Human chromosomes and their analysis – cytogenetics
Cytogenetics – how chromosomes relate to cell behaviour, particularly during mitosis and meiosis
Early history of cytogenetics
• Arnold 1879 – described human cells in division
• Flemming 1882 – cell division in corneal epithelium
• Hauseman 1891 – chromosome counts of 18, 24 and >46
• De Winiwarter 1912 – 47 chr in spermatogonia cell, suggest XX/XO
• Painter 1921 – spermatogonia cell count range from 45-48, although in clearest plates only 46
chromosomes found
• Painter 1923 – concluded 48 correct chromosome number, idea persistence until 1956
• Painter 1924 – confirmed presence of X and Y
• Koller 1937 – presence of Y confirmed in spermatocytes
• Tijo and Levan 1956 – 46 chromosomes in cultured fibroblast cells
• Ford and Hamerton 1956 – 46 chromosomes in meiotic cells
• Lejeune 1956 – first association of chromosome abnormality (CA) with disease (Down Syndrome)
• Ford and Jacobs 1956 – role of Y in sex determination
• Moorhead 1960 – PHA stimulation of human lymphocytes to provide mitotic cells
• Howell and Hungerford 1960 – associated of CA with malignant disease (chronic myeloid
leukaemia)
• Caspersson 1971 – identification of individual chromosomes after staining with quinacrine (Q
banding)
• Yunis 1976 – high resolution banding and microdeletion
• 1990s – development of FISH and molecular cytogenetics
• 2005 – adoption of microarray CGH testing (identify microdeletion syndromes)
Incidence of chromosome abnormalities – Sankaranarayaran 1979
• 10% spermatozoa, 25% mature oocytes
• 50% spontaneous miscarriages
• 20% normal embryos (selective abortion)
• 5% still borne
• 1% live births
• Predates microdeletion syndromes
• Would expect 22x more autosomal Cas in live births than sex chr abnormalities as 22 autosomal chr
pairs and only 1 sex chr pair – not the case as most autosomal CAs not viable for life
Transmission of chromosomes
• Haploid sperm and eggs originate from diploid precursors by meiosis
• In fertilised egg, sperm and egg chr initially form from separate male and female pronuclei – these
combine during first mitosis of embryo
, Cell cycle
• Checkpoints and proliferation decision points monitor
progress of cell through cell cycle
• Interphase comprises of G1, S and G2
o Chromosomes contain 1 DNA double helix from
anaphase until DNA duplicated in S phase
o From S phase until end of mitosis, chromosome
consists of 2 chromatids, each containing a DNA
duplex – 2 DNA double helices per
chromosomes
• DNA content of a diploid cell before S phase is 2C (1 double helix), between S phase and mitosis is
4C (2 double helices)
• Individual chromosomes occupy distinct chromosome territories in the interphase nucleus – can
see with chromosome painting counterstained with DAPI
• Formation of metaphase chromosomes
o From DNA duplex
o Estimate packing ratios (degree of compaction of linear DNA duplex) are 1:6 for
nucleosomes, 1:36 for 30nm fibre, >1:10,000 for metaphase chromosome
• Mitosis
o Deconvolution microscopy with HeLA cells
o DNA stained with DAPI (falsely coloured red)
o Microtubules stained with beta-tubulin antibody (falsely coloured green)
Cytogenetics – how chromosomes relate to cell behaviour, particularly during mitosis and meiosis
Early history of cytogenetics
• Arnold 1879 – described human cells in division
• Flemming 1882 – cell division in corneal epithelium
• Hauseman 1891 – chromosome counts of 18, 24 and >46
• De Winiwarter 1912 – 47 chr in spermatogonia cell, suggest XX/XO
• Painter 1921 – spermatogonia cell count range from 45-48, although in clearest plates only 46
chromosomes found
• Painter 1923 – concluded 48 correct chromosome number, idea persistence until 1956
• Painter 1924 – confirmed presence of X and Y
• Koller 1937 – presence of Y confirmed in spermatocytes
• Tijo and Levan 1956 – 46 chromosomes in cultured fibroblast cells
• Ford and Hamerton 1956 – 46 chromosomes in meiotic cells
• Lejeune 1956 – first association of chromosome abnormality (CA) with disease (Down Syndrome)
• Ford and Jacobs 1956 – role of Y in sex determination
• Moorhead 1960 – PHA stimulation of human lymphocytes to provide mitotic cells
• Howell and Hungerford 1960 – associated of CA with malignant disease (chronic myeloid
leukaemia)
• Caspersson 1971 – identification of individual chromosomes after staining with quinacrine (Q
banding)
• Yunis 1976 – high resolution banding and microdeletion
• 1990s – development of FISH and molecular cytogenetics
• 2005 – adoption of microarray CGH testing (identify microdeletion syndromes)
Incidence of chromosome abnormalities – Sankaranarayaran 1979
• 10% spermatozoa, 25% mature oocytes
• 50% spontaneous miscarriages
• 20% normal embryos (selective abortion)
• 5% still borne
• 1% live births
• Predates microdeletion syndromes
• Would expect 22x more autosomal Cas in live births than sex chr abnormalities as 22 autosomal chr
pairs and only 1 sex chr pair – not the case as most autosomal CAs not viable for life
Transmission of chromosomes
• Haploid sperm and eggs originate from diploid precursors by meiosis
• In fertilised egg, sperm and egg chr initially form from separate male and female pronuclei – these
combine during first mitosis of embryo
, Cell cycle
• Checkpoints and proliferation decision points monitor
progress of cell through cell cycle
• Interphase comprises of G1, S and G2
o Chromosomes contain 1 DNA double helix from
anaphase until DNA duplicated in S phase
o From S phase until end of mitosis, chromosome
consists of 2 chromatids, each containing a DNA
duplex – 2 DNA double helices per
chromosomes
• DNA content of a diploid cell before S phase is 2C (1 double helix), between S phase and mitosis is
4C (2 double helices)
• Individual chromosomes occupy distinct chromosome territories in the interphase nucleus – can
see with chromosome painting counterstained with DAPI
• Formation of metaphase chromosomes
o From DNA duplex
o Estimate packing ratios (degree of compaction of linear DNA duplex) are 1:6 for
nucleosomes, 1:36 for 30nm fibre, >1:10,000 for metaphase chromosome
• Mitosis
o Deconvolution microscopy with HeLA cells
o DNA stained with DAPI (falsely coloured red)
o Microtubules stained with beta-tubulin antibody (falsely coloured green)
