b. the trp operon in e. coli

B. The trp Operon in E. coli

This operon is responsible for making proteins that PRODUCE the amino acid tryptophan.

If [trp] is low, then genes are onAnd trp is made

As [trp] increases, it binds to repressor, ACTIVATING the repressor so that it can bind to the operator – turning genes OFF.

This regulation is also mediated through stem-loop formation in the ‘leader-attenuator’ region of the m-RNA product.


This regulation is mediated through stem-loop formation in the ‘leader-attenuator’ region of the m-RNA product.

If [trp] is low:

Ribosome read to the double trp codons, but [trp] is low, so they are slow to add into the ribosome… so translation stalls in the 1 region; so 2 binds with 3. The 2-3 loop forms does not destabilize transcription – transcription continues and the rest of the m-RNA is made, and translation continues, also....

GENES ON for trp SYNTHESIS

m-RNA

…m-RNA


This regulation is mediated through stem-loop formation in the ‘leader-attenuator’ region of the m-RNA product.

If [trp] is high:

Ribosome read through the trp codons, adding trp amino acid to the protein chain and preventing the 2-3 loop. 3 binds with 4, forming the loop that terminates transcription even before [trp] is high enough to activate the repressor. Transcription is terminated in the attenuator region.

m-RNA

…m-RNA

XII. Gene Regulation

A.The lac Operon in E. coli


C. Regulation in Eukaryotes

Regulation in eukaryotes is more complex, as it regulates the specialization of different tissues, the developmental changes of organisms over time, and genetic responses to the environment.

As such, it is not surprising that the patterns of regulation are much more complex; often as a consequence of greater structural complexity of chromosomes and genes.


- Histones affect accessibility of a gene


- Histones affect accessibility of a gene - Methylation causes condensation of genes and chromosomes

ImprintingHeterochromatin (highly repetitive DNA)Barr Bodies (X inactivation)


- Histones affect accessibility of a gene - Methylation causes condensation of genes and chromosomes - Transcription is regulated by transcription factors that increase (enhancers)

or decrease (silencers) the ability of the RNA polymerase to bind to the promoter and begin transcription.

Because genes are regulated by multiple transcription factors, it means one gene can be turned on by many different signals – and become associated with many enzymatic pathways in a cell/organism. And, binding can be modulated – not just on/off but on by degrees


- Histones affect accessibility of a gene - Methylation causes condensation of genes and chromosomes - Transcription is regulated by transcription factors that increase (enhancers)

or decrease (silencers) the ability of the RNA polymerase to bind to the promoter and begin transcription.

They tend to have particular structures that bind DNA in specific ways: helix-turn-helix, zinc fingers, leucine zippers


- Transcription can also be regulated by small pieces of RNA called “si-RNA” (small interfering RNA) and mi-RNA (microRNA). These molecules are short RNA sequences that bond with m-RNA and either cleave it (si-RNA) or just bind to it and block translation (mi-RNA). They can also initiate methylation of promoters and turn genes off.

- This process is called RNA interference or RNAi

High resolution image (pdf 2,5 Mb) •

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Fire and Mello -1998 (Nobel in Phys or Medicine 2006)

RISC = “RNA-induced Silencing Complex”Binds to m-RNA and splices it.

http://www.nobelprize.org/nobel_prizes/medicine/laureates/2006/medpress_eng.pdf

http://www.nobelprize.org/nobel_prizes/medicine/laureates/2006/medpress_eng.pdf

http://www.addthis.com/bookmark.php?v=250

http://www.nobelprize.org/prog/php/tellfriend/tell.php?head=1&top=1&url=/nobel_prizes/medicine/laureates/2006/press.html

http://www.addthis.com/bookmark.php?v=250

http://www.nobelprize.org/prog/php/tellfriend/tell.php?head=1&top=1&url=/nobel_prizes/medicine/laureates/2006/press.html

High resolution image (pdf 2,5 Mb) •

•


- There are two ways that double-stranded RNA’s naturally occur – through viral infection or transcription of mi-RNA “genes”.

- Unclear whether this evolved first as a mechanism to regulate gene production, or as an adaptation to resist viral infection.


- RITS target promoters of specific genes or larger regions of chromatin. Remodeling of chromatin turns genes off, often by inducing methylation.

- Important in imprinting

- Both pathways can inhibit production of transcription factors, too, and affect other regulatory pathways.

RITS = “RNA induced initation of transcription silencing complex”


- Alternate Splicing Pathways, regulated by intronic ribozymes and spliceosomes, can make different protein products

A calcium regulator in the thyroid

A hormone made in the brain


- The initial polypeptide product can be spliced differently, too, to produce different functional proteins from the same initial polypeptide. And the methionine is cleaved and it is coplexed with other molecules (quaternary protein, lipoprotein, glycoprotein, riboprotein).

Chromatin remodeling

Polymerase binding

M-RNA processing

Post-translational modifications

transcription

translation

zygote

mitosis

Heredity, Gene Regulation, and Development MutationA. Overview

MutationA. Overview

1) A mutation is a change in the genome of a cell.

MutationA. Overview

1) A mutation is a change in the genome of a cell. 2) Some mutations occur during DNA repair, or after DNA is damaged by a mutagen. These changes may affect how that particular cell works. When/if that cell divides, then this defect will be propagated to the daughter cells in that body tissue. These are somatic mutations.

MutationA. Overview

1) A mutation is a change in the genome of a cell. 2) Some mutations occur during DNA repair, or after DNA is damaged by a mutagen. These changes may affect how that particular cell works. When/if that cell divides, then this defect will be propagated to the daughter cells in that body tissue. These are somatic mutations. 3) Some errors occur in DNA replication that precedes cell division; these changes are passed to the daughter cells in that body tissue. These are somatic mutations, too.

MutationA. Overview

1) A mutation is a change in the genome of a cell. 2) Some mutations occur during DNA repair, or after DNA is damaged by a mutagen. These changes may affect how that particular cell works. When/if that cell divides, then this defect will be propagated to the daughter cells in that body tissue. These are somatic mutations. 3) Some errors occur in DNA replication that precedes cell division; these changes are passed to the daughter cells in that body tissue. These are somatic mutations, too. 4) Some mutations occur during meiosis, and produce mutant gametes. These are the heritable mutations that we will focus on.

VI. MutationA.Overview

A change in the genome

Occurs at four scales of genetic organization:

1: Change in the number of sets of chromosomes ( change in ‘ploidy’)

2: Change in the number of chromosomes in a set (‘aneuploidy’)

3: Change in the number and arrangement of genes on a chromosome

4: Change in the nitrogenous base sequence within a gene

VI. MutationA.OverviewB.Changes in Ploidy

- These are the most dramatic changes, adding a whole SET of chromosomes

Triploidy occurs in 2-3% of all human pregnancies, but almost always results in spontaneous abortion of the embryo.

Some triploid babies are born alive, but die shortly after.

Syndactyly (fused fingers), cardiac, digestive tract, and genital abnormalities occur.



1.Mechanism #1: Complete failure of Meiosis - if meiosis fails, reduction does not occur and a diploid gamete is produced. This can occur because of failure of homologs OR sister chromatids to separate in Meiosis I or II, respectively.

Failure of Meiosis I

2n = 4Gametes:2n = 4



1.Mechanism #1: Complete failure of Meiosis - if meiosis fails, reduction does not occur and a diploid gamete is produced. This can occur because of failure of homologs OR sister chromatids to separate in Meiosis I or II, respectively.

Failure of Meiosis II

2n = 4

Normal gamete formation is on the bottom, with 1n=2 gametes. The error occurred up top, with both sister chromatids of both chromosomes going to one pole, creating a gametes that is 2n = 4.



1.Mechanism #1: Complete failure of Meiosis - if meiosis fails, reduction does not occur and a diploid gamete is produced. This can occur because of failure of homologs OR sister chromatids to separate in Meiosis I or II, respectively. - this results in a single diploid gamete, which will probably fertilize a normal haploid gamete, resulting in a triploid offspring.

-negative consequences of Triploidy:1) quantitative changes in protein production and regulation.2) can’t reproduce sexually; can’t produce gametes if you are 3n.

Like this Blue-spotted Salamander A. laterale, which has a triploid sister species, A. tremblayi



1.Mechanism #1: Complete failure of Meiosis-negative consequences of Triploidy:

1) quantitative changes in protein production and regulation.2) can’t reproduce sexually; can’t produce gametes if you are 3n.3) but, some organisms can survive, and reproduce parthenogenetically (mitosis)

A. tremblayi is a species that consists of 3n females that reproduce clonally – laying 3n eggs that divide without fertilization.



1.Mechanism #1: Complete failure of Meiosis2.Mechanism #2: Failure of Mitosis in Gamete-producing Tissue

2n

1) Consider a bud cell in the flower bud of a plant.

2n


4n

2) It replicates it’s DNA but fails to divide... Now it is a tetraploid bud cell.

2n


4n


3) A tetraploid flower develops from this tetraploid cell; eventually producing 2n SPERM and 2n EGG

2n


4n


3) A tetraploid flower develops from this tetraploid cell; eventually producing 2n SPERM and 2n EGG

4) If it is self-compatible, it can mate with itself, producing 4n zygotes that develop into a new 4n species.

Why is it a new species?

How do we define ‘species’?

“A group of organisms that reproduce with one another and are reproductively isolated from other such groups”(E. Mayr – ‘biological species concept’)

How do we define ‘species’?

Here, the tetraploid population is even reproductively isolated from its own parent species…So speciation can be an instantaneous genetic event…

4n

2n

Gametes

2n

4n

Zygote

2n

1n

1n

Gametes

Zygote

2n3n

Triploid is a dead-end… so species are separate



1.Mechanism #1: Complete failure of Meiosis2.Mechanism #2: Complete failure of Mitosis3.Mechanism #3: Allopolyploidy - hybridization

2n = 16

Black Mustard

2n = 18

Cabbage

n = 8

n = 9

gametes

n = 17

Fertilization produces a cell with non-homologous chromosomes

2n = 34

Polyploidy occurs here; creating a cell with homologous sets

New Species

Spartina alterniflora from NA colonized Europe

Spartina maritima native to Europe

X

Spartina anglica – an allopolyploid and a worldwide invasive outcompeting native species

Sterile hybrid – Spartina x townsendii

Allopolyploidy – 1890’s



1.Mechanism #1: Complete failure of Meiosis2.Mechanism #2: Complete failure of Mitosis3.Mechanism #3: Allopolyploidy - hybridization4.The Frequency of Polyploidy

For reasons we just saw, we might expect polyploidy to occur more frequently in hermaphroditic species, because the chances of ‘jumping’ the triploidy barrier to reproductive tetraploidy are more likely. Over 50% of all flowering plants are polyploid species; many having arisen by this duplication of chromosome number within a lineage.

b. the trp operon in e. coli

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