Genetics Chapter 17: Gene Expression

Inducible Promoter

-Opern or other system of gene regulation in which transcription is normally off

-Something must take place for transcription to be induced or turned on

-i.e lac operon

How does eukaryotic gene expression differ from prokaryotic gene expression?

1. Each gene has its own promoter (not organized into operons)

2. Chromatin structure effects eukaryotic gene expression

3. Nuclear membrane separates transcription and translation

Transcriptional Regulation

-Transcription factor binding

-Transcriptional activator and insulator protein binding

-Use of alternative promoters

-DNA methylation

-Modification of chromatin structure

Post Translational Regulation

Covalent modifications to the protein itself

mRNA Processing Regulation

-Addition of the 5' guanine cap and 3' poly A tail

-Intron splicing

Regulation of Mature mRNA

-Translational regulator proteins bind mRNA

-Silencing by interfering RNA blocks translation

-Transport to cytoplasm

-mRNA stability

Translational Regulation

Translate the mRNA into only needed

Transcriptional Regulator Protein

-Combine with the basal transcription apparatus to reach normal levels of transcription

-Transcription factors can be regulators or activators

-Bind DNA in a sequence specific manner

B-Galactosidase

-Catalyzes the breakdown of lactose into glucose and galactose, allowing it to be used by the cell as an energy source

-Breaks lactose down into allolactose, a regulator of lactose metabolism

-Allolactose binds the repressor of the lac operon, allowing the enzymes involved in metabolism to be transcribed

Silencer

Region of DNA with similar properties to those of an enhancer, but represses transcription

Mediator

Complex of proteins that is part of the basal transcription apparatus

Coactivator

-Protein that cooperates with a transcription activator

-Often (in eukaryotes) interacts physically with transcriptional activators and the basal transcription apparatus

Insulator Binding Proteins

Proteins that bind insulators and play a role in their blocking activity

Coordinated Gene Regulation

-Coordination through response elements in the regulatory promoter

-Eukaryotic genes that need to be synchronized can contain the same response elements

Epigenetic Changes

Changes in chromatin that can affect gene expression

1. Histone modifications

2. Chromatin remodeling

3. DNA methylation

Chromatin

-Formed from DNA packaged around nucleosomes

-Presents an obstacle to gene expression (DNA is less accessible)

-Provides many opportunities for regulation

Nucleosome

Histones wrapped around DNA

Histone Structure

Composed of H1 and two each of H2A, H2B, H3, and H4

Histone Modification

-Occurs largely at specific residues of histone H3 and H4 N-tails

-Tails can undergo acetylation, phosphorylation, and methylation of specific lysine residues

Lysine Acetylation

-Associated with actively transcribed DNA

-DNA is less tightly bound and more accessible to transcription factors

-Creates binding sites for specific activators

DNA Methylation

-Leads to inhibition of transcription

-Methylation of CpG (islands) can turn off a gene

Chromatin Remodeling

-Catalytic reaction by chromatin remodeling complexes

-Shifting or removal of nucleosomes results in different DNA becoming accessible

-Requires ATP hydrolysis

N Tails

-Histone tails that wrap around DNA

-Can be acetylated

H3K16me3

Histone H3 lysine 16....

Chromatin Remodeling Complex

Complex of proteins that alters chromatin structure without acetylation of histone proteins

CpG Island

-Region of DNA that contains many copies of cystine followed by guanine

-Found near eukaryotic transcription start sites

-C's are commonly methylated when genes are inactive and de-methylated prior to transcription

Methylation and Imprinting

Epigenetic markers are established in germ cells and maintained through the somatic cell divisions of a developing organisim

ICR

-Imprinting control region

-Insulator that can not function when methylated

-Exhibits differences in chromatin modifications between alleles inherited from the mother and father

RNAi

-Gene silencing by RNA interference

-Post transcriptional gene regulation

-Includes micro-RNA, small interfering RNA, and piwi-interacting RNA

miRNA

-Micro RNA

-Pair imperfectly with mRNA

-Inhibits translation and targets RNA for degradation

-Regulates endogenous genes

siRNA

-Small interfering RNA

-Pairs with complementary sequences on mRNA

-Cleaves mRNA

-Targets RNA for degradation

-Targets DNA for chromatin remodeling

-Offers defense against genomic invaders such as viruses

RISC

-RNA induced silencing complex

-Uses sirna to target mRNA for cleavage and degradation

-Uses miRNA to target mRNA for translation inhibition

RITS

-RNA induced transcriptional silencing complex

-Uses siena to target DNA for chromatin modification or DNA methylation