Chromosomes as organelles in
cell division and gametogenesis
MD Pertile
VCGS Pathology, Royal Children’s Hospital, Parkville, VIC 3052
Chromosomal basis of hereditary
Genetics: A conceptual approach 2002 B A Pearce
DNA and chromosomes
„
Chromosome = “colored body”
Gk, chroma + soma
[Heinrich Wilhelm Waldeyer 1888]
„
Each chromosome consists of
a single molecule of DNA +
histone and non-histone
proteins (= chromatin)
„
Nucleosome is an octomer of
histones 2 x [H2A, H2B, H3
and H4]
„
146-bp DNA + 55-bp DNA
assoc. with linker [H1]
DNA and chromosomes
„
Higher-order packaging of
chromatin into 30 nm fibre
„
Chromosomes highly
condensed during mitosis
(10,000-fold compaction)
„
Chromosomes only visible
cytologically during cell division
Chromosome number
Arabidopsis thaliana
2n = 10
Canis familiaris
2n = 78
• Chromosome number
& morphology varies
between organisms
Homo sapiens
2n = 46
Drosophila
melanogaster
2n = 8
• Common aim is faithful
passage of DNA into
daughter cells
Pan troglodytes
2n = 48
Mus musculus
2n = 40
Human metaphase chromosomes (2n = 46)
Mouse karyotype
2n = 40 (telocentric)
19 pairs of autosomes
X & Y sex chromosomes
C-banding
Centromere
„
Cytologically visible as primary
constriction
„
Typically associated with large
arrays of AT-rich repetitive DNA
(0.5 - 4.0 Mb in humans)
„
No DNA sequence conservation
b/w species (centromeric
chromatin is epigenetically
determined)
„
Site of kinetochore assembly and
spindle attachment
Dawe & Henikoff 2006 Trends Biochem Sci 31(12), 662-669
Centromeric
chromatin
„
CENPA replaces Histone H3
within the centromere associated
nucleosomes
„
Kinetochore assembles just
before and during the early
stages of mitosis
„
Assembles on poleward (CENPA)
face and recruits centromere and
spindle binding proteins
Adapted from Santaguida and Musacchio (2009).
EMBO J 28: 2511-2531.
Mammalian mitotic cell cycle
2n,
2c
„
Cell grows, duplicates DNA and
divides into identical daughter
cells
„
Mammalian cell cycle ~20-24 hrs
2n, 2c
2n,
2c
(G1 12 hr, S 7 hr, G2 4 hr, M 1 hr)
2n, 4c
2c → 4c
Genetics Vol 1 (A-D) 2003 The MacMillan Science Library
„
Checkpoints ensure fidelity of
DNA replication and cell division
„
cyclin-dependent kinases (CDKs)
regulate cell cycle
„
CDK inhibitors (CKIs) cause cell
cycle to halt
Checkpoints demand successful completion of prior phase before next phase proceeds
Mitosis
„
Spindle checkpoint proteins
avert aneuploidy by delaying
anaphase onset until c’somes
align
„
2 identical daughter cells
produced from mitotic division
H3-GFP fusion gene in mouse
embryonic fibroblast cells
Courtesy Dr. Damien Hudson Chromosome & Chromatin Research Lab, MCRI
Meiosis
„
Specialised cell division that
occurs in gonads
„
C’some replication in S phase is
followed by two consecutive cell
divisions
„
Homologous chromosomes pair
and recombine during first meiotic
division (reduction division 2n→n)
„
Second meiotic division is similar
to mitosis (without S phase)
„
Produces haploid gametes (4 x n
in males and 1 x n in females)
Reduction
division
Biology 6th Edition 2001 Raven / Johnson
Biology 6th Edition 2001 Raven / Johnson
Recombination during Prophase of Meiosis I
Prophase of Meiosis I
Biology 6th Edition 2001 Raven / Johnson
„
„
„
Crossing over visualised as chiasmata (visible in diplotene)
Occurs during fetal life in females (14-15 wks pc)
At least one ‘obligatory’ chiasma per arm for each pair of homologues
Normal meiosis
Meiosis I
Meiosis II
Meiosis I non-disjunction
Meiosis I
Non-disjunction
Meiosis II
Disomic gametes
Nullisomic gametes
Meiosis II non-disjunction
Meiosis I
Meiosis II
Normal gametes
Non-disjunction
Disomic
Nullisomic
Maternal age and non-disjunction
Hassold and Hunt 2001 Nat Rev Genet 2:280-287
„
Maternal MI errors predominate amongst
trisomies
„
In females, first division is initiated
prenatally and suspended in dictyotene
until ovulation 12-50 yrs later
„
Second division completed after
fertilization
„
Achiasmate bivalents never engage in
genetic recombination and drift
independently across metaphase plate
[40-50% of all DS conceptions from MI
non-disjunction result from achiasmate
bivalents; freq similar in younger and older
mothers]
Origin of human trisomies
Hassold et al 2007 Hum Mol Genet 16(R2) R202-208
Altered genetic recombination and aneuploidy
„
Significant reductions in recombination found in all MI-derived
trisomies studied (T15, 16, 18, 21, XXX & XXY of mat origin & T21
and XXY of pat origin)
„
Telomeric exchanges contribute to trisomy amongst younger women
but less important in older women (bivalents more susceptible to nondisjoin)
Post zygotic errors of chromosome division
„
Post zygotic non-disjunction or anaphase lag leads to
chromosomal mosaicism [normal + aneuploid cells]
„
Very common in early human conceptions (perhaps 5060% of preimplantation embryos)
„
5% of early miscarriages
„
0.3% at amniocentesis (16+ weeks gestation)
„
May manifest as confined placental mosaicism (CPM),
generalised mosaicism, tissue limited mosaicism (e.g.
Pallister-Killian syndrome)