Development
AP Biology
Big Questions:
1. How does a multicellular organism
2.
3.
4.
AP Biology
develop from a zygote?
How is the position of the parts of an
organism determined?
How does differentiation of cell type
occur in organisms?
How are genetics and development
connected? Genetics and the environment?
Development is Regulated
 The development of

an organism is
coordinated by
sequential changes in
gene expression.
Multicellular
organisms must
regulate:
Cell division
 Differentiation
 Morphogenesis

AP Biology
Early Developmental Pathways
 Cytoplasmic
Determinants

unequal distribution of
mRNA and proteins
from mother
 Embryonic Induction:

Local signaling by
nearby cells send
cells down a specific
developmental path.
AP Biology
AP Biology
Pattern Formation
 Cells need to establish their position in

the developing organism. (head/tail, right
/left etc.)
Cytoplasmic determinants and induction
lead to gradient morphogens.
AP Biology
Cellular Differentiation
 Germ lines established
 Cell type already


determined
Differentiation
accomplished by the
expression of cell typespecific proteins.
There is normally no
turning back.
AP Biology
Differentiation Control
 Genes must be both
turned on and turned off
at appropriate times and
locations.
 Regulated by internal
and external cues
AP Biology
Internal Cues
 DNA packing, micro RNA’s and
epigenetics can switch genes ON or OFF.
 Transcription factors must be present
and can be either stimulatory or
inhibitory.
AP Biology
Internal Cues
 A simple example of transcription factors
and gene regulation: the lac operon

AP Biology
http://www.sumanasinc.com/webcontent/a
nimations/content/lacoperon.html
External Cues (outside the cell)
 External cues signal to cells, causing
cells to alter their gene expression.
AP Biology
Environmental Cues
 The presence of particular molecules
and conditions in the local environment
is required for development to proceed
properly.

Ex. The role of temperature and
moisture in seed development.
Why do you water a seed
after you plant it? Hmm…
AP Biology
Environmental Cues
 Changing the environment around the
cells can alter gene expression.
AP Biology
Apoptosis
 “Programmed cell death”:
Important role in defining
borders and openings in
the developing organism.
AP Biology
Apoptosis
 This has been studied thoroughly in
C. elegans development – Robert
Horvitz and his team won a Nobel Prize
for this research!
http://web.mit.edu/horvitz/www/researchlinks/programmed.html
AP Biology
Apoptosis
 Of the 947 cells generated during C. elegans
development, 131 undergo apoptosis
AP Biology
Apoptosis
 The genes involved in the process of cell death



(ced-3/ced-4) are closely related to the same
genes in humans (Apaf-1)
Genes that PREVENT cell death (ced-9) are
closely related to the same in humans (bcl-2)
This research has had
many implications for
cancer research!
(Thanks, little worm!)
AP Biology
Homeobox (HOX) Genes
 A family of
related genes
that serve as
master
regulators of
animal
development
in all animals
on the planet.
http://study.com/academy/lesson/role-of-homeotic-genes-in-drosophila-development.html
AP Biology
AP Biology
AP Biology
Experimental Evidence
 Mutations in normal developmental pathways lead to
malformations in embryonic development
AP Biology
AP Biology
Experimental Evidence
 Transplantation Experiments: Moving
regions of a developing embryo affects
normal pattern formation.
AP Biology
Experimental Evidence
 Transplantation Experiments: Moving
regions of a developing embryo affects
normal pattern formation.
AP Biology