135
CHAPTER FOUR
Cellular Metabolism
4.7
CHANGES IN GENETIC
INFORMATION
Remarkably, we are more alike than different—human genome
sequences are 99.9 percent the same among individuals. The
tenth of a percent of the human genome that can vary from
person-to-person includes rare DNA sequences that affect
health or appearance, as well as common DNA base varia-
tions that do not exert any observable effects.
Nature of Mutations
The rare distinctions in DNA sequence that affect how we look
or feel are called
mutations
(mu-ta
shunz) More common
genetic variants with no detectable effects are called
single
nucleotide polymorphisms,
abbreviated SNPs (pronounced
“snips”). “Polymorphism” means “many forms.”
To visualize the concept of genetic variability, imagine a
simpliF ed DNA sequence that is part of a particular genome
region:
A A A A A A A A A A A A
A person with a mutation or polymorphism at the fourth
base might have any of the following sequences for this por-
tion of the genome, with the differences highlighted:
A A A C A A A A A A A A
A A A T A A A A A A A A
A A A G A A A A A A A A
TABLE
4.3
|
Protein Synthesis
Transcription (In the Nucleus)
1. RNA polymerase binds to the DNA base sequence of a gene.
2. This enzyme unwinds a portion of the DNA molecule, exposing part
of the gene.
3. RNA polymerase moves along one strand of the exposed gene and
catalyzes synthesis of an mRNA, whose nucleotides are complemen-
tary to those of the strand of the gene.
4. When RNA polymerase reaches the end of the gene, the newly
formed mRNA is released.
5. The DNA rewinds and closes the double helix.
6. The mRNA passes through a pore in the nuclear envelope and enters
the cytoplasm.
Translation (In the Cytoplasm)
1. A ribosome binds to the mRNA near the codon at the beginning of
the messenger strand.
2. A tRNA molecule that has the complementary anticodon brings its
amino acid to the ribosome.
3. A second tRNA brings the next amino acid to the ribosome.
4. A peptide bond forms between the two amino acids, and the F
rst
tRNA is released.
5. This process is repeated for each codon in the mRNA sequence as
the ribosome moves along its length, forming a chain of amino acids.
6. As the chain of amino acids grows, it folds, with the help of chaper-
one proteins, into the unique conformation of a functional protein
molecule.
7. The completed protein molecule (polypeptide) is released. The
mRNA molecule, ribosome, and tRNA molecules are recycled.
TABLE
4.2
|
Codons (mRNA Three Base Sequences)
SECOND LETTER
First Letter
UC
A
G
Third Letter
U
UUU
phenylalanine (phe)
UCU
serine (ser)
UAU
tyrosine (tyr)
UGU
cysteine (cys)
U
UUC
UCC
UAC
UGC
C
UUA
leucine (leu)
UCA
UAA
STOP
UGA
STOP
A
UUG
UCG
UAG
STOP
UGG
tryptophan (trp)
G
C
CUU
leucine (leu)
CCU
proline (pro)
CAU
histidine (his)
CGU
arginine (arg)
U
CUC
CCC
CAC
CGC
C
CUA
CCA
CAA
glutamine (gln)
CGA
A
CUG
CCG
CAG
CGG
G
A
AUU
isoleucine (ile)
ACU
threonine (thr)
AAU
asparagine (asn)
AGU
serine (ser)
U
AUC
ACC
AAC
AGC
C
AUA
ACA
AAA
lysine (lys)
AGA
arginine (arg)
A
AUG
START methionine (met)
ACG
AAG
AGG
G
G
GUU
valine (val)
GCU
alanine (ala)
GAU
aspartic acid (asp)
GGU
glycine (gly)
U
GUC
GCC
GAC
GGC
C
GUA
GCA
GAA
glutamic acid (glu)
GGA
A
GUG
GCG
GAG
GGG
G
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