Download Chapter 1 A View of Life

Chapter 12
RNA and Protein Synthesis: The Expression of
Genetic Information
Lecture Outline
I. DNA is transcribed to form RNA; RNA is translated to
form proteins
A. RNA (ribonucleic acid) is used as an intermediary
between DNA and proteins
1. RNA is a single-stranded nucleotide polymer
(although some areas may be folded)
2. The sugar in RNA is ribose, and uracil
substitutes for thymine
Uracil is a pyrimidine and forms two bonds
with adenine
Difference between DNA and RNA structures (See figures be
Characters
DNA
R
1.
Molecule
Double stranded, helical
2.
Pentose sugar
Deoxyribose
Rib
3.
Pyrimidine base
Thymine
Ur
4.
Complementary
base pairing
Always present and exists between A = T
and G = C
Single stranded, straight or
Normally absent, but m
segments o
If present, pairing is be
5.
Ratio of
Purines:
Pyrimidines
Always 1:1
B. A copy of the DNA is made in the form of
messenger RNA (mRNA) in transcription
Not nece
C. Translation involves mRNA, tRNA (transfer RNA),
and rRNA (ribosomal RNA)
coordinating to produce proteins
D. mRNA has sequences of 3 nucleotides called
codons
E. Codons are read in sequences of 3; this is called
triplet code
Codons are written in 5’ to 3’ fashion
F. Each codon codes for 1 amino acid
Four bases can combine in 43 combinations (64)
– more than enough to code
for the 20 naturally occurring amino acids
G. Each tRNA molecule has a sequence of 3
nucleotides – the anticodon
1. Anticodons base pair with a codon in a
complimentary way
2. Anticodons are written in a 3’ to 5’ fashion
H. Ribosomes are composed of proteins and rRNA
II. Transcription is the synthesis of RNA from a DNA
template
A. RNA is synthesized by DNA – dependent RNA
polymerases, which are
similar to DNA polymerases
B. Messenger RNA contains base sequences that code
for a protein
1. Transcription does not involve a primer; it
begins at a promoter sequence on
the template strand (the promoter is not
transcribed)
2. RNA synthesis proceeds in a 5’→3’ direction
(copying the DNA from the 3’→5’)
a. Upstream refers to sections towards the 5’
end of the mRNA sequence
b. Downstream refers to sections towards
the 3’ end of the mRNA sequence
c. Bacterial promoters are about 40 bases
long and are located in the
DNA just upstream from the starting point
d. Sequences at the end of the gene act as
‘stop’ signals
3. Typically only one strand of the DNA is
transcribed – the template strand
In a single DNA molecule, some of the
template sections are on one strand,
others are on other strands
For an animation of mRNA transcription go
to:
http://www.google.com/imgres?imgurl=http://www.virtualsciencefair.org/2004/m
cgo4s0/public_html/t3/RNAstructure1.jpg&imgrefurl=http://www.virtualsciencefair.org/2004/mcgo4s0/public_html/t
3/RNA.html&h=351&w=292&sz=12&tbnid=30TvlbQZWGYJ::&tbnh=120&tbnw=100
&prev=/images%3Fq%3Drna%2Bstructure%2Bimage&usg=__HWdESNwYOlwzGpiK
x7LB3aWklWA=&sa=X&oi=image_result&resnum=3&ct=image&cd=1
C. Messenger RNA contains additional base
sequences that do not directly code for protein
1. mRNA has a noncoding leader sequence at the
5’ end, important in binding to the ribosome
during translation
2. Coding sequences follow the leader sequence
3. Coding sequences are followed by termination
signals and other noncoding sequences
III. During translation, the nucleic acid message is decoded
A. An amino acid is attached to transfer RNA before
becoming incorporated into a polypeptide
1. To form a polypeptide chain, the amino and
carboxyl groups of amino acids are joined
2. The specific sequence of the amino acids
(primary structure) is dictated by the sequence of
codons of the mRNA
3. tRNA molecules are produced from tRNA
genes
a. tRNA is linked to amino acids by
aminoacyl-tRNA synthetases
b. This is an energy-requiring process
B. Transfer RNA molecules have specialized regions
with specific functions
1. tRNA molecules have attachment sites for
amino acids
2. tRNA molecules have anticodons which will
bind to the mRNA
3. tRNA molecules must be recognized by both
the specific aminoacyl-tRNA synthetase
and the ribosome
4. tRNA molecules are approximately 70
nucleotides long, with some generic sections
and some unique sections
5. The nucleotide chain is folded back upon itself
to form 3 or more loops with
unpaired nucleotides exposed
C. The components of the translational machinery
come together at the ribosomes
Ribosomes are composed of two subunits
1. In E. coli, the small subunit is composed of 21
proteins and one RNA molecule,
the larger subunit is composed of 35 proteins and
two RNA molecules
2. The large subunit has a groove into which the
small subunit fits
3. Ribosomes are transcribed from DNA, but do
not carry information – rather they
function as the physical site of translation, and as
a catalyst
4. The A site of the ribosome is where the
aminoacyl-tRNA binds
5. The P site of the ribosome is where the tRNA
holding the polypeptide chain is positioned
D. Translation includes initiation, elongation, and
termination
To view an animation of translation, including initiation, elongation, and termination click the link below.
Protein Synthesis: at the ribosome
http://www.sciencenetlinks.com/interactives/protein.html
1. Initiation is the first step
a. Initiation factors (proteins) move
initiation tRNA onto the small ribosomal subunit
b. The codon for the initiation is AUG,
which codes for the amino acid
methionine (met)
c. The initiation complex binds to ribosomerecognition sequences on the mRNA,
and aligns the anticodon of the tRNA with
the codon of the mRNA
d. The large ribosomal subunit then binds,
forming the functional ribosome
2. Elongation is the addition of new amino acids
a. As the initiator tRNA is bound to the P
site of the ribosome, the A site is
unoccupied until the next aminoacyl-tRNA
moves in
b. The energy for this process comes from
guanosine triphosphate (GTP)
c. Peptide bond formation takes place
between the amino group of new amino
acid and the carboxyl group of the amino
acid bound in the preceding reaction
d. Protein synthesis proceeds from the
amino acid end to the carboxyl end
e. The tRNA molecule is released from the P
site
This reaction requires the ribosome,
peptidyl transferase
f. Translocation is the movement of the
growing polypeptide chain from the A site
to the P site
Energy for translocation comes from
GTP
g. Translation of the mRNA proceeds from
3’ to a 5’ direction, which is the same
direction as transcription
h. Termination occurs when the mRNA
presents the codons UAA, UGA, or UAG,
as there is no tRNA which is complimentary
to them
1). Release factors recognize these
codons
2). The ribosome dissociates into the
two subunits
At right are
images of the
E. coli
ribosome,
reconstructed
from electron
density maps
obtained by
threedimensional
cryo-electron
microscopy.
TWO VIEWS OF THE STRUCTURE OF THE THERMUS THERMOPHILUS
70S RIBOSOME AT 7.8 � RESOLUTION.
TRANSFER RNAs (YELLOW, GREEN, and RED) OCCUPY A CAVITY
BETWEEN TWO RIBOSOMAL SUBUNITS.
E. A polyribosome is a complex of one mRNA and
many ribosomes
1. In prokaryotes, transcription and translation
simultaneously occur
2. The half-life of the mRNA molecules in
Bacteria is about 2 minutes
IV. Transcription and translation are more complex in
eukaryotes than in prokaryotes
A. Eukaryotic mRNA molecules are processed after
transcription
1. Processing occurs in the nucleus of eukaryotes
2. As the mRNA transcript grows, a cap is added
to the 5’ end
a. The cap is 7-methylguanylate
b. The cap is necessary for binding to
ribosomes
3. Capping may protect mRNA from degradation;
half-lives of eukaryotic mRNA molecules are as
long as 24 hours
4. At the 3’ end of the molecule, a sequence is
added which acts as a signal for the subsequent
addition of a “tail”
5. Enzymes recognize the signal and add 100-250
adenines (the poly-A tail)
B. Both noncoding sequences and coding sequences
are transcribed from eukaryotic genes
1. Coding sequences are exons (expressed
sequences); noncoding sequences are
introns (intervening sequences)
This terminology refers to sequences in both
DNA and mRNA
2. Precursor mRNA (pre-mRNA) contains both
introns and exons
3. Small nuclear ribonucleoprotein complexes
(snRNPs) may bind to the introns
and catalyze excision and splicing
4. RNA may also act as self-acting enzyme and
excise and splice without the
intervention of protein enzymes
V. The genetic code is read as a series of codons in mRNA
A. In 1961 Crick and colleagues determined that the
code was read in a
non-overlapping sequence of bases, forming a 3 base
reading frame
1. Niremberg and Mathaei synthesized poly-U
(UUUUUUU…) and found that it
coded for phenylalanine
2. UAA, UGA, UAG were found to be stop
codons, which do not code for any amino acids
3. AUG is the start codon and codes for
Methionine
4. The genetic code of all 64 possible codons has
been found to be nearly universal
B. The genetic code is redundant
1. Redundant codons have similar base sequences
2. Only methionine (AUG – start codon) and
tryptophan (UGG) have single triplet codes
3. The wobble hypothesis, proposed by Crick,
suggests that the third nucleotide
Of the tRNA anticodon can bind with more than
one nucleotide of the mRNA codon, as cells
typically produce only about 40 different tRNA
molecules
VI. A gene is defined as a functional unit
A gene is a transcribed nucleotide sequence that yields
a product
VII. Mutations are changes in DNA
A. Base-substitution mutations involve the substitution
of one base for another
1. Due to redundancy of the code, this may not
alter the product
2. Missense mutations result in the substitution of
one amino acid for another
The resulting protein may be nonfunctional,
or the mutation may have no effect
on the organism (a silent mutation)
3. Nonsense mutations result in a termination
codon
B. Frameshift mutations involve insertion or deletion
of a base
Frameshift mutations result in an entirely
different sequence of amino acids
(they change the reading frame)
C. Transposons are movable sequences of DNA which
may move into another area of DNA
1. Transposons may disrupt genes, but may also
activate some genes
2. Transposons were discovered by Barbara
McClintock in corn in the 1950s
(she received the Nobel Prize in 1983)
3. Have some similarities to retroviruses
D. Hot spots are regions of DNA more likely to
undergo mutations, and are often regions of repeated
nucleotides, causing the polymerases to “slip”
E. Mutagens are agents which cause mutations,
including ionizing radiation
1. Mutations in somatic cells are not passed on to
the next generation
2. Some mutagens are also carcinogens