Chromosome Structure And Homology

Question 1

Types of DNA and their sizes

Answer 1

1.Unique sequence (single copy)

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

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

Question 2

What is satellite DNA

Answer 2

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

Question 3

Types of satellite DNA

Answer 3

Alpha:
- ~170bp; Located around centromere

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

Micro:
- 2-4bp; Widespread.

Question 4

Interspersed Repetitive DNA

Answer 4

• 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

Question 5

Middle repetitive sequence

Answer 5

• Genes for 18 and 28S ribosomal RNA

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

Question 6

What does chromatin consist of

Answer 6

DNA, proteins, RNA, polysaccharides.

Question 7

2 types of chromatin

Answer 7

1.Euchromatin

2. Heterochromatin:
   - Facultative
   - Constitutive

Question 8

4 features of euchromatin

Answer 8

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

Question 9

5 features of heterochromatin

Answer 9

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

Question 10

What are constitutive heterochromatin

Answer 10

• 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.

Question 11

Which chromosomes contain variably sized constitutive heterochromatin regions

Answer 11

1,9,16,Y.

Question 12

Function of constitutive heterochromatin

Answer 12

Regulation of crossing over.

Question 13

Example of facultative heterochromatin

Answer 13

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

Question 14

Function of RNA-protein enzyme telomerase

Answer 14

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

Question 15

Where are NORs located and describe its stain

Answer 15

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

Question 16

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

Answer 16

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

Question 17

Structure of a karyotype and example

Answer 17

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

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

Question 18

Group A chromosomes

Answer 18

1-3, large metacentric

Question 19

Group B chromosomes

Answer 19

4,5, large sub-metacentric

Question 20

Group C chromosomes

Answer 20

6-12,X, medium sub-metacentric

Question 21

Group D chromosomes

Answer 21

13-15, medium acrocentric + satellites

Question 22

Group E chromosomes

Answer 22

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

Question 23

Group F chromosomes

Answer 23

19,20, short metacentric

Question 24

Group G chromosomes

Answer 24

21,22(short acrocentric + satellites),Y

Question 25

To obtain G-banding, what are slides treated with

Answer 25

Protease, like trypsin

Question 26

Advantages of G-banding

Answer 26

- Simple | - Slides can be kept long time without deteriotion

Question 27

2 types of G-banding

Answer 27

G-dark bands & G-light bands

Question 28

6 things about G-dark bands

Answer 28

- 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

Question 29

6 things about G-light bands

Answer 29

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

Question 30

2 types of cytogenetic changes

Answer 30

Constitutional: From fatal development (zygote stage/embryogenesis) Acquired

Question 31

What is a robertsonian translocation & e.g.

Answer 31

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

Question 32

Characteristics of trisomy 13

Answer 32

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

Question 33

Trisomy 13

Answer 33

Patau syndrome; 47,XY,+13

Question 34

Trisomy 18

Answer 34

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

Question 35

Characteristics of Edwards syndrome

Answer 35

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

Question 36

Trisomy 21

Answer 36

Down syndrome; 47,XY/XX,+21

Question 37

Characteristics of trisomy 21

Answer 37

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

Question 38

Chimerism:

Answer 38

2+ cell lines from 2 seperate zygotes in one individual. | Fusion of dizygotic twins/fertilisation of a retained polar body

Question 39

Mosaic trisomy’s

Answer 39

20,21,8

Question 40

Isochromosome 12p

Answer 40

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

Question 41

Characteristics of isochromosome 12p

Answer 41

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

Question 42

Isochromosome 22q characteristics

Answer 42

``` 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 ```

Question 43

Digyny

Answer 43

2 maternal copies | Diploid egg fertilised by haploid sperm

Question 44

Diandry

Answer 44

2 paternal copies | Formation of partial hydatidiform molar pregnancy

Question 45

Monosomy X and its characteristics

Answer 45

``` 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 ```