Control of Prokaryotic Gene Expression Prokaryotic Regulation of

Control of Prokaryotic Gene Expression

Prokaryotic Regulation of Genes Regulating Biochemical Pathway for Tryptophan Synthesis. 1. Produce something that will interfere with the function of the enzyme in the pathway. 2. Produce a gene regulator that can inhibit the transcription of one biochemical pathway enzymes. 2

1. Eukaryotic cells have many more genes (i. e. 23, 000 in human cells) in their genomes than prokaryotic cells (i. e. average 3000). 2. Physically there are more obstacles to regulate eukaryotic genes because there is so much more DNA to manage. For example, eukaryotic chromatin is wrapped around histone proteins. 3. In addition there are other nonhistone proteins that are used in eukaryotic gene expression that are not used in prokaryotic gene expression. 3

Operon and Prokaryotic Gene Expression • Operon- A group of prokaryotic genes with a related function that are often grouped and transcribed together. In addition, the operon has only one promoter region for the entire operon. PROG 4

Operon and Prokaryotic Gene Expression PROG An operon is composed of the following: • Structural genes- genes that are related and used in a biochemical pathway. • Promoter-The nucleotide sequence that can bind with RNA polymerase to start transcription. This sequence also contains the operator region. • Operator-The nucleotide sequence that can bind with repressor protein to inhibit transcription. 5

Regulator Genes and Repressors • Regulator gene- This gene produces a protein called a repressor that can inhibit the transcription of an operon by attaching to the operator (upstream or downstream). 6

Interaction of Modulators and Repressors • Repressors have allosteric properties. Modulators can bind to the repressor at an allosteric site changing the conformation of the repressor, thereby activating or deactivating the repressor. Usually the modulator is a product of the biochemical pathway. 7

Lactose and the Inducible lac Operon Negative Gene Regulation 1. Inducible operon- the lac operon. This operon has the ability to convert lactose into glucose and galactose. This involves three structural genes • The lac operon is an example of an inducible operon. 8

Animation of the lac Operon and Presence of Lactose 9

Absence of Lactose and the lac Operon • If no lactose or allolactose is present, the repressor protein is active, binding to the operator site. This prohibits the RNA polymerase from transcribing the operon. 10

Animation of the lac Operon and Absence of Lactose 11

Synthesis of Tryptophan and the Repressible trp Operon 12

Animation of the trp Operon and Absence of Tryptophan 13

Tryptophan Present and the Repressible trp Operon 14

Animation of the trp Operon and Presence of Tryptophan 15

Lac and trp Operons-Examples of Negative Gene Regulation • The lac and typ operons are example of negative gene regulation as the repressor protein inhibits transcription of the operons. 16

Example of Positive Gene Regulation 17

Both Lactose and Glucose Present • Lactose present, glucose present (c. AMP level low), little lac m. RNA synthesized 18

Control of Eukaryotic Gene Expression

Eukaryotic Gene Regulation Prokaryotic regulation is different from eukaryotic regulation. 1. Eukaryotic cells have many more genes (23, 700 in human cells) in their genomes than prokaryotic cells (average 3000). 2. Physically there are more obstacles as eukaryotic chromatin is wrapped around histone proteins. 3. There are more non-histone proteins that are used in eukaryotic gene expression than in prokaryotic gene expression. 20

Eukaryotic Gene Regulation in Multicellular Organisms • Almost all the cells in an organism are genetically identical or totipotent. • Differences between cell types result from differential gene expression -- the expression of different genes by cells with the same genome. • Errors in gene expression can lead to diseases including cancer. • Gene expression is regulated at many stages. 21

Organization of DNA 22

Types of Repetitive DNA 23

Altering the Genome is a Form of Gene Regulation • Gene amplification- In amphibian ovum there are over 1 million copies of the r. RNA made from tiny circles of DNA in the nucleoli. • Gene Loss-In gall midges (an insect) during development, all but two cells lose 32 of their 40 chromosomes during the first mitotic divisions. These cells that retain 40 chromosomes will produce gametes. • Transposed genes-The hemoglobin gene has been duplicated, mutated, and transposed to other chromosomes to produce multiple but different copies of the same gene. 24

Gene Duplication and Transposition Regulates Genes 25

Rearrangement Gene Domains Regulates Genes • B lymphocytes can produce millions of different types of antibodies (proteins) that react with millions of different antigens. This happens by rearrangement of AB genes. 26

Epigenetics • Epigenetics refers to processes that influence gene expression or function without changing the underlying DNA sequence. 1. Acetylation 2. Methylation 27

Acetylation • Acetylation of lysine found on the histone decreases the affinity of histones for DNA and other histones, thereby making DNA more accessible for transcription. 28

Methylation • A methyl group can be added to the nitrogenous bases of cytosine that are followed by guanine. Many human genes have upstream CG-rich regions called Cp. G islands. Methylation of a gene's Cp. G island represses gene expression. Different cells have different methylation patterns, which contributes to the differences in gene expression in different cell types. 29

Role of Transcription Factors 30

Role of Transcription Factors 31

Role of Transcription Factors and Activators • This illustrates how different cells have different activators which activate different genes. • The liver cells need the protein albumin and not the protein crystallin and the lens cells do not need albumin but do need crystallin. 32

Role of Transcription Factors and Lactose Persistence • The LCT gene produces the enzyme lactase which digests lactose. Lactose is a disaccharide found in dairy products. In order to be transcribed, the LCT gene needs a regulatory protein coded for by the MCM 6 gene. Most humans after weaning cease to produce the regulatory protein but a mutation in the gene will allow its continued production through adulthood. This mutation causes a condition called lactose persistence. 33

Coordination of Expression of Related Proteins in a Biochemical Pathway 34

Post-Transcriptional Control Alternative Splicing Alternative splicing • Once the immature m. RNA is made, it could be processed in different ways to give rise to different mature m. RNA and thus different proteins. 35

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Example of Alternative Splicing the Same Gene in Humans 37

Post-Transcriptional Control and RNAi Alternative • m. RNA molecules do not remain functional indefinitely. This length of time can affect the number of protein product synthesize. • Small pieces of RNA can interfere (RNAi) with m. RNA by being complementary to a small part of the m. RNA and tagging it for destruction. 38

Post-Transcriptional Control and micro. RNA (mi. RNA) 39

Post-transcriptional control- si. RNA 40

Other Factors that Can Affect the Expression of Genes-Post Transcription • Chemical signals that regulate the m. RNA leaving the nucleus. Nearly half of all mature m. RNA never reaches the cytoplasm. There must be some sort of inhibitor that will allow certain m. RNA to leave and others toremain. • Degrading of the m. RNA that affects its lifespan. The life-span can be associated with the length of the poly-A-tail. As it shortens, it aids other enzymes with the removal of the 5'cap and nucleases break down the m. RNA can last from minutes to weeks. 41

Post-translational control- Ubiquitin 42

Other Factors that Can Affect the Expression of Genes- Post Translation Post-translational control • Modification of protein product. Often amino acids must be removed in order for the protein to be functional. • Proteins are often modified with prosthetic groups to make them functional. 43

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