Lecture 5: Basic Concepts in Developmental Biology

Question 1

control development by determining where and when proteins are
synthesized.

Answer 1

genes

Question 2

• sets up intracellular networks of interactions between proteins and genes, and between proteins and proteins, that give cells their particular properties.
• One of these properties is the ability to communicate with, and respond to, other cells.
• It is these cell–cell interactions that determine how the embryo develops; no developmental process can therefore be attributed to the function of a single gene or single protein.

Answer 2

gene activity

Question 3

Development involves the: (4)

Answer 3

• emergence of pattern
• change in form
• cell differentiation
• growth

Question 4

After fertilization, the embryo undergoes cell division immediately, a stage known as __

Answer 4

cleavage

Question 5

__ divides the fertilized egg into smaller cells without increasing cell size.

Answer 5

Cleavage

Question 6

• involves cycles of DNA replication, mitosis, and cell division.
• There is no significant increase in cell size between its divisions, maintaining the overall size of the embryo.

Answer 6

cleavage

Question 7

size in embryogenesis

• Early embryos remain the same size as the __.
• Substantial growth only occurs if the embryo has access to adequate materials for development.

Answer 7

zygote

Question 8

the emergence of organized structures from an initially very simple group of cells.

Answer 8

development

Question 9

Four main developmental processes (4)

Answer 9

a. Pattern formation
b. Morphogenesis
c. Cell differentiation
d. Growth

These processes occur in roughly sequential order during development, although in reality, they overlap with and influence each other considerably. They are neither independent of each other nor strictly sequential.

Question 10

• Defined as change in form.
• Embryos undergo remarkable changes in three-dimensional form.

Answer 10

morphogenesis

Question 11

Morphogenesis

• At certain stages in development, there are characteristic and dramatic changes in form, of which __ is the most striking.
• Almost all animal embryos undergo this, during which endoderm and mesoderm move inside, the gut is formed, and the main body plan emerges.

Answer 11

• gastrulation

Question 12

Morphogenesis

During gastrulation, cells on the outside of the embryo move inwards and, in animals such as the sea urchin, gastrulation transforms a hollow spherical __ into a __ with a tube through the middle—the __.

Answer 12

• blastula
• gastrula
• gut

Question 13

The process by which cells become structurally and functionally different from each other, ending up as distinct cell types, such as blood, muscle, or skin cells.

Answer 13

Cell differentiation

Question 14

a gradual process, with cells often going through several divisions between the time at which they start differentiating and the time they are fully differentiated.

Answer 14

differentiation

Question 15

Defined as the increase in size.

Answer 15

growth

Question 16

• In general, there is little growth during early embryonic development, and the basic pattern and form of the embryo is laid down on a small scale, always less than a millimeter in extent.
• Subsequent growth can be brought about in various ways: ____ (3), such as, for example, in bone.

Answer 16

• cell multiplication
• increase in cell size
• deposition of extracellular materials

Question 17

Growth can also be __, in that differences in growth rates between organs or between parts of the body can generate changes in the overall shape of the embryo.

Answer 17

morphogenetic

Question 18

provides the link between gene action and developmental processes.

Answer 18

Cell behavior

Question 19

Cell behavior

• __ within cells leads to the synthesis of proteins that are responsible for particular cellular properties and behavior, which in turn determine the course of embryonic development.
• The past and current patterns of gene activity confer a certain state, or identity, on a cell at any given time, which is reflected in its molecular organization—in particular, which proteins are present.
• As we shall see, embryonic cells and their progeny undergo many changes in state as development progresses.

Answer 19

• Gene expression

Question 20

Cell Behavior in Development

Other categories of cell behavior that will concern us are: (4)

Answer 20

• intercellular communication or cell–cell signaling
• changes in cell shape and movement
• cell proliferation
• cell death

Question 21

Cell Behavior in Development

Changing patterns of gene activity during early development are essential for __. They give cells identities that determine their future behavior and lead eventually to their final differentiation.

Answer 21

pattern formation

Question 22

Cell Behavior in Development

The capacity of cells to influence each other’s fate by producing and responding to signals is crucial for development. By their response to signals for cell movement or a change in shape, for example, cells generate the physical forces that bring about __.

Answer 22

morphogenesis

Question 23

Cell Behavior in Development

The curvature of a sheet of cells into a tube, as happens in* Xenopus* and other vertebrates during the formation of the __, is the result of contractile forces generated by cells changing their shape at certain positions within the cell sheet.

Answer 23

neural tube

Question 24

Cell Behavior in Development

An important feature of cell surfaces is the presence of adhesive proteins known as __, which serve various functions: they hold cells together in tissues, enable cells to sense the nature of the surrounding extracellular matrix, and guide migratory cells such as the __ of vertebrates, which leave the neural tube to form structures elsewhere in the body.

Answer 24

• cell-adhesion molecules
• neural crest cells

Question 25

__ control cell behavior by specifying which proteins are made.

Answer 25

Genes

Question 26

# Genes and Cell Behavior What a cell can do is determined very largely by the __ it makes. For example, the hemoglobin in red blood cells enables them to transport oxygen.

Answer 26

proteins

Question 27

# Genes and Cell Behavior In development, we are primarily concerned with those proteins that make cells different from one another, often known as __.

Answer 27

tissue-specific proteins

Question 28

# Genes and Cell Behavior They control development mainly by specifying which proteins are made in which cells and when. In this sense, they are passive participants in development compared with the proteins they encode.

Answer 28

genes

Question 29

# Genes and Cell Behavior the agents that directly determine cell behavior, which includes determining which genes are expressed.

Answer 29

genes

Question 30

# Genes and Cell Behavior Developmental genes typically code for proteins involved in the regulation of cell behavior: __, __ such as __, and gene-regulatory proteins.

Answer 30

- receptors - intercellular signaling proteins - growth factors - gene-regulatory proteins

Question 31

# Genes and Cell Behavior Many of these genes, especially those for receptors and signaling molecules, are used throughout an organism’s life, but others are active only during __.

Answer 31

embryonic development

Question 32

# Gene Expression and Protein Synthesis A __ comprises a coding region—a stretch of DNA that contains the instructions for making the protein—and adjacent DNA sequences that act as a __.

Answer 32

- protein-coding gene - control region

Question 33

# Gene Expression and Protein Synthesis The control region includes the __, to which general transcription factors and the enzyme RNA polymerase bind to start transcription, and a __, consisting of one or more modules, at which other transcription factors bind to switch the gene on or off. This latter control region may be thousands of base pairs away from the promoter.

Answer 33

- promoter region - cis regulatory region

Question 34

# Gene Expression and Protein Synthesis When the gene is switched on, the DNA sequence is transcribed to produce __.

Answer 34

RNA

Question 35

# Gene Expression and Protein Synthesis The RNA formed by transcription is spliced to remove introns and the noncoding region at the start of the gene and is processed within the nucleus to produce __. This is exported from the nucleus to the cytoplasm for translation into protein at the ribosomes.

Answer 35

mRNA

Question 36

# Gene Expression and Protein Synthesis * Control of gene expression and protein synthesis occurs mainly at the level of __ but can also occur at later stages. * Some proteins require a further step of __ to become biologically active. A very common example for this is the addition of carbohydrate side chains (__).

Answer 36

- transcription - post-translational modification - glycosylation

Question 37

# Gene Expression and Protein Synthesis * It is also possible to prevent the expression of a given gene and thus prevent its protein from being made. * This can be done using __, which are small RNA molecules of about 25 nucleotides complementary in sequence to part of a gene or mRNA. * They can block gene function by binding to the gene or its mRNA. * Artificial RNA molecules called __ are used, which are more stable than normal RNA.

Answer 37

- Antisense RNAs - morpholinos

Question 38

# Gene Expression and Protein Synthesis Another technique for blocking gene expression is __ (_), which operates in a similar way as antisense RNAs but uses a different type of small RNA called __ to exploit cells’ own metabolic pathways for degrading mRNAs.

Answer 38

- RNA interference (RNAi) - small interfering RNAs (siRNAs)

Question 39

# Control of Developmental Gene Expression * All the somatic cells in an embryo are derived from the fertilized egg by successive rounds of mitotic cell division. Thus, with rare exceptions, they all contain identical genetic information, the same as that in the zygote. * Differentiated cells still contain all the genetic information required to make a complete animal.

Answer 39

Genomic equivalence

Question 40

# Control of Developmental Gene Expression * The differences between cells must therefore be generated by differences in gene activity that lead to the synthesis of different proteins. * Turning the correct genes on or off in the right cells at the right time becomes the central issue in development. * The genes do not provide a blueprint for development but a set of instructions.

Answer 40

Differential Gene Expression

Question 41

# Control of Developmental Gene Expression * Developmental genes are highly regulated to ensure they are switched on only at the right time and place in development. * To achieve this, developmental genes usually have extensive and complex __ composed of one or more modules. Each module contains multiple binding sites for different transcription factors, and it is the precise combination of factors that binds that determines whether the gene is on or off.

Answer 41

cis-regulatory regions ## Footnote * The modular nature of the regulatory region means that each module can function somewhat independently. * This means that a gene with more than one regulatory module can usually respond quite differently to different combinations of inputs, allowing it to be expressed at different times and in different places within the embryo in response to developmental signals.

Question 42

# Control of Developmental Gene Expression highly regulated to ensure they are switched on only at the right time and place in development.

Answer 42

Developmental genes

Question 43

# Control of Developmental Gene Expression * Different genes can have the same regulatory module, which usually means that they will be expressed together, or different genes may have modules that contain some, but not all, of the binding sites in common, introducing subtle differences in the timing or location of expression. * Thus, an organism’s genes are linked in complex interdependent networks of expression through the modules of their __ and the proteins that bind to them. * Common examples of such regulation are __ and __, in which a transcription factor respectively promotes or represses the expression of a gene whose product maintains that gene’s expression. ## Footnote * Developmental genes are highly regulated to ensure they are switched on only at the right time and place in development. * To achieve this, developmental genes usually have extensive and complex cis-regulatory regions composed of one or more modules. Each module contains multiple binding sites for different transcription factors, and it is the precise combination of factors that binds that determines whether the gene is on or off.

Answer 43

- cis-regulatory regions - positive-feedback loops - negative-feedback loops

Question 44

__ is the ultimate developmental outcome of a cell, while __ is the initial commitment to a particular fate, which can still be reversed, and __ is the irreversible commitment to that fate.

Answer 44

- Cell fate - specification - determination

Question 45

Development is progressive, and the __ of cells become determined at different times.

Answer 45

fates

Question 46

# Progressive Development and Cell Fate Determination * As embryonic development proceeds, the organizational complexity of the embryo becomes vastly increased over that of the fertilized egg. * Many different cell types are formed, spatial patterns emerge, and there are major changes in shape. * In general, the embryo is first divided up into a few broad regions, such as the __ (__, __, and __). * Subsequently, the cells within these regions have their fates more and more finely determined.

Answer 46

future germ layers (mesoderm, ectoderm, and endoderm).

Question 47

# Progressive Development and Cell Fate Determination * __ implies a stable change in the internal state of a cell, and an alteration in the pattern of gene activity is assumed to be the initial step, leading to a change in the proteins produced in the cell. * For example, different mesoderm cells eventually become determined as muscle, cartilage, bone, the fibroblasts of connective tissue, and the cells of the dermis of the skin.

Answer 47

- Determination -

Question 48

# Progressive Development and Cell Fate Determination refers to the direct line of descent of a particular cell.

Answer 48

Lineage

Question 49

# Progressive Development and Cell Fate Determination * Once a cell is __, this stable change is inherited by its descendants. Thus, the __ of a cell has important consequences in terms of what type of cell it can eventually become. * For example, once determined as ectoderm, cells will not subsequently give rise to endodermal tissues, or vice versa.

Answer 49

- determined - lineage

Question 50

# Progressive Development and Cell Fate Determination The __ of a group of cells merely describes what they will normally develop into.

Answer 50

fate

Question 51

# Progressive Development and Cell Fate Determination A group of cells is called __ if, when isolated and cultured in the neutral environment of a simple culture medium away from the embryo, they develop more or less according to their normal fate.

Answer 51

specified

Question 52

# Progressive Development and Cell Fate Determination A general feature of development is that cells in the early embryo are less narrowly __ than those at later stages; with time, cells become more and more __ in their developmental potential.

Answer 52

- determined - restricted

Question 53

# Progressive Development and Cell Fate Determination * When the developmental potential of cells is much greater than their normal fate, they are capable of __ and are described as __. * In contrast, __ have cells that develop strictly according to their early fate from a very early stage.

Answer 53

- regulation - regulative - mosaic embryos

Question 54

# Progressive Development and Cell Fate Determination * The term __ describes eggs and embryos that develop as if their pattern of future development is predetermined, even in the egg, as a __ of different molecules in the cytoplasm. * These __ are distributed in a regular pattern during cleavage, influencing cell lineage fate at an early stage. * In __ development, different parts of the embryo develop independently, and cell interactions may be quite limited.

Answer 54

- mosaic - spatial mosaic - cytoplasmic determinants - mosaic

Question 55

# Progressive Development and Cell Fate Determination * occur when a gene's effect is restricted to the cell expressing it. * Example: Transcription factors that regulate gene expression within the same cell.

Answer 55

Cell-autonomous effects

Question 56

# Progressive Development and Cell Fate Determination * occur when gene expression in one cell influences other cells. * Example: A gene coding for an intercellular signaling protein, which is secreted by one cell and affects others.

Answer 56

Non-cell-autonomous effects

Question 57

# Progressive Development and Cell Fate Determination * A __ is an organism composed of cells with different genetic constitutions, often created by fusing two very early embryos. * Example: If early embryos from a brown-haired and a white-haired mouse are fused, the resulting mouse will have distinct patches of brown and white hair. * The brown cells continue to produce brown pigment even when placed in a white-haired region, showing that pigmentation is a __—it does not affect neighboring cells.

Answer 57

- chimera - cell-autonomous character

Question 58

# Inductive Interactions in Development * a process where one cell or tissue directs the development of another, usually adjacent, cell or tissue. * A signal from one group of cells influences the developmental fate of another group.

Answer 58

Induction

Question 59

# Inductive Interactions in Development * Hans Spemann and Hilde Mangold conducted a transplantation experiment in amphibian embryos. * They demonstrated that a small region of the early newt embryo could induce the formation of a __ when grafted onto another embryo at the same stage. * The transplanted tissue was taken from the __, where gastrulation begins. * This region, known as the __ (or simply __), is crucial for controlling embryonic body organization. * Spemann was awarded the Nobel Prize in Physiology or Medicine in 1935 for this discovery.

Answer 59

- second embryonic body - dorsal lip of the blastopore - Spemann–Mangold organizer (Spemann organizer)

Question 60

# Inductive Interactions in Development Types of Inductive Signaling (3)

Answer 60

- Permissive Induction - Instructive Induction

Question 61

# Inductive Interactions in Development * A cell makes only one kind of response when exposed to a signal. * The response occurs once the signal reaches a sufficient threshold. | Types of Inductive Signaling

Answer 61

Permissive Induction

Question 62

# Inductive Interactions in Development Cells respond differently to different concentrations of a signal. | Types of Inductive Signaling

Answer 62

Instructive Induction