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HIV, a retrovirus.
The Human Immunodeficiency Virus
(HIV) is a retrovirus.
It has the enzyme reverse transcriptase to copy the
RNA genome into DNA, which can be integrated into the host genome as a provirus.
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An RNA virus.
The viral RNA genome functions as a template for synthesis of complementary
RNA strands, which are used to to make copies of viral genome RNA.
The viral genome also serves as a template for synthesis of mRNA,
which is translated into capsid proteins and glycoproteins of the viral envelope.
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Activator proteins bind to distal control elements grouped as an enhancer in the DNA.
A DNA-bending protein brings the bound activators closer to the promoter.
The activators bind to transcription factors and mediator proteins, forming
a transcription initiation complex on the promoter with RNA polymerase.
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Many animal viruses have a membranous envelope with glycoproteins
that bind to specific receptor molecules on the surface of a host cell.
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This fluorescence micrograph shows chromatin packed into the main axis of the chromosome.
Those parts that are being actively transcribed are spread out in loops .
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The chromatin folds further, resulting in the maximally compacted chromosome seen at metaphase.
Each metaphase chromosome consists of two chromatids.
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Single-stranded
microRNAs (miRNAs) are cut by the Dicer and prevent expression of complementary mRNAs.
expression.html: 19_03EukGeneExpRegulation.jpg
Regulation of gene expression may take place at different stages:
chromatin changes,
transcription control,
mRNA degradation,
and protein processing, activation,
and degradation.
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The lytic and lysogenic cycles of λ, a temperate phage.
After entering the host cell and circularizing, the λ (lambda) DNA can enter the
lytic cycle
or integrate into the bacterial chromosome (lysogenic cycle) as a prophage,
where it does not kill the host and may be carried in the host chromosome
for many generations.
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Lytic cycle of T4, a virulent phage.
A virulent phage
such as T4
reproduces only by a lytic cycle which
leads to the death of the host when progeny phages burst (lyse) the host cell.
This T4 phage uses its tail fibers to bind to receptor sites on the surface of an E. coli cell.
The phage then uses host resources for the synthesis of viral genomes and proteins, eventually leading to release
of progeny phages and lysis of the host.
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Degradation of a protein by a proteasome.
A proteasome chops up unneeded proteins in the cell.
In most cases, the proteins attacked by a proteasome
have been tagged with short chains of ubiquitin, a small protein.
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Alternative RNA splicing.
The primary transcripts of some genes can be spliced in more than one way,
generating different mRNA molecules.
In this example one mRNA molecule has ended up with the green exon
and the other with the purple exon.
With alternative splicing,
an organism can produce more than one type of polypeptide from a single gene.
transcription.html: 19_05EukGeneTranscript.jpg
Distal control elements (far from the promoter, where RNA polymerase binds,)
can be grouped together as enhancers and may interact with
activators
or repressors to control initiation of transcription.
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Barbara McClintock
(Nobel 1983) discovered transposable elements (transposons),
which can move by either a cut–and–paste or a copy–and–paste (shown here) mechanism
where a DNA sequence is inserted elsewhere into the genome.
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A simplified viral reproductive cycle.
A virus is an obligate intracellular parasite that needs a host cell to reproduce.
In this simplest of viral cycles, the parasite is a DNA virus with a capsid.
virus.html: 19_03-ViralStructure-L.jpg
Viruses are made up of nucleic acid (DNA or RNA) enclosed in a protein coat (capsid)
which is sometimes wrapped in a membranous envelope.
The protein subunits making up the capsid are called capsomeres.