bio 2c03 chap 17-20

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uniparental inheritance

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1

uniparental inheritance

inheritance pattern where genome is inherited from only mother or only father

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Organellar Inheritance

transmission of genes on mitochondrial/chloroplasts chromosomes

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Erwin Baur

Research on germanium plants, discovered variegated plants having a combination of both mutant and wt chloroplasts, giving it that "variegated colour"

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How is leaf colour transmitted in germanium? (maternal or paternal)

maternal

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Variegated female parent mixed with any male pollen can produce which types of offspring?

Green, white, and variegated offspring

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homoplasmy vs heteroplasmy

Homoplasmy - all copies of organellar genes are the same Heteroplasmy - copies of organellar genes may contain mutants (ex. variegated)

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Nucleoids (In organelles)

Contains multiple copies of the organellar genome, (important in genome replication of organelles)

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replicative segregation

Random segregation of organelles into progeny cells in cell division. An individual nucleoid can be affected by a single mutant organellar genome (the mutant genome can replicate and mutanize the entire nucleoid, which is later passed down)

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Inheritance of organellar genomes (2 methods)

  1. biased inheritance to the parent that contributes more cytoplasm to the zygote, can be uniparental or biparental (equal cytoplasm distribution)

  2. genetically determined inheritance, one parents genome is selected and the others is destroyed

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Mitochondrial inheritance in Mammals

Maternal inheritance bc egg contributes more cytoplasm than sperm

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Importance of Mitochondrial DNA in Evolution

No recombination exists in mtDNA but 10x more mutations (mtDNA has less DNA mutation repair genes). Mutations can trace ancestry and build pedigrees

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LHON disease

Mitochondrial genetic disease where degradation of the optic nerve results in blindness. Incomplete penetrance, not all progeny experience LHON when mother is affected

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Why is LHON disease Incompletely Penetrant?

Heteroplasmy can lead to some mitochondrial genome being mutant and some being wild type. Genomes are randomly passed down to progeny, so they can receive mutant or wild type genomes

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What happens to gene expression when you have 10 000- 100 000 copies of mitochondrial genome in a single organelle?

Their is a lot of variance in the expression and penetration of mtDNA genes. Some mtDNA genes interact with nuclear DNA genes which cause other changes in expression/penetrance ex) LHON can be penetrated and not expressed if the mutant genomes are low in number relative to wild type

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What does UGA code for in mtDNA?

Trp

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What does AGA code for in mtDNA?

Stop

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mtDNA is in a __________ shape in most organisms

circular

Note: mtDNA can exist linear, multiple linear, and multiple circular in a few organisms)

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mtDNA structure and expression

Heavy strand (protein coding genes) and light strand Encode genes for rRNA and mRNA, these genes on mtDNA are flanked by tRNA genes around them

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Chloroplast structure

Double membrane with thylakoids located inside (third membrane)

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thylakoids

A flattened membrane sac inside the chloroplast, completes photosynthesis

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Chloroplast DNA

mainly circular, contains genes for rRNA, tRNA, and genes involved in photosynthesis Transcription/Translation is similar to bacteria

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Endosymbiosis theory

The theory that the eukaryotic cell evolved via the engulfing of one prokaryotic cell by another. Prokaryotic cells were adapted into organelles such as mitochondria and chlorplasts

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Endosymbiosis evidence

  • double membrane

  • organelle size

  • genome structure

  • similar transcription & translation to bacteria

  • similar protein coding genes to bacteria

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Secondary endosymbiosis

Chloroplasts were engulfed into plastids, replacing mitochondria and developing plant like properties

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NUMTS

Nuclear Mitochondrial Sequences, found in nuclear genome appeared to have originated from mtDNA which was transferred

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How was the organellar genome reduced with evolution

Increased interactions with the nuclear genome lead to gene contaminations and transfers

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NUPTS

Nucelar plastid sequences, found in nuclear genome appeared to have originated from chloroplast DNA which was transferred

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Significance of NUMTS and NUPTS (Three)

  1. They represent evolutionary recent transfers of organellar DNA into nuclear genome

  2. Entire organellar genomes were transferred to nuclear genome (multiple times)

  3. Ongoing process, DNA continues to move btwn organelles and nucleus

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Factors influencing organelle DNA transmission

  1. Dynamics among organelles

  2. Dynamics of organelle genome within each organelle

  3. Replication rate of the individual organellar genome

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Selective Degradation of Mitochondria (Mammals)

Ubiquitin mediated degradation of parental (father) mitchondria

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Mitochondrial Inheritance in Cryptococcus neoformans

Uniparental, discovered via Sxi1alpha gene. When gene was interrupted, biparental inheritance occurred

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How were the genes involved in organism development discovered?

Forward genetics (mutagenesis)

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33

totipotent cells

can differentiate into any cell type

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Pluripotent stem cells

"Fully Differentiated" but can still differentiate into a limited number of cell types (based on the tissue)

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How is a cells fate determined (what will it differentiate to) - 2 mechanisms

  1. Cell inherits molecule that specifies its fate

  2. Fate can be determined through interactions with neighboring cells (inducers/inhibitors)

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What is pattern formation (developmental genetics)

Organization of differentiating cells in the developing embryo to establish body-plane axis. Cells develop a characteristic based on their location

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Morphogens

Substances that change the developmental fate of cells based on their concentration in the cell

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What do Morphogen thresholds do?

The threshold concentration is what limits a cell from having one fate instead of the other

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Organizer cells (Developmental Genetics)

cells that determine their neighboring cells fates/differentiations Inducers/Inhibitors

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Inductor organizer cells

Induce neighbors to adopt specific fate

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Inhibitor organizer cells

Prevents neighbors from adopting specific fate

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Asymetric cell division

two daughter cells inherit different cellular constituents and acquire different fates from the parent cell This can occur in developmental patterning formation

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What is unique about the drosophila embryogenesis pathway?

Embryo develops syncytium instead of blastomere

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syncytium (drosophila development)

multinucleate cell produced in drosophila development (instead of blastomere)

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Syncytial blastoderm (drosophila development)

periphery of embryo surrounded by nuclei enclosed in cell membranes and pole cells are developed on the otuside

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Significance of Syncytial blastoderm (drosophila development)

cells become restricted to their developmental potential (sealed fates), segmentations are developed for different cell fates

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Mutated Coordinate genes

Loss of segments and mirror-image duplications of segments

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Mutated Gap genes

Missing large group of segments (that are grouped together)

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Mutated Pair-Rule genes

Missing parts of adjacent segment pairs

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Mutated segment polarity genes

Affects patterning (anterior/posterior regions) of segments

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Mutated Homeotic genes

Affects identity of a segment

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Maternal effect genes

genes supplied by mother that assist directly in embryonic development

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Zygotic genes

genes that are only active in the zygote/embryo they are dependent on maternal genes (genotype)

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Example of maternally active genes

Coordinate gene - bicoid

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bicoid (coordinate gene)

establishes the anterior-posterior axis in drosophila

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Coordinate genes activate ______________________

gap gene expression patterns

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Combination of coordinate and gap genes determines the expression of ______________ and ______________________________ genes

pair-rule, segment polarity

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Individual segments acquire their characteristics through ______________________ genes

Homeobox (Hox)

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How is hunchback gene expressed (drosophila)

bicoid proteins exceed threshold, act as transcription factors to induce hunchback gene in anterior axis

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How is hunchback gene suppressed (drosophila)

nanos transcription repressors suppress hunchback expression in the posterior axis

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homeodomain

180 nucleotides long, containing a helix-turn-helix motif containing the homeobox (hox genes)

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Patterns of Hox gene expression

Dependant on the segment, each hox gene is expressed at different quantities per segment, giving each segment variation and a distinct characteristic during development

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Hox Complexes (Metazoans)

minor deletions/duplications yet similar cluster arrangement to drosophila

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C. elegans

transparent, free-living animal, can study vulva development

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Vulva development (C. elegans)

six vulval precursor cells (P3-P8) 3 of the 6 participate in vulva formation

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Anchor cell (C. elegans)

binds to p6.p primary vulval cell to initiate vulval development Inductive signal to p6.p

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Cells involved in vulval development (C. elegans)

p6 - primary p5/p7 - secondary these cells duplicate and differentiate into vulva

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Cells not involved in vulval development (C. elegans)

P3, P4, P8 will form the hypodermis instead

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Ablated Anchor cell -mutated (C. elegans)

No vulval development, hermaphrodite state adopted by organism

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What protein from the anchor cell binds to the vulval precursor cell (C. elegans)

LIN-3

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LET-23 (C. elegans)

Protein receptor on vulval precursor cell when LIN_3 binds to it, the nucleus will express genes for vulval fate

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Inhibitor signal by p6.p (C. elegans)

lateral inhibition causes p5.p and p7.p to adopt secondary fate for vulval development

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When LIN-3 is at loss of function (C. elegans)

no vulva development

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When Let-23/Let-60 is at gain of function (C. elegans)

Multiple vulva

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Let-60 loss of function and let-23 gain of function

no vulva development

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76

Gene co-options

finding new uses for existing traits, helps with developing patterning and new organ growth

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Evolution through Co-option

Hox genes (mice and chickens) define limb positions

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Limb bud development

AER formation (pre-limb bud) ZPA promotes the formation of digits (phalanges) via SHH gene

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AER

Apical ectodermal ridge, structure that forms ectodermal cells at the distal end of each limb bud

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ZPA

zone of polarizing activity, area in mesenchyme to signal the limb bud development

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SHH gene

Sonic Hedgehog gene (an ortholog of the drosophila hedgehog gene) is a segment polarity gene

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Abnormal Hoxd gene expression

alters digit identity, affects the expression of SHH, formation of extra digits

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Why dont snakes have limbs (Hox genes)

alterations in Hox and SHH gene expression

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Meristems

Pluripotent cells in plants

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Specialty of plants during development

Can add new organs after maturity unlike animals

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Arabidopsis thaliana

Plant which works well for genetics research due to small genome and fast replication

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Concentric whorls of organs

4 organs in Arabidopsis thaliana sepals, petals, stamens, carpels formed via a collection of undifferentiated cells (meristems)

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wt Arabidopsis thaliana

(OUTER) sepals petals stamens carpels (INNER)

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A-Class mutant (Arabidopsis thaliana)

Two outer whorls, (OUTER) carpels stamens stamens carpels (INNER)

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B-Class mutant (Arabidopsis thaliana)

middle two whorls, (OUTER) sepals sepals carpels carpels (INNER)

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C-Class mutant (Arabidopsis thaliana)

3rd/4th whorls (OUTER) sepals petals petals sepals (INNER)

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BC-double mutant (Arabidopsis thaliana)

(OUTER) sepals sepals sepals sepals (INNER)

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ABC double mutant (Arabidopsis thaliana)

(OUTER) carpels (leaf-like) carpels (leaf-like) carpels (leaf-like) carpels (leaf-like) (INNER)

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AC double mutant (Arabidopsis thaliana)

(OUTER) carpels (leaf-like) stamens (petal-like) stamens (petal-like) carpels (leaf-like) (INNER)

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ABC model for flower development

Model to help visualize gene expression in flowering plants

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MADS box genes

Encoded by flowering plants, containing genes for four transcription factors that play a key role in development

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Are MADS Box genes related to HOX genes in animals?

No

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function of MADS box proteins

help distinguish organ identities in the 4 whorl organs of a plant they interact with SEPALLATAS

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Discontinuous (discrete) variation

single-gene traits, produce the same phenotypic/genotypic ratios

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Continuous variation

polygenic and multifactorial traits

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