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BIOL 202 > Lecture 27 > Flashcards

Lecture 27 Flashcards

1
Q

where do histone modifications occur?

A

on N-terminal ends of core histones that protrude from nucleosome

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2
Q

4 examples of histone modifications

A
  1. acetylation
  2. methylation
  3. phosphorylation
  4. ubiquitination
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3
Q

3 types of histone modifiers

A
  1. Writers
  2. Erasers
  3. Readers
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4
Q

what are readers?

A

can recognize and bind modifications –> coupled with writers and erasers –> 1 mark is made which recruits a reader to allow more marks by the associated writer

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5
Q

how does histone acetylation increase transcription? (2)

A
  1. lysine is acetylated to neutralize its (+) charge
    - loosens its interactions with (-) DNA
    - chromatin relaxes, more accessibility, more transcription
  2. makes binding site for histone code readers that promote activation
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6
Q

what is the writer enzyme for histone acetylation?

A

HAT

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7
Q

what proteins typically have HAT activity?

A

transcriptional co/activators have HAT activity

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8
Q

what is the eraser enzyme for histone acetylation?

A

HDAC

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9
Q

what proteins typically have HDAC activity?

A

transcriptional co/repressors have HDAC activity

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10
Q

how does histone methylation reduce transcription? (2)

A
  1. lysine or arginine is methylated (charge is unaffected) to silence genes
  2. makes binding site for histone code readers
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11
Q

what is the writer enzyme for histone methylation?

A

HMTase

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12
Q

does histone methylation promote heterochromatin or euchromatin formation?

A

heterochromatin

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13
Q

how does histone methylation promote heterochromatin formation? (4 steps)

A
  1. Heterochromatin Protein 1 (HP1) binds methylated histones (H3K9Me)
  2. HP1 induces methylation formation and recruits more HMTase
  3. HMTase methylates neighbouring nucleosomes which become new HP1 binding site
  4. heterochromatin spreads!
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14
Q

what stops the spread of heterochromatin?

A
  1. HATs bind a barrier insulator DNA seq
  2. this causes acetylation to counteract the spread of heterochromatin (i.e. sets limit for where heterochromatin is)
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15
Q

definition of epigenetics

A

study of heritable traits that are not explained by changes in DNA sequences due to proteins associated with DNA

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16
Q

can epigenetics be inherited?

A

yes, can be passed to daughter cells in mitosis

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17
Q

how do epigenetics marks get inherited in mitosis?

A
  1. during DNA replication, nucleosomes disassemble to let replication fork pass thru
  2. then, nucleosomes reassemble on daughter strands as a mix of new and old histones
  3. old histones with modifications recruit readers and writers for modifications of new histones
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18
Q

what is the phenomenon that occurs in Drosophila due to epigenetic modifications?

A

Position-Effect Variegation

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19
Q

what is the gene involved in position-effect variegation?

A

white gene –> ABC transporter that brings red pigment to eye

WT = red pigment
mutant = white pigment

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20
Q

what chromosome is the white gene found on?

A

X chromosome

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21
Q

describe a normal, WT drosophila eye

A

there is a barrier insulator, where part is heterochromatin and the other part is euchromatin
- the WT white gene is in euchromatin so it is expressed normally and the eye is fully red

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22
Q

describe a drosophila eye with position-effect variegation

A

mutagenized with X-rays –> creates inversions where white gene is close to heterochromatin

the barrier insulator has moved, so the heterochromatin can randomly spread into the white gene
–> in some cells, there will be more heterochromatin spreading so the gene will be silenced (white)
–> in other cells, there will be less heterochromatin spreading so the gene will be expressed (red)

all cells have the same genotype but different amounts of heterochromatin

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23
Q

what is non-mendelian genetics?

A

traits don’t segregate according to Mendel’s law –> genotype =/= expected phenotype

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24
Q

when does dosage compensation occur?

A

in FEMALE mammalian embryo at 8-32 cell stage

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25
Q

what is dosage compensation?

A

each cell randomly inactivates one of the X-chromosomes and forms a heterchromatic Barr body

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26
Q

what is the purpose of dosage compensation?

A

allows males and females to have equal expression of genes on the X chromosome

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27
Q

why do all descendent cells have the same inactive X chromosomes as the parent cell?

A

epigenetics are inherited in mitosis

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28
Q

describe dosage compensation in tortoiseshell cats

A

females are heterozygous with 1 allele for orange fur and 1 allele for black fur

  • at 8-32 cell stage, one of the alleles is inactivated
  • cat ends up as a mix of both colours

(males are all orange or all black)

29
Q

what causes red-green colour blindness?

A

mutation in OPN1-LW gene on X chromosome

30
Q

who can get red-green colour blindness?

A

males: mut/Y –> always

females: mut/X –> sometimes

31
Q

why can some mut/X females be colourblind and others are normal?

A

due to X-inactivation –> can inactivate mut or WT X chromosome

32
Q

describe 50-50 X-inactivation

A

half the cells inactivate the mutant allele, so half the cells are WT –> normal vision

33
Q

describe skewed X-inactivation

A

more than half the cells inactivate WT so most cells are mutant –> colour blind

34
Q

how does skewed X inactivation affect females?

A

skewed X inactivation causes female carriers of X-linked mutations to display partial symptoms

35
Q

why are some X-linked diseases only found in females?

A

for some diseases, mut/Y males are not viable

need 1 viable gene copy to survive

36
Q

what causes X inactivation in mammals

A

1 X is repressed with increased H3K9Me and DNA methylation, decreased acetylation

37
Q

how do flies achieve dosage compensation?

A

male X is hyperactive

38
Q

how does the male X in flies become hyperactive?

A

increased acetylation

39
Q

what enzyme is responsible for DNA methylation? how does it work?

A

DNMT enzyme adds methyl to C5, mainly at CpG

40
Q

what % of CpG are methylated?

A

60-80% in specific pattern throughout genome

41
Q

where are CpGs typically located?

A

at intergenic regions –> correlated to heterochromatin

42
Q

what are CpG islands?

A

CpG-rich clusters near promoters for 60% of genes

43
Q

are CpG islands methylated or unmethylated? what does this indicate about the transcription activity at these sites?

A

unmethylated –> transcriptionally active

44
Q

2 ways that DNA methylation causes gene silencing

A
  1. DIRECT EFFECT: methylation blocks TF binding
  2. INDIRECT EFFECT: methylation recruits HDACs and HMTs that cause repressive histone modifications
45
Q

is DNA methylation inherited?

A

yes, it is passed on thru mitosis

46
Q

describe the inheritance of DNA methylation in mitosis

A
  1. after replication, the 2 new daughter stands are HEMIMETHYLATED –> old strand has Me, new strand does not have Me
  2. DNMT have high affinity for hemimethylated sites so they add Me on unmethylated strands
47
Q

what allows DNMT to have high affinity for hemimethylated sites?

A

methylation pattern on parental strand

48
Q

role of Igf2 gene?

A

stimulates cell growth

49
Q

is Igf2 sex-linked or autosomal?

A

autosomal

50
Q

what happens to Igf+/- mice?

A

they are small ONLY if mutant allele was inherited from father

51
Q

what is genomic imprinting aka?

A

silencing, Monoallelic Inheritance

52
Q

what is genomic imprinting?

A

only 1 of the paternal or maternal copy is expressed as if there was only 1 copy of the gene in the cell

53
Q

why is genomic imprinting sex-specific?

A

the allele that gets expressed depends ONLY on the sex of the parent it came from

54
Q

what causes the imprinted allele to be silenced?

A

inactivated by methylation

55
Q

why is genomic imprinting inheritable?

A

methylation is inheritable, so all daughter cells will have the same mark

56
Q

what other gene is involved in Igf2 imprinting? why?

A

H19

they are adjacent

57
Q

describe the genomic imprinting with Igf2 and H19 in the MATERNAL allele

A
  1. Imprinting Control Region (ICR) is unmethylated so CTCF binds
  2. CTCF acts as an enhancer-blocking insulator
  3. Enhancer can only reach H19 which is close by

Igf2 is maternally imprinted and only H19 is expressed

58
Q

describe the genomic imprinting with Igf2 and H19 in the PATERNAL allele

A
  1. ICR is methylated so CTCF cannot bind and TF binds at enhancer
  2. H19 promoter is also methylated so its transcription is silenced

H19 is paternally imprinted and only Igf2 is expressed

59
Q

why is the Igf2/H19 imprinting sex-specific?

A

the methylation at ICR is sex-specific –> only occurs in makes

60
Q

when are maternal/paternal imprints established?

A

during gametogenesis

61
Q

describe establishment of imprints in males

A
  1. original DNA methylation is removed in germ cells
  2. ICR is methylated
  3. all sperm receive the methylation
62
Q

describe establishment of imprints in females

A
  1. original DNA methylation and X inactivation is removed in germ cells
  2. ICR is not methylated
  3. all oocytes are unmethylated
63
Q

2 possible reasons why female ICR does not get methylated?

A
  1. missing enzyme
  2. gets methylated then removes the marks
64
Q

why are uniparental embryos/cloned organisms INVIABLE?

A

researcher removes marks from established donor cell but when it develops it will only have male OR female pattern bc only 1 pattern to influence the marks –> organism is unstable

normal embryos have both patterns differentially expressed in different cells

65
Q

difference btwn differentially imprinted maternal and paternal alleles

A

genetically identical but epigenetically different

66
Q

what happens if there is a mutation in an imprinted gene?

A

unaffected bc gene normally silenced

67
Q

what happens if there is a mutation in a non-imprinted gene?

A

affected bc gene normally expressed

68
Q

what causes Silver-Russell Syndrome (dwarfism)?

A

faulty imprinting

69
Q

describe the faulty imprinting in individual affected with Silver-Russell Syndrome?

A

paternal ICR does NOT get methylated so even paternal alleles will only expresse H19, not Igf –> i.e. have another maternal allele copy

BIOL 202 (29 decks)

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