GENE EXPRESSION protein is being made AND - Some are expressed for - - PDF document

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GENE EXPRESSION AND MUTATION

GENE EXPRESSION IN PROKARYOTES

• A gene is being “expressed” or “activated” when a

protein is being made

• Some are expressed for a time and then turned off

How does a cell know how and when to turn

• n and off certain genes?

Discovery of Gene Expression

1961: Francois Jacob & Jacques Monod

• studied bacteria e. coli (normal flora in intestines)
• bacteria will break down lactose (into glucose + galactose)

from dairy products in intestine to use as energy source (will only do so in presence of lactose)

• three enzymes needed to do this (each has a different gene)
• allows bacteria to conserve energy when gene is off
• lac operon: cluster of genes that enables e. coli to build

proteins needed for lactose metabolism when lactose is present

The “Players” in Prokaryotic Gene Expression

• Operon: promoter, operator, structural (functional) genes
• Promoter: control sequence, site where replication starts
• Operator: DNA sequence between promoter and

enzyme genes, acts as on/off switch for genes

• Functional genes: coding sections
• Inducer: protein that initiates gene expression, must be present
• The default mode for the operon is the “off” position
• Gene expression occurs only when the cell needs

specific proteins to be made

Operon

RNA polymerase cannot attach to promoter

Steps of Gene Expression in Prokaryotes

I. Turning on the lac operon A. RNA polymerase attaches to promoter region near the genes “START HERE” B. RNA polymerase moves along chromosome to genes C. Once it hits genes, it produces m RNA (transcription) D. mRNA instructs ribosomes to make enzymes (translation)

The lac operon in “on” mode

Lactose present

Enzymes for lactose utilization DNA mRNA Protein Inactive repressor Lactose

Operon turned on (repressor inactivated by lactose)

RNA polymerase bound to promoter

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• II. Turning off the lac operon

**repressor – protein that inhibits gene from being expressed

• A. repressor attaches to
• perator and sits between

promoter and the genes

• B. repressor blocks access of

RNA polymerase to genes

• C. protein stops being made

The lac operon in “off” mode Operon turned off (default state when no lactose is present)

Lactose absent

Operon Regulatory gene Promoter Lactose-utilization genes DNA mRNA Protein Active repressor RNA polymerase cannot attach to promoter Operator

• III. Reactivation of lac operon

***if cell needs more enzyme***

• A. when inducer enters cell it binds to the repressor
• B. repressor changes shape and cant bend to operator any

longer

• C. repressor falls off operator
• D. RNA polymerase binds to promoter and again forms m RNA

which will instruct ribosome to again make enzyme

• E. when inducer runs out-
• repressor binds to operator again, changes shape & falls off
• operon is turned off

*SYSTEM IS AUTOMATIC AND SELF-REGULATING*

Lac operon animation

GENE EXPRESSION IN EUKARYOTES

• more complex than prokaryotes
• nuclear envelope physically separates transcription

from translation, more opportunities for regulation

• f gene expression
• eukaryotes have DNA on many chromosomes not one

circular DNA

• many different cell types make many different

proteins

Controlling Onset of Transcription

• Gene regulation controls onset of transcription

(when RNA polymerase binds to beginning of gene)

• Requires transcription factors (extra regulatory

proteins)

• help arrange RNA polymerases in correct position on

promoter

Proofreading

• remember that before mRNA goes into cytoplasm to start protein

synthesis, RNA polymerase proofreads strand

• 1976: Philip Sharp & Susan Berget
• discovered that m RNA not exactly complementary to strand of DNA

Introns: non coding, non functional DNA , “junk DNA” Exons: – coding functional sections of DNA (1.5%)

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RNA splicing animation

Steps

• 1. RNA polymerase moves

along gene and transcribes entire gene

• 2. pre mRNA is produced
• 3. A. RNA splicing occurs –

introns are removed and exons are spliced back together

• B. chemical cap and

tail are attached to RNA

• C. pre mRNA now called

mRNA

• 4. molecular “gatekeeper” only

allows processed mRNA to leave and go to cytoplasm to ribosome to make protein

Differences in Prokaryotic and Eukaryotic Gene Expression

Differences in Prokaryotic and Eukaryotic Gene Expression

Prokaryotes

Transcription in cytoplasm Uses operons as functional units Regulatory gene causes inhibitor to make repressor which binds to

• perator

No proofreading- mRNA goes directly to make proteins

Eukaryotes

Transcription in nucleus No operons Regulatory gene recognizes RNA polymerase and starts transcription Proofreading occurs (prevents mutations) DNA Pre mRNA mRNA

Gene Expression Theories:

• Introns have some type of function, but we are not sure what

they are.

• The more complex the organism, the more introns it has.
• It doesn’t make sense for DNA to have introns if there is no

function because it goes to so much work to keep them and remove them.

• Study done where they spliced out introns of a plant leaf and

crossed it: the resulting leaf was very different than original leaf.

• It is thought that introns add evolutionary flexibility.

Gene Expression during Development

During zygote development:

• genes expressed only when

specific proteins are needed

• cell differentiation:

development of different cells with specialized functions

• forms tissues and organs
• morphogenesis: development of

form in an organism enhancer: non-coding control sequence of DNA that facilitates transcription

• located 1000’s of nucleotides away from promoter
• loop can bring enhancer and promoter together
• transcription factors bound to RNA polymerase

and enhancers activate transcription factors bound to promoter and allow transcription to begin

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Homeotic Genes

Homeotic genes: regulatory genes that determine where certain anatomical structures will develop in an organism during morphogenesis

• master genes for development of body
• rganization
• regulate gene expression by turning genes
• n and off
• increases or decreases cell division rates

in areas of developing organism

• every cell in an organism carries, within its

DNA, all of the information necessary to build the entire organism

Homeobox Sequences

• DNA sequences of many genes that control body pattern contain a

common stretch of about 180 nucleotides within its sequence

• homeobox: specific common DNA sequence, codes for

proteins that regulate patterns of development

• only a portion of each gene
• homeotic genes: contain the homeobox
• ex: if the words below were homeotic genes, the

capital letters would represent the homeobox togeTHEr THEoretical gaTHEring boTHEr

MUTATIONS

Mutation: any sudden chemical change in genes or chromosomes (mistake)

• most mutations are recessive
• can occur in any cell
• NOT normal occurrence like recombination
• germ mutation: affect reproductive or germ

cells (inherited)

• somatic mutation: affect body cells (not inherited)

Somatic vs Germ Mutation

Mutant: organism that has a mutation and shows a completely different trait than its parents

• can also carry 1 recessive gene and not

express mutation

• can occur at the level of the chromosome
• r gene

Chromosome mutation: chemical alteration in segments of chrom, whole chrom., or sets of chrom.

• 1. deletion – piece of chrom. is

broken off and information is lost

• 2. duplication - segment of
• chrom. is repeated
• 3. inversion – pieces breaks

from chrom and reattaches to same

• chrom. in reverse order
• 4. translocation – broken piece
• f one chrom. breaks off

and attaches itself to another non- homologous (replicated) chromosome

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Gene Mutation: any chemical change in the base code of DNA molecule

• can affect 1 or many nucleotides
• 1. point mutation - single nucleotide is

affected

• substitution

AUG methionine AUA isoleucine

• 2. frameshift mutation – insertion or deletion
• f a single base
• shifts groupings of codons following

mutation **very serious- will completely change protein made by a single gene**

Cancer

Tumor: abnormal proliferation

• f cell that results

from uncontrolled, abnormal cell division Benign: non cancerous, cells stay within the mass Malignant: uncontrolled dividing cells invade and destroy healthy tissues in body Metastasis: spread of cancer cells beyond

• riginal site

Kinds of Cancer

Carcinomas: grow in skin and tissues that line

• rgans of the body

Sarcomas: grow in bone and muscle tissue Lymphomas: solid tumors that grow in tissues that form blood cells

• cause leukemia

Genetics of Cancer

Oncogenes: genes that cause cancer or other uncontrolled cell proliferation

• proto-oncogene: normal form of oncogene that controls cells

growth and proliferation

• mutation in proto oncongene causes uncontrolled

growth leading to cancer

• tumor suppressor gene: codes for proteins that prevent

uncontrolled cell division

• we have two copies (both must be mutated)
• mutation can cause suppressor expression not to

work leading to uncontrolled growth

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Causes of Cancer

Exposure to the following:

• Radiation
• Viruses
• Chemicals

Study for the test (lots)!!