Week 8.2_Eukaryotic Gene Expression

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Gene Expression II: Eukaryotes Week 8 – Wednesday, October 11, 2023
• Quiz #2 will OPEN • 15 minutes, 10 questions • Open until 11:55 PM tonight 10/14 • Correct answers will be available on Monday Updates and reminders
Week 8 Learning Objectives Understand how prokaryotes use operons to tightly regulate genes Understand how chromatin affects how eukaryotic genes are expressed
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Summarizing the lac operon Prokaryotes organize related genes into operons. lac operon is regulated by a repressor When lactose absent , repressor binds to operator When lactose present, repressor cannot bind operator allolactose lac operon
Summarizing the trp operon When tryptophan is absent , the repressor cannot bind to operator, which enables transcription to occur. Amino acids will work in a similar fashion . trp operon is also regulated by a repressor When tryptophan is present , it binds to the repressor and changes its shape, enabling it to bind to the operator and prevent transcription.
Transcription of Multiple Genes Bacteria are able to transcribe multiple gene simultaneously with a single mRNA. Encoding genes in the same pathway. DNA RNA Proteins
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Eukaryotic gene expression How do Eukaryotic genomes differ from prokaryotes? Eukaryotic gene expression (especially in multicellular species) requires complex regulation. Not organized into operons, so each gene must be regulated independently. More genes than prokaryotes. Regulation of gene expression can happen at any of the stages as DNA is being transcribed into m RNA and mRNA is translated into protein. Can be divided into five levels:
Epigenetic control Epigenetics control involves changes to genes that do not alter the nucleotide sequence of the DNA and are not permanent . Instead, these changes alter the chromosomal structure so that genes can be turned on or off. Although changes are not permanent , they can persist through multiple cell division and even cross generational lines. Behavior and environmental conditions can cause epigenetic changes. A game changer since this means “genes do not totally make you who you are”. Nature AND nurture plays a role. Reason why identical twins can exhibit different behaviors, skills health and achievements.
Environmental Influence on Gene Expression Diet Exercise Smoking Stress Carcinogens The following can influence gene expression:
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Epigenetic control Epigenetic control involves changes to genes that do not alter the nucleotide sequence of the DNA and are not permanent . Instead, these changes alter the chromosomal structure so that genes can be turned on or off. Best studied example is modification of histone proteins. histone DNA nucleosome DNA in the nucleus is wound, folded and compacted into chromosomes called nucleosomes .
Epigenetic control The nucleosomes look like beads (histone proteins) that move along on a string (DNA) and change the structure of the molecule. The string of nucleosome are chromatin : DNA and proteins that make up the chromosomes in a cell. Heterochromatin : tightly compact DNA ”Closed” chromatin RNA polymerase cannot bind the promoter. Cannot “read” the gene and is turned “OFF”. Figure 16.7 DNA Methylation occurs by adding a chemical group to DNA, where it blocks the proteins that attach to DNA that reads the gene. Typically turns gene off
DNA itself can also be methylated which is correlated with gene expression states DNA can also be methylated directly (majority nucleotides are cytosines) Presence of DNA methylation suppresses gene expression in various ways.. cytosine
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How Tightly Packed Chromatin Are Determines Gene Expressed or Not Euchromatin : “Open” DNA is only loosely bound by histones. RNA polymerase can bind the promoter. Gene can be “read” and is transcribed. Figure 16.7 Histone acetylation results in loose packing of nucleosomes. DNA now accessible and be transcribed.
Nucleosome: DNA + histone proteins. DNA wraps around histone octamers to help keep DNA compact. Further folding results in the classic ‘X’ shaped chromosomes seen under a microscope. Chromatin
Chromatin
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There are four different proteins that make up a histone octamer : Two H2A proteins Two H2B proteins Two H3 proteins Two H4 proteins Each histone has a globular region (structured) and an unstructured tail Histone tails have lots of Lysines and Arginines The Histone Octamer
Transcriptional Control Recall from protein synthesis that transcription of a gene requires RNA polymerase and other proteins, transcriptional factors (TFs) to bind to a promoter to initiate transcription. TF are proteins that bind to the promoter sequence and other regulatory sequence to control transcription of target gene. RNA polymerase cannot initiate transcription in eukaryotes. TFs must bind to the promoter region first. Only then can RNA polymerase bind to the promoter for transcription to begin. Activators Enhancer Transcription factors
Enhancers ”Enhance ” Gene Expression of Eukaryotic genes In addition to promoters, enhancers are another DNA element that controls gene expression. Proteins at enhancers interact with transcription factors at promoter. This promotes transcription. Can be found upstream of the promoter, thousands of nucleotides away from promoter. When enhancer is far away from gene, DNA folds, bringing enhancer closer to promoter. Results in looping of chromatin. DNA bending protein Transcription factors and mediator proteins Distal control elements Enhancer
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What sorts of sequence is found “upstream” (before initiation site) of eukaryotic genes? A. Intron B. Poly-A tail C. TATA box D. RNA polymerase
Which of the following are regulators of gene expression in eukaryotes? • A – Enhancers • B – Promoters • C – DNA Methylation • D – All of the Above
Alternative Splicing is a Form of Post - transcriptional Gene Regulation Last week’s lecture we covered RNA splicing out of introns. Alternative splicing results in mRNAs that encode proteins that are slightly different from each other (called isoforms ). So one gene can produce MANY isoforms of same gene. Leads to diversity of gene products.
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Which of the following are post- transcriptional regulators of gene expression in eukaryotes? • A – Enhancers • B – Transcription Factors • C – Alternative Splicing • D – All of the Above
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Other Examples of Epigenetic Gene Regulation Differences in lifespan and health of identical twins (both twins contain the exact same DNA sequence). Coat color in Calico cats and X-chromosome in activation. Thus calico cats are nearly always females.
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Summary Gene expression in prokaryotes controlled by operons. Epigenetics control involves changes to genes that do not alter the nucleotide sequence of the DNA and are not permanent . In eukaryotes, promoter sequences recruit specific transcription factors. Enhancers can enhance transcription. Far up stream from transcription factors. Looping can occur to bring TF closer to enhancer. Expression can occur at any time during protein synthesis. In eukaryotes, chromatin is condensed and packed (heterochromatin) or open (euchromatin). Alternative splicing of introns can give rise to different proteins (isoforms) from same mRNA.
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