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Cytogenetics > Chromosome Structure And Homology > Flashcards

Chromosome Structure And Homology Flashcards

1
Q

Types of DNA and their sizes

A

1.Unique sequence (single copy)

  1. Highly repetitive DNA (>10^5):
    - Tandem
    - Interspersed

3.Middle repetitive (10^2 - 10^4)

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2
Q

What is satellite DNA

A

Tandem repetitive DNA sequences separated from main band of DNA by density gradient centrifugation (Due to differing G-C content).

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3
Q

Types of satellite DNA

A

Alpha:
- ~170bp; Located around centromere

Mini:
- 20-70bp long; Extremely polymorphic; Around sub-telomere can regions.

Micro:
- 2-4bp; Widespread.

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4
Q

Interspersed Repetitive DNA

A
  • SINE: 90-500 bp; High G-C content; Found in G-light bands.

- LINE: 1.5-5kbp; High A-T content; Found in G-dark bands

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5
Q

Middle repetitive sequence

A
  • Genes for 18 and 28S ribosomal RNA

- 100+ copies tandemly arranged on short arms of Afrocentric chromosome

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6
Q

What does chromatin consist of

A

DNA, proteins, RNA, polysaccharides.

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7
Q

2 types of chromatin

A

1.Euchromatin

  1. Heterochromatin:
    - Facultative
    - Constitutive
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8
Q

4 features of euchromatin

A
  1. Unique sequence DNA
  2. Contains coding genes
  3. Extended, uncoiled
  4. Contains active, early replicating genes.
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9
Q

5 features of heterochromatin

A
  1. Tightly wound (contracted)
  2. Few active genes
  3. Replicates late during mitosis (S) phase.
  4. Contains highly repetitive DNA
  5. Located at centromeres
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10
Q

What are constitutive heterochromatin

A
  • Consist of simple repeats of nitrogenous bases located around centromeres of all chromosomes and distal ends of Y chromosome.
  • No transcribed genes, and variation has no effect on phenotype.
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11
Q

Which chromosomes contain variably sized constitutive heterochromatin regions

A

1,9,16,Y.

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12
Q

Function of constitutive heterochromatin

A

Regulation of crossing over.

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13
Q

Example of facultative heterochromatin

A

Lyonisation (1 randomly inactivated X chromosome of every female cell).

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14
Q

Function of RNA-protein enzyme telomerase

A

Rebuilds chromosomal telomeres by repetitively adding 5’-TTAGGG-3’ DNA sequence.

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15
Q

Where are NORs located and describe its stain

A
  • Nucleolar Organising Regions
  • Located on satellite stalks on the short arms of acrocentric chromosomes(13,14,15,21,22)
  • Stains darkly with silver nitrate.
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16
Q

What are landmarks and regions, and what are examples of these

A
  • Consistent and distinct morphological features for identifying chromosomes
  • Telomere, centromere, prominent bands
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17
Q

Structure of a karyotype and example

A
  1. No. of chromosomes.
  2. Sex chromosomes.
  3. Abnormalities (numerical order)

-e.g. 47,XY,t(3;17)(q12;p13),+18

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18
Q

Group A chromosomes

A

1-3, large metacentric

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19
Q

Group B chromosomes

A

4,5, large sub-metacentric

20
Q

Group C chromosomes

A

6-12,X, medium sub-metacentric

21
Q

Group D chromosomes

A

13-15, medium acrocentric + satellites

22
Q

Group E chromosomes

A

16(short metacentric), 17, 18, sub-metacentric

23
Q

Group F chromosomes

A

19,20, short metacentric

24
Q

Group G chromosomes

A

21,22(short acrocentric + satellites),Y

25
Q

To obtain G-banding, what are slides treated with

A

Protease, like trypsin

26
Q

Advantages of G-banding

A
  • Simple

- Slides can be kept long time without deteriotion

27
Q

2 types of G-banding

A

G-dark bands & G-light bands

28
Q

6 things about G-dark bands

A
  • Replicate mid-late S phase
  • Condense early in mitosis
  • Gene poor tissue specific genes
  • A-T rich DNA
  • Enriched in LINES
  • Localised at nuclear periphery
29
Q

6 things about G-light bands

A
  • Replicate early S phase
  • Condense late in mitosis
  • Gene rich housekeeping genes
  • C-G rich DNA
  • Enriched in SINES
  • Located in nucleus interior
30
Q

2 types of cytogenetic changes

A

Constitutional: From fatal development (zygote stage/embryogenesis)

Acquired

31
Q

What is a robertsonian translocation & e.g.

A
  • When 2 q arms of any acrocentric chromosomes join at centromere to produce single metacentric chromosome.
  • e.g. rob(13;14)
32
Q

Characteristics of trisomy 13

A

-Severe MR
-Microcephaly, small eyes
-Brain defects
-Cleft lip
-Heart and kidney abnormal
-6th finger on ulnar side
Cryptorchidism/bicornate uterus

33
Q

Trisomy 13

A

Patau syndrome; 47,XY,+13

34
Q

Trisomy 18

A

Edwards syndrome; 47,XY/(XX),+18

35
Q

Characteristics of Edwards syndrome

A 
Growth deficiency
Severe MR
Rocker-bottom feet
Prominent occiput, low set ears
Clenched fist
Heart defects
36
Q

Trisomy 21

A

Down syndrome; 47,XY/XX,+21

37
Q

Characteristics of trisomy 21

A 
Full life expectancy
Heart abnormalities 50%
10-20% of childhood leukaemia
Variable MR
Commonly from meiosis 1 error
38
Q

Chimerism:

A

2+ cell lines from 2 seperate zygotes in one individual.

Fusion of dizygotic twins/fertilisation of a retained polar body

39
Q

Mosaic trisomy’s

A

20,21,8

40
Q

Isochromosome 12p

A

Pallister-Killian; +i(12)(p10)

41
Q

Characteristics of isochromosome 12p

A

4 copies of 12p (tetrasomy)in affected cells
Mental delay, heart defects, cataracts, hearing loss
Prominent forehead, sparse scalp hair, wide-set eyes

42
Q

Isochromosome 22q characteristics

A 
Cat-eye syndrome
Ocular coloboma
4 copies of 22p
Renal tract abnormal, anal atresia
Variable mental delay
Heart defects, hernia, cleft palate, scoliosis
Preauricular tags
43
Q

Digyny

A

2 maternal copies

Diploid egg fertilised by haploid sperm

44
Q

Diandry

A

2 paternal copies

Formation of partial hydatidiform molar pregnancy

45
Q

Monosomy X and its characteristics

A 
Turners syndrome; 45X
90% miscarriage 
Short stature
Swelled hands and feet
Webbed neck, wide chest, spaced nipples
Infertile (absence of 2ndory sex characteristics)
Normal IQ

Cytogenetics (2 decks)

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