Biomedical Engineering Reference
In-Depth Information
(a)
Regulatory region
Shine-Dalgarno sequence (AGGAGGU, ribosome binding site)
DNA template
Coding sequence
Promoter
Initiation ( binds to
the promoter)
Stop codon
(TAA, TAG, TGA)
Start codon
(ATG)
Elongation: 30-60 nucleotides/sec
(b)
5
3
Exon 1 Intron 1 Exon 2 Intron 2 Exon 3
DNA
5
3
Primary transcript
An
cap
Poly Adenine (A) tail
An
Fully processed mRNA
FIGURE 10.9
Translation process in prokaryotes and eukaryotes. (a) Transcription in prokaryotes. (b) Tran-
scription in eukaryotes.
several hundred nucleotides long. These two modifications are thought to increase m-RNA
stability and facilitate transport across the nuclear membrane.
Reverse transcription. Some viruses (such as HIV, the cause of AIDS) have the ability to
transcribe RNA into DNA. HIV has an RNA genome that is duplicated into DNA. The result-
ing DNA can be merged with the DNA genome of the host cell. The main enzyme responsible
for synthesis of DNA from an RNA template is called reverse transcriptase. In the case of
HIV, reverse transcriptase is responsible for synthesizing a cDNA strand to the viral RNA
genome. An associated enzyme, ribonuclease H, digests the RNA strand, and reverse tran-
scriptase synthesises a complementary strand of DNA to form a double-helix DNA structure.
This cDNA is integrated into the host cell's genome via another enzyme (integrase) causing
the host cell to generate viral proteins that reassemble into new viral particles. In HIV, subse-
quent to this, the host cell undergoes programmed cell death, apoptosis of T cells. However,
in other retroviruses, the host cell remains intact as the virus buds out of the cell.
Some eukaryotic cells contain an enzyme with reverse transcription activity called telome-
rase. Telomerase is a reverse transcriptase that lengthens the ends of linear chromosomes.
Telomerase carries an RNA template from which it synthesizes DNA repeating sequence,
or “junk” DNA. This repeated sequence of DNA is important because, every time a linear
chromosome is duplicated, it is shortened in length. With “junk” DNA at the ends of
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