Transcript The Genetic Basis of Development

The Genetic Basis
of Development
How do cells with the same
genes grow up to be so
different?
Three Procceses of Development

The transformation from a zygote into
an organism

Results from three interrelated processes:
cell division, cell differentiation, and
morphogenesis
Figure 21.3a, b
(a) Fertilized eggs
of a frog
(b) Tadpole hatching
from egg

Through a succession of mitotic cell divisions


In cell differentiation


The zygote gives rise to a large number of cells
Cells become specialized in structure and function
Morphogenesis encompasses the processes

That give shape to the organism and its various
parts
Some key stages of development in
animals and plants
(a) Animal development. Most
animals go through some
variation of the blastula and
gastrula stages. The blastula is
a sphere of cells surrounding a
fluid-filled cavity. The gastrula
forms when a region of the blastula
folds inward, creating a
tube—a rudimentary gut. Once
the animal is mature,
differentiation occurs in only a
limited way—for the replacement
of damaged or lost cells.
Cell
movement
Zygote
(fertilized egg)
Eight cells
Blastula
(cross section)
Gut
Gastrula
Adult animal
(cross section)
(sea star)
Cell division
Morphogenesis
(b) Plant development. In plants
with seeds, a complete embryo
develops within the seed.
Morphogenesis, which involves
cell division and cell wall
expansion rather than cell or
tissue movement, occurs
throughout the plant’s lifetime.
Apical meristems (purple)
continuously arise and develop
into the various plant organs as
the plant grows to an
indeterminate size.
Observable cell differentiation
Seed
leaves
Shoot
apical
meristem
Zygote
(fertilized egg)
Root
apical
meristem
Two cells
Embryo
inside seed
Plant
Differential gene expression


Nearly all the cells of an organism have
genomic equivalence, that is, they have
the same genes
Differences between cells in a
multicellular organism


differences in gene expression
not from differences in the cells’ genomes
Cell Differentiation
yields
a variety of cell types
 each expressing a different combination
of genes
 multicellular eukaryotes

cells become specialized as a zygote
develops into a mature organism
Cell Diferentiation

Different types of cells

Make different proteins because different
combinations of genes are active in each type
Muscle cell
Pancreas cells
Blood cells
Differentiated cells

may retain all of their genetic potential


Most retain a complete set of genes
May be totipotent
Totipotency in Plants
EXPERIMENT
Fragments cultured in
nutrient medium;
stirring causes
single cells to shear off
into liquid.
A single
Somatic (nonreproductive) carrot
cell developed into a mature carrot
plant. The new plant was a genetic
duplicate(clone) of the parent plant.
RESULTS
Transverse
section of
carrot root
2-mg
fragments
Single cells
free in
suspension
begin to
divide.
Embryonic
plant develops
from a cultured
single cell.
Plantlet is
cultured on agar
medium. Later
it is planted
in soil.
Adult plant
CONCLUSION
At least some differentiated (somatic) cells in plants are toipotent, able
to reverse their differentiation and then give rise to all the cell types in a mature plant.
DNA packing in a eukaryotic
chromosome

This beaded fiber is further wound
and folded
DNA packing tends to block gene
expression
 Presumably by preventing
access of transcription proteins
to the DNA
DNA
double
helix
(2-nm
diamet
er)
Histones
Linker
“Beads on
a string”
Nucleosome
(10-nm diameter)
Tight helical fiber Supercoil
(30-nm diameter) (300-nm diameter)
TEM

Wound around clusters of histone
proteins, forming a string of beadlike
nucleosomes
TEM

700
nm
Metaphase chromosome
An Extreme Example of DNA Packing
X chromosome inactivation in the cells of
female mammals
Early embryo
X
chromosomes
Cell division
and random
X
chromosome
inactivation
Two cell populations
in adult
Active X
Orange
fur
Inactive X
Inactive X
Allele for
orange fur
Active X
Allele for
black fur
Black fur
Nuclear Transplantation
The nucleus of an unfertilized
egg cell or zygote is replaced
with the nucleus of a
differentiated cell
How to Clone a Sheep
Bill Ritchie (Produced for the 1997 Royal Agricultural Show)

The Egg


The Cell



The unfertilized eggs are flushed out of a sheep which has
been induced to produce a larger than normal number of
eggs.
Previously a sample of tissue was from the udder
of a six year old ewe was taken and cultured in a
dish (Dolly 1).
The cultured cells are starved to send them into a
resting or quiescent state.
The fusion

A cell is placed beside the egg and an electric current used
to fuse the couplet.
How to Clone a Sheep

Culture


The reconstructed embryo is put into culture and
grows for seven days.
Development


Embryos which grow successfully are taken and
transferred to a sheep which is at the the same
stage of the oestrus cycle as the egg.
The sheep becomes pregnant and produces a lamb
after 21 weeks (Dolly).
“Copy Cat”

Was the first cat ever cloned
Figure 21.8
The Stem Cells of Animals

A stem cell
Is a relatively unspecialized cell
 Can reproduce itself indefinitely
 Can differentiate into specialized cells of
one or more types, given appropriate
conditions

Embryonic and Adult Stem Cells
Embryonic stem cells

Stem cells
can be
isolated


From early
embryos at
the
blastocyst
stage
Adult stem
cells

pluripotent,
able to give
rise to
multiple but
not all cell
types
Early human embryo
at blastocyst stage
(mammalian equivalent of blastula)
Cultured
stem cells
Adult stem cells
From bone marrow
in this example
Pluripotent
cells
Totipotent
cells
Different
culture
conditions
Different
types ofLiver cells
differentiated
cells
Nerve cells
Blood cells
Transcriptional Regulation of Gene
Expression During Development
Complex
assemblies of proteins control
eukaryotic transcription

A variety of regulatory proteins interact
with DNA and with each other

To turn the transcription of eukaryotic
genes on or off
Transcription Factors
Assist in initiating eukaryotic transcription
Enhancers
Promoter
Gene
DNA
Activator
proteins
Transcription
Other
factors proteins
RNA polymerase
Bending
of DNA
Transcription
Determination and differentiation of muscle cells
Nucleus
Master control gene myoD
Other muscle-specific genes
DNA
Embryonic
precursor cell
OFF
OFF
Determination and differentiation of muscle cells
Nucleus
Master control gene myoD
Other muscle-specific genes
DNA
OFF
Embryonic
precursor cell
1
Myoblast
(determined)
Determination. Signals from other
cells lead to activation of a master
regulatory gene called myoD, and
the cell makes MyoD protein, a
transcription factor. The cell, now
called a myoblast, is irreversibly
committed to becoming a skeletal
muscle cell.
OFF
OFF
mRNA
MyoD protein
(transcription
factor)
Determination and differentiation of muscle cells
Nucleus
Master control gene myoD
Other muscle-specific genes
DNA
OFF
Embryonic
precursor cell
1
Myoblast
(determined)
2
Determination. Signals from other
cells lead to activation of a master
regulatory gene called myoD, and
the cell makes MyoD protein, a
transcription factor. The cell, now
called a myoblast, is irreversibly
committed to becoming a skeletal
muscle cell.
Differentiation. MyoD protein stimulates
the myoD gene further, and activates
genes encoding other muscle-specific
transcription factors, which in turn
activate genes for muscle proteins. MyoD
also turns on genes that block the cell
cycle, thus stopping cell division. The
nondividing myoblasts fuse to become
mature multinucleate muscle cells, also
called muscle fibers.
OFF
OFF
mRNA
MyoD protein
(transcription
factor)
mRNA
MyoD
Muscle cell
(fully differentiated)
mRNA
Another
transcription
factor
mRNA
mRNA
Myosin, other
muscle proteins,
and cell-cycle
blocking proteins
Cytoplasmic Determinants and Cell-Cell
Signals in Cell Differentiation

Cytoplasmic determinants in the
cytoplasm of the unfertilized egg

Regulate the expression of genes in the zygote
that affect the developmental fate of embryonic
cells
Molecules of
Sperm
Unfertilized egg cell
Sperm
Molecules of a
a cytoplasmic
determinant
Zygote
(fertilized egg)
Fertilization
another cytoplasmic determinant
Nucleus
Mitotic cell division
Two-celled
embryo
Induction

Signal molecules from embryonic cells cause
transcriptional changes in nearby target cells
Early embryo
(32 cells)
NUCLEUS
Signal
transduction
pathway
Signal
receptor
Signal
molecule
(inducer)
(b) Induction by nearby cells. The cells at the bottom of the early embryo depicted here are releasing
chemicals that signal nearby cells to change their gene expression.
Pattern Formation


Pattern formation in animals and plants results
from similar genetic and cellular mechanisms
Pattern formation



Is the development of a spatial organization of
tissues and organs
Occurs continually in plants
Is mostly limited to embryos and juveniles in animals
Cell Positioning

Positional information
Consists of molecular cues that control
pattern formation
 Tells a cell its location relative to the
body’s axes and to other cells

THE GENETIC CONTROL OF
EMBRYONIC DEVELOPMENT
Cascades of gene expression and cell-to-cell
signaling direct the development of an animal
Early understanding of the relationship between
gene expression and embryonic development

Came from studies of mutants of the fruit fly
Drosophila melanogaster
Eye
Antenna
SEM 50

Head of a normal fruit fly
Leg
Head of a developmental mutant
Key Developmental Genes are Very
Ancient

Homeotic genes contain nucleotide sequences,
called homeoboxes

That are very similar in many kinds of organisms
Fly chromosome
Mouse chromosomes
Fruit fly embryo (10 hours) Mouse embryo (12 days)
Adult fruit fly
Adult mouse
Follicle cell
Egg cell
developing within
ovarian
follicle
Nucleus
Egg cell
Nurse
cell
Fertilization
Laying of egg
Fertilized egg
Egg
shell
Nucleus
Embryo
Multinucleate
single cell
Early blastoderm

Key developmental
events in the life
cycle of Drosophila
Plasma
membrane
formation
Yolk
Late blastoderm
Cells of
embryo
Body
segments
Segmented
embryo
0.1 mm
Hatching
Larval stages (3)
Pupa
Metamorphosis
Head
Thorax
Abdomen
Adult fly
0.5 mm
Dorsal
Figure 21.12
BODY
AXES
Anterior
Posterior
Ventral
Bicoid Mutation
Tail
Head
T1
T2
T3
A1 A2 A3
A4
A7
A5 A6
A8
Wild-type larva
Tail
Tail
A8
A8
A7
A6
A7
Mutant larva (bicoid)
(a) Drosophila larvae with wild-type and bicoid mutant phenotypes. A mutation
in the mother’s bicoid gene leads to tail structures at both ends (bottom larva).
The numbers refer to the thoracic and abdominal segments that are present.
Summary of Gene Activity During
Drosophila Development
Hierarchy of Gene Activity in Early Drosophila Development
Maternal effect genes (egg-polarity genes)
Gap genes
Pair-rule genes
Segment polarity genes
Homeotic genes of the embryo
Other genes of the embryo
Segmentation genes
of the embryo
C. elegans: The Role of Cell Signaling
The complete cell lineage

Of each cell in the nematode roundworm
C. elegans is known
Zygote
0
Time after fertilization (hours)

First cell division
Nervous
system,
outer
skin, musculature
10
Outer skin,
nervous system
Musculature,
gonads
Germ line
(future
gametes)
Musculature
Hatching
Intestine
Intestine
Eggs
Vulva
Figure 21.15
ANTERIOR
POSTERIOR
1.2 mm
Induction

As early as the four-cell stage in C. elegans

Cell signaling helps direct daughter cells down the
appropriate pathways, a process called induction
2
Anterior
Posterior
1
4
3
Receptor
EMBRYO
4
3
Signal
Anterior
daughter
cell of 3
Posterior
daughter
cell of 3
Will go on to
form muscle
and gonads
Will go on to
form adult
intestine
Figure 21.16a
(a)
Signal
protein
Induction

also critical later in nematode development

As the embryo passes through three larval
stages prior to becoming an adult
Epidermis
Signal
Gonad Anchor cell protein
Vulval precursor cells
ADULT
Outer vulva
Inner vulva
Epidermis
Figure 21.16b
Programmed Cell Death (Apoptosis)

In apoptosis

Cell signaling is involved in programmed cell death
Figure 21.17
2 µm

In vertebrates

Apoptosis is essential for normal
morphogenesis of hands and feet in
humans and paws in other animals
Interdigital tissue
1 mm
Figure 21.19
Plant Development: Cell Signaling and
Transcriptional Regulation

Thanks to DNA technology and clues
from animal research

Plant research is now progressing rapidly
Mechanisms of Plant Development

In general, cell lineage


Is much less important for pattern
formation in plants than in animals
The embryonic development of most
plants

Occurs inside the seed
Pattern Formation in Flowers

Floral meristems

Contain three cell types that affect flower
development
Stamen
Carpel
Petal
Cell
layers
L1
L2
L3
Sepal
Floral meristem
Anatomy of a flower
Figure 21.20
Tomato flower
Organ Identity Genes

Organ identity genes


Determine the type of structure that will
grow from a meristem
Are analogous to homeotic genes in animals
Figure 21.22
Wild type
Mutant