1. What is the Central Dogma?

2. How does prokaryotic DNA compare to eukaryotic DNA?

3. How is DNA organized in eukaryotic cells?

1. Draw and label the 3 parts of an operon.

2. Contrast inducible vs. repressible operons.

3. How does DNA methylation and histone acetylation affect gene expression?

1. List and describe the 3 processes that are involved in transforming a zygote.

2. Compare oncogenes, proto-oncogenes, and tumor suppresor genes.

3. What are the roles of the ras gene and the p53 gene?

Chapter 18

Transcription

Bacteria need to respond quickly to changes in their environment◦ if they have enough of a product,

need to stop production why? waste of energy to produce more how? stop production of enzymes for synthesis

◦ if they find new food/energy source, need to utilize it quickly why? metabolism, growth, reproduction how? start production of enzymes for digestion

STOP

GO

Feedback inhibition◦ product acts

as an allosteric inhibitor of 1st enzyme in tryptophan pathway

◦ but this is wasteful production of enzymes

= inhibition-

-

Gene regulation ◦ instead of blocking

enzyme function, block transcription of genes for all enzymes in tryptophan pathway saves energy by

not wasting it on unnecessary protein synthesis

= inhibition-

--

Cells vary amount of specific enzymes by regulating gene transcription◦ turn genes on or turn genes off

turn genes OFF exampleif bacterium has enough tryptophan then it doesn’t need to make enzymes used to build tryptophan

turn genes ON example if bacterium encounters new sugar (energy source), like lactose, then it needs to start making enzymes used to digest lactose

STOP

GO

OperonOperon: cluster of related genes with on/off switch

Three Parts:1. Promoter – where RNA polymerase attaches2. Operator – “on/off”, controls access of RNA

poly3. Genes – code for related enzymes in a pathway

operatorpromoter

DNATATA

RNApolymerase

repressor

repressor = repressor protein

Operon: Promoter, Operator & Genes they controlserve as a model for gene regulation

gene1 gene2 gene3 gene4RNApolymerase

Repressor protein turns off gene by blocking RNA polymerase binding site.

1 2 3 4mRNA

enzyme1 enzyme2 enzyme3 enzyme4

Repressor protein◦ binds to DNA at operator site ◦ blocking RNA polymerase◦ blocks transcription◦ Produced by regulatory gene

Regulatory geneRegulatory gene: produces repressorrepressor protein that binds to operator to block RNA poly

Normally ON Anabolic (build organic molecules) Organic molecule product acts as

corepressor binds to repressor to activate it

Operon is turned OFF Eg. trptrp (tryptophan) operon (tryptophan) operon

mRNA

enzyme1 enzyme2 enzyme3 enzyme4

operatorpromoter

DNATATA

RNApolymerase

tryptophan

repressor repressor protein

repressortryptophan – repressor proteincomplex

Synthesis pathway modelWhen excess tryptophan is present, it binds to trp repressor protein & triggers repressor to bind to DNA◦ blocks (represses) transcription

gene1 gene2 gene3 gene4

conformational change in repressor protein!

1 2 3 4

repressortrpRNApolymerase

trp

trp

trp trp

trp trp

trptrp

trp

trp

trp

What happens when tryptophan is present?Don’t need to make tryptophan-building enzymes

Tryptophan is allosteric regulator of repressor protein

trptrp operon operon

Normally OFF Catabolic (break down food for energy) Repressor is active inducerinducer binds to and

inactivates repressor Operon is turned ON Eg. laclac operon operon

mRNA

enzyme1 enzyme2 enzyme3 enzyme4

operatorpromoter

DNATATARNApolymerase

repressor repressor protein

repressorlactose – repressor proteincomplex

lactose

lac repressor gene1 gene2 gene3 gene4

Digestive pathway model When lactose is present, binds to lac repressor protein & triggers repressor to release DNA

◦ induces transcription

RNApolymerase

1 2 3 4

lac lac

laclac

lac

lac

lac

conformational change in repressor protein!

lac

lac

What happens when lactose is present?Need to make lactose-digesting enzymes

Lactose is allosteric regulator of repressor protein

laclac operon operon

Repressible operon ◦ usually functions in anabolic pathways

synthesizing end products

◦ when end product is present in excess,cell allocates resources to other uses

Inducible operon ◦ usually functions in catabolic pathways,

digesting nutrients to simpler molecules

◦ produce enzymes only when nutrient is available cell avoids making proteins that have nothing to do,

cell allocates resources to other uses

Many stages

Typical human cell: only 20% of genes expressed at any given time

Different cell types (with identical genomes) turn on different genes to carry out specific functions

Differences between cell types is due to differential gene expressiondifferential gene expression

Chromatin Structure: Tightly bound DNA less

accessible for transcription

DNA methylation: methyl groups added to DNA; tightly packed; transcription

Histone acetylation: acetyl groups added to histones; loosened; transcription

Modifications on chromatin can be passed on to future generations

Unlike DNA mutations, these changes to chromatin can be reversed (de-methylation of DNA)

Explains differences between identical twins

Transcription Initiation: Control elements bind

transcription factors Enhances gene expression

EnhancerEnhancer regions bound to promoterpromoter region by activatorsactivators

Regulation of mRNA:• micro RNAs micro RNAs

(miRNAs) (miRNAs) and small small interfering RNAs interfering RNAs (siRNAs) (siRNAs) can bind to mRNA and degrade it or block translation

Section 18.4

1. Cell Division: large # identical cells through mitosis

2. Cell Differentiation: cells become specialized in structure & function

3. Morphogenesis: “creation of form” – organism’s shape

Cytoplasmic determinants: maternal substances in egg distributed unevenly in early cells of embryo

Induction: cells triggered to differentiate

Cell-Cell Signals: molecules produced by one cell influences neighboring cells◦ Eg. Growth factors

Section 18.5

1. Proto-oncogene = stimulates cell division2. Tumor-suppressor gene = inhibits cell

division

Mutations in these genes can lead to cancer

Proto-Oncogene Oncogene

Gene that stimulates normal cell growth & division

Mutation in proto-oncogene

Cancer-causing gene

Effects: Increase product of

proto-oncogene Increase activity of

each protein molecule produced by gene

Ras gene: stimulates cell cycle (proto-oncogene)◦Mutations of ras occurs in 30% of

cancers p53 gene: tumor-suppresor gene

◦Functions: halt cell cycle for DNA repair, turn on DNA repair, activate apoptosis (cell death)

◦Mutations of p53 in 50+% of cancers

Cancer results when mutations accumulate (5-7 changes in DNA)

Active oncogenes + loss of tumor-suppressor genes

The longer we live, the more likely that cancer might develop

Embryonic development occurs when gene regulation proceeds correctly

Cancer occurs when gene regulation goes awry