Genetics Lesson 10: Regulation of Gene Expression in Eukaryotes

Similarities and differences between eukaryotic and prokaryotic regulation example

similarity: DNA binding regulates transcription of RNA polymerase

Differences: DNA must unwind from histone protein before transcription, each structural gene has its own promoter

Chromosomes

Consist of DNA and histone proteins which form a nucleosome

Histones have positive charged and attract negatively charged phosphate on DNA. This can reduce length of DNA to about 1/3

Nucleosomes fold up and compress to become a chromatin fiber.

Chromatin fibers form chromatids which build/constitute a chromosome

Proteins in chromatin

2 each of H2A, H2B, H3, and H4 in the core, making a histone octamer

There is one H1 histone between nucleosomes

Interaction between histone positive amine group and negative DNA keep them tightly bound to eachother

DNA packed into all levels step by step into chromosome

Chromatin is simplest level of double helix.

This is complexed to become nucleosomes whcih have 8 proteins, DNA wraps 1.65 times around the nucleosome core

These fold up to produce a fiber and form loops which then compress to make a thicker fiber then there is tight coiling of the fiber producing the chromatid of a chromosome.

Euchromatin

Deconsensed chromosome regions

constitutes the majority of the chromosome material and includes most of the genes

Heterochromatin

Condensed chromosome regions containing mostly noncoding DNA.

Also is the centromere and telomere regions and has long sequences of tandem repeats

mRNA decay in eukaryotes vs prokaryotes

Eukaryotic mRNA has a much longr half life because it was processed.

Chromatin structure alteration for regulation

Chromatin structure repressed gene expression: condensed and coiled DNA is inacessible for RNA polymerrase

Chromatin structure is opened and exposes the promoter region

The processes affect gene regulation by alterin chromatin structure:

• Chromatin remodeling

• modification of histone proteins

  • DNA methylation

Chromatin remodeling definition

Some transcription factors (TF) and regulatory proteins alter chromatin structure without altering chemical structure of histone directly

Chromatin remodeling complex

Binds directly to DNA and repositions nucleosomes, allowing TFs and RNA polymerase to bind to promoters and initiate transcription.

Chromatin remodeling complexes work together with enzymes to change chromatin structure and expose DNA for transcription

Modification of Histone Histone Acetylation

Acetylation stimulates transcription as it causes open configuration of chromatin

Acetyl group destabilize chromatin structure

Acetyltransferase enzyme adds acetyl, deacetylases remove it

Acetylation of histone tails weakens their interaction with DNA phosphate and permit transcription factors to bind

Step by step histone protein acetylation

Acetyl groups destabilize chromatin

Regulater protein is encoded because it is transcribed.

Once it is translated it encodes a deacetylase enzyme that removes acetyl groups and restores structure, stopping transcription of regulator protein

DNA methylation

Most common on cytosines that are adjacent to guanine.

Methylation of DNA represses transcription

Transcriptional level regulation, what on DNA stimulates transcription normally?

Transcription factos, RNA polymerase and transcriptional activator proteins bind to DNA to stimulate transcription

Basal transcription apparatus: TFs, and RNA polymerase bind to core promoter TATA box downstream of transcriptional activator protein

GAL4 in Yeast (transcriptional Activator)

Regulates transcription of genes whose products metabolize galactose.

Binds to upstream activating sequence (UAS) (enhacner)

When there is no galactose, GAL80 binds to GAL4 causing no transcription

When theres galactose, GAL4 stays unbound and transcription is stimulated

Inducible vs repressible systems

Inducible systems transcription are turned ON by an inducing signal; repressible systems transcription are turned OFF by a corepressor

Transcriptional Repressors

Bind to SILENCERS

Insulator: DNA sequence that blocks or insulates the effect of enhancers

Gene regulation at mRNA processing level

mRNA alternative splicing: Variation in which exons are put together depending on the tissue, developmental stage or sex

Offers flexibility since one gene can encode different variants of a protein

T antigen gene of the mammalian virus SV40

One gene encodes two different proteins: large T and small T antigens

Splicing Factor 2 enhances the mRNA splicing of small T antigen

RNA interference for Gene regulation

RNA interference (RNAi) ALA RNA silencing and post-transcriptional gene silencing are tiggered by microRNAs and small interfering RNAs which are only about 22 nucleotide RNA

30% of genes regulated by RNAi

Origin and target of siRNA

Origin: mRNA, transposon, or virus

Target: Genes from which they were transcribed

origin and target of miRNA

Origin: RNA transcribed from a distinct gene

Target: Genes other than those from which they were transcribed

miRNA and siRNA step by step origin

Inverted repeat, the sequence between them is transcribed, the repeat bind to eachother to make a double stranded RNA which is cleaved by enzyme DICER to produce mi or siRNAs

siRNA can arise from RNA viruses or long hairpins

4 mechanisms of RNAi for gene regulation and general info for each

Cleavage of mRNA: a RNA induced silencing complex (RISC) is formed by mi or siRNA with protein “slicer” and cleaves mRNA, decreasing amount of protein.

Inhibition of translation: miRNA in RISCs match the mRNA imperfectly and inhibit the translation of targeted mRNA

Transcriptional Silencing: some siRNA form complex called RNA induced transcriptional silencing(RITS) and binds to DNA causing methylation and stopping transcription

Degradation of mRNA: miRNAs regulate gene expression and mRNA degradation required DICER and RISC

siRNA cleavage of mRNA

RISC pairs with complementary sequences on mRNA, cleaving mRNA, later degrading the RNA

miRNA inhibition of Translation step by step

miRNAs combine with RISC complex, pairing imperfectly with mRNA and lead to inhibition of translation

siRNA transcriptional silencing

siRNA in RITS attaches to complementary DNA sequence and attracts methylation which inhibit transcription