Chapter 21

Hox genes

homeotic genes that control formation of segment-specific structures - clusters of Hox genes occur in virtually every animal examined to date - number of Hox genes varies widely among species, however, their chromosomal organization is similar to that of the fly homeotic genes - Hox genes are important in identifying the position of cells along the head-to-tail axis of the body (anterior-posterior)

clone

identical genetic copy - cellular differentiation doesn't involve changes in genetic makeup of cells. Instead, it results from differential gene expression

homeotic genes

identifies each segment's structural role - triggers development of structures that are appropriate to each type of segment, such as antennae, wings, or legs - proteins required are produced by effector genes that are regulated by Hox genes

morphogens

molecules that provide spatial information during early embryonic development, via concentration gradient in both plants and animals - bicoid and auxin are morphogens that have a fundamental impact on early development - provide cells with information about their position along the anterior - posterior or the apical - basal body axis using concentration gradient

homeosis

type of replacement of one structure by another structure - ie. replaces halteres of an adult fruit fly with another pair of wings - occurs when cells get incorrect information about where they are in the body

cell differentiation

undifferentiated cells specialize at specific times and places in a stepwise fashion - cells that don't undergo differentiations are called stem cells in animals - many plant cells are capable of de-differentiating

stem cells

undifferentiated cells that continue to proliferate in animals - embryonic stem cells can give rise to almost any differentiated cell type in the body - juvenile and adult stem cells are found in specific locations in the body where they proliferate to replace skin and blood and gut cells that die, repair wounds, and create a constant supply of disease-fighting cells in the immune system

meristems

undifferentiated cells that continue to proliferate in plants - present in same locations in embryonic and adult plants - perform same function - giving rise to the stems, roots, leaves, flowers, and other structures that develop throughout life

master regulator in plants

unlike bicoid, plant master regulator is not a transcription factor. Instead it is a hormone. - a signaling molecule that travels through the body and acts on distant target cells - auxin enters cells and triggers the production of transcription factors that affect differentiation

Regulatory Gene Cascade in Fruit Flies

- Master Regulator (Establish anterior-posterior gradient of embryo) - Gap genes (Organize cells into groups of segments along anterior/posterior axis) - Pair-rule genes (Organize cells into individual segments) - Segment polarity genes (Establish anterior/posterior gradient within each segment) - Homeotic genes (Trigger development of structures) - Effector genes (Change proliferation, death, movement, and differentiation of cells)

Shared Developmental Processes

- a few fundamental principles are common to all developmental sequences observed in multicellular organisms cells divide, move, or expand in a directed way - express certain genes rather than others - signal to each other about where they are, what they are doing and what type of cell they are becoming - selected cells die in a regulated manner during development

master regulator in animals

- bicoid gets the cell differentiation process underway, by providing information on where cells are in the body, extremely early in development

5 essential developmental processes

- cell proliferation - programmed cell death - cell movement or differential expansion - cell differentiation - cell-cell interactions

cell movement or differential expansion

- cells move past one another within a block of animal cells, causing drastic shape changes in the embryo - certain cells can break away from a block of animal cells and migrate to new locations - plant cells can divide along certain planes and expand in specific directions, causing dramatic changes in shape (don't move)

Wingless genes

15 Wnt ("wint") genes that code for proteins responsible for cell-cell signaling during development - sets up anterior-posterior axis in embryo - signals from mammalian Wnt genes contribute to the formation of -- back muscles -- midbrain region -- limbs -- gonads (testes or ovaries) -- hair follicles -- parts of the intestine - structures inside kindey

bicoid

Bicoid is a maternal effect gene that is important in patterning of anterior body parts - without bicoid protein, no anterior body parts form - without bicoid protein, two tailed embryo forms - signals anterior, middle or posterior depending on concentration levels of bicoid

cell movement

Later in development, animal cells migrate to new locations in embryo and give rise to: - germ cells (sperm and egg) - pigment-contianing cells - precursor of blood cells - certain nerve cells if any of these cell movements are inhibited or end up in the wrong place, the embryo is likely to be deformed or die before development is complete

differential cell expansion

Plant cells are encased in stiff cell walls and do not move. Instead, the direction cell division and cell expansion takes place is carefully regulated. - change in direction of cell growth causes proper formation of straight and bent stems, leaf veins, and other structures - change in cell size can make an individual "move" - cells that receive light give signals to expand on that side (plants grow toward light)

embryo

a young, developing organism - risen from a fertilized egg

regulatory genes

act in a sequence, triggering gene cascades that provide progressively detailed information about where cells are located in time and space - this positional information causes changes in cell proliferation, death, movement, differentiation and interaction - signals and transcription factors vary in identity and concentration along the three major body axes, cells in different locations receive unique positional information - each level in a regulatory cascade provides a more specific level of information about where a cell is - as regulatory cascades proceed over time, a cell's fate becomes more and more finely determined

auxin

cell-cell signal produced in meristematic cells at the tip or apex of the growing embryo (what will become the top of the stem) and is transported toward the base (what will become the root) - high concentration of auxin means "you're near the top" - when auxin accumulates at the root it singlas, "you're near the base of the root"

gastrulation in animal cells

cells in different parts of the mass rearrange themselves into three distinct layers which then give rise to: - skin (ectoderm, mesoderm, endoderm) - gut - other basic parts of the body

Cell proliferation

divide and make more cells - location, timing, and extent of cell division must be tightly controlled - in eukaryotes: mitosis and cytokinesis

gap genes

early in development, gap genes define the general position of head, thorax, and abdominal regions

Cell-Cell interactions

embryonic cells divid, die, grow, move or differentiate in response to signals from other cells.

segmentation genes

establisht he boundaries of each segment - gap genes - pair-rule genes - segment polarity genes

pattern formation

events that determine spatial organization of an embryo - if a molecule signals that a target cell is in the embryo's head, tail, dorsal, or ventral side, that molecule is involved in pattern formation - progressive, early signals act as master regulators that set up the general anterior-posterior, dorsal-ventral, and left-right axes of an embrio. - genes activated by master regulators send signals with more specific information about cells' physical location - as growth continues, process repeats - new signals arrive and activate genes that specify finer and finer control over what a cell becomes

in situ hybridization

finds where bicoid mRNAs are located in embryos by adding a label to single stranded copies of DNA or RNA molecules - obtain a single-stranded DNA or RNA probe complementary in sequence to target mRNA - Add label to probe (a radioactive atom or an enzyme that catalyzes a color-producing reaction) - Add many copies of probe to preserved cells of tissue made permeable to probe - Probe binds to target mRNA. Labeled probe that does not bind to target mRNA is washed away - Observe location of probe. In this case, target mRNAs are concentrated in anterior end of embryo (label shows up as black).

Transcription

fundamental level of control in differential gene expression during development - in eukaryotes, transcription is controlled primarily by presence of regulatory transcription factors that influence chromatin remodeling and bind to: -- promoter-proximal elements -- enhancers -- silencers -- other regulatory sites in DNA

"pinch" analogy

in development, as in human communication, the context in which a signal is sent and received - its location, timing and intensity - has a major effect on the signal's meaning and consequences

pair-rule genes

later in development, pair-rule genes demarcate the edges of individual segments

common tool kit

multicellular organisms have a tool kit of common signals, signal-trandsuction pathways, and regulatory proteins that are used over and over during development - can direct development of dramatically different structures because the tools are deployed at different times and different locations

apoptosis

programmed cell death - cell-suicide genes found in cells that undergo apoptosis - normal apoptosis is important in the development of human embryos - abnormal apoptosis (either too much or too little) causes diseases in adults. (i e. ALS - Lou Gehrig's disease)

segment

region of an animal body that contains a distinct set of structures and is repeated along its length

evolutionary-developmental biology (evo-devo)

research field focusing on understanding how changes in developmentally important genes have led to the evolution of new phenotypes such as the flower, leaf, the limbs found in tetrapods and the limbs of arthropods.

MADS-box genes

sequences analogous to homeotic genes of animals

social controls

signals from other cells - each stage of cell cycle has checkpoints that are carefully regulated - tells cells to continue or stop growin

segment polarity genes

still later, segment polarity genes delineate boundaries within individual segments

programmed cell death

the timing location and amount of cell death are regulated - also commit suicide in response to signals from other cells - apoptosis ("falling away") - occurs as certain tissues and organs take shape - i.e. skin between toes in animals and leaves falling from trees in plants

Four Axes

time (current stage of development) and 3 spatial dimensions (body axes) - one axes runs anterior (toward the head) to posterior (toward the tail) - one axes runs ventral (toward the belly) to dorsal (toward the back) - one axes runs left to right