Download Genetics Part 2B 2015

Regulation of metabolic pathways
Bacterial control of gene expression
Operon: cluster of related genes with on/off switch
Three Parts:
1. Promoter – where RNA polymerase attaches
2. Operator – “on/off”, controls access of RNA poly
3. Genes – code for related enzymes in a pathway
Regulatory gene: produces repressor
protein that binds to operator to block
RNA poly
Repressible Operon (ON  OFF)
Repressible Operon
Normally ON
Anabolic (build organic molecules)
Organic molecule product acts as corepressor
 binds to repressor to activate it
Operon is turned OFF
Eg. trp operon
trp operon
Inducible Operon
Normally OFF
Catabolic (break down food for energy)
Repressor is active  inducer binds to and
inactivates repressor
Operon is turned ON
Eg. lac operon
lac operon
• Typical human cell: only 20% of genes
expressed at any given time
• Different cell types (with identical genomes)
turn on different genes to carry out specific
functions
• Differences between cell types is due to
differential gene expression
Eukaryotic gene
expression
regulated at
different stages
Chromatin Structure:
• Tightly bound DNA less
accessible for
transcription
• DNA methylation:
methyl groups added to
DNA; tightly packed; 
transcription
• Histone acetylation:
acetyl groups added to
histones; loosened; 
transcription
Transcription Initiation:
• Control elements bind
transcription factors
• Enhances gene expression
Transcription Initiation Complex
Enhancer regions
bound to
promoter region
by activators
Regulation of mRNA:
• micro RNAs (miRNAs) and
small interfering RNAs
(siRNAs) can bind to
mRNA and degrade it or
block translation
Embryonic Development:
Zygote  Organism
1. Cell Division: large # identical cells through
mitosis
2. Cell Differentiation: cells become specialized in
structure & function
3. Morphogenesis: “creation of form” – organism’s
shape
Determination: irreversible series of events that
lead to cell differentiation
• Cytoplasmic determinants:
maternal substances in
egg distributed unevenly in
early cells of embryo
• Induction: cells triggered
to differentiate
• Cell-Cell Signals:
molecules produced by one
cell influences neighboring
cells
– Eg. Growth factors
Pattern formation: setting up the body plan
(head, tail, L/R, back, front)
Morphogens: substances that establish an
embryo’s axes
Homeotic genes: master control genes that
control pattern formation (eg. Hox genes)
Control of Cell Cycle:
1. Proto-oncogene = stimulates cell division
2. Tumor-suppressor gene = inhibits cell division
• Mutations in these genes can lead to cancer
Proto-oncogene  Oncogene
Genes involved in cancer:
• Ras gene: stimulates cell cycle (protooncogene)
– Mutations of ras occurs in 30% of cancers
• p53 gene: tumor-suppresor gene
– Functions: halt cell cycle for DNA repair,
turn on DNA repair, activate apoptosis
(cell death)
– Mutations of p53 in 50+% of cancers
• Cancer results when mutations accumulate (5-7
changes in DNA)
• Active oncogenes + loss of tumorsuppressor genes
• The longer we live, the more likely that
cancer might develop
Bacteria vs.Viruses
Bacteria
• Prokaryotic cell
• Most are free-living (some
parasitic)
• Relatively large size
• Antibiotics used to kill
bacteria
Virus
• Not a living cell (genes
packaged in protein shell)
• Intracellular parasite
• 1/1000 size of bacteria
• Vaccines used to prevent
viral infection
• Antiviral treatment
Viruses
• Very small (<ribosomes)
• Components = nucleic acid + capsid
– Nucleic acid: DNA or RNA (double or
single-stranded)
– Capsid: protein shell
– Some viruses also have viral envelopes
that surround capsid
• Limited host range (eg. human cold virus
infects upper respiratory tract)
• Reproduce within host cells
Simplified viral replicative cycle
Bacteriophage
• Virus that infects bacterial cells
Lytic Cycle of T4
Phage
Bacteriophage Reproduction
• Lytic Cycle:
– Use host machinery to make copies of virus
– Death of host cell by rupturing it (lysis)
– Virulent phages replicate by this method
• Lysogenic Cycle:
– Phage DNA incorporated into host DNA and
replicated along with it
– Phage DNA = prophage
• Temperate Phage: uses both methods of
replication
Lytic Cycle vs. Lysogenic Cycle
Animal viruses have a membranous envelope
• Host membrane forms
around exiting virus
• Difficult for host
immune system to
detect virus
Retrovirus
• RNA virus that uses reverse transcriptase (RNA 
DNA)
• Newly made viral DNA inserted into chromosome
of host
• Host transcribes viral DNA (= provirus) to make
new virus parts
• Example: HIV (Human Immunodeficiency Virus)
HIV =
Retrovirus
HIV
◦ Infects white blood cells
◦ HIV+: provirus (DNA
inserted)
◦ AIDS: active viral
reproduction
Vaccines
• Weakened virus or part of pathogen that
triggers immune system response
Emerging viruses = mutation of existing
viruses
Tools of Genetic Engineering
 Restriction enzymes (restriction endonucleases): used to
cut strands of DNA at specific locations (restriction sites)
 Restriction Fragments: have at least 1 sticky end (singlestranded end)
 DNA ligase: joins DNA fragments
 Cloning vector: carries the DNA sequence to be cloned
(eg. bacterial plasmid)
Using a restriction
enzyme (RE) and
DNA ligase to
make recombinant
DNA
Gene Cloning
Applications of Gene Cloning
PCR (Polymerase Chain
Reaction): amplify (copy)
piece of DNA without use
of cells
Gel Electrophoresis: used to separate DNA molecules on
basis of size and charge using an electrical current (DNA 
+ pole)
Gel Electrophoresis: used to separate DNA molecules on
basis of size and charge using an electrical current (DNA 
+ pole)
Cloning Organisms
• Nuclear transplantation: nucleus of egg is
removed and replaced with nucleus of body
cell
Nuclear Transplantation
Problems with Reproductive Cloning
• Cloned embryos exhibited various defects
• DNA of fully differentiated cell have epigenetic
changes
Stem Cells
• Stem cells: can reproduce itself indefinitely and
produce other specialized cells
– Zygote = totipotent (any type of cell)
– Embryonic stem cells = pluripotent (many cell types)
– Adult stem cells = multipotent (a few cell types) or
induced pluripotent, iPS (forced to be pluripotent)
Embryonic
vs. Adult
stem cells
Applications of DNA Technology
1. Diagnosis of disease – identify alleles, viral
DNA
2. Gene therapy – alter afflicted genes
3. Production of pharmaceuticals
4. Forensic applications – DNA profiling
5. Environmental cleanup – use
microorganisms
6. Agricultural applications - GMOs
Human DNA
• 3 billion base pairs
• ~20,000 genes
• Only 1.5% codes for
proteins (or RNA)
• Repetitive DNA:
sequences present in
multiple copies
Transposable Elements
Make up 75% of repetitive DNA
Can be moved from one location to another
in genome
Discovered by Barbara McClintock – corn
breeding experiments
2 Types:
Transposons
Retrotransposons
Transposons
Moves within genome via DNA intermediate
“cut & paste” or “copy & paste” mechanisms
Requires enzyme transposase
Retrotransposons
 Move by means of RNA intermediate
 Leaves copy at original site
 Involves enzyme reverse transcriptase
Multigene Families
• Collections of 2 or more identical or very
similar genes
• Eg. hemoglobin: -globin and -globin gene
families
Transpositions  chromosomal rearrangements
Transposable elements contribute to
evolution
• Promote recombination, disrupt genes or
control elements, & carry genes to new
locations
• May be harmful or lethal, but can also have
small beneficial effects
• Provides raw material for natural selection
Evolutionary Development
(Evo-devo)
• Compares developmental processes to understand
how changes can lead to evolution of organisms
Homeotic genes: master regulatory genes
• Control placement and spatial organization of
body parts
Homeobox: widely conserved 180-nucleotide
sequence within homeotic (Hox) genes
• Found in many groups (fungi, animals, plants)
• Hints at relatedness between all life forms
Conservation of
homeotic genes
Genetic Diversity in Prokaryotes
Factors:
1.Rapid reproduction (binary fission)
2.Mutations – errors in replication
3.Genetic recombination
Genetic Recombination in Bacteria
1. Transformation: uptake of foreign DNA from
surroundings
2. Transduction: viruses transfer genes
between prokaryotes
3. Conjugation: DNA transferred from one to
another
Transformation
• Uptake of foreign DNA from surroundings
• Observed by Griffith (bacteria & mice)
Plasmids
Small ring of DNA that carries a few genes
Replicates separately from bacterial chromosome
Can carry genes for antibiotic resistance
Used frequently in genetic engineering for gene
cloning
Using plasmids and bacteria in genetic
engineering
Transduction
• Viruses
(bacteriophages)
carry bacterial
genes from one
host cell to another
• Recombine DNA of
donor and
recipient cell
Conjugation
• One cell donates DNA to another
• Donors cell extends a sex pilus (“mating
bridge”) through which DNA is transferred
• Requires the presence of a piece of DNA
called the F factor to produce the pilus