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 ▫ Euchromatin- lighter proteinencoding area Chromosome Structure: Telomeres • Chromosomal tips • Human chromosomes have repeats of TTAGGG • Telomeres shorten during each cell division in most cells Chromosome Structure: Centromere • Constricted region of chromosome • Region of attachment of spindle fibers during mitosis and meiosis • Contains many repeats of a 171base sequence called the alpha satellite • Centromere associated proteins ▫ Some form kinetochore during mitosis and meiosis ▫ Centromere protein A (CENPA)- surround centromere; are divided and continue with each chromatid during anaphase Chromosome Structure • Moving away from the centromere ▫ Gene-rich regions ▫ Subtelomeres Gene start to taper off More repeats start to occur 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 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 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 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 Obtaining a Sample • Blood sample or cheek cell sample • Embryo ▫ ▫ ▫ ▫ Amniocentesis Chorionic Villus Sampling Fetal Cell Sorting Pre-implantation Testing 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 Karyotype Analysis Shorthand • 46,XX • First number- number of chromosomes • Sex chromosomes • Extra information ▫ Extra chromosomes (+#) 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 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 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 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 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 Chromosomal Alterations • Structural problems of chromosomes • Include ▫ ▫ ▫ ▫ Deletions Duplications Translocations Inversions 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-) Duplication • Chromosome with repeated sections • Usually repeats have to be large to be symptomatic • Fragile X syndromeexpanding triplet repeat adds extra material to X 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 Robertsonian Translocation Reciprocal Translocation 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 Inversion 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 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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