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 Drosophila • Chromosome number melanogaster & morphology varies 2n = 8 between organisms • Common aim is faithful passage of DNA into Homo sapiens daughter cells 2n = 46 Mus musculus 2n = 40 Pan troglodytes 2n = 48 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 Cell grows, duplicates DNA and 2n, divides into identical daughter 2c 2n, 2c cells Mammalian cell cycle ~20-24 hrs 2n, (G1 12 hr, S 7 hr, G2 4 hr, M 1 hr) 2c Checkpoints ensure fidelity of DNA replication and cell division cyclin-dependent kinases (CDKs) 2n, 4c regulate cell cycle 2c → 4c CDK inhibitors (CKIs) cause cell Genetics Vol 1 (A-D) 2003 The MacMillan Science Library 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 Reduction division 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) 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 Non-disjunction Normal gametes Disomic Nullisomic Maternal age and non-disjunction 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 Hassold and Hunt 2001 Nat Rev Genet 2:280-287 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 non- disjoin) 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 50- 60% 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) .