1. 7.2 Transcription & gene expression

2. Definition of “transcribe”
To make a written copy of

3. Application: The promoter as an example of non-coding DNA with a function
= non-coding sequence that is located near a gene
= binding site of RNA polymerase
= not transcribed into mRNA, but plays a role in transcription

5. Understanding: Gene expression is regulated by proteins that bind to specific base sequences in DNA
Gene expression = the production of a protein from genetic instructions
regulated in prokaryotes because of variations in environmental factors
E.g. Expressing the antibiotic resistance gene only in presence of antibiotic
E.g. lac operon
Operon = cluster of genes under the control of one promoter
Lac = lactose
Default condition (if no = no gene expression for enzymes that break down & uptake lactose due to repressor gene
In presence of lactose, repressor gene is deactivated, allowing for gene expression to break down the lactose

6. Lac operon in E. coli

7. Gene regulation in eukaryotes
Regulated in response to variations in environmental conditions
Critical part of cell differentiation and embryonic development
Enhancers = non-coding sequences that increase rate of transcription (Activator proteins bind to enhancers to increase trans)
Silencers = non-coding sequences that decrease rate of transcription (Repressor proteins bind to silencers to decrease trans)
Promoter-proximal elements = non-coding sequences close to promoter; initiate transcription when proteins bind to them

10. Nature vs Nurture debate
Understanding: The environment of a cell and of an organism has an impact on gene expression
Nature vs Nurture debate
Environmental factors can affect gene expression
E.g. production of melanin during exposure to sunlight
In embryonic development, embryo contains uneven distribution of chemicals called morphogens
Concentrations of morphogens affect gene expression

11. Siamese Cats!
In cats, the “C” gene codes for tyrosinase, part of production of fur color
Mutant allele “c” allows fur color production only at temps below body temp
Selective breeding of Siamese cats  c allele selected for to produce less pigment

12. Understanding: Nucleosomes help to regulate transcription in eukaryotes
Chemical modification of histone tails is important factor in determining whether a gene will be expressed or not.
Chemical mods can either activate or deactivate genes by decreasing or increasing accessibility of gene to transcription factors.
Modifications:
Acetyl group
Methyl group
Phosphate group

13. Methylation of histones

14. Gene not being expressed
Gene expression = process by which the info encoded in a gene is used to assemble a protein
Gene being expressed
Gene not being expressed

16. Skill: Analysis of changes in DNA methylation patterns (DBQ p358)
Methyl groups (CH3-) are added directly to the DNA base cytosine to help regulate gene expression.

17. Methylation Amount of DNA methylation varies during lifetime Amount of DNA methylation is affected by environment
Of histones
Of DNA
can promote or inhibit transcription
tends to decrease transcription

18. Understanding: Transcription occurs in a 5’ to 3’ direction
Transcription = synthesis of mRNA
3 stages:
Initiation
Elongation
Termination
Begins near promoter region
RNA polymerase binds to promoter, then unwinds & unzips DNA, then synthesizes RNA

19. Steps in transcription

20. Understanding: Eukaryotic cells modify mRNA after transcription (in nucleus)
Introns = intravening sequences = non-coding regions = removed!
(prokaryotes do not contain introns)
Exons = coding sequences = spliced together once introns are removed
Methyl-guanine (mG) cap added to 5’ end
Poly-A tail (AAAAAAAA) added to 3’ end

21. Understanding: Splicing of mRNA increases the number of different proteins an organism can produce
Sometimes, not all exons get included in the splicing of mature mRNA
Therefore, multiple different proteins can be produced from one gene
E.g. tropomyosin
Gene has 11 exons
5 different forms of tropomyosin, depending on type of muscle it is found in
E.g. Dscam protein in fruit flies – guides growing nerve cells to their targets
38,000 different mRNAs possible

22. Nature of Science: looking for patterns, trends & discrepancies: There is mounting evidence that the environment can trigger heritable changes in epigenetic factors.
Epigenetics = study of heritable regulation of gene expression
Epigenetic tags = chemical modifications added to histones & DNA (acetyl, methyl, phosphate)
Epigenome = sum of all the epigenetic tags – can change in utero and throughout your life!
When sperm and egg meet, most epigenetic tags removed = “reprogramming”
About 1% epigenome not erased = “gene imprints”
E.g. mother has gestational diabetes (high blood glucose during pregnancy)
High level of glucose in fetal blood
Triggers epigenetic changes in fetal DNA
Now fetus is predisposed to develop obesity and/or type II diabetes

23. How genes switch on and off