D2.2  Gene Expression

Theme:  Continuity and Change

Nearly every cell in a multicellular organism contains the exact same set of genetic instructions, regardless of its function.

• DNA consistency ensures that the organism’s basic biological identity is preserved throughout its lifespan and across all its tissues.
• Housekeeping genes are expressed continuously in all cells. These code for essential functions like the enzymes used in cell respiration or the structure of the ribosome. 
• While most epigenetic tags are removed during reproduction, some marks can be passed from parent to offspring. This preserved environmental "memory" for the next generation without changing the DNA sequence itself.

Cell specialize and respond to its environment by altering its protein profile. 

• By using transcription factors to "turn on" or "turn off" specific genes, a generic stem cell changes into a specialized cell. 
• Adding methyl groups to DNA usually "silences" genes, causing a long-term change in expression. 
• Adding acetyl groups to histone tails neutralizes their positive charge, causing the DNA to wrap more loosely. This structural change allows RNA polymerase to access genes, activating transcription.
• The phenotype of an organism can change in response to external stimuli without any change to the genotype. 

Guiding Questions:  
Guiding questions help students view the content of the syllabus through the conceptual lenses of both the themes and the levels of biological organization.

• How is gene expression changed in a cell?
• How can patterns of gene expression be conserved through inheritance?

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​Linking Questions:  
Linking questions strengthen students’ understanding by making connections between topics.  The ideal outcome of the linking questions is networked knowledge.

• What mechanisms are there for inhibition in biological systems? 
• In what ways does the environment stimulate diversification? 

Key Terms to Know: all are higher level only

Air Pollution
Amino Acid
Base Sequence
Cystine
Differentiation
Deoxyribonucleic Acid (DNA)
Enhancer
Epigenetic
Epigenetic Tag
Gene Expression
Genome
Genotype

Histone
Hormone
Lactose
Liger
Meiosis
Methyl Tag
Methylation
Mitosis
Monozygotic Twin
mRNA 
Multicellular
Nuclease

Nucleosome
Ovum
Phenotype
Promoter
Protein
Proteome
Sperm
Tigon
Transcription
Transcription Factor
Transcriptome
Tryptophan

AHL ​​​​​​​D2.2.1— Gene expression as the mechanism by which information in genes has effects on the phenotype.

• Define genotype and phenotype.
• Define gene expression.  
• List the most common stages in the process of gene expression.​

AHL ​​​​​​​D2.2.2— Regulation of transcription by proteins that bind to specific base sequences in DNA. 

• Explain the regulation of transcription by a transcription factor binding to the promoter sequence of DNA.
• Compare the function of enhancer and silencer DNA sequences. 
• Compare the function of repressor and activator transcription factors.

AHL ​​​​​​​D2.2.3-- Control of the degradation of mRNA as a means of regulating translation.

• Outline how the length of the poly-A tail on mRNA impacts degradation of mRNA by nucleases.
• Outline how the degradation of mRNA is a mechanism of regulating translation.

AHL ​​​​​​​D2.2.4- Epigenesis as the development of patterns of differentiation in the cells of a multicellular organism.  

• Define epigenesis and epigenome.​

AHL ​​​​​​​D2.2.5— Differences between the genome, transcriptome and proteome of individual cells.

• Compare the genome, transcriptome and proteome.​

AHL ​​​​​​​D2.2.6- Methylation of the promoter and histones in nucleosomes as examples of epigenetic tags.

• Describe the impact of methylation of DNA in the promoter of DNA on gene expression.
• Describe the impact of acetylation and methylation of histone proteins on gene expression.

 AHL ​​​​​​​​​​​D2.2.7- Epigenetic inheritance through heritable changes to gene expression.

• Describe the inheritance of epigenetic tags in differentiated cells of a multicellular organism.
• Discuss the consequences of reprogramming and imprinting of epigenetic tags in haploid gametes.

AHL ​​​​​​​D2.2.8- Examples of environmental effects on gene expression in cells and organisms.

• Outline the impact of air pollution on the epigenetic regulation of genes associated with the immune response. 

AHL ​​​​​​​D2.2.9- Consequences of removal of most but not all epigenetic tags from the ovum and sperm.

• Discuss how imprinting of epigenetic tags impacts gene expression in a diploid cell.
• Outline the epigenetic origins in the difference in size between tigons and ligers (lion–tiger hybrids).

AHL ​​​​​​​D2.2.10- Monozygotic twin studies.

• Explain the reason why monozygotic twin studies are often used to measure the impact of the environment on gene expression.​

AHL ​​​​​​​D2.2.11- External factors impacting the pattern of gene expression.

• Outline the mechanism by which the presence of lactose regulates the expression of genes related to digestion and use of lactose in E. coli. 
• Outline the mechanism by which the presence of oestrogen in a cell’s environment regulates the expression of genes related to endometrium development and maintenance during the uterine cycle.  ​