6.6Gene Expression and Regulation
Gene Expression and Cell Specialization — AP BIOLOGY Revision Notes & Topical Questions
5 min read 8 parts Lesson 7 of 9 1 quiz
Lesson summary
6.6 Gene Expression and Cell Specialization — Notes — AP Biology notes: Explain how transcription factors binding promoters affects gene expression and phenotype, and how cells with the same genome specialise.Genotype — the set of alleles an organism carries; phenotype — the obse…
- Differentiation
- Cytoplasmic determinant
- Pluripotent
- Pathway — an allele sets a protein sequence, the protein affects a cell process, and many processes add up to a trait
- Equation — phenotype = genotype + environment + their interaction; most traits involve several genes
- Negative regulation — regulatory molecules inhibit expression by binding DNA and blocking transcription
Section 1 of 8From genotype to phenotype
1
Genotype — the set of alleles an organism carries; phenotype — the observable characteristics produced by expression of those genes in an environment.
2
Pathway — an allele sets a protein sequence, the protein affects a cell process, and many processes add up to a trait.
3
Equation — phenotype = genotype + environment + their interaction; most traits involve several genes.
4
Combination and level — the phenotype depends on which genes are expressed and at what levels,
- which gives different proteins and controls how much of each is made.
5
Environmental influence — the environment changes gene expression or enzyme activity, not the DNA sequence (e.g. Himalayan rabbit pigment enzyme, hydrangea colour, etiolated seedlings, reptile sex, lac operon).
Section 2 of 8Regulating transcription
1
Promoters and enhancers — RNA polymerase and transcription factors bind these DNA sequences to start transcription; they can be upstream or downstream of the start site,
- and promoters are binding sites, not coding sequences.
2
Negative regulation — regulatory molecules inhibit expression by binding DNA and blocking transcription.
3
Combination — a gene is expressed only when the right combination of factors is present.
4
Small RNAs — microRNAs and siRNAs also regulate expression after transcription by binding mRNA.
Section 3 of 8Cell differentiation
1
Same genome — all body cells derive from a zygote by mitosis and contain the same DNA, and differ through differential gene expression.
2
Cell differentiation = the process by which a cell becomes specialised in structure and function by expressing a particular set of genes.
3
Unique combination — each specialised cell has its own combination of transcription factors,
- which decides which genes are transcribed and translated to tissue-specific proteins.
4
Examples — muscle cells express myosin and actin genes, red blood cell precursors express globin genes (and later lose the nucleus), pancreatic beta cells express the insulin gene,
- and a neuron switches on ion channel and neurotransmitter genes while keeping muscle genes off.
5
No gene loss — a differentiated nucleus placed into an egg with its nucleus removed can direct development of a whole organism (cloning),
- and plant cells grown in tissue culture can form whole plants.
Section 4 of 8Signals that direct differentiation and development
1
Development — a fertilised egg becomes an organism, needing the right genes at the right time and place and in the right sequence, with early regulators activating later ones.
2
Cytoplasmic determinants — maternal mRNAs and proteins unevenly distributed in the egg make daughter cells of early divisions express different genes.
3
Induction — signalling molecules from neighbouring cells trigger transduction pathways that activate transcription factors in target cells.
- Steroid hormones are lipid-soluble, cross the membrane and bind receptors that act as transcription factors.
- Water-soluble signals bind surface receptors and trigger pathways ending in phosphorylation of transcription factors.
4
Morphogen gradients — switch on different genes at different concentrations, patterning the embryo.
5
Cascades — one master regulator (e.g. MyoD) activates many downstream genes.
6
Hox genes — homeotic genes encode transcription factors that specify where body parts form along an axis; a mutation can put legs where antennae should grow in fruit flies,
- and similar Hox genes in many animals indicate common ancestry.
7
Mis-timed expression of a regulator can alter the body plan, which is why related animals differ in segment number or limb type.
8
Maintenance — transcription factors, non-coding RNAs and epigenetic marks keep a differentiated cell specialised through later divisions.
Section 5 of 8Stem cells
- Stem cells — unspecialised cells that can divide and differentiate.
| Type | Potential | Source | Example |
|---|---|---|---|
| Totipotent | All cells including placenta | Zygote, early embryo | Fertilised egg |
| Pluripotent | Almost all body cells | Embryo | Embryonic stem cells |
| Multipotent | Several related cell types | Adult tissue | Blood stem cells |
Section 6 of 8More from the specification
1Kinetic regulation of mRNA synthesis rates
- Kinetic regulation of mRNA synthesis rates epigenetic histone acetylation alters chromatin compaction, directly controlling RNA polymerase elongation rates.
2Lactose
- A disaccharide of glucose and galactose.
3Repressor
- A protein that binds the operator and stops transcription.
4Operator
- Region where the repressor binds.
5Prokaryote
- An organism whose cells have no nucleus.
6Histone
- Protein that DNA coils around.
Section 7 of 8Key terms
| Term | Meaning |
|---|---|
| Differentiation | specialisation of a cell through a particular set of expressed genes |
| Promoter | DNA sequence where RNA polymerase and transcription factors bind |
| Enhancer | regulatory sequence that can be distant from the gene |
| Cytoplasmic determinant | unevenly distributed egg molecule directing early cell fate |
| Induction | neighbour signals triggering gene expression in target cells |
| Hox gene | regulatory gene controlling body plan along an axis |
| Pluripotent | able to become almost all body cell types |
| Stem cell | unspecialised cell that can divide and differentiate |
Section 8 of 8Exam technique
1
Saying specialised cells have different DNA or lost genes — they keep the whole genome and express a subset.
2
Saying promoters code for proteins — they are binding sites for transcription factors.
3
Mixing up totipotent and pluripotent — only totipotent cells can also form placenta.
4
Saying the environment changes the DNA sequence to alter phenotype — it changes gene expression or enzyme activity.
5
Saying Hox genes code for structures directly — they code for transcription factors.
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