Bi-212-001: Principles Of Biology Ii (Winter 2017)

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Five general processes occur throughout cellular development

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1. cell division -proliferation2. cell-cell interactions3. differential gene expression (cell differentiation)4. cell movement, expansion and shape formation5. programmed cell death (apoptosis)

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Cell proliferation

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in which during Mitosis & cytokinesis;timing, location, amount regulated

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Cell-cell interactions

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Cells divide, die, grow, move, differentiate in response to cell-cell signals

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Cell differentiation

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Undifferentiated cells specialize at specific times/places; stem cells; meristem cells

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Cell movement or differential expansion

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Move past one another, break away and migrate, divide in specific directions

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Programmed cell death

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Timing, location, amnt. regulated

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Proliferate

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Divides and make more cells; allows for formation of multicellular organisms

•Location, timing, and extent of cell divisions tightly controlled by interacting layers of regulation

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•Most cells stop proliferating at maturity–Some specialized, undifferentiated cells continue proliferating ?

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throughout the organism’slife

•In plants: meristems

•In animals: stem cells

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Apoptosis

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Programmed cell death; normal part of development -carefully regulated

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Gastrulation

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cells in different parts of an early embryo rearrange themselves into three distinctive types of embryonic tissues which later form specific organs

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Plant Cells

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Because of their cell walls, plant cells do not move; instead they control how the cleavage plane is oriented during cell division and the direction of the subsequent cell growth (differential cell growth).

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Cell Differentiation

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Most cells undergo differentiation to become a specialized type of cell during development.

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Differentiation

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progressive, step-by-step process–cells initially committed to a specific developmental pathway but become differentiated later

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Stem cells (animals):

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retain the ability to divide and give rise to an array of specialized cell types

–Skin

–Bone marrow

(blood)

–Gut

–Immune system

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Meristems (plants)

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give rise to various structures that develop throughout life

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totipotent (“all-powerful”)

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Many plant cells are capable of de-differentiating even after they have specialized

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Totipotency

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Important difference between plant and animal cells! Once differentiated animal cells cannot de-differentiate and re-differentiate. They are what they are (with some exceptions…)!

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Cell-Cell Interactions

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Cell-cell interactions involve sending and receiving signals

•Cell-cell signals change patterns of gene expression

•Are essential for changing cell activity during development.

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Differential gene expression:

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expression of different genes in different cell types–key to cell differentiation during development

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Are differentiated animal cells genetically equivalent?

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Early experiments showed that transplanting nuclei from diploid frog cells into unfertilized eggs without nuclei resulted in development of normal tadpoles.

•Nuclear transfer experiments in sheep reinforced these results.

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All cells of an organism contain the same genes but....

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but expressonly a specialized subset.

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Mammals can be cloned by...

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transplanting nuclei from mature cells

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In animals the best cells to use for cloning are stem cells, which are programmed to make ????

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asymmetrical cell divisions.

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Onedaughter cell is identical to the parent, i.e., a stem cell, and...

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and the otherdaughter cell is more differentiated.

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In adults, stem cell populations in…-the gut-skin-bone marrowOf the stem cells, the very best are......

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embryonic ones.

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During Gene Expression

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-The process of cellular differentitation does not involve changes in the genetic makeup of cells

-is based on differential gene expression

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Gene regulation occurs at multiple levels:

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chromatin modification, transcription, RNA processing, translation, and post-translation.

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Transcription:

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Fundamental level of control in differential gene expression during development.

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Eukaryotes:

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transcription controlled primarily by proteins called regulatory transcription factors.

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In Gene Expression: Regulatory Transcription Factors:

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-influence chromatin remodeling

-bind to promoter-proximal elements

-enhancers

-silencers

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What Triggers Differential Gene Expression?

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-Timing: the current stage of development of the organism

-Spatial location: where it is in the body of the organism

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Spatial location determined by three major body axes:

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-anterior-posterior

-ventral-dorsal

-left-right

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Cells know their position in time and space by interacting with other cells via:

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cell-cell signaling.

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? Setup the Major Body Axes

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Master Regulators

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Pattern Formation:

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series of events that determine spatial organization of embryo

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Master Regulators

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Certain early signals set up major body axes of embryo.

-activate network of genes that sends signals with more specific information about spatial location of cells.

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Major body Axes

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-As development proceeds, a series of signals arrive and activate or deactive genes that specify finer and finer control over what a cell becomes.

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Bicoid

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two-tailed.

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Segment

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Distinct region of animal body that is repeated along its length

-fly embryo segments are groupsed into three regions: head, thoracic, and abdominal.

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The bicoid gene:

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Is expressed not in the embryos but in mothers-the eggs contain bicoid product, either mRNA or protein

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Bicoid mRNA highly localized in anterior of ?

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egg

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Bicoid protein is made from ?

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mRNAs in the anterior end and diffused away from that end of the embryo.

-Tis produces a steep concentration gradient from the anterior to the posterior end.

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The concentration gradient formed by bicoid protein provides ?

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cells with information abou their position along the anterior-posterior axis. It also turns on genes responsible for forming anterior structures. The absense of bicoid contributes to the formation of posterior structures.

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The bicoid factor gradient determines ?

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the anterior-posterior axis of the embryo.

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Segmentation genes

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Organize cells and tissues into distinct segments (gap-genes, pair-rule genes, segment-polarity genes, pages)

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Homeotic genes

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Trigger development of structures appropriate to each type of segment (hox-genes)

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Homeosis

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Occurs when cells get incorrect information about where they are in the body.

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Hox Genes

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Are highly conserved through evolutionary time. (homologous genes.)

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Evo-devo

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Research field of evolutionary-development biology

-Focuses on understanding how changes in developmentally important genes have led to the evolution of new phenotypes.

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Hoxc6 and Hoxc8

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led to the evolutionary loss of the forelimb in snakes

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Hoxc6 abd Hoxc8 are always expressed ?

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Together, so no forelimb is formed.