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Eukaryotic Gene Expression
Chapter 19
Why control gene expression
To respond to their external
environment
To respond to their internal
environment
Direct the cellular differentiation
necessary to create specialized cells
Packing of DNA
Each chromosome is long-all 46
chromosomes are about 2 m long
Multilevel folding helps determine gene
activity
Histones are small positively charged
proteins
Histones bind to DNA to make up chromatin
These form “beads” each of which consists
of a nucleosome of DNA wound around a
protein core of 8 histones
Higher Levels of DNA packing
30 nm chromatin fiber is a repeating array of
6 nucleosomes organized around histone
molecules (H1)
Looped domain is a fold in the 30 nm
chromatin fiber that may contain genes that
are expressed in a coordinated fashion
Looped domains may coil and fold,
compacting the chromatin
Highly compacted heterochromatin is not
transcribed
Open form of chromatin, euchromatin, is
transcribed
Role of DNA Packing and
Methylation
Barr bodies are examples of
heterochromatin that is not transcribed
DNA methylation of cytosine residues
are not expressed
Drugs that inhibit methylation can
induce gene reactivation
DNA methylation may cause the double
helix to assume the left-hand Z form,
which may function in gene regulation
Transcriptional Control
Chromosome puffs appear along polytene
chromosomes in larvae
These DNA loops make the DNA more accessible
to RNA polymerase
Loops are regions of active RNA synthesis
Shifting locations of puffs indicated that genes
are turned on and off
Ecdysone, insect hormone that initiates molting,
can induce changes in puff patterns
Gene regulation is responsive to chemical signals
Gene activation
Most genes are inactive
Gene regulatory proteins interact with
the nucleotide sequences, or regulatory
sites, to activate transcription of genes
Promoter regions
2 types of promoter regions have been identified
that are specifically bound by RNA polymerase
There are 3 different RNA polymerases which
transcribe rRNA, mRNA, & tRNA
Enhancers are DNA sequences that greatly increase
the activity of nearby genes
Enhancers are recognition sites for proteins that
increase DNA’s accessibility to RNA polymerase
Transcriptional Control Summary
DNA packing
Polytene puffs
Gene regulatory proteins
Promoter regions
Enhancers
Posttranscriptional Control
Gene expresseion is meausred by the
number and types of proteins that are
made
Gene expression can be blocked or
enhanced at any posttranscriptional step
Translation is separated by nuclear
membrane from transcription
Posttranscriptional Control in
nucleus
Removal of intron DNA segments
Splicing together of exons
Poly U caps and poly A tails
Posttranscriptional Control in
cytoplasm
Eukaryotic mRNA can last hours or even weeks
Length of time before degraded by cellular enzymes is
related to quantity of protein synthesis they can direct
Translation of mRNA can be delayed until control
signal initiates it
Initiation factors may be required for translation
Polypeptides are extensively cleaved to produce an
active protein
Selective degradation of protins and metabolic
regulation of enzymes serves as control mechanism in
cell
Multigene families
These are collections of similar or identical genes
that code for RNA products
Identical genes coding for the major rRNA
molecules are arranged one after another
hundreds or thousands of times, forming huge
tandem arrays
Tandem arrays enable cells that are actively
synthesizing proteins to produce millions of
ribosomes required
Beta polypeptide chains of hemoglobin are an
example of a multigene family
Origin of multigene families
Families of identical genes may have risen from
repeated gene dupliation
Nonidentical families probably arose from
mutation in duplicated genes
Pseudogenes, sequences of DNA that resemble
real genes but lack signals for expression, may be
present in gene families
Globin pseudogenes lack introns and have poly A
tails, may have been transcribed from RNA by
reverse transcriptase and may have moved about
the genome by transposition
Highly repetitive sequences
Highly repetitive short sequences may make
up 10-25% of total DNA
Called satellite DNA because of their base
compositions may be sufficiently different
from rest of the cell’s DNA to isolate them by
ultracentrifugation
This DNA is located at centromeres and may
be structural rather than genetic role in cell
Cell differentiation
Regulation of gene expression leads to genes
producing their particular set of proteins
exclusively
Zygote is totipotent
Differentiation of cells is signaled by changes in
cellular structure, appearance of characteristic
proteins, or initially, by accumulation of specific
mRNA molecules
Environmental cues or chemical turn on the genes
that allow cells to become specialists at making the
particular proteins associated with their functions
Genomic Equivalence
All cells contain the full complement of genes
Differentiation is reversible
Determination affects the ability of a nucleus
to express all of its genome
Regeneration is replacement of lost parts in
an organism
Many vertebrates can regenerate wholes from
fragments
Cancer: Development and
differentiation out of control
No contact inhibition
Tumor forms (the HeLa cell line has been reproducing in
culture for 30 years!)
Benign tumors remain at original site and may be
completely removed by surgery
Malignant tumors formed by cells that lose ability to
recognize each other and lose sticking qualities to other
cells
Cells may then travel throughout body-metastasis
Carcinogens: physical or chemical agents or viruses
may induce cancer
Carcinogens may activate protooncogens to become
oncogenes
If a gene has multiple copies, has been transposed, or
translocated, it may become an oncogene
Homeotic genes and homeoboxes
Homeotic genes play major role in
control of developental patterns in all
organisms
Homeoboxes are translated into amino
acid sequences, called homeodomains,
which are able to bind to DNA
Proteins containing homeodomains are
regulatory proteins
Gene amplification
Multiple copies of rRNA genes are
present in the genomes of most
eukaryotic cells – form nucleolus
Gene amplification is the selective
synthesis of extra DNA
Selective Gene loss
Chromosome diminution is elimination
of whole or parts of chromosomes from
cells early in development
Occurs in some organisms (like flies)
Rearrangements in the genome
Rearrangements may activate or
inactivate genes
Much of the variation in antibodies arises
from different combinations of variable
and constant regions of 4 polypeptides
that form the complete antibody molecule
This is an example of genomic
nonequivalence
Aging
Organisms change by growth and maturation,
metamorphosis, wound healing, and regeneration of
body parts
Genes program aging and death of selected parts of
organisms throughout development
Webbing of fingers of human embryo, production of
mature xylem, loss of tadpole tails
Human fibroblasts in cell culture stop reproducing and
degenerate and die after 50 divisions other cell types
have different number
Aging may be accumulation of mutations and other
problems resulting in functional decline of cells
Aging and death may be programmed
Epigenesis versus preformation
Preformation – egg has preformed mini
me
Epigenesis forma emerges gradually
through progressive development