133
CHAPTER FOUR
Cellular Metabolism
From Science to Technology 4.3 describes another type
of transcriptional control—microRNAs.
PRACTICE
24
How is genetic information carried from the nucleus to the cytoplasm?
25
How are protein molecules synthesized?
26
How is gene expression controlled?
and they control which proteins a cell produces and how
many copies form under particular conditions. A connective
tissue cell might have many mRNAs representing genes that
encode the proteins collagen and elastin; a muscle cell would
have abundant mRNAs encoding contractile proteins, such
as actin and myosin. Extracellular signals such as hormones
and growth factors activate transcription factors.
AT
UA
G
C
G
C
G
C
CG
CG
CG
Messenger
RNA
1
DNA
information
is copied, or
transcribed,
into mRNA
following
complementary
base pairing
2
mRNA leaves
the nucleus
and attaches
to a ribosome
3
Translation begins as tRNA anticodons
recognize complementary mRNA codons,
thus bringing the correct amino acids into
position on the growing polypeptide chain
4
As the ribosome
moves along the
mRNA, more amino
acids are added
5
At the end of the mRNA,
the ribosome releases
the new protein
6
tRNA molecules
can pick up another
molecule of the
same amino acid
and be reused
Amino acids
attached to tRNA
Polypeptide
chain
Cytoplasm
DNA
double
helix
DNA
strands
pulled
apart
Transcription
(in nucleus)
Translation
(in cytoplasm)
Nucleus
D
ir
e
c
tio
no
f“re
adin
g
C
Codon 1
Codon 2
Codon 3
Codon 4
Codon 5
Codon 6
Codon 7
G
G
G
G
G
A
A
A
U
U
C
C
C
C
C
C
G
G
G
A
Methionine
Glycine
Amino acids
represented
Serine
Alanine
Threonine
Alanine
Glycine
D
i
r
e
c
t
i
o
n
o
f
r
e
a
d
i
n
g
DNA
strand
Messenger
RNA
A
T
A
A
T
T
T
AT
AT
AT
A
AT
UA
UA
UA
G
C
C
GC
G
C
G
C
G
C
G
C
G
C
G
G
C
C
G
C
C
G
U
A
CG
C
G
G
G
G
G
G
G
G
G
G
C
C
C
C
C
C
C
C
C
C
A
A
A
A
A
T
TA
AT
AT
AT
AT
CG
GC
GC
GC
TA
TA
TA
CG
AT
GC
TA
CG
TA
CG
CG
GC
AT
TA
CG
GC
T
T
G
CG
CG
CG
CG
CG
CG
CG
C
G
Nuclear
pore
FIGURE 4.23
Protein synthesis. DNA information is transcribed into mRNA, which in turn is translated into a sequence of amino acids. The inset
shows some examples of the correspondence between mRNA codons and the speciF
c amino acids that they encode.
TABLE
4.1
|
A Comparison of DNA and RNA Molecules
DNA
RNA
Main location
Part of chromosomes, in nucleus
Cytoplasm
5-carbon sugar
Deoxyribose
Ribose
Basic molecular structure
Double-stranded
Single-stranded
Nitrogenous bases included
Cytosine, guanine, adenine, thymine
Cytosine, guanine, adenine, uracil
Major functions
Contains genetic code for protein
synthesis, replicates prior to mitosis
Messenger RNA carries transcribed DNA information to cytoplasm and acts as template
for synthesis of protein molecules; transfer RNA carries amino acids to messenger RNA;
ribosomal RNA provides structure for ribosomes
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