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Chromosomes

Chromosomes. Chromosome. Tightly coiled DNA form Found during mitosis and meiosis Made mostly of DNA and proteins Centromere - point of attachment Chromatids - each arm of a chromosome in replicated form Banding Heterochromatin- dark area with repetitive sequences

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Chromosomes

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  1. Chromosomes

  2. Chromosome • Tightly coiled DNA form • Found during mitosis and meiosis • Made mostly of DNA and proteins • Centromere- point of attachment • Chromatids- each arm of a chromosome in replicated form • Banding • Heterochromatin- dark area with repetitive sequences • Euchromatin- lighter protein-encoding area

  3. Chromosome Structure: Telomeres • Chromosomal tips • Human chromosomes have repeats of TTAGGG • Telomeres shorten during each cell division in most cells

  4. Chromosome Structure: Centromere • Constricted region of chromosome • Region of attachment of spindle fibers during mitosis and meiosis • Contains many repeats of a 171-base sequence called the alpha satellite • Centromere associated proteins • Some form kinetochore during mitosis and meiosis • Centromere protein A (CENP-A)- surround centromere; are divided and continue with each chromatid during anaphase

  5. Chromosome Structure • Moving away from the centromere • Gene-rich regions • Subtelomeres • Gene start to taper off • More repeats start to occur

  6. Chromosome Shorthand • Shorthand used to locate gene on chromosome • First number is chromosome number • Letter (p or q) refers to the arm • Second number refers to the band on that arm • Number gets higher as the band is farther from the centromere

  7. Chromosomal Differences • Size Difference • Chromosomes are numbered from 1 (largest)-22 (smallest) • Banding differences • # of Genes • Some chromosome have very few genes for their size, and some have a lot of genes for their size • Location of Centromere

  8. Location of the Centromere • Metacentric- located in the middle; divides chromosomes into two equal arms • Submetacentric- located slightly off of center; divides the chromosome into a long (q) arm and a short (p) arm • Acrocentric- located near one end of the chromosome; divides the chromosome into a long (q) arm and a chort (p) arm • Telocentric- located at the end; humans do not have these

  9. Karyotype • Pictorial representation of chromosomes • Autosomal chromosomes are lined up by number (from largest to smallest) • Sex Chromosomes are arranged last • What does it tell you? • Sex of individual • Presence of chromosomal abnormality

  10. Obtaining a Sample • Blood sample or cheek cell sample • Embryo • Amniocentesis • Chorionic Villus Sampling • Fetal Cell Sorting • Pre-implantation Testing

  11. Creating a Karyotype • Cell division is stopped with colchicine • Cell is ruptured • Cell with the most spread out chromosomes is used • Chromosomes are identified with stain or FISH • Staining- use chromosome specific stains • FISH (fluorescence in situ hybridization)- fluorescent probes are attached to chromosomes; each chromosome fluoresces a different color

  12. Karyotype Analysis Shorthand • 46,XX • First number- number of chromosomes • Sex chromosomes • Extra information • Extra chromosomes (+#)

  13. Chromosomal Abnormality: Polyploidy • Extra set of chromosomes • Triploid (3 sets) • Ovum is fertilized with two sperm • One gamete remains diploid after meiosis • Usually spontaneous abortion

  14. Chromosomal Abnormailty: Aneuploidy • Have a missing or extra chromosome • Most are spontaneously aborted • Some types can survive until birth • Monosomy- missing one chromosome • Only one viable is XO (Turner Syndrome) • Trisomy- extra chromosome

  15. Nondisjunction • Most aneuploidy results from nondisjunction • Chromosomes fail to separate during meiosis • More commonly seen in females (especially with older maternal age) • Results in some gametes missing chromosomes and some having extra

  16. Mosaics • Aneuploidy occurs during mitosis • Individual has some cells that are normal and some that are aneuploid • Severity of disorder depends on when during development that it happens

  17. Common Aneuploids • Autosomal • 47, X__ +21– Down Syndrome • 47, X__ +13- Patau Syndrome • 47, X__ +18- Edward Syndrome • Sex Chromosome • 45, XO- Turner Syndrome • 47, XXX- triplo-X • 47, XXY- Klinefelter’s Syndrome • 47,XYY- Jacobs Syndrome

  18. Chromosomal Alterations • Structural problems of chromosomes • Include • Deletions • Duplications • Translocations • Inversions

  19. Deletions • Deletion of part of a chromosome • The larger the deletion, the more severe the disorder • Microdeletions- small deletions that can create symptoms (ex: impair fertility if on Y chromosome) • Cri-du-chat- deletion of part of the short arm of chromosome 5 (5p-)

  20. Duplication • Chromosome with repeated sections • Usually repeats have to be large to be symptomatic • Fragile X syndrome- expanding triplet repeat adds extra material to X

  21. Translocations • Two nonhomologous chromosomes exchange or combine parts • Robertsonian translocation- short arms are removed from two chromosomes; long arms stick together to form one long chromsome • Problems for future generations • Reciprocal translocation- two different chromosomes exchange parts • Problem for individual if gene is broken • Problem for future generations • Translocation carrier- asymptomatic carrier of translocation

  22. Robertsonian Translocation

  23. Reciprocal Translocation

  24. Inversion • Chromosome with some genes inverted • Paracentric inversion- • Does not include centromere • Crossing over can result in • Normal chromosomes • Dicentric chromosome (two centromeres) • Acentric chromosome (no centromere) • Pericentric inversion- • Does include centromere • Crossing over can result in • Normal chromosomes • Abnormal chromosomes with one centromere

  25. Inversion

  26. Isochromosomes and Ring Chromosomes • Isochromosome • Chromosome splits the wrong way during meiosis • Chromosome end up with identical arms • Ring Chromosome • Removal of telomeres creates sticky end • Sticky ends attach to each other creating a ring

  27. Uniparental Disomy • Individual has a chromosome set where both chromosomes come from one parent • Can cause problems with recessive alleles or imprinting • Occurs if • One parent has nondisjunction resulting in a gamete with that chromosome missing • One parent has nondisjunction resulting in a gamete with both homologs of that chromosome • These two fertilize each other

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