Presentation on theme: "Chromosomes An overview"— Presentation transcript:
1 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.
2 Chromosomes This presentation includes: The anatomical structure of chromosomesClassification of chromosomal anomaliesDescription of chromosomal anomaliesExamples of chromosomal anomaliesExplanation of normal and abnormal karyotypesChromosomal findings in early miscarriages.
3 Chromosomes Chromosomes are made of DNA. 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 parentChromosome pairs 1 – 22 are called autosomes.The 23rd pair are called sex chromosomes: XX is female, XY is male.In the slide you can see chromosomes arranged in pairs, this is called a karyotype (chromsome complement)One of each pair is inherited from the mother, the other from the fatherAsk participants - what sex is this individual? (Male, XY)Gene for sickle cell disease (chromosome 11)
5 Chromosomes as seen at metaphase during cell division Telomere DNA and protein cap Ensures replication to tip Tether to nuclear membraneLight bands Replicate early in S phase Less condensed chromatin Transcriptionally active Gene and GC richShort arm p (petit)Centromere Joins sister chromatidsEssential for chromosome segregation at cell division 100s of kilobases of repetitive DNA: some non-specific, some chromosome specificLong arm qDark (G) bands Replicate late Contain condensed chromatin AT richTelomere
6 Human chromosome banding patterns seen on light microscopy 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 shownFZD2AKAP10ITGB4KRTHA8Genes are arranged in linear order on chromosomesWD1SOSTMPP3MLLT6STAT3BRCA1breast cancer 1, early onsetGFAPNRXN4NSFNGFRCACNB1HOXB9HTLVRABCA5CDC6ITGB3Chromosome 17 source: Human Genome Project
9 Chromosome anomaliesCause 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 trisomy Polyploidy - triploidyStructural (may be due to de novo error in meiosis or inherited) Translocations - reciprocal Robertsonian (centric fusion) Deletions Duplications InversionsDifferent cell lines (occurs post-zygotically) Mosaicism
11 Anomalies of chromosome structure TranslocationsDeletionsDuplicationsRing chromosomesRobertsonianReciprocal
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.
14 The KaryotypeA normal male chromosome pattern would be described as:46,XY.46 = total number of chromosomesXY = sex chromosome constitution(XY = male, XX = female).Any further description would refer to any abnormalities orvariants found (see following slide for examples).
15 The Karyotype: an international description Total number of chromosomes,Sex chromosome constitution,Anormalies/variants.46,XY47,XX,+2147,XXX69,XXY45,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,XXYKaryotypes are described by an internationally recognised format.The number refers to the number of chromosomes whilst the X and Y refer to the sex chromosomes, e.g. 46 XX would be a healthy female,46 XY a healthy male.Thereafter any chromosomal abnormalities or findings are described by recognised abbreviations and location is given of the affected part of the chromosome.e.g. 46,XX,dup(2)(p13p22) describes a female with 46 chromosomes and a duplication of the short arm of chromosome 2 from bands
16 The Karyotype: an international description Total number of chromosomes,Sex chromosome constitution,Anomalies/variants.46,XY47,XX, Trisomy 21 (Down syndrome)47,XXX Triple X syndrome69,XXY Triploidy45,XX,der(13;14)(p11;q11) Robertsonian translocation46,XY,t(2;4)(p12;q12) Reciprocal translocation46,XX,del(5)(p25) Deletion tip of chromosome 546,XX,dup(2)(p13p22) Duplication of part of short arm Chr 246,XY,inv(11)(p15q14) Pericentric inversion chromosome 1146,XY,fra(X)(q27.3) Fragile X syndrome46,XY/47,XXY Mosaicism normal/Klinefelter syndrome
17 Chromosomal findings in early miscarriages 40% apparently normal60% 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 numbere.g. trisomy 21 Down syndrome; Edwards’ syndrome; Turner syndrome.Usually an isolated occurrence.Change in structuree.g. translocationsMay be inherited.Down’s SyndromeAn example of a chromosomal abnormality which causes learning difficultiesThe karyotype shows a case of Down’s syndrome (trisomy 21)For more information about Down’s syndrome, visit:[NOTES FOR TRAINING FACILITATORS IN CASE OF QUESTIONS ONLYIn total, there are normally 23 pairs of chromosomes, but in a trisomy there is an extra chromosome (tri somy = three bodies)A normal karyotype is written 46,XY or 46,XXThe karyotype, or complement of chromosomes, seen in a regular trisomy 21 is written as 47,XX +21 if female or 47, XY +21 if maleNote that Down’s syndrome can be inherited, although this is rare. These cases are due to a change in chromosome structure (a translocation)]Trisomy 21