Ch. 19 Developmental Genetics
How does the bicoid gene product accumulate in the anterior region of the oocyte?
-specialized nurse cells supply oocytes with the products from maternal effect genes -bicoid is transcribed in the nurse cells and the mRNA is then transported from the nurse cells into the anterior end of the oocyte and trapped there -before fertilization, the bicoid mRNA is concentrated near the anterior end of the oocyte, and after it is translated and a gradient of bicoid protein is est. across the zygote -gradient starts a progression of developmental events that will provide positional info that cause the end with bicoid to become anterior end
cell response to positional information
1. cell division 2. cell migration 3. cell differentiation 4. cell death
induction
1. in development, the process by which a cell or group of cells governs the developmental fate of neighboring cells 2. in molecular genetics, refers to the process by which transcription has been turned on by the presence of a small effector molecule
developmental stages of the fruit fly
1. oocyte- elongated cell that contains positional information and already has anterior/posterior ends est. 2. embryonic development- formation of a segmented body 3. Larva- a free living organism that is morphologically very different from the adult (fly has three larval stages, some mammals have none) 4. at 3rd larval stage, larva becomes pupa, transitional state between larva and adult 5. metamorphosis- larva transforms into a pupa and then becomes mature adult
why were these two organisms chosen?
1. researchers have exposed the fly to mutagens and identified many mutant organisms with altered developmental pathways 2. in all life stages, fly has distinct morphological features and is large enough to easily identify the effects of mutations 3. worm used for simplicity
plant morphology
1. root-shoot axis- most growth occurs via cell division in the tips of the shoots and the roots 2. growth occurs in well-defined radial pattern, occurs in concentric rings of tissue
two common characteristics of stem cells
1. they have the capacity to divide 2. their daughter cells can differentiate into one or more specialized cell types -important to replenish cells that have finite life span
formation of a body
Transcription factors are what carry out each step -many morphogens, particularly those that act during early phases of development, function as transcription factors 1.organizes the body along major axes 2. the body becomes organized into smaller regions, a process called segmentation 3.cells within the segments organize themselves in ways that will produce particular body parts 4 cells change their morphologies and become differentiated -these phases are overlapping
homebox
a 180-bp sequence within the coding sequence of homeotic genes -encodes a region of the protein called the homeodomain
cell differentiation
a cell's morphology and function have changed into a highly specialized cell type
homeotic genes
a gene that controls the developmental fate of particular segments or regions of an animal's body -mutations can lead to body parts being replaced by other parts
segmentation genes
a gene that controls the segmentation pattern of an animal embyro -3 types: gap genes, pair rule genes, and segment-polarity
maternal effect genes
a gene that follows a maternal effect inheritance pattern
homeodomain
a region of homeotic protein that functions in binding DNA -arrangement of alpha helices promotes the binding
products of maternal effect genes such as bicoid...
activate gap genes, then the products from gap genes and maternal effect genes function as transcription factors to activate the pair rule genes and their products then regulate the segments polarity genes
multipotent
adults have these, stem cells that can differentiate into several cell types but less than pluripotent (bone marrow cells to red blood cells)
unipotent
adults have these, stem cells that produces daughter cells that differentiate into only one cell type
how does pattern formation occur in plants?
after fertilization, the 1st cellular division is asymmetrical and produces a smaller apical cell and a larger basal cell -apical cell gives rise to most of the embryo and later develops into the shoot of the plant -basal gives rise to the root, along with a suspensor which channels nutrients from the parent plant to the young embryo
model organisms
an organism studied by many different researchers so they can compare their results and determine scientific principles that apply more broadly to other species -ex. fruit fly, nematode worm, mouse and zebrafish
cell adhesion
another mechanism to convey positional information between neighboring cells; a vital function of the cell membrane that allows cells to bind to each other. Cell adhesion is critical in the formation of multicellular organisms
apical basal patterning genes
are important in early stages of plant development -mutations cause dramatic effects in one of these three regions
central zone
area where undifferentiated stem cells are always maintained
bicoid
bicoid gene product accumulates in the anterior region of the oocyte, and later acts as a morphogen to cause the development of the anterior end of the embryo -bicoid negative (mutant)- both ends of the larva develop spiracle which is usually only found in posterior end -the function is as a transcription factor and it functions highest at the ant. end of the embryo -this allows it to play a role in segmentation
determined
cell is committed to become a particular cell type such as a muscle of intestinal cell
plant development
cell migration does not occur during plant development -does not rely on morphogens -entirely new individual can be regenerated from many types of somatic cells (cells that do not give rise to gametes-Totipotent) Like animals -use mechanism of differential gene regulation to coordinate the development of a body plan -developmental program relies on the use of transcription factors
Cell adhesion molecules (CAMs)
collection of surface receptors that enable it to adhere to other cells and to the ECM -positioning of cells within a multicellular organism is strongly influenced by the combination of contacts it makes with other cells and with the ECM
heart stage
composed of only about 100 cells, the basic organization of the plant has been est. -progresses to the formation of a seedling that has two cotyledons which are embryonic leaves that store nutrients for the developing embryo and seedling
peripheral zone
contains dividing cells that eventually differentiate into plant structures
stem cell research
controversial but research has found that induced pluripotent cells can differentiate into all cell types when injected into mouse blastocysts -results indicates that adult cells can be programmed to become embryonic stem cells
central region
creates the stem
pair rule gene
defect causes alternating segments or parts of segments to be absent
zygote
diploid cell formed by a sperm and egg
how does an organism develop the correct morphological features or pattern?
during development, cells receive positional information, molecules that provide a cell with information regarding its location relative to other cell of the body
Development of segments
each segment of the embryo gives rise to unique morphological features in the adult
blastocyst
early embryonic structure that contains embryonic stem cells which are located in the inner cell mass
pluripotent
embryonic stem cells are this and means they can also differentiate into every or nearly every cell type of the body, however a single one of these, itself, has lost the ability to produce an entire individual
organizing center
ensures the proper organization of the meristem and preserves the correct number of actively dividing stem cells
phase 1 pattern formation of Fly
est. of the body axes -morphogens needed are distributed before fertilization
totipotent
fertilized egg is said to be this because it produces all of the cell types in the adult organism
dorsoventral axis
front(ventral) and back(dorsal) of animal
apical cell
gives rise to most of the embryo and later develops into the shoot of the plant
phase 3 pattern formation
homeotic genes control the development of segment characteristics -segments begin to develop its own unique characteristics
How do homeotic genes work at the molecular level?
homeotic genes encode homeotic proteins that function as transcription factors
morphogens
impart positional info and promote developmental changes at the cellular level, can be proteins or small signaling molecules -act in a concentration-dependent manner -distributed asymmetrically along a concentration gradient
hox genes
in animals, a class of genes involved in pattern formation in early embryos -evolutionarily related among mammals -play a role in determining the fates of regions along the anteroposterior axis
embryonic germ cells
in early fetal stage of development, cells that later give rise to sperm or egg cells, pluripotent
thale cress
model organism for plants -annual plant: lives out its entire life cycle during a single growing cycle -occurs naturally in temperate regions -short generation time -small genome size
segment-polarity gene
mutation causes portions of segments to be missing and cause adjacent regions to become mirror images of each other
antennapedia mutation
mutation on antp gene where legs grow instead of antennae
plant formation
organized along a root-shoot axis: roots grow down and shoot grows up, and in a radial pattern in which cells found in roots and shoots form concentric rings of tissue
body pattern
plants and animals produces a body plan, which is due to arrangement of cells and their differentiation
meristems
plants have organized groups of actively dividing stem cells -produces offshoots of proliferating and differentiating cells
homeotic proteins
primary function- activate the transcription of specific genes that promote developmental changes in the animal -homeodomain protein binds to DNA sequences called enhancers -also contain a transcriptional activation domain -after homeodomain binds to an enhancer, the transcriptional activation domain of the homeotic protein activated RNA polymerase to begin transcription
pattern formation
process gives rise to the formation of a body with a particular morphology -usually along three axes: dorsoventral axis, anteroposterior axis, and left-right axis
shoot apical meristems
produces all aerial parts of the plant, which include the stem as well as lateral structures such as leaves and flowers -organized into three areas- the organizing center, central zone, and the peripheral zone
apical region
produces leaves and flowers of the plant
basal region
produces the roots
root apical meristem
region of rapidly dividing cells at plant root tips
MyoD
research has found this gene belongs to a small group of genes that initiate muscle cell development -encode transcription factors
causes of asymmetrical cell division
sperm carries mRNA molecules that are critical for it
phase 4 pattern formation
stem cells can divide and differentiate into specialized cell types
fate
term used to describe the ultimate morphological features that a cell or group of cells adopts
threshold concentration
the concentration above which a morphogen will exert its effects but below which it is ineffective
segmentation
the division of an animals's body into clearly defined regions
embryo
the early stages of development in a multicellular organism during which the organization of the organism is largely formed
differential gene regulation
the phenomenon in which the expression of genes differs under various environmental conditions and in specialized cell types
colinearity rule
the phenomenon whereby the order of homeotic genes along the chromosome correlates with their expression along the anteroposterior axis of the body
morphological difference between two cells
these differences arise through gene regulation -a neuron cell and a muscle cell have same set of genes but they regulate the expression of their genes in different ways by expressing different proteins
phase 2 pattern formation
to make a segment, particular genes act sequentially to govern the fate of a given region of the body -involves development of segments
anteroposterior axis
top to bottom, head to tail
how does positional information lead to the development of a body plan?
two main molecular mechanisms are commonly used to communicate positional information -morphogens and cell-to-cell contact/adhesion
stem cells
undifferentiated cells that divide and supply the cells that constitute the bodies of all animals and plants ultimate stem cell is the fertilized egg(can give rise to whole organism) -both daughter cells cannot differentiate into specialized cell types (only one at a time)
gap gene
when a mutation inactivates this gene, several adjacent segments are missing in the larva