Lecturea 2-4 Flashcards
how does a new multi cellular organism arise
an aggregation of many single cells.
cellular slime molds
these form multicellular aggregates in which cells are separated by their membranes
example of cellular slime mold
dictyostelium discoideum
fruiting body
fungal structures that contiain spores
asexula reproduction
bacteria and simple animals such as hydra can reproduce by budding.
mechanisms of asexual reproduction
fission - (separation of an organism into two or more individuals of approx equal size)
fission
fission - (separation of an organism into two or more individuals of approx equal size)
budding
budding - new individuals arise from existing ones. (found only in invertebrates0
fragmentation
two step process, breaking of the body into pieces, all or some of which develop into adults, followed by regeneration. (regrowth of lost body parts (annelid worms)
parthenogenesis
females produce offspring from unfertilised eggs (mainly invertebrates )
bees wasps and ants
zygote
a fertilised egg cell
driving force behind evolution
variation
general principles of development
cell division, cell differentiation, morphogenesis
gametogenesis
Spermatogenesis- the production of mature sperm is a continuous process in the adult
male. Sperm structure is very consistent throughout the animal kingdom.
Oogenesis, the development of a mature egg, is a prolonged process
why dont we study development in humans
Observation is difficult
• Morally and ethically- no experiments on human embryos (following implantation).
• We wouldn’t want to (nor could we) breed humans to look at effects of gene mutations
on embryos.
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model organism
an organism who has very similar developmental mechanisms to the organism being studied
aspects to be considered when choosing a model organism
- biological considerations
- practical considerations
- historical considerations
organism for observing development and morphological analysis
Advantages in this case would be large eggs, accessible embryos, short development time, and easy to keep in the lab. So the organisms of choice for this kind of work would be the amphibian (frog) and the chicken. Arabidopsis in the case of plants.
for manipulating the embryo (experimental biology)
larg accesible embryos, robust embryos that can tolerate manipulation.
can be grown in a dish, in culture.
frog or chick
vertebrate
The frog Xenopus laevis (Chp 46 – first page)
Advantages
•Ease of access and manipulation of the egg and embryo
allowing experimental manipulation, assay inductive interactions
•Robust embryos, can tolerate manipulation in relatively simple lab facilities.
•Parts of the embryo can easily be cultured.
•Rapid development, 4 days to a free swimming tadpole.
•A well-established system
Disadvantages
•No genetics. Complex, uncharacterised genome.
•Animals cannot be bred for multiple generations in the lab.
The mouse Mus musculus has a long history as a mammalian model of development.
Advantages
•Much is known about its biology, including its genes.
•The genome is well characterized and almost completely sequenced
•Sophisticated and elaborate techniques exist to manipulate mouse genes:
transgenic mice and
mice in which particular genes are “knocked out” by mutation.
Disadvantages:
Mice are complex animals with a genome as large as ours, and their embryos develop in the
BYU11102 Organisms to Ecosystems: Multicellularity and Development
Dr. Rebecca Rolfe – Lecture 2
mother’s uterus, hidden from view.
•The embryo develops in utero and is inaccessible.
-we are restricted to snap shots of development
•Embryo culture is difficult and limited.
•The generation interval is long (3 months)
•Difficult to find genes by mutational screens, because of the above.
But the mouse is still an important model system because we want to be able to study
development in a mammal. It serves as a good model for human development.
invertebrate
INVERTEBRATE
The nematode worm Caenorhabditis elegans normally lives in the soil but is easily
grown in petri dishes.
•Only a millimeter long, it has a simple, transparent body with only a few cell types and
grows from zygote to mature adult in only three and a half days.
•Its genome has been sequenced.
•It is easy to identify mutant animals: Because individuals are hermaphrodites, it is easy to
detect recessive mutations: Self-fertilization of heterozygotes will produce some homozygous
recessive offspring with mutant phenotypes.
•The huge advantage is its simplicity: every adult C. elegans has exactly (only) 959 somatic
cells. These arise from the zygote in virtually the same way for every individual.
•By following all cell divisions with a microscope, biologists have constructed the
organism’s complete cell lineage, a type of fate map.
A fate map traces the development of an embryo – Cell lineage analysis. (
For Developmental genetics:
Advantages would be ease of breeding in the laboratory, short generation interval (time
from fertilisation to sexual maturity), simple or small genome and ease of observing embryos
to see mutant effects. Organisms that have been widely used for this are the fruit fly
(Drosophila), the zebrafish (Danio), the nematode worm (Caenorhabditis elegans), The plant
Arabidopsis thaliana, the mouse (despite major disadvantages, see below).
transcription factors
In molecular biology, a transcription factor is a protein that controls the rate of transcription of genetic information from DNA to messenger RNA, by binding to a specific DNA sequence
actin and myosin
In summary, myosin is a motor protein most notably involved in muscle contraction. Actin is a spherical protein that forms filaments, which are involved in muscle contraction and other important cellular processes
neuroreceptors
in neurons receive cell signals
globulin genes
make haemoglobin which carry oxygen
repressor in prokaryotes
a repressor is encoded by the lacl gene which binds to the operator which prevents RNA polymerase from binding to the promoter and transcribing the genes downstream
promoter
a regulatory sequence at the start of the coding sequence that are needed for the transcriptional machinery to assemble and begin to transcribe the DNA sequence into an RNA message or transcript
promoters in eukaryotes
much more complicated in eukaryotes, often has upstream control elements called enhancers which can influence prooter function.
can be either proximal or distal, or upstream or downstream or placed within an intron
general transcription factors
these control transcription initiation by binding to the enhancers.
form a transcription initiation complex
cell-specific transcription factors
also known as activators.
these influence the efficiency with which the general transcription factors assemble on the promoter sequence and initiate the expression of a gene.
determination
irreversibly commits a cell to become a particular cell type
MyoD protein
capable of starting a cascade of events that changes non-muscle cells into muscle cells
master regulatory genes
the genes that encode transcription factors that control cellular differentiation
gene expression analysis
allows researchers to examine where a particular gene is expressed in the embryo
plant gene expression and cellular differentiation
funtions based on gene expression
however, in plants gene activation depends on the cell to cell communication
eg. arabidopsis root epidermis forms root hairs or hairless cells depending and the number of cortical cells it is touching.
GLABRA - 2 gene
maternal affect gene
Maternal-effect genes are transcribed in the mother and their mRNA influences development of oocytes and embryos.
one source of early information in fertilised egg cells is proteins and transcription factors. this information is transcribed by genes known as maternal affect genes
differential gene expression
the specialisation of cells thorugh gene regulation - the turning on and off of genes to allow them to specialise.
the cells take on characteristics appropriate to their location within the embryo and their ultimate function within the cell
three types of genes
structural genes
housekeeping genes
regulatory genes
developmental mutants
organisms in which genes have been altered
feasability of a mutant screen
space to house a large number of animals
short generation time
does the mutation show in the phenotype
two ways developmental mutants have been classically identified
spontaneous mutations
large scale mutant screens
maternal effect genes
genes that are active in the mother not the embryo
the gene product is supplied to the egg/early embryo by the mother
bicoid gene
forms the front half of the embryo
in the absence of bicoid, embryos with no head,and posterior structures at both ends are formed
morphogen
a gene that establishes body plan according to a graded concentration
