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Chromosomes An overview

Chromosomes An overview. This PowerPoint file contains a number of slides that may be useful for teaching of genetics concepts. You may use these slides and their contents for non-commercial educational purposes. Chromosomes. This presentation includes: The anatomical structure of chromosomes

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Chromosomes An overview

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  1. ChromosomesAn overview This PowerPoint file contains a number of slides that may be useful for teaching of genetics concepts. You may use these slides and their contents for non-commercial educational purposes.

  2. Chromosomes This presentation includes: • The anatomical structure of chromosomes • Classification of chromosomal anomalies • Description of chromosomal anomalies • Examples of chromosomal anomalies • Explanation of normal and abnormal karyotypes • Chromosomal findings in early miscarriages.

  3. Chromosomes Gene for cystic fibrosis (chromosome 7) • Chromosomes are made of DNA. • Each contains genes in a linear order. • Human body cells contain 46 chromosomes in 23 pairs – one of each pair inherited from each parent • Chromosome pairs 1 – 22 are called autosomes. • The 23rd pair are called sex chromosomes: XX is female, XY is male. Gene for sickle cell disease (chromosome 11)

  4. Chromosomes p Centromere q Chromosome 5

  5. Chromosomes as seen at metaphase during cell division TelomereDNA and protein capEnsures replication to tipTether to nuclear membrane Light bandsReplicate early in S phaseLess condensed chromatinTranscriptionally activeGene and GC rich Short armp (petit) CentromereJoins sister chromatids Essential for chromosome segregation at cell division100s of kilobases of repetitive DNA: some non-specific, some chromosome specific Long armq Dark (G) bandsReplicate lateContain condensed chromatinAT rich Telomere

  6. Human chromosome banding patterns seen on light microscopy Chromosome 1 Different chromosome banding resolutions can resolve bands, sub-bands and sub-sub-bands

  7. A pair of homologous chromosomes (number 1) as seen at metaphase Locus (position of a gene or DNA marker) Allele (alternative form of a gene/marker)

  8. Total Genes On Chromosome: 723 373 genes in region marked red, 20 are shown FZD2 AKAP10 ITGB4 KRTHA8 Genes are arranged in linear order on chromosomes WD1 SOST MPP3 MLLT6 STAT3 BRCA1 breast cancer 1, early onset GFAP NRXN4 NSF NGFR CACNB1 HOXB9 HTLVR ABCA5 CDC6 ITGB3 Chromosome 17source: Human Genome Project

  9. Chromosome anomalies • Cause their effects by altering the amounts of products of the genes involved. • Three copies of genes (trisomies) = 1.5 times normal amount. • One copy of genes (deletions) = 0.5 times normal amount. • Altered amounts may cause anomalies directly or may alter the balance of genes acting in a pathway.

  10. Classification of chromosomal anomalies • Numerical (usually due to de novo error in meiosis)Aneuploidy - monosomy - trisomyPolyploidy - triploidy • Structural (may be due to de novo error in meiosis or inherited)Translocations - reciprocal - Robertsonian (centric fusion) Deletions Duplications Inversions • Different cell lines (occurs post-zygotically)Mosaicism

  11. Anomalies of chromosome structure Robertsonian Reciprocal Translocations Deletions Duplications Ring chromosomes

  12. Chromosomal deletions and duplications (not caused by translocations) • Are usually “one off”/de novo events occurring in meiosis. • Have a very low recurrence risk in future pregnancies.

  13. Most frequent numerical anomalies in liveborn Autosomes Down syndrome (trisomy 21: 47,XX,+21) Edwards syndrome (trisomy 18: 47,XX,+18) Patau syndrome (trisomy 13: 47,XX+13) Sex chromosomes Turner syndrome 45,X Klinefelter syndrome 47,XXY All chromosomes Triploidy (69 chromosomes)

  14. The Karyotype A normal male chromosome pattern would be described as: 46,XY. 46 = total number of chromosomes XY = sex chromosome constitution (XY = male, XX = female). Any further description would refer to any abnormalities or variants found (see following slide for examples).

  15. The Karyotype: an international description Total number of chromosomes, Sex chromosome constitution, Anormalies/variants. 46,XY 47,XX,+21 47,XXX 69,XXY 45,XX,der(13;14)(q10;q10) 46,XY,t(2;4)(p12;q12) 46,XX,del(5)(p25) 46,XX,dup(2)(p13p22) 46,XY,inv(11)(p15q14) 46,XY,fra(X)(q27.3) 46,XY/47,XXY

  16. The Karyotype: an international description Total number of chromosomes, Sex chromosome constitution, Anomalies/variants. 46,XY 47,XX,+21 Trisomy 21 (Down syndrome) 47,XXX Triple X syndrome 69,XXY Triploidy 45,XX,der(13;14)(p11;q11) Robertsonian translocation 46,XY,t(2;4)(p12;q12) Reciprocal translocation 46,XX,del(5)(p25) Deletion tip of chromosome 5 46,XX,dup(2)(p13p22) Duplication of part of short arm Chr 2 46,XY,inv(11)(p15q14) Pericentric inversion chromosome 11 46,XY,fra(X)(q27.3) Fragile X syndrome 46,XY/47,XXY Mosaicism normal/Klinefelter syndrome

  17. Chromosomal findings in early miscarriages 40% apparently normal 60% abnormal: Trisomy (47 chromosomes – one extra) 30% 45,X (45 chromosomes – one missing) 10% Triploidy (69 chromosomes – three sets) 10% Tetraploidy (92 chromosomes – four sets) 5% Other chromosome anomalies 5% (e.g. structural anomalies)

  18. Summary of Chromosome Anomalies • Change in number e.g. trisomy 21 Down syndrome; Edwards’ syndrome; Turner syndrome. Usually an isolated occurrence. • Change in structure e.g. translocations May be inherited. Trisomy 21

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