt by noncoding regions.
The Concept of the Gene
• The gene includes DNA sequence that codes for all
exons, introns, and those sequences at the beginning
and end of the RNA that are not translated into a protein,
including the entire transcription unit – the promoter, the
RNA coding sequence, and the terminator.
14.2 mRNAs are modified after transcription in
eukaryotes
• The structure of messenger RNA:
• A mature mRNA contains 5′ untranslated region (5′
UTR, or leader sequence)
Shine–Dalgarno consensus sequence in bacteria
• Protein coding region: codons that specify the aa
sequence of the protein. Start codon and stop codon
• 3′ untranslated region (3’UTR or trailer): affects the
stability of the mRNA and the translation of the mRNA
protein-coding sequence.
Three primary regions of mature mRNA in bacteria.
Concept Check 1
Which region of mRNA contains the Shine–Dalgarno
sequence?
a.
b.
c.
d.
5′ UTR
3′ UTR
Protein coding region
All three regions
Pre-mRNA processing
•
The addition of the 5′ cap:
- An extra nucleotide is added to the 5’ end and
- Methylation to the base of new nucleotide and to the
2’-OH group of the sugar of one or more nucleotides at
the 5’end.
•
The addition of the poly(A) tail:
50 ~ 250 adenine nucleotides are added to the 3′-end of
the mRNA by polyadenylation.
Pre-mRNA processing
• RNA splicing: removal of introns in the nucleus
• Consensus sequences:
– 5′ splice site: GU and end with AG
– 3′ consensus sequence: CAGG
– Branch point: the adenine “A”: 18 ~ 40
nucleotides upstream of 3′-splicing site
• Spliceosome: 5 RNA molecules + 300 proteins
• The process of splicing
Concept Check 2
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?
a. It would be shorter than normal.
b. It would be longer than normal.
c. It would be the same length but would have different
amino acids.
• Intron removal, mRNA processing, and transcription take
place at the same site in the nucleus.
• Self-splicing introns happen in some rRNA genes in
protists and in mitochondria genes in fungi.
• There are alternative processing pathways for
processing pre-mRNA.
The Dscam gene of Drosophila can produce 38,016 different
types of proteins by alternative nRNA splicing
The Drosophyla genome is thought to contain only 14,000 genes,
but here is a single gene that encodes three times that
number of proteins.
RNA editing
• RNA editing: The coding sequence of an mRNA
molecule is altered after transcription.
• Insertion or deletion of nucleotides or the conversion of
one base into another.
• gRNAs (guide RNAs) contain sequences that are
partially complementary to segments of the pre- edited
RNA.
• After the mRNA is anchored to the gRNA, the mRNA
undergoes cleavage and nucleotides are added, deleted,
or altered according to the template provided by gRNA.
Concept Check 3
Alternative 3′ cleavage sites result in:
a.
b.
c.
d.
multiple genes of different length.
multiple genes of pre-mRNA of different length.
multiple mRNAs of different length.
all of the above.