Biology Chapter on Fertilization and Development

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Questions and Answers

What is the primary function of the promoter in a eukaryotic gene?

It serves as the termination site for transcription.

It is where the poly-A tail is added.

It is the site where RNA polymerase binds to initiate transcription. (correct)

It contains sequences that are not translated.

Which type of chromatin is characterized by tightly packed structure and inactive genes?

Heterochromatin (correct)

Transcriptionally active chromatin

Euchromatin

Promoter region

What occurs to introns during RNA processing?

They are transcribed but removed before translation. (correct)

They serve as promoters for gene expression.

They are translated into protein.

They remain in the mRNA after transcription.

Why is the poly-A signal important in mRNA processing?

It is crucial for the stability and efficiency of translation. Signup and view all the answers

Which proteins are responsible for aiding transcription regulation alongside RNA polymerase?

Transcription factors Signup and view all the answers

What role do transcription factors (TFs) play in gene expression?

They determine which genes are expressed in a cell. Signup and view all the answers

What is the function of activators in the context of transcription?

They bind to enhancers to promote transcription. Signup and view all the answers

Which statement accurately describes repressors?

They interact with silencers to inhibit transcription. Signup and view all the answers

What is NOT a characteristic of transcription factors (TFs)?

They are produced in the same chromosome they operate on. Signup and view all the answers

Which is an example of a transcription factor's function?

Binding to the TATA box in the promoter. Signup and view all the answers

What role do general transcription factors (TFs) play in transcription?

They are required for RNA polymerase to bind to the promoter. Signup and view all the answers

Why do liver and lens cells express different genes despite having the same DNA?

The presence of specific transcription factors allows for gene expression. Signup and view all the answers

What distinguishes regulatory transcription factors from general transcription factors?

Regulatory TFs control the rate of transcription. Signup and view all the answers

Which of the following statements is true regarding enhancers and silencers?

They interact with regulatory transcription factors. Signup and view all the answers

How do transcription factors facilitate the connection between distal elements and genes?

By looping DNA to bring distant elements together. Signup and view all the answers

Which proteins are categorized as general transcription factors?

Proteins such as TFIID, TFIIA, and TFIIB. Signup and view all the answers

What is the primary outcome of RNA processing?

To modify mRNA before it undergoes translation. Signup and view all the answers

Which of the following functions do silencers perform?

They bind with regulatory TFs to inhibit transcription. Signup and view all the answers

What determines whether a gene is expressed in a particular cell type?

The specific transcription factors present in that cell type. Signup and view all the answers

What happens when the necessary transcription factors for a gene are absent in a cell?

The gene is repressed regardless of other factors. Signup and view all the answers

What is the primary function of DICER in the processing of miRNA and siRNA?

To cleave miRNA and siRNA precursors Signup and view all the answers

How does the mechanism of siRNA differ from that of miRNA in gene regulation?

miRNA allows for imprecise matching with mRNA Signup and view all the answers

What role do maternal mRNAs play during development?

They control developmental processes until zygotic genes are activated. Signup and view all the answers

What happens to mRNA when bases are complementary to miRNA or siRNA?

It is degraded. Signup and view all the answers

Which modification is NOT mentioned as part of post-translational regulation?

Degradation of RNA Signup and view all the answers

In which cellular compartment do miRNA precursors undergo processing?

Both nucleus and cytoplasm Signup and view all the answers

What is the consequence of less than complete matching between miRNA and mRNA?

Translation of the mRNA is blocked. Signup and view all the answers

What type of RNA primarily serves as the precursor for siRNA?

Double-stranded RNA from viruses or laboratory synthesis Signup and view all the answers

Which of the following processes is a required step for some proteins before they attain functionality?

Covalent modification, such as phosphorylation Signup and view all the answers

What type of structure do miRNA precursors form that is crucial for their processing?

Hairpin loops Signup and view all the answers

What process leads to the formation of different cell types from the same set of genes?

Differential gene expression Signup and view all the answers

Which of the following is NOT a level of control for gene expression?

Protein folding Signup and view all the answers

What is the outcome of somatic cell nuclear transfer demonstrated by the cloning of Dolly?

The nucleus of a somatic cell can direct the development of a complete organism Signup and view all the answers

What directly follows transcription in the gene expression process?

RNA transportation Signup and view all the answers

What do enhancers and repressors do in gene expression?

They regulate the transcription process. Signup and view all the answers

What is the result of differential mRNA stability?

Variation in protein expression levels Signup and view all the answers

Which germ layer is NOT formed from the zygote during embryonic development?

Dermatoderm Signup and view all the answers

Which aspect of gene expression involves modifications to chromatin structure?

Transcription Signup and view all the answers

In the context of differential gene expression, which statement is true?

All differentiated cells have identical DNA. Signup and view all the answers

What is a key characteristic of mRNA after transcription before translation?

It must undergo splicing and modifications. Signup and view all the answers

Differential gene expression can lead to which of the following?

Development of distinct cell types Signup and view all the answers

Which of the following processes occurs during RNA processing?

Addition of a 5' cap Signup and view all the answers

What does genomic equivalence suggest about somatic cells?

They contain the same number of chromosomes as zygotes. Signup and view all the answers

Which mechanism allows for selective localization of mRNA?

Cytoplasmic transport processes Signup and view all the answers

What is the role of bicoid protein in relation to hunchback mRNA?

Activates the transcription of hunchback mRNA Signup and view all the answers

How do maternal effect proteins influence gene expression in early embryos?

They regulate the spatial and temporal expression of zygotic genes Signup and view all the answers

What is indicated by the presence of caudal mRNA in the early embryo?

It activates genes necessary for abdomen formation Signup and view all the answers

Which of the following correctly describes the function of terminal genes?

They generate unsegmented extremities of the body plan Signup and view all the answers

What is one of the key outcomes of differential transcription of zygotic genes during early embryonic development?

Establishment of tissue-specific gene expression patterns Signup and view all the answers

What is the role of maternal effect genes in Drosophila development?

Provide genetic products that regulate traits in offspring Signup and view all the answers

What function do nurse cells serve in Drosophila embryonic development?

They synthesize and transfer nutrients to the developing oocyte. Signup and view all the answers

How does the maternal protein gradient influence axis formation in Drosophila?

It determines the polarity of the developing embryo. Signup and view all the answers

What is the significance of segmentation in early Drosophila development?

It defines the thoracic and abdominal segments of the adult fly. Signup and view all the answers

What occurs shortly after fertilization in Drosophila development?

Distinct segmentation patterns are established. Signup and view all the answers

Which genes primarily control the segmentation of Drosophila embryos?

Segmentation genes Signup and view all the answers

What defines the terminal genes in Drosophila development?

They influence the formation of head and tail structures. Signup and view all the answers

What defines the specification of the anterior-posterior axis in Drosophila?

Distribution of maternal proteins and gradients. Signup and view all the answers

What is primarily controlled by the nuclear gene products in maternal effect genes?

The phenotype of the offspring traits. Signup and view all the answers

What role do maternal effect genes play in early Drosophila development?

They regulate the expression of zygotic genes through mRNA and proteins. Signup and view all the answers

What is the primary function of nurse cells in the Drosophila egg chamber?

To provide nutrients and regulatory factors to the developing oocyte. Signup and view all the answers

How do protein gradients influence early Drosophila development?

They establish the spatial organization of nuclear divisions. Signup and view all the answers

During which stage is the diploid zygote nucleus formed in Drosophila development?

By the fusion of parental gamete nuclei after fertilization Signup and view all the answers

What is the outcome of the syncytial blastoderm formation?

It results in a multinucleated cell with a shared cytoplasm. Signup and view all the answers

At approximately 2.5 hours after fertilization, what significant cellular event occurs in Drosophila development?

Nuclei migrate to the periphery of the egg and undergo additional divisions. Signup and view all the answers

What does the term 'maternal effect genes' refer to in the context of early Drosophila embryonic development?

Genes that the maternal organism expresses, influencing offspring development. Signup and view all the answers

What is the significance of the incomplete cytokinesis that occurs during the development of Drosophila germline cells?

It creates interconnected nurse cells that support oocyte development. Signup and view all the answers

What is the primary role of nurse cells in early embryonic development?

Produce mRNAs and proteins for transportation to the oocyte Signup and view all the answers

How do the protein products from maternal genes impact follicle cell differentiation?

They signal follicle cells to differentiate into posterior follicle cells Signup and view all the answers

What is the significance of the maternal protein gradient in embryonic development?

It initiates the formation of the anterior-posterior axis Signup and view all the answers

What process ensures that some mRNAs do not reach the posterior end of the oocyte?

Binding by translation inhibitors, leading to degradation Signup and view all the answers

Which morphogen is responsible for establishing the head/anterior structure of the embryo?

Bicoid mRNA Signup and view all the answers

What result occurs when bicoid mRNA is absent in an embryo?

It forms two tail portions Signup and view all the answers

What happens to the concentration of maternal effect genes towards the posterior end of the embryo?

It decreases progressively towards the anterior Signup and view all the answers

What interaction occurs between posterior follicle cells and the oocyte during embryonic development?

They send signals to organize the microtubules in the oocyte Signup and view all the answers

What characteristic of nanos mRNAs is crucial for their role in embryonic development?

They diffuse passively to the posterior pole Signup and view all the answers

Which of the following statements about cytoplasmic polarity is accurate?

It represents the distribution gradient of maternal effect genes Signup and view all the answers

What is the role of Patched in the Hedgehog signaling pathway?

To inhibit the activity of GAP protein. Signup and view all the answers

What occurs to Smoothened when Hh is present?

Smoothened is activated and promotes Gli release. Signup and view all the answers

What is the fate of Gli proteins in the absence of Hh?

Gli complexes with proteins and is cleaved. Signup and view all the answers

Which mechanism is involved in Smoothened's action within the Hedgehog pathway?

Inactivation of cleavage proteins. Signup and view all the answers

What happens to the ERK protein within the Hedgehog signaling cascade?

ERK is phosphorylated by active MEK and enters the nucleus. Signup and view all the answers

What is a primary role of FGF1 in vertebrate development?

Regeneration and tissue homeostasis Signup and view all the answers

Which Hedgehog protein is crucial for specifying motor neuron identities in the neural tube?

Sonic hedgehog (shh) Signup and view all the answers

What condition can result from the misregulation of Hedgehog proteins in development?

Skin cancer Signup and view all the answers

How does cholesterol affect Hedgehog protein secretion?

Low levels cause rapid diffusion before receptor binding Signup and view all the answers

What is one of the specific functions of FGF8 in development?

Limb development Signup and view all the answers

In which cellular structure do Hedgehog signaling pathways primarily occur?

Primary cilium Signup and view all the answers

What happens to uncomplexed β-catenin in the presence of active Hedgehog signaling?

It remains bound to cell membrane proteins Signup and view all the answers

Which of the following is NOT a process associated with Hedgehog proteins during development?

Blood vessel formation Signup and view all the answers

What is the role of fibroblast growth factors (FGFs) in relation to receptor tyrosine kinases (RTKs)?

FGFs bind to RTKs to initiate the tyrosine kinase signaling pathway. Signup and view all the answers

What best describes the function of heparan sulfate proteoglycans (HSPGs) in the context of FGFs and RTKs?

They stabilize FGFs and assist in their binding to receptors. Signup and view all the answers

How do paracrine factors typically operate within cellular signaling?

They diffuse over small distances to influence nearby cells. Signup and view all the answers

Which component is involved as a molecular switch in the RTK pathway?

Ras protein Signup and view all the answers

What determines the specificity of the epithelial response to mesenchymal signals?

The genome of the responding epithelium. Signup and view all the answers

What is the primary role of GSK3 in the Wnt signaling pathway?

To phosphorylate β-catenin for degradation Signup and view all the answers

What occurs during the initial steps of the RTK pathway when FGFs bind to RTKs?

RTKs undergo phosphorylation to activate signaling. Signup and view all the answers

What occurs when Wnt binds to its receptors Frizzled and LRP5/6?

β-catenin is released from degradation Signup and view all the answers

Which part of the Wnt pathway is essential for the activation of target genes?

The formation of the Wnt-receptor complex Signup and view all the answers

How does the signaling range of paracrine factors compare to that of endocrine signaling?

Paracrine factors act over much shorter distances compared to endocrine factors. Signup and view all the answers

What is the result of axin and GSK3 binding to the Wnt-receptor complex?

Promotion of LEF/TCF transcription factor activity Signup and view all the answers

Which statement about regional specificity in cutaneous structure formation is true?

The same epithelium can form diverse structures based on mesenchymal origin. Signup and view all the answers

Which of the following is the correct sequence of events following FGF binding to RTKs?

Dimerization, phosphorylation, and activation of downstream signaling. Signup and view all the answers

What happens to β-catenin when Wnt is not present in the signaling pathway?

It is targeted for degradation Signup and view all the answers

What characterizes the interaction of mesenchyme with the overlying epithelium?

Mesenchyme can influence epithelial responses across species boundaries. Signup and view all the answers

Which of the following is NOT a key component of the Wnt signaling pathway?

Akt Signup and view all the answers

What is the role of LEF/TCF transcription factors in the Wnt signaling pathway?

They regulate the transcription of Wnt-responsive genes Signup and view all the answers

Which ions are released in the Wnt/Calcium signaling pathway to activate cellular responses?

Calcium ions Signup and view all the answers

What initiates the cellular responses in the Wnt signaling pathway upon ligand binding?

Formation of a multimeric receptor complex Signup and view all the answers

Which process is directly influenced by the Wnt signaling pathway?

Transcriptional activation of target genes Signup and view all the answers

Study Notes

Fertilization and Development

Fertilization is the fusion of sperm and egg (gametes) to form a single cell (zygote).
The zygote undergoes mitosis to become a blastula and then morphogenetic movements to become a gastrula.
The dividing cells of a fertilized egg form three distinct embryonic germ layers: ectoderm (outer layer), mesoderm (middle layer), and endoderm (internal layer).

Differential Gene Expression

Cells with the same genes can express different sets of genes based on their specific functions.
Different proteins produced due to differential gene expression give rise to different cell types.
The percentage of the genome expressed in a cell varies, and a portion of the RNA synthesized in each cell is unique.

Genomic Equivalence

All somatic cells in an organism have the same DNA as the zygote due to mitosis.
Each somatic cell nucleus has the same number of chromosomes and genes as other somatic nuclei.
The nucleus of a somatic cell contains all necessary information to generate all cells in the body, as demonstrated by somatic cell nuclear transfer.

Somatic Cell Nuclear Transfer

Somatic cell nuclear transfer involves transplanting the nucleus of a differentiated cell into an enucleated egg.
The transplanted nucleus can direct the development of the organism, as shown by the cloning of Dolly the sheep.

Three Postulates of Differential Gene Expression

The DNA of all differentiated cells is identical.
Unused genes in differentiated cells are neither destroyed nor mutated.
Specific gene activation and inactivation define the identity of a cell.

Levels of Control of Gene Expression

Transcription:
- RNA polymerase binds to DNA to initiate transcription.
- Enhancers/activators and silencers/repressors regulate transcription.
- Chromatin structure (histone modification, DNA methylation/demethylation) influences gene accessibility.
RNA Processing:
- mRNA is modified through splicing and transported out of the nucleus.
Translation:
- Some mRNA may be degraded or not translated.
- Differential mRNA stability, selective inhibition/activation, localization, and RNA-induced silencing regulate translation.
- Cytoplasmic localization controls RNA translation.
Post-Translational Control:
- Modifications like cleavage, transport, subunit assembly, ion binding, and covalent modification (phosphorylation, acetylation, glycosylation) control protein activity.

Transcription Factors

Transcription factors (TFs) are proteins that bind to DNA sequences (cis-acting elements).
TFs influence RNA polymerase's ability to transcribe genes.
General TFs are required for RNA polymerase binding to the promoter, while regulatory TFs control the rate of transcription by binding to enhancers or silencers.
Activators increase transcription, while repressors decrease transcription.
Different sets of TFs in a cell determine which genes are expressed.

RNA Processing

RNA processing involves modifications to mRNA before translation.
miRNA (microRNA) and siRNA (small interfering RNA) are involved in RNA silencing.
miRNA and siRNA associate with DICER and RISC complexes to regulate mRNA degradation and translation.
miRNAs can target hundreds of endogenous mRNAs with imprecise matching, allowing for broader regulation.
siRNAs have more precise targeting, specifically targeting specific sequences.
miRNA plays a role in the degradation of maternal mRNAs during gastrulation, allowing for the expression of zygotic genes.

Early Stages of Embryonic Development

After fertilization, a diploid zygote nucleus is formed via the fusion of parental gamete nuclei.
Nine rounds of nuclear divisions result in a multinucleated syncytium (single cell with many nuclei within a common cytoplasm).
About 2.5 hours after fertilization, nuclei migrate to the periphery of the egg and undergo four further divisions.
Pole cells (precursors of germ cells) form at the posterior pole.
Around 3 hours after fertilization, nuclei become enclosed in membranes, forming a single layer of cells over the embryo surface, known as the syncytial blastoderm.
About 10 hours after fertilization, segmentation pattern is clearly established: T1-T3 for thoracic segments, A1-A8 for abdominal segments, Acron for head-forming region.

Maternal Effect Genes

Genes expressed in the mother that direct the early development of the offspring.
Encode transcription factors, receptors, and proteins that regulate the expression of zygotic genes.

Egg Chamber of Drosophila

Oogonium undergoes four rounds of mitotic division with incomplete cytokinesis, producing 16 interconnected germline cells (15 nurse cells and 1 oocyte).
Nurse cells produce mRNAs and proteins that are transported to the developing oocyte.
Posterior follicle cells send signals back to the oocyte to organize its microtubules.

Maternal Protein Gradient

Initiates the formation of the anterior-posterior axis.
Maternal effect: The offspring's phenotype for a trait is controlled by nuclear gene products in the unfertilized egg.
Two maternal mRNAs / morphogens (determine cell fate by concentration) initiate formation of the A-P axis:
- Bicoid mRNAs: head/anterior morphogen actively transported along microtubules to the anterior end of the oocyte by Dynein protein.
- Nanos mRNAs: tail/posterior morphogen diffuse passively to the posterior pole of the oocyte and anchored by proteins.

Terminal Genes

Encode proteins that generate the unsegmented extremities.
- Acron: terminal portion of the head.
- Telson: extreme posterior end (tail).

Zygotic Genes

Genes transcribed in the embryonic nuclei formed after fertilization.
Differential transcription is regulated by the distribution of maternal effect proteins.

Segmentation Genes

Hunchback and caudal mRNAs are regulated by bicoid and nanos proteins:
- Hunchback mRNA: Expression activated by bicoid protein, translation inhibited by nanos protein.
- Caudal mRNA: Translation inhibited by bicoid protein; activates zygotic genes important in the formation of the abdomen.

Homeotic Genes (Hox)

Control the identity of segments and their development into various structures.
Different Hox genes are expressed in different regions of the embryo, defining the identity of each segment.
Mutations in Hox genes can cause dramatic transformations in body structures.

Progressive Restriction of Cell Fate

As development proceeds, the number of potential fates for each cell decreases.
This is primarily regulated by the expression of different genes at different times and places.
This process ensures the proper development of body plan.

Hedgehog (Hh) Signaling Pathway

Hh proteins are paracrine factors involved in regulating Gli, a transcription factor.
The Hh pathway occurs in the primary cilium of vertebrate cells and plays a crucial role in signaling and development.
Cholesterol is essential for proper Hh secretion. If cholesterol levels are low, Hh diffuses too quickly before binding to its receptor.

Examples of Hh proteins and their Functions

Sonic hedgehog (shh): Involved in motor neuron development, feather pattern formation, and various other developmental processes.

Developmental Events Where Hh Proteins Are Active:

Limb patterning
Neural differentiation and pathfinding
Retinal and pancreas development
Cranial morphogenesis
Feather formation in the chick embryo
Hair formation in mammals
Skin cancer (Hh pathway misregulation)

Wnt/β-catenin Signaling Pathway:

Regulates cell division, shape, and migration.
When Wnt is present:
- Wnt binds to its receptors (Frizzled and LRP5/6), forming a multimeric complex.
- This complex binds to axin and GSK3, preventing them from degrading β-catenin.
- Accumulated β-catenin enters the nucleus and activates Wnt-responsive genes.

Fibroblast Growth Factor (FGF) Family:

FGFs are paracrine factors activating the receptor tyrosine kinase (RTK) pathway.
RTKs have an extracellular domain that binds to ligands and an intracellular domain with kinase activity.

Types of FGFs and Their Functions:

FGF1 (acidic FGF): Regeneration and tissue homeostasis.
FGF2 (basic FGF): Blood vessel formation.
FGF7 (keratinocyte growth factor): Skin development.
FGF8: Segmentation, limb development, and lens induction.

Notch Signaling Pathway:

Regulates cell fate decisions in development.

Pathway When Hh Is Not Present:

Patched (Hh receptor) is not activated, inhibiting the GAP protein, which keeps Ras active for a longer time.
Smoothened is inhibited, leading to the retention of the Gli/Cubitus interruptus transcription factor in the cytoplasm.
Gli is cleaved and acts as a repressor, inhibiting transcription.

Pathway When Hh Is Present:

Hh binds to Patched and is endocytosed, degrading Patched and allowing Smoothened to become active.
Active Smoothened inactivates cleavage proteins, releasing Gli from microtubules.
Gli enters the nucleus and acts as an activator, promoting transcription.

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Description

Explore the process of fertilization and the developmental stages that follow, including the formation of the zygote and differentiation of germ layers. Understand how differential gene expression leads to the variety of cell types despite genomic equivalence in somatic cells.