Silence of the Genes

Genetics The study of inheritance

Gregor Mendel ( )

Mendelian Ratios

Mutation ATGCGAGCGAGTATGCGATCGAGT Genotype Phenotype

Epigenetics Heritable changes in gene expression that do not involve changes in DNA sequence.

Epi-mutation ATGCGAACGAGTATGCGATCGAGT Genotype Phenotype DNA Modifications Histone Modifications Proteins

Nucleosome DNA histones

DNA Methylation histones CH 3

Histone Methylation histones CH 3

Examples of Epigenetics

X-Inactivation inactivation malefemale XYXX unequal expression equal expression XYX X Barr Body

Calico Cat XX oO X X o O X X o O orange allele black allele black sector orange sector

Other Examples of Epigenetics

Imprinting Imprinted genes are expressed differently depending on whether they are inherited through the maternal or paternal parent.

Horses and Donkeys male female

Gynogenotes Embryos containing two female genomes –do not develop normally –fail due to underdeveloped extraembryonic placental tissue

Androgenotes Embryos containing two paternal genomes –result in abnormal (often overgrown) embryo –display overdeveloped extraembryonic placental tissue

Maternal vs. Paternal Imprinting Male genome wants to promote growth. Female genome wants to inhibit growth.

Resetting Methylation Patterns Primordial Germ Cells male female methylation developmental time

Battle for Maintaining DNA Methylation

Battle to Maintain Methylation methylation developmental time Fertilization

Female Strategy Genes that promote growth Fertilization

Male Strategy Antisense Genes that inhibit growth Fertilization

Resetting Imprints is Important for Proper Development

Dolly First mammal to be cloned from an adult cell. Developed illness common in older sheep (arthritis) Probably due to abnormal imprinting

Gene Silencing

Heterochromatin Densely staining condensed chromosomal regions; believed to be transcriptionally inert. Euchromatin A chromosomal region that stains normally; thought to contain the normally functioning genes.

Heterochromatin Paul Fransz B. McClintock IV

Heterochromatin Centromeric regions Telomeric regions

Position Effect Variegation W+W+ W+W+ W+W+ W+W+ heterochromatin spreading suppressed enhanced

Post-Transcriptional Gene Silencing

Anti-Sense

Sense Also Works? Guo and Kemphues, (1995)

What is Causing Silencing? ?

dsRNA Post Transcriptional ? Fire et al. (1998)

RNAi ? Fire et al. (1998)

RNAi A A A A A A Dicer RISC dsRNA

A A A A A A Dicer RISC dsRNA RNAi

RNA Silencing cosuppression

quelling RNA Silencing

Post Transcriptional Gene Silencing (PTGS) RNA Silencing

RNA Interference (RNAi)

RNA Interference

DNA Methylation Mette et al. (2000)

RNAi and Transcriptional Silencing

Transcriptional Silencing Mette et al. (2000)

Micro RNAs Grishok et al. (2001) dicer

RNAi Genes in S. pombe?

RNAi Genes in pombe? Yes!! Mutation of these genes results in loss of centromeric silencing. Loss of RNAi reveals centromeric transcripts. dsRNA from centromere targets transcriptional silencing.

Cen

RNAi Small RNAs

Cen RNAi Silencing Machinery Small RNAs

Cen RNAi Silencing Machinery Small RNAs

Cen

RNAi Is important for initiation and maintenance of heterochromatin at the centromere. Could be involved in other silencing phenomena

Where else may RNAi be functioning to silence genes?

X Chromosome Inactivation (RNAi?) Xist Tsix Heard et al (2001)

Xist Tsix Heard et al (2001) RNAi Silencing Machinery X Chromosome Inactivation (RNAi?)

Xist Tsix Heard et al (2001) RNAi Silencing Machinery X Chromosome Inactivation (RNAi?)

Imprinting (RNAi?) Antisense RNAi Silencing Machinery Small RNAs

Imprinting (RNAi?) Antisense

Imprinting (RNAi?) RNAi Silencing Machinery

Heterochromatin Densely staining condensed chromosomal regions; are not necessarily transcriptionally inert. RNAi is important for its initiation and maintenance. Euchromatin Chromosomal regions that do not densely stain; thought to contain functioning genes that may be transcriptionally active.