L1: Cytogenetics

Introductions

Chromosome

Characteristics

  • composed of thin chromatin threads called chromatin fibres

    • undergo folding coiling and supercoiling during prophase → thicker and smaller: observable under light microscope

  • telomere: chromosome ends

  • centromere: site of spindle attachment

  • nucleosome: DNA double helix wrapped around histone proteins

Chromosomal Locations

  • arm: p and q

  • region: 1 and 2

  • band:

    • p: 3, 2, 1

    • q: 1, 2, 3, 4

  • suband

    • p23, q11, q12, q24: 1-3

    • p22: 1-2

    • p11: 1-5

    • q13: 1-4

Chromosome morphology

  • metacentric

    • centrally located centromere

    • two arms of equal length

    • chromosomes 1,3, 16, 19, 20

  • submetacentric

    • slightly off-center centromere

    • a short arm (p) and a long arm (q)

    • chromosome 17 and 18

  • acrocentric

    • centromere located near the end of a chromosome

    • a long arm and a short arm (satellite and stalk)

    • chromosomes 13, 14, 15, 21, and 22

Definitions

  • Karyotype

    • the general morphology of the somatic chromosome

    • represent by arranging in the descending order of size

    • centromeres are kept in a straight line

  • Ideogram

    • diagrammatic representation of karyotype of a species

    • shows all morphological features of the chromosomes

Chromosome Abnormalities

Numerical abnormalities

Down syndrome

  • trisomy 21

  • arise from

    • non disjunction in the first meiotic division

    • mosaic: non dissection during early zygotic division

    • inheritance of a translocation chromosome from a parent who is a balanced carrier

  • symptoms

    • facial dysmorphology

    • IQ less than 50

    • moderate to severe mental handicap

Patau syndrome

  • trisomy 13

  • Mostly due to

    • maternal non-disjuction

    • a parent who is a translocation carrier

  • 50% of babies die within 1st month and very few service beyond the first tear

  • symptom

    • small head

    • small or missing eyes

    • hear defects

    • extra fingers

    • abnormal genitalia

    • mentally retarded

    • cleft palate

Edwards syndrome

  • trisomy 18

  • not inherited: random event in sperm or egg

Turner syndrome

  • 45, X

  • # 23 monosomy non-disjunction

  • X inactivation occurs randomly and is not inherited

  • symptoms

    • very short

    • infertile

    • broad chest

    • widely spaced nipples

    • webbed neck

    • unaffected IQ and lifespan

Kleinfelter’s Syndrome

  • 47, XXY

  • # 23 trisomy non-disjunction

  • symptoms

    • scarce beard

    • longer fingers and arms

    • sterile: testes are small and fail to produce normal levels of testosterone→ breast growth

    • delicate skin

    • low mental ability

    • slight IQ reduction

    • normal lifespan

Non-disjunction

  • unequal distribution of chromosome due to failure of separation

  • occurs during mitosis/ meiosis/ embryogenesis

  • mitotic non-disjunction

    • occurs after fertilisation

    • happen during 1st or 2nd cleavage

    • → one blastomere receive 45 chromosomes, the other receive 47

  • meiotic non-disjunction

    • occur during gametogenesis

    • one type contain 22 chromosomes, the other contain 24

Structural abnormalities

  • Deletion

    • terminal deletion of short arm

    • terminal deletion of long arm

    • deletion of long arm

    • ring chromosome

  • Insertion

    • a segment of one chromosome breaks off and inserts into another chromosome

  • Duplication and deletion

    • unequal crossing over during meiosis.

    • due to misalignment of homologous chromosomes

    • → one chromatid gaining extra genetic material while the other loses some.

  • Translocation

    • reciprocal translocation

      • two non-homologous chromosomes exchange segments

      • happen during meiosis when chromosomes break and rejoin incorrectly

    • robertsonian translocation

      • two acrocentric chromosomes fuse at the centromere, forming a single chromosome

      • → loss of short arm

  • Inversion

    • a piece of chromosome has been lifted out, turned around and reinserted

    • paracentric: exclude centromere

    • pericentric: include centromere

Abbreviation used in describing chromosomes and karyotypes

Cytogenetic technology

Peripheral blood lymphocyte culture

  • Collect blood sample

  • Stimulate lymphocytes with PHA

  • Culture cells in RPMI 1640 for 72 hours

  • Analyze metaphase chromosomes under a microscope

Banding: Giemsa

  • 350–550 bands per haploid set during metaphase

  • 850 bands in prometaphase

  • G-band (dark): AT-rich, fewer transcribed gene

  • R-band (light): GC-rich, more transcribed genes

FISH Technique

  • Fluorescence In Situ Hybridization

  • Purpose: detect and localize the presence or absence of specific DNA sequences on chromosomes

  • metaphase or interphase

  • procedure

    • denaturation of chromosome

    • denaturation of probes

    • hybridisation

    • fluorescence staining

    • examination of slides