Chromosome Structure And Homology Flashcards
Types of DNA and their sizes
1.Unique sequence (single copy)
- Highly repetitive DNA (>10^5):
- Tandem
- Interspersed
3.Middle repetitive (10^2 - 10^4)
What is satellite DNA
Tandem repetitive DNA sequences separated from main band of DNA by density gradient centrifugation (Due to differing G-C content).
Types of satellite DNA
Alpha:
- ~170bp; Located around centromere
Mini:
- 20-70bp long; Extremely polymorphic; Around sub-telomere can regions.
Micro:
- 2-4bp; Widespread.
Interspersed Repetitive DNA
- 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
Middle repetitive sequence
- Genes for 18 and 28S ribosomal RNA
- 100+ copies tandemly arranged on short arms of Afrocentric chromosome
What does chromatin consist of
DNA, proteins, RNA, polysaccharides.
2 types of chromatin
1.Euchromatin
- Heterochromatin:
- Facultative
- Constitutive
4 features of euchromatin
- Unique sequence DNA
- Contains coding genes
- Extended, uncoiled
- Contains active, early replicating genes.
5 features of heterochromatin
- Tightly wound (contracted)
- Few active genes
- Replicates late during mitosis (S) phase.
- Contains highly repetitive DNA
- Located at centromeres
What are constitutive heterochromatin
- 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.
Which chromosomes contain variably sized constitutive heterochromatin regions
1,9,16,Y.
Function of constitutive heterochromatin
Regulation of crossing over.
Example of facultative heterochromatin
Lyonisation (1 randomly inactivated X chromosome of every female cell).
Function of RNA-protein enzyme telomerase
Rebuilds chromosomal telomeres by repetitively adding 5’-TTAGGG-3’ DNA sequence.
Where are NORs located and describe its stain
- Nucleolar Organising Regions
- Located on satellite stalks on the short arms of acrocentric chromosomes(13,14,15,21,22)
- Stains darkly with silver nitrate.
What are landmarks and regions, and what are examples of these
- Consistent and distinct morphological features for identifying chromosomes
- Telomere, centromere, prominent bands
Structure of a karyotype and example
- No. of chromosomes.
- Sex chromosomes.
- Abnormalities (numerical order)
-e.g. 47,XY,t(3;17)(q12;p13),+18
Group A chromosomes
1-3, large metacentric
Group B chromosomes
4,5, large sub-metacentric
Group C chromosomes
6-12,X, medium sub-metacentric
Group D chromosomes
13-15, medium acrocentric + satellites
Group E chromosomes
16(short metacentric), 17, 18, sub-metacentric
Group F chromosomes
19,20, short metacentric
Group G chromosomes
21,22(short acrocentric + satellites),Y
To obtain G-banding, what are slides treated with
Protease, like trypsin
Advantages of G-banding
- Simple
- Slides can be kept long time without deteriotion
2 types of G-banding
G-dark bands & G-light bands
6 things about G-dark bands
- 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
6 things about G-light bands
- Replicate early S phase
- Condense late in mitosis
- Gene rich housekeeping genes
- C-G rich DNA
- Enriched in SINES
- Located in nucleus interior
2 types of cytogenetic changes
Constitutional: From fatal development (zygote stage/embryogenesis)
Acquired
What is a robertsonian translocation & e.g.
- When 2 q arms of any acrocentric chromosomes join at centromere to produce single metacentric chromosome.
- e.g. rob(13;14)
Characteristics of trisomy 13
-Severe MR
-Microcephaly, small eyes
-Brain defects
-Cleft lip
-Heart and kidney abnormal
-6th finger on ulnar side
Cryptorchidism/bicornate uterus
Trisomy 13
Patau syndrome; 47,XY,+13
Trisomy 18
Edwards syndrome; 47,XY/(XX),+18
Characteristics of Edwards syndrome
Growth deficiency Severe MR Rocker-bottom feet Prominent occiput, low set ears Clenched fist Heart defects
Trisomy 21
Down syndrome; 47,XY/XX,+21
Characteristics of trisomy 21
Full life expectancy Heart abnormalities 50% 10-20% of childhood leukaemia Variable MR Commonly from meiosis 1 error
Chimerism:
2+ cell lines from 2 seperate zygotes in one individual.
Fusion of dizygotic twins/fertilisation of a retained polar body
Mosaic trisomy’s
20,21,8
Isochromosome 12p
Pallister-Killian; +i(12)(p10)
Characteristics of isochromosome 12p
4 copies of 12p (tetrasomy)in affected cells
Mental delay, heart defects, cataracts, hearing loss
Prominent forehead, sparse scalp hair, wide-set eyes
Isochromosome 22q characteristics
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
Digyny
2 maternal copies
Diploid egg fertilised by haploid sperm
Diandry
2 paternal copies
Formation of partial hydatidiform molar pregnancy
Monosomy X and its characteristics
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
