Chapter 19: The Organization and Control of Eukaryotic Genomes

Gene regulation is more complex than in prokaryotes because eukaryotes have a
larger, more complex genome.
Chromosome structure:
 Chromatin- complex of DNA and protein that make up a eukaryotic chromosome
o DNA is wrapped around histone proteins to form a nucleosome- this may
control gene expression by controlling the access of transcription proteins
to the DNA
o Chromatin fiber- tightly wound coil with 6 nucleosomes per turn
o Looped domains are attached to a non-histone protein scaffold (20,000 to
100,000 base pairs per loop)
 These looped domains are then folded onto one another to form a chromosome
 Heterochromatin- chromatin that remains condensed during interphase and that is
not actively transcribed
 Euchromatin- chromosome that is less condensed during interphase and is
actively transcribed, becomes highly condensed during mitosis
Genome organization
 The eukaryotic genome is plastic, or changeable, in ways that affect the
availability of specific genes for expression
1. Tandemly repetitive DNA (satellite DNA)
 10- 25 % of the total DNA is satellite DNA (short 5 to 10 nucleotide
sequences that are tandemly repeated thousands of times
 Mostly located at the centromeres
 Is not transcribed as its functions is still unknown
 Could not function in a structural role during chromosome replication and
separation during mitosis and meiosis
 Also found in telomeres and may help prevent chromosomes from shortening
with each replication cycle.
2. Interspersed repetitive DNA
 25-40 % of DNA in mammals
 Large amounts of repeated units of DNA (100s or 1000s of base pairs long)
dispersed at random intervals throughout the genome
 Function still unknown but are abundant and variable
 Most in mammals seem to be transposons ( I piece of DNA that can move
from one location to another within a genome
3. Multigene families- a collection of genes that are similar or identical in sequence
and presumably of common ancestral origin


Families of identical genes – arise from a single ancestral gene that has
undergone repeated duplication. Ex. Genes for major RNA molecules to
make millions of ribosomes
Families of non-identical genes – arise from mutations that accumulate in
duplicated genes
Altering a cell’s genome:
 Gene amplification- selective synthesis of DNA, which results in multiple copies
of a single gene, ex. Amphibian rRNA genes in the oocyte, to produce numerous
ribosomes
 Rearrangement
o Transponsons- by inserting into the middle of a coding sequence of
another gene, can prevent gene from functioning normally, most are
retrotransposons (fig 19.5, transcribe transposon, translate to a reverse
transcriptase, reverse transcription of RNA to DNA, synthesis of second
DNA strand, insertion into original DNA)
Control of Gene Expression:
 Only express a small % of genes
 Cellular differentiation- divergence in structure and function of different cell
types, as become more specialized during an organism’s development
 Must regulate gene expression for the long term
 Highly specialized cells express only a small % of genes so transcription enzymes
must locate the right genes at the right time
 DNA binding proteins regulate gene activity, particularly transcription
 Overview for the control of gene expression (fig. 19.7)
1. Pre- transcriptional control- chromatin modification= controls which DNA regions are
available for transcription

DNA menthylation- addition of methyl groups (- CH3) to
bases of DNA
 Usually cytosine ( 5% methylated)
 Genes heavily methylated usually not expressed (Barr Bodies)
 Drugs that inhibit methylation can induce gene reactivation
 Must occur for normal cell differentiation to occur, DNA methylation
ensures genes that must be off, stay off.
ii. Histone acetylation- attach (-COCH3) groups to certain amino acids of
histone proteins, will bind less tightly to DNA so easier to transcribe
because transcription factors have easier access
2. Transcriptional Control (Fig, 19.8 )
 Control elements – segments for noncoding DNA that help regulate the
transcription of a gene by binding specific proteins (transcription factors)



Transcription factors- assist the binding of RNA polymerase to the promoter, can
attach to DNA or to RNA polymerase or to each other, specifically, RNA
polymerase cannot bind without attachment of transcription factor to TATA box
How does it work? (fig, 19.9)
a. Activator proteins bind to enhancer sequences in the DNA
b. DNA bending brings the bound activators closer to the promoter, other
transcription factors and RNA polymerase are nearby
c. Protein- binding domains on the activators attach to certain transcription
factors and help them form an active transcription initiative complex on
the promoter
Coordinately controlled genes- no operons in eukaryotes but still need to
coordinate expression of polypeptides and enzymes associated with a pathway,
have specific type if transcription factors that are associated with specific
regulatory DNA sequences of enhancers so these genes are transcribed
simultaneously, examples: heat shock response, steroid hormone action
3. Post transcriptional control- supporting role
 mRNA modification and splicing of introns- example: can produce different
mRNA transcripts depending on which RNA segments are treated as introns and
which ax exons
 mRNA degradation- longevity of mRNA affects how much protein synthesis it
directs, some can last for minutes, other for a week, degradation could begin with
the removal of the 5’ cap
 Control of translation- repress initiation of translation by:
a. Repressor proteins that bind to specific sequences or structures within the
leader region at the 5’ ends, this prevents ribosome attachment
b. Inactivating necessary transcription factors (global control)
 Protein processing and degradation
a. many polypeptides need modification before they can properly function
including attachment of sugars, lipids, and phosphates, removal of one or
more amino acids form the leading end, cleave into to pieces (ex. Insulin
is cut in half and held together with disulfide bridges)
b. alter targeting of a protein- if protein cannot reach the target site, it will
not function
c. selective degradation- attach ubiquitin to the protein to mark it for cell
destruction, large proteasomes recognize the ubiquitin and degrades the
tagged protein ( could lead to cancer if cell cycle proteins become
impervious to proteasomes)