Microbiology, 2021

458 11 • Summary
living organisms.
• Prokaryotic (70S) and cytoplasmic eukaryotic
(80S) ribosomes are each composed of a large
subunit and a small subunit of differing sizes
between the two groups. Each subunit is
composed of rRNA and protein. Organelle
ribosomes in eukaryotic cells resemble
prokaryotic ribosomes.
• Some 60 to 90 species of tRNA exist in bacteria.
Each tRNA has a three-nucleotide anticodon as
well as a binding site for a cognate amino acid.
All tRNAs with a specific anticodon will carry the
same amino acid.
• Initiation of translation occurs when the small
ribosomal subunit binds with initiation factors
and an initiator tRNA at the start codon of an
mRNA, followed by the binding to the initiation
complex of the large ribosomal subunit.
• In prokaryotic cells, the start codon codes for N-
formyl-methionine carried by a special initiator
tRNA. In eukaryotic cells, the start codon codes
for methionine carried by a special initiator
tRNA. In addition, whereas ribosomal binding of
the mRNA in prokaryotes is facilitated by the
Shine-Dalgarno sequence within the mRNA,
eukaryotic ribosomes bind to the 5’ cap of the
mRNA.
• During the elongation stage of translation, a
charged tRNA binds to mRNA in the A site of
the ribosome; a peptide bond is catalyzed
between the two adjacent amino acids, breaking
the bond between the first amino acid and its
tRNA; the ribosome moves one codon along the
mRNA; and the first tRNA is moved from the P
site of the ribosome to the E site and leaves the
ribosomal complex.
• Termination of translation occurs when the
ribosome encounters a stop codon, which does
not code for a tRNA. Release factors cause the
polypeptide to be released, and the ribosomal
complex dissociates.
• In prokaryotes, transcription and translation
may be coupled, with translation of an mRNA
molecule beginning as soon as transcription
allows enough mRNA exposure for the binding
of a ribosome, prior to transcription
termination. Transcription and translation are
not coupled in eukaryotes because transcription
occurs in the nucleus, whereas translation
occurs in the cytoplasm or in association with
the rough endoplasmic reticulum.
• Polypeptides often require one or more posttranslational
modifications to become
biologically active.
11.5 Mutations
• A mutation is a heritable change in DNA. A
mutation may lead to a change in the aminoacid
sequence of a protein, possibly affecting its
function.
• A point mutation affects a single base pair. A
point mutation may cause a silent mutation if
the mRNA codon codes for the same amino acid,
a missense mutation if the mRNA codon codes
for a different amino acid, or a nonsense
mutation if the mRNA codon becomes a stop
codon.
• Missense mutations may retain function,
depending on the chemistry of the new amino
acid and its location in the protein. Nonsense
mutations produce truncated and frequently
nonfunctional proteins.
• A frameshift mutation results from an
insertion or deletion of a number of nucleotides
that is not a multiple of three. The change in
reading frame alters every amino acid after the
point of the mutation and results in a
nonfunctional protein.
• Spontaneous mutations occur through DNA
replication errors, whereas induced mutations
occur through exposure to a mutagen.
• Mutagenic agents are frequently carcinogenic
but not always. However, nearly all carcinogens
are mutagenic.
• Chemical mutagens include base analogs and
chemicals that modify existing bases. In both
cases, mutations are introduced after several
rounds of DNA replication.
• Ionizing radiation, such as X-rays and γ-rays,
leads to breakage of the phosphodiester
backbone of DNA and can also chemically
modify bases to alter their base-pairing rules.
• Nonionizing radiation like ultraviolet light may
introduce pyrimidine (thymine) dimers, which,
during DNA replication and transcription, may
introduce frameshift or point mutations.
• Cells have mechanisms to repair naturally
occurring mutations. DNA polymerase has
proofreading activity. Mismatch repair is a
process to repair incorrectly incorporated bases
after DNA replication has been completed.
• Pyrimidine dimers can also be repaired. In
nucleotide excision repair (dark repair),
enzymes recognize the distortion introduced by
the pyrimidine dimer and replace the damaged
strand with the correct bases, using the
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