Three major types of insect herbivores

Phloem feeders, such as aphids and whiteflies, cause little damage to the epidermis and mesophyll cells. Phloem feeders insert their narrow stylet, which is an elongated mouthpart, into the phloem sieve tubes of leaves and stems. 

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#Unit 3. Fundamental Processes

Regardless of how many X chromosomes are present, all but one will be inactivated. 

In normal females there are of course two X chromosomes, but in rare cases where nondisjunction has generated a 3X or greater genotype, only one X chromosome remains active

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#Unit 3. Fundamental Processes

Xic is a cis-acting locus that contains the information necessary to count X chromosomes and inactivate all copies but one. Inactivation spreads from Xic along the entire X chromosome.

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#Unit 3. Fundamental Processes

Xic is a complex genetic locus that expresses several long noncoding RNAs (ncRNAs). The most important of these is a gene called Xist (X  (X inactive specific transcript), which is stably expressed only on the inactive X chromosome.

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#Unit 3. Fundamental Processes

The Xist transcript is regulated in a negative manner by Tsix, its antisense partner.

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#Unit 3. Fundamental Processes

To modify DNA at a new position requires the action of a de novo methyltransferase, which recognizes DNA by virtue of a specific sequence. It acts only on unmethylated DNA to add a methyl group to one strand.

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#Unit 3. Fundamental Processes

There are two de novo methyltransferases (Dnmt3A and Dnmt3B) in the mouse; they have different target sites, and both are essential for development.