20.1-20.4 Flashcards
homeotic mutations
a mutant in which an apparently normal organ or body part develops in the wrong place
totipotent
a property of embryonic cells
means they have the potential to differentiate into any tissue or cell type the animal can produce.
all genes have the potential to be expressed given the appropriate queues
embryonic stem cells
totipotent cells of all early embryos in vertebrates
differentiated
cells having taken on different morphologies and undertaking different physiological activities
Differentiation is characterized by hanges in the patterns of gene expression that progressively limit which genes continue to be expressed by each cell type.
pluripotent
cells that have retained the potential to give rise to many different types of descendants, but not all.
development
a progressive process during which totipotent cells differentiate into specialized cell types through a series of genetically controlled steps that place ever more restrictive limits on their developmental potential.
positional information
inthe combinations of internal and external signals that a cell perceives during development provides information on the cells location within an organism
morphogens
substances whose presence in different concentrations directs developmental fates.
discreet boundaries can be established if activation or repression of gene expression is dependent on threshold concentrations of a morphogen
induction
once a cell has acquired a specific identity, it may induce its neighbors to acquire a certain fate
organizer
cells in an organizer region possess the ability to organize cells in the surrounding tissue.
inhibition
a cell that acquires a specific fate may then produce an inhibitory substance that prevents its neighbors from acquiring a certain fate
can be used to produce patterns of regularly spaced cells of a particular fate within a field of cells that would otherwise all differentiate in the same manner.
syncytium
early in embryonic development in Drosophila proceeds by nuclear division without division of cytoplasm, forming a syncytium.
A multinucleate cell in which the nuclei are not seperated by cell membranes.
syncytial blastoderm
after syncytium formation from nine mitotic nuclear divisions, the nuclei migrate to the periphery of the embryo. Then about 10 pole cells, from which the germ line will be derived are set aside at the posterior end of the embryo.
the somatic cells undergo another four rounds of mitotic divisions at the periphery, forming a syncytial blastoderm containing about 6000 nuclei.
cellular blastoderm
by about 3 hours after egg laying, cellularization of the syncytium occurs by the assembly of cell membranes that seperate nuclei into individual cells, thus forming a cellular blastoderm.
cells already have become committed to differentiate into particular tissues when the cellular blastoderm has been formed.
housekeeping genes
genes that have essential cellular functions, mutations result in lethality.
coordinate genes
defects affect an entire pole of the larva
gap genes
mutants are missing large contiguous groups of segments
pair-rule genes
mutants are missing part of adjacent segment pairs in alternating segments
expression follows that of gap genes and produces seven stripes in the embryo (for Drosophila)
segment polarity genes
defects affect patternng within each of the 14 segments
occurs in 14 polar stripes (i.e. each stripe has anterior and posterior “poles”) one for each embryo segment
homeotic genes
defects affect the identity of one or more segments
last genes to be expressed
affect broad domains of contiguous parasegments along the anterior-posterior axis.
parasegments
domains of gene expression controlled by the pair-rule gene
expression
maternal effect genes
in animals, the mother often supplies critical gene products to the egg tat subsequently direct embryo development.
the genotype of the mother determines the phenotype of the zygote
zygotic genes
genes that are only active in the zygote or embryo.
for these genes, the genotype of the embryo determines the phenotype.
Antennapedia complex
consists of five genes
one of two homeotic clusters on the third chromosome for Drosophila
(mutants have legs on head)
bithorax complex
consists of 3 genes
one of two homeotic clusters on the third chromosome for Drosophila
homeobox
all Homeotic genes share a conserved sequence of DNA of 180 nucleotides called the homeobox, encoding a 60-amino acid protein domain, termed the homeodomain.
homeodomain
has a helix-turn-helix motif. made
Hox genes
homeobox genes that share both molecular and functional similarity as well as having a common evolutionary origin
realizator genes
genes whose expression is determined by the parasegment the cell is contained in.
They are expressed with the right combination of Hox gene expression
inductive signal
a molecule produced by a cell controlling differentiation.
If it is disseminated in a gradient, the cell closest to the anchor cell could acquire a different fate than cells that are more distant.
lateral inhibition
applies to inductive signaling
after the reception of a signal, a second signal is sent to another cell that inhibits other cells from differentiating the same way as the cell receiving the signal. It reinforces initial asymmetry.
co-option
a common theme in the evolutionary history of all genes, and particularly those influencing development, is the co-option of genes and genetic modules to direct the patterning or growth of novel organs.
evo-devo
the study of the evolution of development
zone of polarizing activity (ZPA)
acts as an organizer that promotes digit formation at the distal ends of lim buds through the production of a morphogen, now known to be a small secreted signaling protein called sonic hedghog (shh).
