Eukaryotic Genomes 15 November, 2002 Text Chapter 19

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Eukaryotic Genomes15 November, 2002Text
Chapter 19
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Chromatin
In eukaryotes, DNA is present as several
different molecules. Each DNA molecule, along
with its associated proteins is one chromosome.
Chromosomes are in the extended conformation
while they are being transcribed. They are at
their most condensed during nuclear division.
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Eukaryotic DNA
In most eukaryotes, only about 5 of the genome
actually codes for protein or RNA sequence.
10 - 15 of the genome is satellite DNA, repeats
of the sequence GTTAC. This DNA is mainly
structural, clustering at telomeres and
centromeres.
About 40 of the genome is interspersed
repetitive DNA, mostly from replicative
transposition.
10 - 15 of the genome is made up of introns and
other transcribed spacers.
5 to 10 of the genome is regulatory sequence.
About 5 of the genome is pseudogene sequence.
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The globin gene family arose by duplication and
differentiation. Duplication can occur due to
unequal crossing over.
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Immunoglobin genes are rearranged in immune cells
to specify immune diversity.
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Regulation of Gene Expression
Changing the level of active protein to suit cell
type and environmental conditions is the goal of
gene regulation.
This regulation can occur at any of the steps in
gene expression. Regulation of transcription is
most common, important examples of each type of
regulation exist.
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Eukaryotic genes differ form prokaryotic genes in
several important respects. First, these genes
are never in operons. Each is transcribed
independently. Introns exist, and are spliced
out of the primary transcript. Finally, control
elements, the binding sites for transcription
factors, may be close to or far from the promoter.
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How can transcription factors bound to distal
control elements (activators) influence
transcription?
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Proteins recognize specific DNA sequences by
forming weak interactions between amino acids on
the protein and bases in the DNA molecule.
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Cancer results from genetic changes that affect
the cell cycle.
These changes include abnormal activation of
oncogenes and inactivation of tumor suppressor
genes.
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Usually, a cancerous transformation in any cell
requires that several oncogenes be activated and
several tumor suppressors be inactivated.
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The genetic changes that cause colon cancer are
well understood.
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