VL7 - Drosophila melanogaster Flashcards
Drosophila melanogaster
Characteristics:
* Single pair of anterior wings and posterior halters
* Holometabolous, larve mostly without legs
Why Drosophila:
* short life cycle (24h embryogenesis, 9days until fertile adult)
* high reproductivity rate
* simple and well-characterized genetics
* inexpensive, small and easy to breed
* Evolutionary conservation of biological processes
Drosophila Life cycle
- Female mate once and store sperm for the rest of their life
- Male:
-continous pigmentationof posterior end
-round shape of abdomen
-diffrent genital apparatus
-sex combs on forelegs
Genome feature:
* only 4 chromosomes
* fully sequenced in 2000
* Genome size around 180 Mb
* 14692 predicted protein-coding genes
* 2873 non-coding genes
* 50% show alternative splicing
* 45% of genes encode more than one protein isoform
- Fertilized Egg –> cleavage
- Syncytial lastoderm –> Gastrulation
- Germband elongation
(Extension and elongation of the germ band, with cells rearranging along the AP axis. Beginning of AP Axis formation ) - Germ band retraction
(contraction and retraction of the germ band. Continued AP-Axis formation) - Head involution and dorsal closure
(Folding of the head region and closure of the dorsal side of the embryo. DV Axis formation) - Embryo –> hatching
- Larve (3 Stages)
- Pupa –> Metamorphosis
- Adult Fly
Mb = Megabaspairs
Founders of Drosophila genetics and their experiments
Thomas Morgan, Alfred Sturtevant and Calvin Bridges = founders of drosophila genetics
Mendelian Inheritance:
* elucidation of Mendelian inheritance principle
* short life span allowed researchers to undertsand the patterns of gene transmission
Sex-Linked Inheritance:
* Studies on eye color and wing morphology provided critical evidence supporting the idea that genes are located on sex chromosomes.
Genetic Mapping:
* Researchers used recombination frequencies to map the locations of genes on chromosomes, laying the foundation for genetic linkage studies.
What is the balanced system
Drosophila
- Arrangement of mutations
Balancer Chromosome:
* Prevention of homozygous lethal or sterile mutations from being lost
* Prevention of uncontrolled recombination events
* Inversion breakpoints (CCrossovers do not occur in the region of inversion breakpoints, because synapse formation is inhibited.)
- A recessive lethal or sterile mutation in one gene can be maintained in a population if it is combined with a recessive lethal or sterile mutation in another gene on the homologous chromosome
- dominant or visible mutations allow you to tell which progency inherit the balancer in crosses
- Dominant visible mutations allow you to tell which progeny inherit the balancer in crosses
- Many mutations with dominant visible phenotypes are also recessive lethal, so they contribute to the balanced system.
What is genetic linkage analysis?
Gentic linkage analysis is a tool for mapping genes on chromosomes and understanding the inheritance of traits
- Detect the chromosomal locatiion of genes
- Genes that reside physically close on a chromosome remain linked during meiose
basic 2 point mapping:
–> determine the likelihood of genetig likage between two genteic markers or genes
* key point: recombination frequency (percentage of offsprings that show a recombination event between two markes)
* The farther two genes are apart, the more likely that a crossover will occure and the higher the proportion of recombinants products will be
2 genes are linked (on the same chromosome) when
* Two equally non-recombinant classes totaling >
50
* Two equally recombinant classes totaling < 50 %
Which mechanism are involved in the pattering of the embryo?
Drosophila
- Oranogenesis
- Segmentation
- Segregation of imaginal discs
- Neuroblast differentiation
The Larva stage
How is the animal body plan set up?
Drosophila blastoderm cells differentiate into two kinds of cells:
1) Those that will give rise to larval tissues and 2) Those that will develop into tissues and organ.
–> Certain group of undifferentiated cells form structures called imaginal discs.
–>17 imaginal discs form, that give rise to specific organ of the adult fly.
–> The process by which cells of imaginal discs make irreversible commitments to
specific patterns of differentiation is called determination.
Imaging discs: Sheets of epidermal cells tha will form adult structures
- 3 thoraric segments
- 8 abdominal segments
- telson
- senticles bands on ventral side
What are the principals of Hox Gene?
1. Colinearity principle
Activity of homeotic genes along the A-P axis matches the order in the chromosomal complex
- Hox genes code for transcription factors activating or repressing whole developmental pathways. Repress anterior genes.
- Homeotic mutations transform the identity of serially iterated structures (in accordance to colinearity principle)
- Homeotic genes are functionally conserved across different metameric metazoan species
Anterior-posterior pattering of the Embryo
The main body axes are established through morphogen gradients. Morphogene gradient provide positional information along the a-p and d-v axes.
Segmentation gene cascade
1. Maternal effect genes e.g: Bicoid or Dorsal
- laid down by femal in the egg & provide first positional information on body axis
- Bicoid protein provides an anteroposterior gradient and acts as a morphogen
- maternal bicoid mRNA anchored at anterior end off egg, by fertilization transition into biocoid protein and diffusion in posterior direction
- produces a gradient)
- Zygotic genes are regulated by different concentrations of Bicoid
Segmentation genes
2. Gap genes e.g: Hunchback
- define segmental regions in the embryo
- create an anatomical gap along the anterior-posterior axis
- Each Gap gene is expressed in characteristic regions in the early embryo under the control of the maternal-effect genes (response to different levels of Bicoid and other transcription factors)
3. Pair-rule genes e.g: Even-skipped & fushi tarazu
- These genes define a pattern of segments within the embryo
- regulated by gap genes and are expressed in seven alzternating bands, or stripes, along the a-p axis
- dividing the embryo into 14 distinct zones, or parasegments
- Each pair-rule gene is expressed
in 7 stripes, each a few cells
wide
4. Segment polarity genes e.g: Wingless (wg)
- refine pattern of parasegments
- define the anterior and posterior compartments of individual segments along the anterior-posterior axis
- Many of the segment-polarity genes are expressed in 14 narrow bands along the anterior- posterior axis.
Hox-Genes
5. Homeotic selector genes e.g: abdomimal-A
- determine segment identity
- proteins bind to regulatory sequences in the DNA, which in turn act to determine the segmental identities of individual cells.
- These genes, along with pair rule and segment polarity genes dtermine th indentities of individual segments in developing embryo
