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