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LECTURE
CONNECTIONS
14 | RNA Molecules and RNA Processing
© 2009 W. H. Freeman and Company
14.3 Transfer RNAs, Which Attach to Amino
Acids, Are Modified after Transcription in
Bacteria and Eukaryotic Cells
•
Each tRNA attaches to a particular aa and carries it
to the ribosome.
•
Each tRNA is capable of attacjing to only one type of
aa.
tRNAAla - attaches to aa alanine
•
20 aa are found in proteins, so there must be a
minimum of 20 different types of tRNA
The Structure of tRNA
• Rare modified RNA nucleotide bases
• Ribothymine
• Pseudouridine
• Modified bases arise from chemical changes made to
the four standard bases after transcription.
(tRNA-modifying enzymes)
• Common secondary structure – the cloverleaf structure
The Structure and processing of tRNA
• All tRNA molecules in both bacterial and eukaryotic cells
undergo processing after transcription.
• Eukaryotic cells usually have many copies of each tRNA
gene.
• Different tRNAs are processed in different ways, so a
generic processing pathway is not possible.
• Some eukaryotic and archeal tRNA genes possess
introns of variable length that must be removed in
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
14.4 Ribosomal RNA (rRNA)
•
Ribosomal RNA is a component of the ribosome and
is also processed after transcription
•
Within ribosomes, the genetic instructions contained
in mRNA are translated into the aa sequences of
polypeptides.
The Structure of the Ribosome
• Ribosome is one of the most abundant organelles in the
cell;
• Ribosomes contain typically about 80% of total cellular
RNA;
• Ribosomes are complex organelles (more than 50
proteins and RNA molecules);
• A functional ribosome consists of 2 subunits:
Large ribosome subunit
Small ribosome subunit
Each ribosome unit consists of one or more pieces of
RNA and a number of proteins.
A measure of how rapidly an object
sediments in a centrifugal field.
Ribosomal RNA Gene Structure and Processing
•
The genes for rRNA can be present in multiple
copies and the number vary among species;
•
In bacteria, rRNA genes are dispersed, while in
eukaryotic cells, they are clustered;
•
Ribosonal RNA is processed in both bacterial
and eukaryotic cells.
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
• Antisense RNA: single-stranded RNA molecules that bind
to mRNA and inhibit translation
• siRNA and microRNAs: doubled-stranded RNA that
regulate gene expression by a process called RNA
interference.
14.5 Small RNA Molecules Are Present
Extensively in Eukaryotes and Participate in a
Variety of Functions
• Both siRNA and miRNA molecules combine with proteins
to form an RNA-induced silencing complex (RISC).
• The RISC pairs with a mRNA molecule that possesses a
sequence complementary to its siRNA or miRNA
component and either:
- cleaves the mRNA (leading to degradation or mRNA) or
- represses translation of the mRNA