Download 1. Compare the organization of prokaryotic and eukaryotic genomes.

Chapter 19 Reading Quiz
1.
2.
3.
4.
5.
What are the proteins called around which
DNA winds?
What is the basic unit of DNA packing?
The attachment of methyl groups to DNA
bases after DNA is synthesized is known
as..?
What general effect does this process have
on DNA?
What are “oncogenes”?
1. Compare the organization of prokaryotic and
eukaryotic genomes.
•
•
•
•
Eukaryotic
Prokaryotic
• Complexed with a large
Usually circular
amount of protein to
Smaller
form chromatin
Found in the nucleoid • Highly extended and
tangled during
region
interphase
Less elaborately
• Found in the nucleus 
structured and
folded
2. Describe the current model for progressive
levels of DNA packing.
• Nucleosome  basic unit of DNA packing [formed
from DNA wound around a protein core that consists
of 2 copies each of the 4 types of histone (H2A, H2B,
H3, H4)]
• A 5th histone (H1) attaches near the bead when the
chromatin undergoes the next level of packing
• 30 nm chromatin fiber  next level of packing; coil
with 6 nucleosomes per turn
• the 30 nm chromatin forms looped domains, which
are attached to a nonhistone protein scaffold
(contains 20,000 – 100,000 base pairs)
• Looped domains attach to the inside of the nuclear
envelope 
3. Explain how histones influence folding in
eukaryotic DNA.
• Histones  small proteins rich in basic
amino acids that bind to DNA, forming
chromatin
• Contain a high proportion of positively
charged amino acids which bind tightly
to the negatively charged DNA 
4. Distinguish between heterochromatin and
euchromatin.
Heterochromatin
• Chromatin that
remains highly
condensed during
interphase and is
NOT actively
transcribed
Euchromatin
• Chromatin that is
less condensed
during interphase
and IS actively
transcribed
• Becomes highly
condensed during
mitosis 
5. Describe where satellite DNA is found and
what role it may play in the cell.
• Satellite DNA  highly repetitive DNA
consisting of short unusual nucleotide
sequences that are tandemly repeated 1000’s
of times
• It is found at the tips of chromosomes and
the centromere
• Its function is not known, perhaps it plays a
structural role during chromosome replication
and separation 
6. Describe the role of telomeres in solving the
end-replication problem with the lagging DNA
strand.
• Telomere  series of short tandem repeats
at the ends of eukaryotic chromosomes;
prevents chromosomes from shortening with
each replication cycle
• Telomerase  enzyme that periodically
restores this repetitive sequence to the ends
of DNA molecules 
7. Using the genes for rRNA as an example,
explain how multigene families of identical
genes can be advantageous for a cell.
• Multigene family  a collection of genes that
are similar or identical in sequence and
presumably of common ancestral origin
• Include genes for the major rRNA molecules,
huge tandem repeats of these genes enable
cells to make millions of ribosomes during
active protein synthesis 
8. Using  -globin and  -globin genes as examples,
describe how multigene families of nonidentical genes
probably evolve, including the role of transposition.
• They arise over time from mutations that
accumulate in duplicated genes
• Can be clustered on the same chromosome or
scattered throughout the genome
• Original α & β genes evolved from duplication
of a common ancestral globin gene
• Transposition separated the α globin and β
globin families, so they exist on different
chromosomes 
9. Explain gene amplification.
• Gene amplification  the selective replication
of certain genes that is a potent way of
increasing expression of the rRNA genes,
enabling more ribosomes to be made
• Selective gene loss  in certain tissues,
genes are selectively lost and may be
eliminated from certain cells 
10. Describe the effects of transposons and
retrotransposons.
• Transposons  jump and interrupt the normal
functioning may increase or decrease production of
one or more proteins
- can carry a gene that can be activated when
inserted downstream from an active promoter and
vice versa
• Retrotransposons  transposable elements that move
within a genome by means of an RNA intermediate, a
transcript of the retrotransposon DNA
- to insert it must be converted back to DNA by
reverse transcriptase 
11. Explain immunoglobin genes.
• Immunoglobin genes  genes that encode
antibodies
• Basic immunoglobin molecule  consists of
four polypeptide chains held by disulfide
bridges
- each chain has 2 regions: constant and
variable
- variable gives each antibody its unique
function 
12. Explain the chromatin modifications of DNA
methylation, genomic imprinting, and histone
acetylation.
• DNA methylation  the attachment of methyl groups
(-CH3) to DNA bases
-Inactive DNA is usually highly methylated (adding
methyl groups inactivates DNA)
• Genomic imprinting  where methylation permanently
turns off either the maternal or paternal allele of
certain genes at the start of development
• Histone acetylation  the attachment of acetyl
groups (-COCH3) to certain amino acids of histone
proteins
- transcription proteins have easier access to genes in
acetylated regions of DNA 
13. Explain the potential role that promoters
and enhancers play in transcriptional control.
Promoters
• Include the proximal
control elements
• Produces a low rate of
initiation with few RNA
transcripts
• Unless  DNA
sequences can improve
the efficiency by
binding additional
transcription factors
Enhancers
• The more distant
control elements
• Bending of the DNA
enables the
transcription factors
bound to enhancers to
contact proteins of the
transcription-initiation
complex at the
promoter 
14. Compare the arrangement of coordinately
controlled genes in prokaryotes and eukaryotes.
Prokaryotic
• Prokaryotic genes that
are turned on and off
together are often
clustered into operons
which are transcribed
into one mRNA molecule
and translated together
Eukaryotic
• Eukaryotic genes
coding for enzymes
of a metabolic
pathway are often
scattered over
different
chromosomes and
are individually
transcribed 
15. Explain how eukaryotic genes can be coordinately
expressed and give some examples of coordinate gene
expression in eukaryotes.
• Associated with specific regulatory DNA sequences
or enhancers that are recognized by a single type of
transcription factor that activates or represses a
group of genes in synchrony
- heat shock response  series of proteins that help
stabilize and repair
- Steroid hormone action  steroids activate protein
receptors which activate genes
- Cellular differentiation  the genes produce
particular sets of proteins which go on and off 
16. Explain why the ability to rapidly degrade
mRNA can be an adaptive advantage for
prokaryotes.
• Prokaryotic mRNA molecules are
degraded by enzymes after only a few
minutes  thus bacteria can quickly
alter patterns of protein synthesis in
response to environmental changes 
17. Describe the importance of mRNA
degradation in eukaryotes, describe how it can
be prevented.
• The longevity of a mRNA affects how
much protein synthesis it directs; those
that are viable longer can produce more
of their protein
• Control mechanisms of gene expression
can help prevent degradation 
18. Explain how gene expression may be
controlled at translation and post-translation.
• Translational  binding of translation repressor
protein to the 5’ end of a particular mRNA can
prevent ribosome attachment
- translation of all mRNAs can be blocked by the
inactivation of certain initiation factors
• Posttranslational  last level
- many eukaryotic polypeptides must be modified or
transported before becoming biologically active by
adding phosphates, chemical groups, etc.
- selective degradation of particular proteins and
regulation of enzyme activity are also control
mechanisms of gene expression 
19. Describe the normal control mechanisms
that limit cell growth and division.
• Proto-oncogenes  gene that normally
codes for regulatory proteins
controlling cell growth, division, and
adhesion 
20. Briefly describe the four mechanisms that
can convert proto-oncogenes to oncogenes.
1.
2.
3.
4.
Movement of DNA within the genome 
chromosomes have been broken and rejoined
Gene amplification  sometimes more copies of
oncogenes are present in a cell than is normal
Point mutation  a slight change in the nucleotide
sequence might produce a growth-stimulating
protein that is more active or more resistant to
degradation than the normal protein
Changes in tumor-suppressor genes that normally
inhibit growth can promote cancer 
21. Explain how changes in tumor-suppressor
genes can be involved in transforming normal
cells into cancerous cells.
• Frequency of
mutation is close to
50% for the p53
tumor suppressor
gene 
22. Explain how oncogenes are involved in virusinduced cancers.
• Viruses might add oncogenes to cells,
disrupt tumor-suppressor genes’ DNA,
or convert proto-oncogenes to
oncogenes 