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The Genetic Basis of Development. How do cells with the same genes grow up to be so different?. (b) Tadpole hatching from egg. (a) Fertilized eggs of a frog. Figure 21.3a, b. Three Procceses of Development. The transformation from a zygote into an organism

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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

(b) Tadpole hatching

from egg

(a) Fertilized eggs

of a frog

Figure 21.3a, b

Three Procceses of Development
  • The transformation from a zygote into an organism
    • Results from three interrelated processes: cell division, cell differentiation, and morphogenesis
slide4
Through a succession of mitotic cell divisions
    • The zygote gives rise to a large number of cells
  • In cell differentiation
    • 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.

Gut

Cell

movement

Zygote

(fertilized egg)

Eight cells

Blastula

(cross section)

Gastrula

(cross section)

Adult animal

(sea star)

Cell division

Morphogenesis

Observable cell differentiation

(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.

Seed

leaves

Shoot

apical

meristem

Root

apical

meristem

Zygote

(fertilized egg)

Two cells

Embryo

inside seed

Plant

Some key stages of development in animals and plants
differential gene expression
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

0

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

Muscle cell

Pancreas cells

Blood cells

0

Cell Diferentiation
  • Different types of cells
    • Make different proteins because different combinations of genes are active in each type
differentiated cells

0

Differentiated cells
  • may retain all of their genetic potential
    • Most retain a complete set of genes
    • May be totipotent
totipotency in plants

CONCLUSION

EXPERIMENT

RESULTS

Totipotency in Plants

Transverse

section of

carrot root

Fragments cultured in nutrient medium; stirring causes

single cells to shear off into liquid.

2-mg

fragments

Plantlet is

cultured on agar

medium. Later

it is planted

in soil.

Embryonic

plant develops

from a cultured

single cell.

Single cells

free in

suspension

begin to

divide.

A single

Somatic (nonreproductive) carrot

cell developed into a mature carrot

plant. The new plant was a genetic

duplicate(clone) of the parent plant.

Adult plant

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

DNA doublehelix(2-nmdiameter)

Histones

Linker

“Beads ona string”

TEM

Nucleosome(10-nm diameter)

Supercoil

(300-nm diameter)

Tight helical fiber(30-nm diameter)

700

nm

TEM

Metaphase chromosome

DNA packing in a eukaryotic chromosome
  • Wound around clusters of histone proteins, forming a string of beadlike nucleosomes
  • 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
an extreme example of dna packing

0

An Extreme Example of DNA Packing

X chromosome inactivation in the cells of female mammals

Two cell populationsin adult

Early embryo

Cell divisionand randomX chromosomeinactivation

Active X

Orangefur

X chromosomes

Inactive X

Inactive X

Black fur

Active X

Allele fororange fur

Allele forblack fur

nuclear transplantation
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
How to Clone a SheepBill Ritchie (Produced for the 1997 Royal Agricultural Show)
  • The Egg
    • The unfertilized eggs are flushed out of a sheep which has been induced to produce a larger than normal number of eggs.
  • The Cell
    • 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
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

Figure 21.8

“Copy Cat”
  • Was the first cat ever cloned
the stem cells of animals
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

Early human embryo

at blastocyst stage

(mammalian equiva-

lent of blastula)

Adult stem cells

From bone marrow

in this example

Pluripotent

cells

Cultured

stem cells

Different

culture

conditions

Different

types of

differentiated

cells

Blood cells

Embryonic and Adult 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

Totipotent

cells

Liver cells

Nerve cells

transcriptional regulation of gene expression during development

0

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

0

Transcription Factors

Assist in initiating eukaryotic transcription

Enhancers

Promoter

Gene

DNA

Activatorproteins

Transcriptionfactors

Otherproteins

RNA polymerase

Bendingof DNA

Transcription

determination and differentiation of muscle cells

Nucleus

Master control gene myoD

Other muscle-specific genes

DNA

OFF

OFF

Embryonicprecursor cell

Determination and differentiation of muscle cells
determination and differentiation of muscle cells1

1

Nucleus

Master control gene myoD

Other muscle-specific genes

DNA

OFF

OFF

Embryonicprecursor cell

Determination. Signals from othercells lead to activation of a masterregulatory gene called myoD, andthe cell makes MyoD protein, atranscription factor. The cell, nowcalled a myoblast, is irreversiblycommitted to becoming a skeletalmuscle cell.

OFF

mRNA

MyoD protein(transcriptionfactor)

Myoblast (determined)

Determination and differentiation of muscle cells
determination and differentiation of muscle cells2

1

2

Nucleus

Master control gene myoD

Other muscle-specific genes

DNA

OFF

OFF

Embryonicprecursor cell

Determination. Signals from othercells lead to activation of a masterregulatory gene called myoD, andthe cell makes MyoD protein, atranscription factor. The cell, nowcalled a myoblast, is irreversiblycommitted to becoming a skeletalmuscle cell.

OFF

mRNA

MyoD protein(transcriptionfactor)

Myoblast (determined)

Differentiation. MyoD protein stimulatesthe myoD gene further, and activatesgenes encoding other muscle-specifictranscription 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, alsocalled muscle fibers.

mRNA

mRNA

mRNA

mRNA

Myosin, othermuscle proteins,and cell-cycleblocking proteins

MyoD

Anothertranscriptionfactor

Muscle cell(fully differentiated)

Determination and differentiation of muscle cells
cytoplasmic determinants and cell cell signals in cell differentiation
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

another cyto-

plasmic deter-

minant

Sperm

Unfertilized egg cell

Sperm

Molecules of a

a cytoplasmic

determinant

Fertilization

Nucleus

Zygote

(fertilized egg)

Mitotic cell division

Two-celled

embryo

induction

(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.

Induction
  • Signal molecules from embryonic cells cause transcriptional changes in nearby target cells

Early embryo

(32 cells)

Signal

transduction

pathway

NUCLEUS

Signal

receptor

Signal

molecule

(inducer)

pattern formation
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
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

Eye

Antenna

Leg

SEM 50

Head of a developmental mutant

Head of a normal fruit fly

0

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
key developmental genes are very ancient

Fly chromosome

Mouse chromosomes

Fruit fly embryo (10 hours)

Mouse embryo (12 days)

Adult fruit fly

Adult mouse

0

Key Developmental Genes are Very Ancient
  • Homeotic genes contain nucleotide sequences, called homeoboxes
    • That are very similar in many kinds of organisms
slide34

Follicle cell

Nucleus

Egg cell

developing within

ovarian

follicle

Egg cell

Nurse

cell

Fertilization

Laying of egg

Fertilized egg

Egg

shell

Nucleus

Embryo

Multinucleate

single cell

Early blastoderm

Plasma

membrane

formation

Yolk

Late blastoderm

Cells of

embryo

Body

segments

Segmented

embryo

0.1 mm

Hatching

Larval stages (3)

Pupa

Metamorphosis

Head

Abdomen

Thorax

Adult fly

0.5 mm

Dorsal

Anterior

Posterior

BODY

AXES

Figure 21.12

Ventral

  • Key developmental events in the life cycle of Drosophila
bicoid mutation

Tail

Head

T1

A8

T2

A7

T3

A6

A1

A5

A2

A4

A3

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.

Bicoid Mutation
summary of gene activity during drosophila development

Hierarchy of Gene Activity in Early Drosophila Development

Maternal effect genes (egg-polarity genes)

Gap genes

Segmentation genes

of the embryo

Pair-rule genes

Segment polarity genes

Homeotic genes of the embryo

Other genes of the embryo

Summary of Gene Activity During Drosophila Development
c elegans the role of cell signaling
C. elegans: The Role of Cell Signaling

The complete cell lineage

Of each cell in the nematode roundworm C. elegans is known

Zygote

0

First cell division

Germ line

(future

gametes)

Outer skin,

nervous system

Musculature,

gonads

Nervous

system,

outer

skin, mus-

culature

Time after fertilization (hours)

Musculature

10

Hatching

Intestine

Intestine

Eggs

Vulva

ANTERIOR

POSTERIOR

1.2 mm

Figure 21.15

induction1

2

Posterior

1

Anterior

Signal

protein

4

3

Receptor

EMBRYO

3

4

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

(a)

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

Figure 21.16a

induction2

Epidermis

Signal

protein

Gonad

Anchor cell

Vulval precursor cells

Outer vulva

ADULT

Inner vulva

Epidermis

Figure 21.16b

Induction
  • also critical later in nematode development
    • As the embryo passes through three larval stages prior to becoming an adult
programmed cell death apoptosis

2 µm

Figure 21.17

Programmed Cell Death (Apoptosis)
  • In apoptosis
    • Cell signaling is involved in programmed cell death
slide42

Interdigital tissue

1 mm

Figure 21.19

  • In vertebrates
    • Apoptosis is essential for normal morphogenesis of hands and feet in humans and paws in other animals
plant development cell signaling and transcriptional regulation
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
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

Carpel

Stamen

Petal

L1

L2

Cell

layers

L3

Sepal

Floral meristem

Anatomy of a flower

Tomato flower

Figure 21.20

Pattern Formation in Flowers
  • Floral meristems
    • Contain three cell types that affect flower development
organ identity genes

Wild type

Mutant

Figure 21.22

Organ Identity Genes
  • Organ identity genes
    • Determine the type of structure that will grow from a meristem
    • Are analogous to homeotic genes in animals