Transcript Document

Announcements
1.Student-initiated genetics study group to meet Thursdays
at 4pm in either Brooks 203 or 204 for problem solving.
Review of Last Lecture
1. Finished linkage: mapping human chromosomes
2. Conjugation
-F factor is a plasmid that allows for fertility and for
a cell to act as a donor of genetic material
F+ x F-
F+
F+
F-
F+
F+
Hfr is special F+
Hfr x FF-
F+
Hfr
F+
F-
Outline of Lecture 15
I. Bacterial genetics, continued
II. Extrachromosomal Inheritance
Chloroplast DNA
Mitochondrial DNA
Maternal Effect Genes
Circular
Map of
E. coli
~2000 genes
Scaled in minutes,
100 minutes total
Learning check
A strain of Hfr cells that is sensitive to the antibiotic streptomycin (strs) has
the genotype gal+ his+ bio+ pur+ gly+. These cells are mixed with an Fstrain that is resistant to streptomycin (strr) and that is gal- his- bio- purgly-. Cells are allowed to undergo conjugation. At regular intervals, a
sample of cells was removed and conjugation was interrupted by placing
the sample in the blender. The cells are plated on medium that contain
streptomycin. Cells that grew were tested for the presence of genes
transferred from the Hfr strain. Genes from the donor Hfr strain first
appeared in the F- strain at the times listed.
Give the order of the genes on the bacterial chromosome and indicate the
minimum distances between them.
gly+ 3 min.
his+ 14 min.
bio+ 35 min.
gal+ 36 min.
pur+ 38 min.
Transformation: a different process of
recombination, can be used to map genes
* Relative frequency of
genes being
cotransformed indicates
distance between the
two genes
Bacteriophages are viruses that use
bacteria as hosts
• Known as phages
• Nobel prize awarded to Delbruck, Luria
and Hershey in 1969 for their work on
the genetic structure of phages
T4 bacteriophage
Virulent or
temperate phage
(e.g.  phage)
Lytic and
Lysogenic
Cycles
Recombinants from
transduction can be
propagated
Lederberg-Zinder experiment - Salmonella
Could this recombination
be due to conjugation or
transformation?
Transduction: virus-mediated
bacterial DNA transfer
Viruses
can
package
bacterial
DNA
Transduction
Recombination in Phage
Large, dark
recombinants
Small, light
• Strains with different plaque
morphologies “crossed” by
coinfection of bacteria:
•
h r + X h+ r
– h mutant plaques are
darker than h+
– r mutant plaques are
larger than r+
• Results: parental (h r+ and h+
r) and recombinant (h+ r +
and h r) plaques.
Small, dark
parental
Large, light
• # recombinants/total X 100%
= recomb. frequency
Learning check
Given the following results of a cross between phage
T2 strain A (h r+) and strain B (h+ r), determine the
recombination frequency and distance between the h
and r genes:
Genotype
Plaques
h r+
h+ r
h+ r+
h r
42
34
12
12
rII locus
T4 Map
From Recombination Analysis
II. Extrachromosomal inheritance
Some inheritance patterns are non-Mendelian and the
phenotype is not transmitted by nuclear genes located on
chromosomes of both parents
II.A. Chloroplast DNA
Chloroplasts contain their own genomes, are inherited
maternally in the cytoplasm of the egg
First example in 1908: Correns discovered branch leaf
color in the four o’clock plant was affected by phenotype
of ovule only, not pollen.
Maternal Inheritance
• Phenotype of maternal parent is solely responsible for
phenotype of offspring
• In Four o’clocks:
– F white X M green >
– F white X M variegated >
– F green X M white >
– F green X M variegated >
– F variegated X M green >
– F variegated X M white >
white offspring
white offspring
green offspring
green offspring
variegated, green or white
variegated, green or white
Explanation of Results
Variegated
II.B. Mitochondrial Heredity
• About 30,000 genes on 23 human chromosomes (3.3
billion base pairs/haploid cell)
• Mitochondrial Genome contains 37 genes:
– 13 code for some of the proteins involved in
oxidative respiration
– 22 tRNA genes
– 2 rRNA genes
– some proteins involved in mt protein synthesis
– 16,569 base pairs, circular
Human Mitochondrial Diseases
• Are maternally inherited: only offspring of affected
mothers are affected
• Show deficiency in mitochondrial function
• Are caused by a mutation in a mitochondrial gene
• Example: myoclonic epilepsy and ragged red fiber
disease (MERRF)
– Deafness, dementia, seizures
– Point Mutation in a mitochondrial tRNA
Other Human Mitochondrial
Diseases
• Leber’s Hereditary Optic Neuropathy (LHON)
– Sudden bilateral blindness
– Point mutation in small subunit of NADH
dehydrogenase
• Kearns-Sayre Syndrome (KSS)
– Symptoms in eyes, muscles, heart, brain
– Deletion mutation in mtDNA
Mitochondrial DNA
and
Diseases
KSS
LHON
MERRF
II.C. Maternal Effect: Snail Coiling
• Pattern of offspring determined by genotype of mother,
regardless of phenotype
Although D is
dominant to d,
phenotypes of
Dd and dd
depend on
mother’s
genotype.
Embryological Origins of Coiling Pattern:
Orientation of Mitotic Spindles
D product is deposited in cytoplasm during oogenesis.
Maternal Effect Genes in Drosophila
Development
• Discovered by Edward Lewis, Christiane NussleinVolhard and Eric Weischaus (Nobel Prize in
Physiology or Medicine, 1995)
• Gene products deposited in egg by cells of mother’s
ovary.
• These genes set up the first embryonic axes: dorsalventral and anterior-posterior and control later zygotic
genes.