Download 14.1 Many Genes Have Complex Structures

14.1 Many Genes Have Complex
Structures
Gene Organization
• The concept of colinearity and noncolinearity
Gene Organization
• Introns
– Regions of RNA that are not reflected in protein
sequence
• INTervening sequences
• Exons
– Regions of RNA that are reflected in protein
sequence
• EXpressed sequences
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 Messenger RNAs, Which Encode the
Amino Acid Sequences of Proteins, 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 sequence
• Protein coding region
• 3′ untranslated region
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
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:
•
•
•
A nucleotide with 7-methylguanine; 5′-5′
bond is attached to the 5′-end of the RNA.
The addition of the poly(A) tail:
•
50 ~ 250 adenine nucleotides are added to
the 3′-end of the mRNA.
Pre-mRNA processing
– RNA splicing:
• Consensus sequences:
–5′ consensus sequence: GU A/G AGU:
5′ splice site
–3′ consensus sequence: CAGG
–Branch point: the adenine “A”: 18 ~ 40
nucleotides upstream of 3′-splicing site
• Spliceosome: five RNA molecules + 300
proteins
• The process of splicing
• 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.
•RNA editing
• RNA editing: The coding sequence of an mRNA
molecule is altered after transcription.
• Guide RNA is used to serve as template for RNA
editing
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.
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.
14.3 Transfer RNAs, Which Attach to Amino
Acids, Are Modified after Transcription in
Bacteria and Eukaryotic Cells
The Structure of Transfer RNA
• Rare modified RNA nucleotide bases
• Ribothymine
• Pseudouridine
• Common secondary structure – the cloverleaf
structure
The Structure of Transfer RNA
• Transfer RNA gene structure and processing
Concept Check 3
How are rare bases incorporated into tRNAs?
a.
b.
c.
d.
Encoded by guide RNAs
By chemical changes in one of the standard bases
Encoded by rare bases in DNA
Encoded by sequences in introns
Concept Check 3
How are rare bases incorporated into tRNAs?
a.
b.
c.
d.
Encoded by guide RNAs
By chemical changes in one of the standard bases
Encoded by rare bases in DNA
Encoded by sequences in introns
14.4 Ribosome RNA, a Component of the
Ribosome, Also Is Processed after Transcription
The Structure of the Ribosome
• Large ribosome subunit
• Small ribosome subunit
Ribosomal RNA Gene Structure and
Processing
Concept Check 5
What types of changes take place in rRNA
processing?
a.
b.
c.
d.
Methylation of bases
Cleavage of bases
Nucleotides are trimmed from the ends of rRNAs.
All of the above
Concept Check 5
What types of changes take place in rRNA
processing?
a.
b.
c.
d.
Methylation of bases
Cleavage of bases
Nucleotides are trimmed from the ends of rRNAs.
All of the above
14.5 Small RNA Molecules Are Present
Extensively in Eukaryotes and Participate in a
Variety of Functions
• RNA interference: limits the invasion of foreign
genes and censors the expression of their own
genes
14.5 Small RNA Molecules Are Present
Extensively in Eukaryotes and Participate in a
Variety of Functions
• Model Genetic Organism – the Nematode Worm