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Eukaryotic
Genomes
12 November, 2004
Text Chapter 19
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.
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.
The globin gene family arose by duplication and differentiation.
Duplication can occur due to unequal crossing over.
Immunoglobin
genes are
rearranged in
immune cells to
specify immune
diversity.
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.
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.
How can transcription factors bound to distal control elements
(activators) influence transcription?
Proteins recognize specific DNA sequences by forming weak
interactions between amino acids on the protein and bases in the DNA
molecule.
Cancer results from genetic changes that affect the cell cycle.
These changes include abnormal activation of oncogenes and
inactivation of tumor suppressor genes.
Usually, a cancerous
transformation in
any cell requires that
several oncogenes
be activated and
several tumor
suppressors be
inactivated.
The genetic changes that cause colon
cancer are well understood.