Centromeres and telomeres Flashcards

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1
Q

Centromeres

A

Constricted region of heterochromatin in each eukaryotic chromosome
Includes spindle attachment site
Ensures correct segregation of chromosomes and prevents chromosome breakage

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2
Q

Types of centromeres

A
Point centromeres (S. cerevisae). 1 microtubule attaches. Specific DNA sequence
Regional centromeres (S. pombe, humans). Repetitive DNA sequences
Dispersed/holocentric (C. elegans)

Epigenetic structures

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3
Q

S. cerevisiae centromeres

A
CEN elements (define centromeres)
CEN elements consist of short, conserved sequences (centromere DNA elements)
CDE-I/III flank AT rich region CDE-II
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4
Q

CDE

A

DNA at CDE-II wound around alternative nucleosome containing Cse4
CBF3 tetrameric protein complex binds CDE-III
Cbf1 homodimer binds CDE-I
Proteins bound at CDE-I/II/III interact with Ctf19 complex (links centromeric complex to kinetochore)
Proteins act as assembly platform for kinetochore

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5
Q

Regional centromeres

A

Core sequence flanked by repetitive DNA
Recruit specific histones (CENP-A) into nucleosomes of central region
Whole structure specified as heterochromatin

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6
Q

Formation of centromeric heterochromatin in S. pombe

A

Centromeric repeats transcribed by RNA pol II
dsRNA formed by convergent transcription and RNA-dependent RNA polymerase (RDRP)
siRNAs produced by dicer and loaded into RNA-induced transcriptional silencing complex (RITS)
RITS binds nascent transcripts and recruits HMT and RDRP
H3K9 methylation recruits HP1
Heterochromatin spreading

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7
Q

Human centromere

A

Long arrays of simple tandem repeats
Higher order repeat of alpha-I satellite DNA (copied thousands of times to form chromatin)
Pericentromere is divergent repetitive sequences and retrotransposons (alpha-II satellite DNA)

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8
Q

Telomere structure

A
TTAGGG repeat sequence in vertebrates
G rich 5'-3' towards telomere
C rich 5'-3' towards centromere
Variable length
G-rich 3' ss tail results from post-replicative processing of C-rich strand to remove primers and iDNA
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9
Q

Chromosome capping

A

3’ single stranded G-rich strand forms T-loop
Displaces complementary sequence in an upstream region of the telomere
Catalysed by TRF2

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10
Q

Telomerase

A

Only active in stem and cancer cells

Ribonucleoprotein complex composed of telomerase reverse transcriptase (TERT) and RNA template (TERC)

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11
Q

Shelterin

A

Coats telomeres and protects from DNA damage repair pathways
Regulates telomere length
Inhibits telomerase

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12
Q

Shelterin complex

A

Telomeric ds repeat binding factors 1/2 (TRF1/2)
TRF2-interacting factor repressor and activator protein 1 (Rap1)
Telomeric protection factor (POT1)
Bridging molecules TIN2 and TPP1

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13
Q

Telomere heterochromatin

A
Resembles pericentric chromatin
H3K9, H4K20 methylation
HP1 binding (reader)
Telomeric DNA cannot be methylated (no CpG)
Sub-telomeric DNA methylated
May regulate telomere length
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14
Q

TERRA RNAs

A

Telomere repeat containing RNA (lncRNA)
5’-UUAGGG-3’ repeats at 3’ end
Base pairs with C or G-rich strand of telomeres
Promotes recruitment of enzymes (heterochromatin formation and DNA replication)
Control telomere length (inhibit telomerase)

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15
Q

Importance of telomeres

A

Senescence (limiting lifespan of normal cells). Limits mutation accumulation in dividing cells
Promote genome stability

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