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

F+ x F- F+

Hfr is special F+Hfr x F- F-

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+ Hfr F-F+

F-F+ F+ F+

Outline of Lecture 15

I. Bacterial genetics, continued

II. Extrachromosomal Inheritance

Chloroplast DNA

Mitochondrial DNA

Maternal Effect Genes

Circular Map ofE. coli

~2000 genes

Scaled in minutes,100 minutes total

Learning checkA 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 F- strain that is resistant to streptomycin (strr) and that is gal- his- bio- pur- gly-. 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

Lytic and Lysogenic

Cycles

Recombinants fromtransduction can bepropagated

Virulent or temperate phage

(e.g. phage)

Lederberg-Zinder experiment - Salmonella

Could this recombination be due to conjugation or transformation?

Transduction: virus-mediated bacterial DNA transfer

Transduction

Viruses can package bacterial DNA

recombinantsLarge, dark

Small, light

Small, dark

parentalLarge, light

Recombination in Phage

• 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.

• # 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+ 42h+ r 34h+ r+ 12h r 12

T4 Map

rII locus

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 > white offspring

– F white X M variegated > white offspring

– F green X M white > green offspring

– F green X M variegated > green offspring

– F variegated X M green > variegated, green or white

– F variegated X M 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 DNAand

Diseases

MERRFLHON

KSS

II.C. Maternal Effect: Snail Coiling

• Pattern of offspring determined by genotype of mother, regardless of phenotype

Although D isdominant to d,phenotypes ofDd and dddepend onmother’sgenotype.

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 Nusslein-Volhard 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: dorsal-ventral and anterior-posterior and control later zygotic genes.