Transcript CHAPTER 12

The importance of telomerase in maintaining
chromosome integrity
The Nucleus of a Eukaryotic Cell
(18)
• Centromeres
– The centromere is located at the site markedly
indented on a chromosome.
– Centromeres contain constitutive
heterochromatin.
– Centromeric DNA is the site of microtubule
attachment during mitosis.
– DNA sequence is not important for centromere
structure and function.
A centromere is marked by
a distinct indentation
The Nucleus of a Eukaryotic Cell
(19)
• Epigenetics: There’s More to Inheritance than
DNA
– Epigenetic inheritance depends on factors other
than DNA sequences.
– Parental histones determine the chemical
modifications found in the newly synthesized
histones.
The Nucleus of a Eukaryotic Cell
(20)
• The Nucleus as an
Organized Organelle
– Chromatin fibers are
concentrated at
specific domains
within the nucleus.
The Nucleus of a Eukaryotic Cell
(21)
• Chromosome ordering is directed by the
nuclear envelope proteins.
• In the nucleus, mRNAs are synthesized as
discrete sites.
• DNA sequences that participate in a common
biological response but reside on different
chromosomes interact within the nucleus.
Interaction between distantly located genes
Interaction between distant genes (continued)
Nuclear compartmentalization of the cell’s
mRNA processing machinery
The Nucleus of a Eukaryotic Cell
(22)
• The Nuclear Matrix
– The nuclear matrix is a network of proteincontaining fibrils.
– It serves as more than a skeleton to maintain the
shape of the nucleus and anchoring the
machinery involved in nuclear activities.
The nuclear matrix
The nuclear matrix
The Human Perspective: Chromosomal
Aberrations and Human Disorders (1)
• A chromosomal aberration is loss or exchange
of a segment between different
chromosomes, caused by exposure to DNAdamaging agents.
• Chromosomal aberrations have different
consequences depending on whether they are
in somatic or germ cells.
The Human Perspective: Chromosomal
Aberrations and Human Disorders (2)
• Inversions involve the
breakage of a
chromosome and
resealing of the
segment in a reverse
order.
The Human Perspective: Chromosomal
Aberrations and Human Disorders (3)
• Translocations are the
result of the attachment
of all or one piece of
one chromosome to
another chromosome.
The Human Perspective: Chromosomal
Aberrations and Human Disorders (4)
• Deletions result
when thee is loss of
a portion of a
chromosome.
• Duplications occur
when a portion of a
chromosome is
repeated.
12.2 Control of Gene Expression in
Bacteria (1)
• Bacterial cells selectively express genes to use
the available resources effectively.
– The presence of lactose in the medium indices the
synthesis of the enzyme β-galactosidase.
– The presence of tryptophan in the medium
represses the genes that encode enzymes for
tryptophan synthesis.
The kinetics of β-galactosidase induction
Control of Gene Expression in
Bacteria (2)
• The Bacterial Operon
– An operon is a functional complex of genes
containing the information for enzymes of a
metabolic pathway. It includes:
• Structural genes – code for the enzymes and are
translated from a single mRNA.
• Promoter – where the RNA polymerase binds.
• Operator – site next to the promoter , where the
regulatory protein can bind.
Control of Gene Expression in
Bacteria (3)
• The Bacterial Operon (continued)
– An operon includes:
• A repressor which binds to a specific DNA sequence to
determine whether or not a particular gene is
transcribed.
• The regulatory gene encodes the repressor protein.
Organization of a bacterial operon
Gene regulation
by operons
Control of Gene Expression in
Bacteria (4)
• The lac Operon
– It is an inducible operon, which is turned on in the
presence of lactose (inducer).
• The lac operon contains three structural genes.
• Lactose binds to the repressor, changing its
conformation and making it unable to bind to the
operator.
• A repressor protein can bind to the operator and
prevent transcription in the absence of lactose.
Control of Gene Expression in
Bacteria (5)
• The lac Operon (continued)
– Positive Control by Cyclic AMP
• The lac repressor exerts negative control.
• The glucose effect is an example of positive control.
• Cyclic AMP (cAMP) acts by binding to a cAMP receptor
protein (CRP).
• Binding of CRP-cAMP to the lac control region changes
the conformation of DNA thus allowing RNA
polymerase to transcribe the lac operon.
The nucleotide sequence of binding sites in the
control region of the lac operon
Control of Gene Expression in
Bacteria (6)
• The trp Operon
– It is a repressible operon, which is turned off in the
presence of tryptophan.
– The trp operon repressor is active only when it is
bound to a corepressor such as tryptophan.
Control of Gene Expression in
Bacteria (7)
• Riboswitches
– A number of bacterial mRNAS can bind to a small
metabolite, which in turn alters the gene involved
in the production of such metabolite.
– These mRNAs are called riboswitches because
they undergo a conformational change and can
suppress gene expression.
– Riboswitches allow bacteria to regulate gene
expression in response to some metabolites.
12.3 Control of Gene Expression in
Eukaryotes (1)
• Cells of a complex eukaryote exist in many
differentiated states.
– Differentiated cells retain a full set of genes.
– Nuclei from cells of adult animals are capable of
supporting the development of anew individual,
as demonstrated in experiments.
Cloning demonstrates that nuclei retain a
complete set of genetic information
Control of Gene Expression in
Eukaryotes (2)
• Genes are turned on and off as a result of
interaction with regulatory proteins.
– Each cell type contains a unique set of proteins.
– Regulation of gene expression occurs on three
levels:
• Transcriptional-level control
• Processing-level control
• Translational-level control
Overview of levels of control of gene expression
12.4 Transcriptional-level control
(1)
1. Differential
transcription is the
most important
mechanism by which
eukaryotic cells
determine which
proteins are
synthesized.
Transcriptional-level control (2)
2. DNA microarrays can monitor the expression
of thousands of genes simultaneously.
a) Immobilized fragments of DNA are
hybridized with fluorescent cDNAs.
b) Genes that are expressed show up as
fluorescent spots on immobilized genes.
c) Microarrays a provide a visual picture of
gene expression.
The construction of
a DNA microarray
DNA microarrays and their use in monitoring
gene transcription
Transcriptional-level control (3)
• The Role of Transcription Factors in Regulating
Gene Expression
– Transcription factors are the proteins that either
acts as transcription activators or transcription
inhibitors.
• A single gene can be controlled by different regulatory
proteins.
• A single DNA-binding protein may control the
expression of many different genes.
Interactions between transcription factors
bound to different regions of a gene
Transcriptional-level control (4)
• The Role of Transcription Factors in
Determining a Cell’s Phenotype
– Embryonic stem (ES) cells and are pluripotent,
capable of differentiating into all of the different
types of cells.
– The importance of transcription factors in ES cells
was demonstrated when these factors were
introduced and shown to reprogram these cells.
Transcriptional-level control (5)
• The Structure of Transcription Factors
– Transcription factors contain a DNA-binding
domain and an activation domain.
• Transcription Factor Motifs
– The DNA-binding domains of most transcription
factors have related structures (motifs) that
interact with DNA sequences.
– Most of the motifs contain a segment that binds
to the major groove of the DNA.
Interaction between a transcription factor and
its DNA target sequence
Transcriptional-level control (6)
• Transcription factors
(continued)
– The zinc finger motif –
the zinc ion of each
finger is held in place by
two cysteines and two
histidines.
Transcriptional-level control (7)
• Transcription Factors
(continued)
– The helix-loop-helix
(HLH) motif – has two
α-helical segments
separated by a loop.
– The leucine zipper
motif – has a leucine at
every seventh amino
acid of an α-helix.
Transcriptional-level control (8)
• HLH-containing transcription factors play a key role
in the differentiation of certain tissues.
• HLH-containing transcription factors also participate
in the control of cell proliferation and cancer.
Transcriptional-level control (9)
• DNA Sites Involved in Regulating Transcription
– The TATA box regulates the initiation of
transcription.
– The core promoter, from the TATA box to the start,
is where the initiation complex assembles.
– The CAAT and the GC box are upstream and are
required for initiation.
– Alternative promoters allow some genes to be
transcribed at more than one site.
Identifying promoter sequences