Biomedical Engineering Reference
In-Depth Information
5'
Sugar-phosphate backbone
3'
5'
T
3'
C
T
A
GGTC T
A
G
G
TCCAGA
TCC
A
G
5'
3'
A
5'
Sugar-phosphate backbone
(a)
(b)
3'
3'
5'
A
GGTC T
T
CC A G A
5'
3'
3'
5'
A
GGT
C
T
TCCAGA
5'
3'
(c)
FIGURE 3.13 During replication, DNA helicase shown as a black wedge in (b) unzips the double helix (a).
Another enzyme, DNA polymerase, then copies each side of the unzipped chain in the 5' to 3' direction. One side
of the chain (5' to 3') can be copied continuously, while the opposite side (3' to 5') is copied in small chunks in the
5' to 3' direction that are bound together by another enzyme, DNA ligase. Two identical double strands of DNA are
produced as a result of replication.
After being transcribed, the mRNA moves out into the cytoplasm through the nuclear
pores and binds to specific sites on the surface of the two subunits that make up a ribosome
(Figure 3.15). In addition to the ribosomes, the cytoplasm contains amino acids and another
form of RNA: transfer RNA (tRNA). Each tRNA contains a triplet of bases, called an
anticodon, and binds at an area away from the triplet to an amino acid that is specific for
that particular anticodon. The mRNA that was produced from the gene in the nucleus
also contains bases in sets of three. Each triplet in the mRNA is called a codon. The four
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