Chapter 18 Gene Expression I What are Genes

Chapter 18 Gene Expression

I) What are Genes? A) Sequences of nucleotides that determine the sequence of amino acids in proteins B) Two Types of Genes Regulatory Genes Structural Genes

I) What are Genes? A) Sequences of nucleotides that determine the sequence of amino acids in proteins B) Two Types of Genes Regulatory Genes Structural Genes • Control transcription and expression of other genes

I) What are Genes? A) Sequences of nucleotides that determine the sequence of amino acids in proteins B) Two Types of Genes Regulatory Genes Structural Genes • Control transcription and expression of other genes • Function as “on/off” switches

I) What are Genes? A) Sequences of nucleotides that determine the sequence of amino acids in proteins B) Two Types of Genes Regulatory Genes Structural Genes • Control transcription and expression of other genes • Function as “on/off” switches • Some code for RNA

I) What are Genes? A) Sequences of nucleotides that determine the sequence of amino acids in proteins B) Two Types of Genes Regulatory Genes Structural Genes • Control transcription and • Determine the structure expression of other genes of a protein • Function as “on/off” switches • Some code for RNA

I) What are Genes? A) Sequences of nucleotides that determine the sequence of amino acids in proteins B) Two Types of Genes Regulatory Genes Structural Genes • Control transcription and expression of other genes • Function as “on/off” switches • Some code for RNA • Determine the structure of a protein • Control amino acid sequences

I) Differentiation A) The process in which cells become specialized as specific genes are activated 1. Function of a cell is dictated by DNA – Not all genes are activated in every cell – Some genes are only active at specific points in the life cycle of a cell

III) Prokaryotic Gene Regulation A) OPERONS - Specific genes within DNA that control the transcription of RNA and control protein synthesis 1) Operons contain four components a) Regulatory Gene – codes for either a repressor protein or an activator protein a. Repressor protein prevents RNA transcription (turns genes off) b. Activator protein starts RNA transcription (turns genes on) b) Promoter – region of DNA where RNA polymerase binds and begins transcription (the Start Codon) c) Operator – region of DNA to which a repressor or activator protein binds – this is the “ON/OFF SWITCH” d) Structural Gene – DNA sequences that determine the amino acid sequence for a specific protein or proteins

Inducible Genes that are usually off and need to be activated for short periods of time Repressible Genes

Inducible Genes that are usually off and need to be activated for short periods of time Require an activator to remove a repressor protein Repressible Genes

Inducible Genes that are usually off and need to be activated for short periods of time Require an activator to remove a repressor protein • lac operon Repressible Genes

Inducible Genes that are usually off and need to be activated for short periods of time Require an activator to remove a repressor protein • lac operon Repressible Genes that are usually on all the time and need to be inactivated for brief periods

Inducible Genes Repressible Genes that are usually off and need to be activated for short periods of time Genes that are usually on all the time and need to be inactivated for brief periods Require an activator to remove a repressor protein Require a repressor to block transcription • lac operon

Inducible Genes Repressible Genes that are usually off and need to be activated for short periods of time Genes that are usually on all the time and need to be inactivated for brief periods Require an activator to remove a repressor protein Require a repressor to block transcription • lac operon • trp operon

IV) Eukaryotic Gene Regulation A) Control of Transcription 1) Transcription Activation a) Transcription Factors a) b) c) Proteins that bind to promoters and affect RNA polymerase activity Coactivator proteins then bind to the transcription factors (the activators) and helps initiate RNA transcription These enhance or promote transcription 2) Inhibition of Transcription a) Methylation 1) 2) Methyl groups (-CH 3) attach themselves to the cytosines that reside in CG (cytosine-guanine) doublets on DNA. Methylation near a gene promoter on DNA will generally prevent transcription of that gene.

Transcription Factor Promoter

Regulatory Switches and Gene Regulation

B) Post-transcriptional Control in Eukaryotes 1) Gene regulation after RNA transcription 2) RNA interference (RNAi) Small RNAs such as mi. RNAs and si. RNAs block RNA translation a) mi. RNA: – micro. RNAs (mi. RNAs) are a class of naturally occurring, small non-coding RNA molecules, about 21– 25 nucleotides in length. – mi. RNAs are partially complementary to one or more messenger RNA (m. RNA) molecules, and their main function is to down-regulate (supress) gene expression – mi. RNA forms double stranded RNA to prevent translation into protein – mi. RNA base pairing is not exact – results in many more possible points of interference

si. RNA (small interfering RNA) • Double Stranded (ds. RNA) is cleaved by an enzyme, Dicer, into double stranded small interfering RNAs (si. RNA) • the si. RNAs become integrated into a multi-subunit protein complex, commonly known as the RNAi induced silencing complex (RISC), which guides the si. RNAs to the target RNA sequence • At some point the si. RNA and RISC directs degradation of the complementary m. RNA sequence • Many genes are thought to be regulated this way • Prevents RNA translation into protein • si. RNA has exact base pairing

EPIGENETICS • Gene expression regulated by environmental factors. • Changes do not involve nucleotide sequence changes. • Changes can be passed to future generations. • Diet, stress, and other factors play a role. – Epigenetics is regulated by DNA methylation – Methylation inhibits gene expression while demethylation induces gene expression

Effects of Bis-Phenol A (BPA) • BPA causes demethylation which activates genes known to induce obesity in GENETICALLY IDENTICAL MICE • BPA is linked to diabetes, obesity, and developmental problems in children • BPA is an estrogen-like substance that affects cell growth and differentiation. • Effects of BPA can be seen in fourth generation offspring

When pregnant yellow agouti mothers were fed BPA, more yellow, unhealthy babies were born than normal. Exposure to BPA during early development had caused decreased methylation of the agouti gene. However, when BPA-exposed, pregnant yellow mice were fed methyl-rich foods, the offspring were predominantly brown. The maternal nutrient supplementation had counteracted the negative effects of exposure

Lick Your Rats! The nurturing behavior of a mother rat during the first week of life shapes her pups' epigenomes. The epigenetic pattern that mom establishes tends to stay put, even after the pups become adults. These patterns can be reversed through the introduction of methyl in the diet or a removal of methyl using chemical compounds

BARR BODIES A Barr body (named after discoverer Murray Barr) is the inactive X chromosome in a female somatic cell. A normal human female has only one barr body per somatic cell, while a normal human male has none.