D2.2 Gene expression (HL) Notes

D2.2.1 gene expression


Gene Expression Determines How Genes Effect the Phenotype

Definition

PhenotypeThe phenotype of an organism refers to its observable traits or characteristics, which result from the interaction between its genetic makeup (genotype) and the environment.

  1. Imagine two siblings with the same genes: one spends all day outdoors in the sun, while the other stays inside.

  2. Over time, one develops a tan while the other doesn't.

  3. Even though their DNA is the same, differences in gene expression, how genetic instructions are activated respond to their environments, shaping their traits.

Tip

Phenotype = Genotype + Environment

Gene Expression Occurs in Three Main Stages, Linking Genes to the Phenotype

  1. Gene expression is the mechanism that bridges genotype (genetic code) and phenotype(observable traits).

  2. It involves three key stages: transcription, translation, and protein function.

1. Transcription: DNA to RNA

  1. The first step in gene expression is transcription, where a specific gene is "read" and converted into messenger RNA (mRNA).

  2. This process occurs in the nucleus and involves three sub-stages:

    1. Initiation

      1. The enzyme RNA polymerase binds to a specific region of DNA called the promoter.

      2. The promoter acts as a "start signal," marking where transcription should begin.

    1. Elongation

      1. RNA polymerase unwinds the DNA and synthesizes a complementary RNA strand using one strand of DNA as a template.

      2. The RNA strand grows in the 5' to 3' direction, matching DNA bases with RNA bases:

      3. Adenine (A) pairs with Uracil (U) in RNA.

      4. Thymine (T) pairs with Adenine (A).

    1. Termination

      1. Transcription ends when RNA polymerase reaches a termination sequence on the DNA.

      2. The newly synthesized mRNA detaches and undergoes processing:

      3. A 5' cap and a poly-A tail are added to protect the mRNA and aid in its transport.

      4. Non-coding regions (introns) are removed through splicing, leaving only coding regions (exons).

Tip

Promoters often contain conserved sequences like the TATA box, which help RNA polymerase recognize the starting point.

Example

If the DNA template reads TACG, the RNA strand will read AUGC.

2. Translation: RNA to Protein

  1. The next step, translation, occurs in the cytoplasm on ribosomes, where the mRNA sequence is decoded to produce a polypeptide chain (a sequence of amino acids).

  2. Translation involves three main stages:

    1. Initiation

      1. The ribosome binds to the start codon (AUG) on the mRNA, which codes for methionine.

      2. Transfer RNA (tRNA) molecules carry amino acids to the ribosome, matching their anticodons with the mRNA codons.

    1. Elongation

      1. The ribosome moves along the mRNA, adding amino acids to the growingpolypeptide chain.

      2. Each tRNA delivers the correct amino acid by matching its anticodon with the codon on the mRNA.

    1. Termination

      1. Translation ends when the ribosome reaches a stop codon (UAA, UAG, or UGA).

      2. The ribosome releases the completed polypeptide, which detaches from the mRNA.

Analogy

Think of tRNA as a delivery truck, bringing the correct amino acid to the ribosome based on the mRNA "address."

Example

If the mRNA sequence is AUG-CGU-AAA, the corresponding amino acids will be methionine-arginine-lysine.

Warning

  • Don't confuse codons (on mRNA) with anticodons (on tRNA).

  • They are complementary, not identical.

3. Protein Function: The Phenotypic Effect

  1. Once translated, the polypeptide folds into a functional protein, which can perform various roles:

    1. Structural Proteins: Form part of the organism's physical structure (e.g., collagen in skin).

    2. Enzymes: Catalyze biochemical reactions (e.g., lactase breaks down lactose).

    3. Transport Proteins: Move molecules across cell membranes (e.g., hemoglobin carries oxygen).

Example

The ability to digest lactose depends on the enzyme lactase. If the gene coding for lactase is expressed, lactose can be broken down into glucose and galactose. Without this expression, lactose intolerance occurs.

Self Review

  • Can you explain how a change in the DNA sequence might affect the protein produced?

  • What impact could this have on the phenotype?

Gene Expression Regulates Phenotype

  1. The phenotype of an organism is shaped by which genes are expressed, where, and when.

  2. Not all genes are active in every cell, gene expression is highly regulated to ensure the appropriate proteins are produced under the right conditions.

Why Is Gene Expression Important?

  1. Adaptation to Environment: Gene expression responds to external factors like temperature, light, or diet. For instance, melanin production in skin increases with sun exposure.

  2. Cell Differentiation: Specialized cells express unique genes to perform distinct functions (e.g., nerve cells vs. muscle cells).

  3. Health and Disease: Abnormal gene expression can lead to diseases, such as cancer or metabolic disorders.

Gene Expression Drives Diversity

  1. Gene expression demonstrates how organisms with the same genetic material can exhibit diverse traits.

  2. For example:

    1. Identical twins may develop differently due to environmental factors influencing gene expression.

    2. A single organism can have cells as varied as neurons and liver cells, each expressing a unique subset of genes.

Example

  • In humans, the gene for melanin (a pigment) is expressed in skin cells but not in liver cells.

  • This selective expression determines skin color.

Tok

  • How does the regulation of gene expression challenge the idea that DNA alone determines an organism's traits?

  • What role might the environment play?


D2.2.2 regulation of transcription by proteins


Why Transcription Needs To Be Regulated

  1. Every cell in an organism contains the same genome, but different cells express different genes.

  2. A muscle cell produces muscle proteins; a nerve cell produces neurotransmitters.

  3. This specificity is achieved by regulating transcription, controlling which genes are transcribed into mRNA.

  4. Proteins bind to specific DNA sequences to turn genes on or off.

Key DNA Sequences That Control Transcription

1. Promoters Are The Starting Point

Definition

Promoter A promoter is a specific DNA sequence located near the start of a gene. It acts as a binding site for RNA polymerase and other proteins needed to initiate transcription.

  1. The promoter marks where transcription begins.

  2. RNA polymerase binds to the promoter to start transcribing the gene.

  3. A common promoter sequence in eukaryotes is the TATA box (TATAAA), which helps position RNA polymerase accurately.

Analogy

The promoter is like a runway where RNA polymerase "lands" to begin the transcription process.

2. Enhancers Are Sequences That Increase Transcription

Definition

Enhancers Enhancers are DNA sequences that increase the rate of transcription when activator proteins bind to them.

  1. Enhancers can be located far away from the gene they regulate, sometimes thousands of base pairs upstream or downstream.

  2. They work by DNA looping, bringing activator proteins bound to the enhancer close to the promoter.

  3. This increases RNA polymerase activity and boosts transcription.

Example

The β-globin gene, responsible for hemoglobin production, is regulated by enhancers located over 50,000 base pairs away.

3. Silencers Are Sequences That Decrease Transcription

Definition

SilencerA DNA sequence that decreases the rate of transcription when repressor proteins bind to it.

  1. Silencers work similarly to enhancers but have the opposite effect, they reduce or block transcription.

  2. Repressor proteins bind to silencers and prevent RNA polymerase from effectively transcribing the gene.

Key Proteins That Regulate Transcription

Definition

Transcription factorsTranscription factors are proteins that control transcription by binding to specific DNA sequences.

  1. Transcription factors can be:

    1. Activators:

      1. Bind to enhancers.

      2. Increase the rate of transcription by helping RNA polymerase bind to the promoter or work more efficiently.

    1. Repressors:

      1. Bind to silencers.

      2. Decrease or block transcription by preventing RNA polymerase from accessing the promoter or functioning properly.

Tip

  • Enhancers and silencers are regulatory sequences, not part of the gene itself.

  • They control when and how much the gene is transcribed.

Self Review

  • What is a promoter and where is it located?

  • What is the role of RNA polymerase in transcription?

  • What is an enhancer and how does it increase transcription?

  • What is a silencer?

  • What are transcription factors?

  • What is the difference between activators and repressors?


D2.2.3 control of the degradation of mrna


mRNA Degradation Ensures Proteins Are Produced in The Correct Amounts and At The Right Time

Definition

mRNA degradationmRNA degradation is the process by which mRNA molecules are broken down after they have served their purpose in protein synthesis.

  1. This process is tightly regulated to control how long an mRNA molecule remains active in the cell.

  2. This mechanism allows cells to adapt protein synthesis to their immediate needs, ensuring efficiency and precision in response to internal and external cues.

The Process of Gene Regulation

Hint

Think of this process as a project manager determining which priorities are most urgent.

The Timing of mRNA Degradation is Very Important

  1. The lifespan of mRNA molecules plays a critical role in regulating protein production.

  2. Some mRNAs persist only for minutes to allow rapid changes in protein levels, while others last for days to ensure sustained protein synthesis.

  3. This controlled degradation enables the cell to:

    1. Regulate Protein Production: Quickly degrade mRNA for proteins that are no longer needed, preventing overproduction.

    2. Adapt to Environmental Changes: Degrade mRNA rapidly in response to changingconditions, such as stress or nutrient availability.

    3. Conserve Energy: Stop unnecessary protein synthesis to save cellular resources.

Example

Short-lived mRNAs often encode proteins involved in immediate responses, like cell signaling, while longer-lasting mRNAs may encode structural or housekeeping proteins.

Key Mechanisms of mRNA Stability and Degradation

1. Poly-A Tail is A Timer for mRNA Lifespan

  1. After transcription, a poly-A tail (a sequence of adenine nucleotides) is added to the 3′ end of the mRNA.

  2. The poly-A tail serves as a protective barrier against degradation, but over time, it is gradually shortened by enzymes called deadenylases.

    1. When the poly-A tail becomes too short, the mRNA is marked for degradation.

    2. The length of the poly-A tail helps determine whether an mRNA persists for minutes or days, acting like a "timer" for its lifespan.

Tip

  • Think of the poly-A tail as a protective cap.

  • The longer the cap, the longer the mRNA can survive and be used for protein synthesis.

2. Decapping and Exonuclease Activity

  1. Once the poly-A tail is shortened, the 5′ cap of the mRNA is removed in a process called decapping.

  2. This exposes the mRNA to nucleases, which break it down from both ends:

    1. 5′ to 3′ Exonuclease: Degrades the mRNA from the exposed 5′ end.

    2. 3′ to 5′ Exosome Complex: Breaks down the remaining RNA from the 3′ end.

Poly-A tail, mRNA regulation

Examples of mRNA Lifespan in Action

  1. Short-Lived mRNAs (Minutes): Encode proteins for rapid, transient responses, such as those involved in signalling pathways.

  2. Long-Lived mRNAs (Days): Encode proteins needed for sustained cellular functions, such as structural components or enzymes.

Example

  • Cyclins (proteins that regulate the cell cycle) are produced only briefly during specific phases of cell division.

  • Casein mRNA (a milk protein) persists during lactation to ensure continuous milk production.


D2.2.4 epigenesis


Epigenesis is The Development of Patterns of Differentiation in Multicellular Organisms

  1. Consider an artist who gradually transforms a blank canvas into a complex painting.

  2. Similarly, a multicellular organism develops from a single, undifferentiated cell into a highly organized system of specialized cells, tissues, and organs.

  3. This process is guided by epigenesis, where gene expression is regulated without altering the DNA sequence itself.

What Is Epigenesis?

Definition

EpigenesisEpigenesis refers to the development of patterns of differentiation in cells, enabling them to perform specific functions.

  1. This is achieved through epigenetic modifications, which act as chemical "tags" on DNA or histone proteins, influencing which genes are active or silent.

Note

  • Epigenetic changes do not alter the DNA base sequence.

  • Instead, they modify how the genetic code is read and expressed.

How Epigenesis Works

1. DNA Methylation

  1. Methylation involves adding a methyl group () to cytosine bases in DNA.

  2. This modification often occurs near promoter regions, silencing gene expression by blocking RNA polymerase binding.

Example

In mammals, methylation of genes involved in cell division helps prevent uncontrolled growth, reducing the risk of cancer.

2. Histone Modification

  1. DNA is wrapped around histone proteins, forming nucleosomes.

  2. Modifications to histone tails, such as acetylation or methylation, affect how tightly DNA is packed.

    1. Acetylation: Loosens DNA, allowing gene expression.

    2. Methylation: Can either activate or repress genes, depending on the context.

Tip

Think of histone modifications as a volume knob for gene expression turning it up or down without changing the underlying DNA sequence.

Epigenesis and Cell Differentiation

  1. During development, cells become specialized through differentiation.

  2. This process is guided by epigenetic modifications, ensuring that only the necessary genes are expressed in each cell type.

Example

In muscle cells, genes for muscle proteins like actin and myosin are activated, while genes for neuron-specific proteins are silenced.

Analogy

  • Think of differentiation as a library where each cell is a librarian with access to specific books (genes) needed for its role.

  • Epigenetic tags determine which books are open and which are locked away.

Epigenesis vs. Genotype: Altering Phenotype, Not DNA

  1. A key feature of epigenesis is that it alters the phenotype (observable traits) without changing the genotype (DNA sequence).

  2. This distinction is crucial because it allows organisms to adapt to environmental changes without permanent genetic alterations.

Example

Identical twins share the same DNA but can develop different traits due to epigenetic differences influenced by their environments.

Why Epigenesis Matters

  1. Epigenesis enables organisms to adapt to environmental cues during development, enhancing their ability to respond to changing conditions.

  2. Although most epigenetic tags are erased during the formation of sperm and eggs, some persist, allowing environmentally influenced traits to be inherited by the next generation.

  3. Epigenetic changes are also associated with diseases like cancer, diabetes, and neurological disorders, offering insights that could pave the way for targeted therapies.

Example

Plants exposed to drought may develop deeper root systems due to epigenetic changes.

Example

The Dutch Hunger Winter study showed that children of mothers who experienced famine during pregnancy had altered glucose metabolism, likely due to epigenetic inheritance.

Tok

  • How does the concept of epigenetic inheritance challenge traditional views of genetics?

  • Does it suggest that experiences can shape future generations?

Challenges and Limitations of Epigenesis

  1. Reversibility: Unlike genetic mutations, epigenetic changes are often reversible, making them difficult to study over long periods.

  2. Complexity: The interplay between different epigenetic modifications is intricate and not yet fully understood.

  3. Environmental Influence: While epigenetic changes allow adaptability, they can also make organisms vulnerable to harmful environmental factors, such as pollution or poor diet.

Tok

To what extent do you think environmental factors should be considered in discussions about inheritance and evolution? How might this influence ethical decisions in fields like medicine or agriculture?


D2.2.5 differences between the genome, transcriptome and proteome of individual cells


Comparing the Genome, Transcriptome, and Proteome

  1. Every cell in an organism contains the same genome (complete set of genetic instructions).

  2. However, no cell expresses all of its genes.

Tip

The pattern of gene expression in a cell determines how it differentiates.

The Genome, Transcriptome, and Proteome

  1. Genome: The complete set of DNA in a cell (same in all cells).

  2. Transcriptome: The complete set of RNA molecules transcribed from the genome at a specific time (varies between cells).

  3. Proteome: The complete set of proteins produced by a cell at a specific time (varies between cells).

Feature

Genome

Transcriptome

Proteome

Definition

All genetic material in a cell

All RNA molecules transcribed

All proteins produced

Variability

Consistent across cells

Varies between cells and over time

Varies between cells and over time

Components

DNA (coding and non-coding)

mRNA, rRNA, tRNA, etc.

Functional proteins

Stability

Stable throughout life

Dynamic and responsive

Dynamic and responsive

Function

Blueprint for all cellular functions

Reflects active gene expression

Determines cell function

Why Do Cells Differ?

  1. Selective gene expression: Cells only transcribe and translate the genes needed for their specific functions.

  2. Cell differentiation: Specialized cells (e.g., muscle cells, neurons) express unique sets of genes to perform specific roles.

Self Review

Why do all cells in an organism have the same genome, yet differ in structure and function?

Explain how the pattern of gene expression determines cell differentiation.


D2.2.6 methylation of the promoter and histones in nucleosomes as examples of epigenetic tags


Epigenetic Tags Lead To The Methylation of Promoters and Histones

  1. Gene expression is more than just the DNA sequence and also about how that sequence is regulated.

  2. This regulation is influenced by epigenetic tags, chemical modifications that alter gene activity without changing the DNA sequence itself.

  3. Two key examples are the methylation of promoters and histones.

Methylation Of DNA Promoters Represses Transcription

Definition

DNA methylationDNA methylation involves the addition of a methyl group (-CH₃) to a cytosine base in DNA.

  1. This modification often occurs in regions called CpG islands, which are rich in cytosine-guanine pairs and are frequently found near promoters.

  2. It affects transcription by:

    1. Blocking transcription factor binding: Methylation of the promoter region can prevent RNA polymerase and other transcription factors from binding to the DNA, blocking the initiation of transcription.

    2. Recruiting repressor proteins: Methylated DNA can attract proteins that compact the chromatin structure, making it inaccessible to the transcription machinery.

Example

In cancer cells, hypermethylation of tumor suppressor gene promoters can silence these critical genes, allowing uncontrolled cell growth.

Methylation of Histones Modulates Gene Expression

Definition

HistoneA protein that provides structural support for DNA in eukaryotic cells. Histones are positively charged, allowing them to interact with negatively charged DNA, helping to compact it into a condensed form called chromatin.

Methylation can occur on specific amino acids in the tails of histone proteins (e.g., lysine or arginine), either activating or repressing transcription, depending on the specific site methylated.

  1. Repressive Methylation

    1. Methylation at certain sites (e.g., H3K9 or H3K27) leads to a tightly packed chromatin structure, making DNA inaccessible to transcription factors.

    2. This represses transcription.

  1. Activating Methylation

    1. Methylation at other sites (e.g., H3K4) results in a looser chromatin configuration, facilitating transcription.

    2. This activates transcription.

Tip

  • DNA methylation of promoters: Represses transcription of the gene downstream.

  • Histone methylation: Can either repress or activate transcription, depending on the site methylated.

  • You are not required to know the detailed mechanisms of how methylation achieves these effects.

Analogy

  • Think of histone methylation as a dimmer switch for gene expression.

  • Depending on where the methyl group is added, it can either brighten (activate) or dim (repress) the transcriptional activity.

Self Review

  • How does methylation of a promoter region affect gene expression?

  • What is the difference between repressive and activating histone methylation?

  • How can environmental factors influence epigenetic tags?


D2.2.7 epigenetic inheritance through heritable changes to gene expression


Epigenetic Tags Are Chemical Modifications That Determine Which Genes Are Active or Silent

  1. Imagine a library where some books are locked away, while others are open for reading.

  2. The books represent genes, and the locks are epigenetic tags.

  3. Unlike changes to the DNA sequence, these tags can be influenced by the environmentand sometimes passed on to future generations.

  4. Two primary mechanisms regulate these tags: DNA methylation and histone modification.

Definition

Epigenetic tag An epigenetic tag is a chemical modification to DNA or histone proteins that regulates gene expression without altering the underlying DNA sequence.

Tip

Epigenetic tags act as "instructions" that determine whether specific genes are turned on or off.

Key Mechanisms of Epigenetic Regulation

1. DNA Methylation

  1. DNA methylation involves the addition of a methyl group () to cytosine bases in DNA, typically occurring in CpG islands near promoter regions.

  2. This process silences genes by:

    1. Blocking Transcription Factors and RNA Polymerase: Methylation prevents these molecules from binding to the promoter, halting transcription of the downstream gene.

    2. Recruiting Repressor Proteins: Methylated DNA attracts proteins that compact chromatin, making it inaccessible for transcription.

Example

Tumor suppressor genes are often silenced by hypermethylation in cancer cells.

2. Histone Modifications

  1. Chemical changes (e.g., acetylation, methylation) to histone proteins.

  2. Modify how tightly DNA is wrapped around histones, affecting gene accessibility.

    1. Acetylation: Loosens chromatin to activate transcription.

    2. Methylation: Can activate or repress transcription depending on the site.

Analogy

  • Again, think of histone methylation as a dimmer switch.

  • Depending on where the methyl group is added, it can either brighten (activate) or dim (repress) gene expression.

Example

In embryonic stem cells, activating methylation marks keep pluripotency genes active, while repressive marks silence differentiation genes until they are needed.

Epigenetic Tags Are Inherited During Mitosis and Meiosis

1. Inheritance During Mitosis

  1. When cells divide, their epigenetic patterns are passed to daughter cells.

  2. This ensures that specialized cells, like muscle or nerve cells, maintain their unique functions over an organism's lifetime.

Example

Skin cells continue to produce keratin because epigenetic tags keep keratin genes active.

Hint

Keratin is a protein that helps form hair, nails and your skin's outer layer (epidermis), supporting your skin, healing wounds, and keeping your nails and hair healthy.

2. Inheritance During Meiosis

  1. Most epigenetic tags are erased during the formation of sperm and eggs, a process that resets the genome for the next generation.

  2. However, some tags persist, allowing transgenerational epigenetic inheritance.

Hint

  • Epigenetic mechanisms enable organisms to adapt to environmental changes without altering their DNA.

  • This is referred to as phenotypic plasticity.

Limitations and Challenges of Epigenetic Inheritance

  1. Reversibility: Unlike DNA mutations, epigenetic changes are often reversible, which maylimit their long-term impact.

  2. Incomplete Erasure: While most tags are erased during gamete formation, some escapethis process, leading to unpredictable transgenerational effects.

  3. Complexity: The exact mechanisms that determine which tags persist during inheritance remain poorly understood, making it a challenging field of study.

Example

Enzymes like demethylases can remove methyl groups, restoring gene activity.

Real-World Applications

  1. Medicine: Epigenetic therapies target abnormal DNA methylation and histone modifications to treat diseases like cancer and neurological disorders.

  2. Agriculture: Understanding epigenetic inheritance can aid in developing crops that are more resilient to environmental stressors, such as drought or salinity.

Reflection and Review

  1. Epigenetic inheritance highlights the intricate relationship between genetics and the environment.

  2. It shows how phenotypic changes, influenced by epigenetic tags like DNA methylationand histone modifications, can be passed to daughter cells and, in some cases, offspring.

  3. While much is still unknown, this field offers groundbreaking potential for understanding biology, improving human health, and addressing global challenges.

Tok

  • How does the concept of epigenetic inheritance challenge traditional views of inheritance?

  • Could this influence ethical decisions in fields like genetic engineering or medicine?


D2.2.8 examples of environmental effects on gene expression in cells and organisms


DNA Methylation is a Response to Air Pollution

  1. Air pollution, particularly fine particulate matter (PM2.5) and harmful gases like nitrogen dioxide (NO₂), has been shown to alter DNA methylation patterns.

  2. Exposure to these pollutants triggers oxidative stress and inflammation, leading to epigenetic modifications in key genes.

Key Findings from Research

Hypermethylation of Tumor Suppressor Genes

  1. Studies have shown that long-term exposure to air pollution can cause hypermethylation of tumor suppressor genes, silencing their expression.

  2. This increases the risk of developing cancers such as lung cancer.

Hypomethylation of Pro-Inflammatory Genes

  1. Inflammatory pathways are often activated by hypomethylation of genes involved in cytokine production.

  2. People exposed to high levels of PM2.5 show increased activity in genes associated with chronic inflammation.

  3. This is a precursor to cardiovascular and respiratory diseases.

Note

  • Children growing up in areas with high air pollution show altered methylation patterns in immune-related genes.

  • These changes increase their susceptibility to asthma by promoting an overactiveinflammatory response.

Other Examples of Environmental Effects on Gene Expression

1. Diet

  1. A diet rich in folate enhances DNA methylation, supporting normal development.

  2. Folate deficiency, on the other hand, reduces methylation, potentially leading to neural tube defects during fetal development.

Hint

  • Folate, also known as vitamin B9, is a water-soluble B vitamin that helps with cell growth and function, especially in red blood cells.

  • Foods high in folate include dark green leafy vegetables, beans, nuts, fruits, and eggs.

2. Stress

  1. Chronic stress reduces methylation in genes associated with the stress hormone receptor, leading to heightened stress responses.

  2. This has been observed in individuals who experienced trauma during childhood.

3. Temperature

  1. In certain reptiles, the temperature of the environment during egg incubation affects the methylation of sex-determining genes.

  2. This influences whether the offspring develops as male or female.

Why Do These Changes Matter?

While some changes are temporary and reversible, others may persist long-term or even be passed to the next generation throughepigenetic inheritance:

  1. Short-Term Adaptation: Epigenetic changes allow organisms to adapt to their immediate environment.

  2. Long-Term Risks: Persistent exposure to harmful environments can "lock in" harmful epigenetic changes, increasing the risk of chronic diseases like cancer, asthma, and cardiovascular disorders.

Hint

  • Recall how epigenetic inheritance allows organisms to "remember" environmental conditions experienced by previous generations.

  • This means some epigenetic tags, like DNA methylation patterns, can persistthrough meiosis and be passed on to offspring.

  • This therefore influences their gene expression and

.

Warning

  • Avoid assuming that all epigenetic changes are permanent.

  • Many are reversible and depend on ongoing environmental conditions.

Reflection and Review

  1. The environment plays a critical role in shaping gene expression through epigenetic modifications.

  2. These changes influence health, adaptability, and even traits passed to future generations.

Tok

  • How does the concept of transgenerational epigenetic inheritance challenge traditional ideas about heredity?

  • What ethical questions arise when considering the long-term effects of environmental exposures?

  • To what extent should governments prioritize environmental policies based on their impact on public health through epigenetic changes?


D2.2.9 consequences of removal of most but not all epigenetic tags from the ovum and sperm


Epigenetic Tags Are Mostly Erased During The Formation of Gametes

  1. This erasure ensures a "clean slate" for the next generation.

  2. Chemical modifications, such as DNA methylation and histone acetylation, do this by without altering the DNA sequence.

  3. However, about 1% of these tags persist, leading to genomic imprinting a process where certain genes are silenced based on their parental origin.

Definition

Genomic imprinting Genomic imprinting is a form of epigenetic inheritance, where the expression of a gene depends on whether it is inherited from the mother or the father.

Epigenetic Tags Persist To Regulate Genes Involved in Growth, Development, and Metabolism

  1. These tags ensure that imprinted genes are expressed in a parent-specific manner.

  2. The differences in ligers and tigons highlight how epigenetic tags inherited from parents can influence gene expression in offspring.

  3. In lions and tigers, these tags have evolved to suit their reproductive strategies:

    1. Lions: Males compete to sire the largest cubs, so their genes promote growth.

    2. Tigers: Females raise cubs alone, so their genes limit growth to conserve resources.

Hint

  • Think of genomic imprinting as a tug-of-war between parental genes.

  • In ligers, the father's genes win, promoting growth.

  • In tigons, the mother's genes balance the effect, resulting in a smaller size.

Tok

  • How does the concept of epigenetic inheritance challenge traditional Mendelian genetics?

  • What ethical considerations arise when studying epigenetic changes in humans?


D2.2.10 monozygotic twin studies


Monozygotic Twin Studies Are A Unique Opportunity to Explore Environmental Impacts on Gene Expression

Definition

Monozygotic twins Monozygotic twins (identical twins) arise when a single fertilized egg splits into two embryos.

  1. They share 100% of their DNA, making them ideal for studying the influence of the environment on gene expression.

  2. This is because by comparing identical twins, scientists can isolate environmentaleffects from genetic ones.

Gene Expression and the Environment

  1. Gene expression is the process by which information in DNA is used to produce proteins, which determine an organism's traits.

  2. While the genetic code remains constant, the environment can influence which genes are active or inactive.

Mechanisms of Environmental Influence

  1. Epigenetic Modifications:Chemical changes to DNA or histones that regulate gene activity without altering the genetic code.

    1. Examples include DNA methylation and histone acetylation.

  1. Transcription Factors: Proteins that bind to DNA and control the transcription of specific genes.

    1. Environmental signals can activate or deactivate these proteins.

  1. RNA Stability: Environmental factors can affect how long mRNA molecules persist, influencing protein production.

Warning

  • A common misconception is that identical twins remain genetically identical throughout their lives.

  • While their DNA sequence remains the same, epigenetic changes can lead to significant differences in gene expression.

Long-Term Effects of Environmental Exposure

  1. Lifestyle Choices: Diet, exercise, and smoking can lead to epigenetic changes that influence health outcomes.

  2. Stress and Mental Health: Chronic stress can alter gene expression related to mood and cognition.

    1. Identical twins exposed to different stress levels may show varying susceptibility to mental health disorders.

  1. Disease Susceptibility: Epigenetic changes can increase or decrease the risk of diseases like cancer or diabetes.

Example

One twin who smokes may develop lung disease, while the non-smoking twin remains healthy.

Limitations of Twin Studies

  1. Sample Size: Identical twins are rare, limiting the number of participants in studies.

  2. Complex Interactions: It can be challenging to separate the effects of specific environmental factors from genetic influences.

  3. Generalizability: Findings from twin studies may not apply to the broader population.


D2.2.11 external factors impacting the pattern of gene expression


Gene Expression Responds Dynamically to Environmental Signals

  1. This is the process where information from a gene is used to synthesize functional products, such as proteins.

  2. This then determines an organism's traits.

Hormonal Regulation: The Role of Oestrogen

Definition

HormonesHormones are chemical messengers that travel through the bloodstream to regulate various physiological processes.

  1. Oestrogen, a steroid hormone, plays a critical role in the female reproductive system, influencing the expression of specific genes.

Tip

  • Remember, oestrogen's ability to regulate gene expression is tissue-specific.

  • It affects only cells with oestrogen receptors, such as those in the uterus, breast, and bone.

Biochemical Regulation: The Lac Operon in Bacteria

  1. In prokaryotes like Escherichia coli, gene expression is tightly regulated to conserve energy and resources.

  2. A cluster of genes under the control of a single promoter, allowing coordinated regulation.

The Lac Operon is A Model of Gene Regulation

  1. The lac operon consists of three structural genes:

    1. lacZ: Encodes β-galactosidase, an enzyme that breaks down lactose into glucose and galactose.

    2. lacY: Encodes permease, a protein that transports lactose into the cell.

    3. lacA: Encodes transacetylase, an enzyme with a less well-understood role.

  1. These genes are controlled by a promoter, an operator, and a regulatory gene () that produces a repressor protein.

How the Lac Operon Works

  1. No Lactose Present:

    1. The repressor protein binds to the operator, blocking RNA polymerase from transcribing the structural genes.

    2. The operon is "off," and the enzymes for lactose metabolism are not produced.

  1. Lactose Present:

    1. Lactose acts as an inducer by binding to the repressor protein, causing it to change shape and detach from the operator.

    2. RNA polymerase can now bind to the promoter and transcribe the genes, enabling the bacterium to metabolize lactose.

Warning

  • Don't confuse the role of the repressor and RNA polymerase.

  • Remember, the repressor blocks transcription by binding to the operator.

  • RNA polymerase initiates transcription by binding to the promoter.

Why These Mechanisms Matter

  1. Efficiency: Cells conserve energy by producing proteins only when needed. For example, E. coli activates the lac operon only in the presence of lactose.

  2. Adaptation: Hormones like oestrogen enable multicellular organisms to respond to internal and external changes, such as preparing the uterus for pregnancy.

  3. Survival: These regulatory mechanisms enhance an organism's ability to survive and reproduce in changing environments.

Tok

  • How do environmental factors like hormones and nutrients challenge the idea of genetic determinism?

  • Could this influence our understanding of human traits and behaviors?

Self Review

Explain how the presence of lactose in the environment affects the expression of the lac operon in E. coli.