1
Exam III Material:Bio 118
Chapter 21: Early Development
Development of an organism
-from single-cell zygote is most incredible processes
1. Cell Proliferation and Death
-cells divide as embryo grows in size
-with directed divisions, to acquire a particular shape
-ex. with part of the embryo is dividing and how fast or whichever direction daughter
cells bud off
A number of control sites for regulating:
-how fast the cell cycle proceeds
-so how fast a region grows
-also mechanisms for regulating
-the direction of cell divisions
As important as cell division is cell death
-it is essential at certain times, in certain places
-cells can die
-cell death that is controlled and regulated is called apoptosis
Apoptosis: controlled cell death
-ex. with formation of fingers: initially, hand is solid, cells between fingers die
off, sometimes babies born with fused
-ex. difference between chicken feet, where apoptosis takes place and duck feet,
where it doesn’t web
-Figure 21.1
2. Cell Movement
-cells move in developing embryo, not randomly
-1st movement turns the early embryo outside in
-cells roll up to make nervous system
-certain cell types migrate around embryo following particular pathways
-Cells change their neighbors:
-key feature cells moving change their neighbors
-interact with a new neighbor can be a signal to a cell
-to differentiate along a particular path: cells communicate with their neighbors
-Table 21.1: The Essential Developmental Processes
3. Cell Differentiation
-occurs in steps: 1st cells become determined; committed to follow a particular developmental
path
-early in this process, it can be reversible
-but at some point it becomes irreversible
-yet, some cells never differentiate or become determined
-in animals, these are the stem cells
-in plants, these are meristems
4. Cell-cell Interactions
-as mentioned in cell migration
, 2
-cells communicate with each other when they are touching or nearby
-cells in new location can signal cells already there or receive signals from cells already
there
-this is a critical factor in development
Differential Gene Expression
-key to all of these processes
-cells follow different developmental paths, look different, and with different functions
-because they’re expressing different genes
-differential gene expression uses mechanisms we have discussed
-such as enhancers, repressors, specific promoters
-with differential gene expression
-cells produce very different sets of proteins
-but don’t forget all cells have the same set of genes
-cells are not different because one cell has a gene and one does not
Cloning Dolly the sheep
-see this fact by the phenomenon of cloning
-where the nucleus of a body cell can be transplanted into an egg cell that had its nucleus
removed
-and a new animal grows
-plants are easier to clone; root cutting etc.
-Figure 21.2: cloning Dolly
-this happens because all cells have all the genes for any cell type
-what a cell looks like and what it does
-depends only on which genes it expresses
-ex. differential gene expression
How differential gene expression occurs
-major concepts of developmental biology
-not all are known
-but it’s due to presence or absence of transcription factors (TF)
-which are proteins that regulate transcription
-also, as proteins, their presence or absence is in turn regulated TFs
-ex. TFs regulating expression of TF
Now consider some fundamental regulations of gene expression
-and how this sets up the early embryo in a very basic way
-this is the creation of the front (anterior) to back (posterior)
-axis of the embryo
-in a human, anterior is head, posterior is butt
-Figure 21.3, three major body axes
Outline (barely) how a cell knows where it is
-how a cell knows it is
-in the head
-or in the center
-or in the posterior of the body
, 3
-if a cell knows where it is, it will know what it needs to differentiate into
-consider embryonic development of fruit flies
-but what is true in the fly is probably universal
-maybe even the same genes-exactly; as we will see
-the gene bicoid sets up the anterior of the embryo
-when it is mutant (not working)
-the embryo has 2 posteriors, no head
-Figure 21.4, bicoid mutant
Bicoid directs the anterior
-if a female fly that lacks bicoid
-she can’t put this protein into her eggs
-the eggs lack bicoid, and when fertilized, can’t develop heads
When expression patterns for bicoid are examined
-using in situ hybridization
-which tracks specific mRNAs
-using complementary DNA/RNA hybridization
-this study finds the mRNA for bicoid is present only in the anterior of the embryo
-see Figure 21.5: bicoid mRNAs
The egg contains the mRNA for bicoid
-tethered in anterior of egg
-so here the protein is translated and in the early fly embryo
-high anteriorly
-lower as you move posteriorly
-and absent in the posterior most embryo
Given the bicoid gradient from anterior to posterior
-so cells are exposed to different amounts of bicoid protein as development proceeds and what
part of the body a cell becomes depends on how concentrated bicoid is in that place
How does the concentration of bicoid dictate that position?
-bicoid protein is a transcription factor
-it triggers formation of anterior structures, so bicoid is in the anterior, triggers transcription of
genes encoding anterior structures
-while bicoid absence in the posterior means no transcription of anterior structures
-Figure 21.6, anterior gradient for bicoid protein
Bicoid activates transcription of another set of genes
-called gap genes
-which also encode for transcription factors
-there are 5 of them, expressed in wide bands
-each at a specific anterior to posterior position
-Figure 21.7, part A: gap genes in early development define the general position of the head,
thorax, and abdominal regions
, 4
Gap genes direct expression of “Pair Rule Genes”
-pair rule genes, which are expressed later
-the interactions of these genes specifies particular segments
-and they have different features anterior to posterior based on a specific gap gene
-figure 21.7, part B
The Pair Rule direct expression of Segment Polarity Genes
-ex. the polarity within each segment
-and by their interactions specify the anterior and posterior region of each segment
-Figure 21.7, Part C: segment polarity genes delineate boundaries within individual segments
Next: segment identity
-exactly what each segment is, whether head, trunk or abdomen
-dictated by Homoetic or HOX gene
-mutations of homeotic genes lead to scrambled segments
-a fly mutant for one of these has extra wings
-mutants with extra legs where they don’t belong
-even replacing antennae with legs!
-Figure 21.8, segment identity mutants
Hox genes are found in all animals
-including humans
-they’re arranged along chromosome
-in the order along the body
-that they are expressed
-they are universal
-a fruit fly that is mutant for a HOX gene can be rescued by inserting into the fly the
same HOX gene from a mouse!
-Figure 21.10
Summary
-regulatory gene cascade in fruit flies
-transcription factors directing other transcription factors (see figure 21.9)
Chapter 22: Animal Development
-development is circular, you could start talking about it from any point
-we’ll start at formation of gametes
-Figure 22.1
-egg and sperm contribute equally to offspring’s chromosomes
-but egg has lots of cytoplasm
-it contributes more things to developing embryo
-ex. bicoid, and yolk
Exam III Material:Bio 118
Chapter 21: Early Development
Development of an organism
-from single-cell zygote is most incredible processes
1. Cell Proliferation and Death
-cells divide as embryo grows in size
-with directed divisions, to acquire a particular shape
-ex. with part of the embryo is dividing and how fast or whichever direction daughter
cells bud off
A number of control sites for regulating:
-how fast the cell cycle proceeds
-so how fast a region grows
-also mechanisms for regulating
-the direction of cell divisions
As important as cell division is cell death
-it is essential at certain times, in certain places
-cells can die
-cell death that is controlled and regulated is called apoptosis
Apoptosis: controlled cell death
-ex. with formation of fingers: initially, hand is solid, cells between fingers die
off, sometimes babies born with fused
-ex. difference between chicken feet, where apoptosis takes place and duck feet,
where it doesn’t web
-Figure 21.1
2. Cell Movement
-cells move in developing embryo, not randomly
-1st movement turns the early embryo outside in
-cells roll up to make nervous system
-certain cell types migrate around embryo following particular pathways
-Cells change their neighbors:
-key feature cells moving change their neighbors
-interact with a new neighbor can be a signal to a cell
-to differentiate along a particular path: cells communicate with their neighbors
-Table 21.1: The Essential Developmental Processes
3. Cell Differentiation
-occurs in steps: 1st cells become determined; committed to follow a particular developmental
path
-early in this process, it can be reversible
-but at some point it becomes irreversible
-yet, some cells never differentiate or become determined
-in animals, these are the stem cells
-in plants, these are meristems
4. Cell-cell Interactions
-as mentioned in cell migration
, 2
-cells communicate with each other when they are touching or nearby
-cells in new location can signal cells already there or receive signals from cells already
there
-this is a critical factor in development
Differential Gene Expression
-key to all of these processes
-cells follow different developmental paths, look different, and with different functions
-because they’re expressing different genes
-differential gene expression uses mechanisms we have discussed
-such as enhancers, repressors, specific promoters
-with differential gene expression
-cells produce very different sets of proteins
-but don’t forget all cells have the same set of genes
-cells are not different because one cell has a gene and one does not
Cloning Dolly the sheep
-see this fact by the phenomenon of cloning
-where the nucleus of a body cell can be transplanted into an egg cell that had its nucleus
removed
-and a new animal grows
-plants are easier to clone; root cutting etc.
-Figure 21.2: cloning Dolly
-this happens because all cells have all the genes for any cell type
-what a cell looks like and what it does
-depends only on which genes it expresses
-ex. differential gene expression
How differential gene expression occurs
-major concepts of developmental biology
-not all are known
-but it’s due to presence or absence of transcription factors (TF)
-which are proteins that regulate transcription
-also, as proteins, their presence or absence is in turn regulated TFs
-ex. TFs regulating expression of TF
Now consider some fundamental regulations of gene expression
-and how this sets up the early embryo in a very basic way
-this is the creation of the front (anterior) to back (posterior)
-axis of the embryo
-in a human, anterior is head, posterior is butt
-Figure 21.3, three major body axes
Outline (barely) how a cell knows where it is
-how a cell knows it is
-in the head
-or in the center
-or in the posterior of the body
, 3
-if a cell knows where it is, it will know what it needs to differentiate into
-consider embryonic development of fruit flies
-but what is true in the fly is probably universal
-maybe even the same genes-exactly; as we will see
-the gene bicoid sets up the anterior of the embryo
-when it is mutant (not working)
-the embryo has 2 posteriors, no head
-Figure 21.4, bicoid mutant
Bicoid directs the anterior
-if a female fly that lacks bicoid
-she can’t put this protein into her eggs
-the eggs lack bicoid, and when fertilized, can’t develop heads
When expression patterns for bicoid are examined
-using in situ hybridization
-which tracks specific mRNAs
-using complementary DNA/RNA hybridization
-this study finds the mRNA for bicoid is present only in the anterior of the embryo
-see Figure 21.5: bicoid mRNAs
The egg contains the mRNA for bicoid
-tethered in anterior of egg
-so here the protein is translated and in the early fly embryo
-high anteriorly
-lower as you move posteriorly
-and absent in the posterior most embryo
Given the bicoid gradient from anterior to posterior
-so cells are exposed to different amounts of bicoid protein as development proceeds and what
part of the body a cell becomes depends on how concentrated bicoid is in that place
How does the concentration of bicoid dictate that position?
-bicoid protein is a transcription factor
-it triggers formation of anterior structures, so bicoid is in the anterior, triggers transcription of
genes encoding anterior structures
-while bicoid absence in the posterior means no transcription of anterior structures
-Figure 21.6, anterior gradient for bicoid protein
Bicoid activates transcription of another set of genes
-called gap genes
-which also encode for transcription factors
-there are 5 of them, expressed in wide bands
-each at a specific anterior to posterior position
-Figure 21.7, part A: gap genes in early development define the general position of the head,
thorax, and abdominal regions
, 4
Gap genes direct expression of “Pair Rule Genes”
-pair rule genes, which are expressed later
-the interactions of these genes specifies particular segments
-and they have different features anterior to posterior based on a specific gap gene
-figure 21.7, part B
The Pair Rule direct expression of Segment Polarity Genes
-ex. the polarity within each segment
-and by their interactions specify the anterior and posterior region of each segment
-Figure 21.7, Part C: segment polarity genes delineate boundaries within individual segments
Next: segment identity
-exactly what each segment is, whether head, trunk or abdomen
-dictated by Homoetic or HOX gene
-mutations of homeotic genes lead to scrambled segments
-a fly mutant for one of these has extra wings
-mutants with extra legs where they don’t belong
-even replacing antennae with legs!
-Figure 21.8, segment identity mutants
Hox genes are found in all animals
-including humans
-they’re arranged along chromosome
-in the order along the body
-that they are expressed
-they are universal
-a fruit fly that is mutant for a HOX gene can be rescued by inserting into the fly the
same HOX gene from a mouse!
-Figure 21.10
Summary
-regulatory gene cascade in fruit flies
-transcription factors directing other transcription factors (see figure 21.9)
Chapter 22: Animal Development
-development is circular, you could start talking about it from any point
-we’ll start at formation of gametes
-Figure 22.1
-egg and sperm contribute equally to offspring’s chromosomes
-but egg has lots of cytoplasm
-it contributes more things to developing embryo
-ex. bicoid, and yolk
