Chapter 19:Chapter 19:

Eukaryotic Eukaryotic Genomes- Genomes-

Organization, Organization, Regulation, & Regulation, &

EvolutionEvolution

Objectives for this lecture

1. Understand how DNA is organized, and how that organization helps us control gene expression

2. Understand how regulation of transcription helps us control gene expression

3. Understand how regulation of translation helps us control gene expression

TermsTerms definitionsdefinitions

1. Gene expression2. Regulation3. Transcription4. Translation5. mRNA6. Protein

A. Controls, rulesB. Produced during

transcriptionC. DNA RNA

protein cell product

D. The product of translation

E. DNA mRNAF. mRNA protein

Warm up, matching

for your notes…

Underlined and colored text most critical

HOW DNA IS ORGANIZED

Section 1

•Eukaryotic genomes have much more DNA than prokaryotes do

•Eukaryotic DNA must be specially organized to maintain efficiency

Chromatin: DNA wrapped around protein

Chromatin Structure is Based on Successive Levels of DNA

Packing

DNA in a eukaryotic chromosome

White center = main axis of the chromosome

Red loops = DNA that is being actively transcribed

•A special problem: All prokaryotic & eukaryotic cells must be able to express certain genes as needed

•All cells contain the same, complete set of DNA•Cells are different because they express different genes

DNA organization changes throughout the cell cycle:

•Histones are proteins that are like spools that DNA wraps around

•A nucleosome is a section of DNA wrapped around 2 histones.

Higher Levels of DNA Packing

•The 10nm fiber continues to coil to 30nm as linker DNA attracts to histone tails and the H1 histone

Higher Levels of DNA Packing•The 30nm fiber then forms looped domains•Protein scaffolds are made of non-histone proteins•Looped domains compact further to form the characteristic metaphase chromosome

Regulation of Chromatin StructureRegulation of Chromatin Structure

•Genes in highly condensed heterochromatin are usually not expressed •Genes in the looser packing of euchromatin usually are expressed

DNA MethylationDNA Methylation

•Genes that are not expressed are usually heavily methylated

Acetyl groups bind to histone tails, keep the DNA loose so transcription can happen

Histone AcetylationHistone Acetylation

QUICK THINKQUICK THINK

Draw the levels of organization of DNA, putting methyl and acetyl groups

REGULATION OF GENE EXPRESSION BY CONTROL OF TRANSCRIPTION

Section 2

First, a review of genes & their transcripts:

promoter1. Transcription begins when a cluster of proteins calleda transcription initiation complex binds to the of the gene

RNA pol II2. The enzyme transcribes the gene into a molecule ofpre-mRNA

a 5’ cap & poly-A tail3. RNA processing includes the addition of as well as cutting out the introns

Control elements : sequences of non-coding DNA that regulate transcription by binding to certain proteins.

*They are often found upstream of eukaryotic genes

Proximal control elements• _________________________ are close to the promoter

•__________________________ , AKA enhancers, are farther away and can even be within introns

Distal control elements

LE 19-6

Distal controlelement Activators

Enhancer

DNA

DNA-bendingprotein

TATAbox

PromoterGene

Generaltranscriptionfactors

Group ofmediator proteins

RNApolymerase II

RNApolymerase II

RNA synthesisTranscriptionInitiation complex

RNA ProcessingThe original RNA transcript can produce various mRNAs depending on which part of the pre-mRNA is treated as introns & which are treated as exons

mRNA DegradationmRNA Degradation

Enzymes may remove the 5’ cap & the poly-A tail. This exposes the mRNA to nucleases that break it down.

microRNAs (miRNAs)

miRNAs are single-stranded RNA molecules that bind to mRNA to either degrade the mRNA or block translation

miRNA starts off as a segment of RNA that is H-bonded to itself, then it gets chopped into smaller pieces by an enzyme called dicer

REGULATION AFTER TRANSCRIPTION HAS OCCURRED

Section 3

Post-Transcriptional RegulationPost-Transcriptional Regulation

Protein production can be controlled at other points before the protein becomes fully functional

Regulation of Translation

Regulatory proteins may bind to the 5’ end of mRNA, which makes the ribosome unable to attach

Protein Processing & Degradation

•Proteins that are to be degraded are tagged with a ubiquitin marker protein

•Giant protein complexes called proteasomes recognize and break down the tagged proteins

LE 19-3Signal

NUCLEUS

DNA

RNA

Chromatin

Gene availablefor transcription

Gene

Exon

Intro

Transcription

Primary transcript

RNA processing

Cap

Tail

mRNA in nucleus

Transport to cytoplasm

CYTOPLASM

mRNA in cytoplasm

Translation

Degradationof mRNA

Polypeptide

CleavageChemical modificationTransport to cellular

destination

Degradation of protein

Active protein

Degraded protein

Eukaryotic gene expression can be controlled at multiple steps

1.

2.

3.

4.

5.

6.

QUICK THINK

Once mRNA is in the cytoplasm, describe some ways that regulate the amount of active protein in the cell