AP Biology Chapters 18-20 Study Guide

Chapter 18

Chapter 18

Operon

-Entire stretch of DNA

-Includes operator, promoter, and genes they control

Promoter

Section of an operon where RNA Polymerase attaches

-TATA box: DNA Sequence located in promoter

Regulatory Gene

Segment of DNA that regulates expression of other genes

-Repressor is the product of this

Operator

-DNA segment that switches functionally related genes on-or-off

-Positioned within the promoter

Repressor

A protein produced by a regulatory gene

-Off Switch for an operon

-Prevents gene transcription by binding to operator and blocking RNA polymerase

-Trp operon

Corepressor

-Helps "turn off" genes

-Molecule that cooperates with repressor protein to switch an operon off

Trp Operon (Bacterial)

-Bacterial Regulation

-Stretch of DNA, promoter + genes of operon

-Genes are trp E -> trp A

-Repressible (ON unless turned off)

-Creates enzymes that create Tryptophan (amino acid)

-Regulatory gene is trp R

-Repressor is a protein

-Corepressor is Tryptophan

Trp Operon (ON)

ON:

-Repressor Inactive, no Tryptophan

-RNA Polymerase binds along promoter, transcribing mRNA by running down operon trpE->trpA

-mRNA is created for each gene, 1 mRNA for 1 gene (trpE -> trpA)

-mRNA codes for polypeptide subunits (E,D,C, B,A) that make up enzymes -> Make tryptophan (amino acid)

Trp Operon (OFF)

OFF:

-Repressor active, corepressor (tryptophan) present

-Active repressor binds to repression site on operator

-RNA polymerase cannot bind to operator

-NO tryptophan is made

Regulating Gene (Trp)

trp R is the regulatory gene for Trp Operon

Lac Operon (Bacterial)

-Bacterial Gene Regulation

-Contains genes that code for enzymes that hydrolize lactose (Add water -> break down lactose, dissacharide)

-Induceable

-Contains genes lacZ, lacY, & lacA

-Regulatory gene is lac Ⅰ -> codes mRNA -> codes repressor (Allolactose = isomer of lactose), always active unless acted upon by inducer

Lac Operon (OFF)

OFF (Default position)

-Regulatory gene lac Ⅰ codes for mRNA -> codes for repressor protein

-No inducer present -> repressor is active and binds to repression site on the operator

-RNA Polymerase cannot bind

-No lactose is broken down

Lac Operon (ON)

ON

-lac Ⅰ codes for mRNA -> codes for repressor

-Inducer (allolactose) is present, binds to protein-> inactivates repressor

-RNA Poylmerase binds to promoter -> runs down lac operon (lacZ -> lacA)

-Creates enzymes that break down lactose: B-Galactosidase (Cuts lactose in half), Permease (brings lactose into cell), & Transacetylase

CAP

-Catabolite Activator Protein

-Activator protein of transcription

-Activated when Lactose is HIGH and Glucose is LOW

-Binds to cAMP, attaches to promoter of lac operon & increases RNA polymerase transcription rate

cAMP

CAP binds to this when glucose is scarce and lactose is high

Inducer

Binds to a repressor so that the repressor no longer binds to the operator

-Inactivates the repressor to turn Lac operon on

Lac Operon Positive Feedback

cAMP binds to CAP, activating it -> attaches to promoter at CAP binding site & increases affinity for RNA polymerase transcription (increases transcription rate)

-High Lactose & Low Glucose = HIGH cAMP -> High lac mRNA

-High Lactose & High Glucose = Low cAMP -> Low lac mRNA

Differential Gene Expression

Expression of different genes by cells with the same genome

-Most cells are identical, gene expression is how they are differentiated in function

-Abnormalities lead to diseases such as cancer

Gene Expression Regulation (Eukaryotes)

-Eukaryotic gene expression is regulated at many stages

-Transcription: Can occur or not

-RNA processing: Alternative Splicing

-Transportation to Cytoplasm: Degradation

-Translation: Can occur or not, protein can be changed

Chromatin Modification

-Initial control of gene expression, makes region of DNA more or less able to bind the transcription machinery

-DNA Methylation (Red Light): Addition of methyl groups to DNA -> inactivation/reduced transcription

-Histone Acetylation (Green Light): Addition of acetyl groups to lysines in histones -> increases transcription rate

DNA Methylation (Red Light)

-Example of Chromatin Modification

-Addition of methyl groups to certain bases in DNA

-REDUCES transcription

-Can cause long-term inactivation of genes in cellular differentiation

Histone Acetylation (Green Light)

-Example of Chromatin Modification

Acetyl (COCh3) groups are added to positively charged lysines in histone tails (proteins)

-INCREASES transcription, spreads out histones

-Promotes loose chromatin structure -> permits transcription

Epigenetic inheritance

Inheritance of traits transmitted by mechanisms not directly involving the nucleotide sequence

Control Elements

-Part of DNA

-Segments of noncoding DNA that serve as binding sites of transcription factors that help regulate transcription

-Critical to precise regulation of gene expression in different cell types

Proximal Control Elements

-Part of DNA

-Located close to promoter

Distal Control Elements (Enhancers)

-Part of DNA

-Enhancers (groups of distal control elements)

-Far away from a gene or located in an intron

Activator Protein

-Such as CAP/CRP

-Protein that attach to ehancer (group of distal control elements) and activates transcription in a gene

-2 domains: one that binds DNA & one that activates transcription

-Bound activators facilitate a sequence of protein-protein interactions, result in transcription of given gene

Repressors

Prevents genes from being turned on

-Can influence chromatin structure to promote or silence transcription

-Bind to enhancers

Mediator Proteins

Enter bent DNA strip & bind to activators & RNA Polymerase

DNA-bending Protein

Protein that helps bend DNA and brings together RNA polymerase + mediator proteins + activators

General Transcription Factor

-Protein regulating gene expression

-Essential for the transcription of all protein-coding genes

Specific Transcription Factor

-Protein regulating gene expression