Genetics Karyotyping & Chromosome PDF

Genetics Karyotyping & Chromosome LECTURE (4) Karyotyping & chromosome DR. El-Sawy 1  Genetics...

Genetics Karyotyping & Chromosome LECTURE (4) Karyotyping & chromosome DR. El-Sawy 1  Genetics Karyotyping & Chromosome  karyotype (chromosome analysis): Definition Description of number and structure of chromosomes.  Can be carried out in dividing cells arrested in metaphase as following ① Sample of blood is collected ② Then add anticoagulant as heparin ③ The blood is then centrifuged to separate lymphocytes from other cells ④ The lymphocytes are then incubated in a suitable culture medium ⑤ Phyto-haemagglutinin (mitogenic agent), is added to the culture medium to stimulate division. Steps ⑥ The cultured lymphocytes are incubated for three days. ⑦ We stop cell division of lymphocytes at metaphase by the addition of colchicine. ⑧ The cultured cells are then treated with hypotonic solution. ⑨ Samples from the cultured cells are then spread on the slides by the drop method. ⑩ The chromosomes are then fixed, stained, examined and photographed. ⑪ Photographed chromosomes are cut, matched into pairs by image analyser DR. El-Sawy 2  Genetics Karyotyping & Chromosome DR. El-Sawy 3  Genetics Karyotyping & Chromosome Definitions:  Thread-like structures. Chromosome  It is consisted of DNA molecule packed tightly and coiled around a specific protein called histones.  Region of repetitive nucleotide sequences  Located at each end of a chromosome, Telomere  Function to stabilize and protects end of chromosome from deterioration or from fusion with neighboring chromosomes Structure Of Human Chromosome  Each D-chromosome is formed of 2 identical chromatids connected together at the centromere.  Each chromatid has : Structure a) Long arm called (q) arm b) Short arm called (P) arm.  The Telomeres are the terminal ends of the chromatids. DR. El-Sawy 4  Genetics Karyotyping & Chromosome Chromosomes classification : A. According to position of centromere  Centromere is nearly in middle Metacentric  The two arms are of the same length.  Centromere is between the center of the chromatids and Submetacentric their upper ends.  One arm is longer than the other  Centromere is close to one end meaning that one arm is Acrocentric much shorter than the other.  No short arm Telocentric  Not present in human (satellite)  Chromosome has only one arm. DR. El-Sawy 5  Genetics Karyotyping & Chromosome B. According to their gene complement:- ① Autosomes (somatic) : 22 pairs of identical homologues. ② Sex chromosomes  One pair of a) identical homologues (XX female) b) unidentical homologues (XY male). C. Denver classification: Based on their total length and position of its centromere into 7 groups in descending manner: Group Chromosome Number Features A 1-3 B 4-5 plus, X C 6-12 chromosome D 13-15 E 16-18 F 19-20 plus, Y G 21-22 chromosome DR. El-Sawy 6  Genetics Karyotyping & Chromosome Clinical Importance Of Chromosomal Studies: ① Diagnosis of certain diseases and syndromes as: Down syndrome. ② Diagnosis of certain malignant diseases as chronic myeloid leukemia. ③ Karyotyping of fetal cells helps in prenatal diagnosis of certain diseases and in determination of the sex of the fetus (either male or female). Causes: ① Radiation: causes some chromosomal damage. ② History: in parents or in their families. ③ Infection with German measles :(its virus causes fragmentation of chromosomes). ④ Pregnancy in old women: may give children with congenital syndromes. ⑤ Auto-immune diseases: may cause chromosomal abnormalities. Types: Numerical Structural  Part of individual  Abnormal number of chromosomes: chromosome is missing, a) Missing (monosomy) extra, switched to b) Extra chromosome normal pair (Trisomy) another chromosome Sex chromosomes Autosomes Males Females Monostomy Wolf syndrome Klinefelter’s Turner’s Trisomoy (Down’s Cri –du-chat syndrome syndrome syndrome) Philadelphia chromosome (47xxy) (45 xo) DR. El-Sawy 7  Genetics Karyotyping & Chromosome Klinefelter’s syndrome Turner’s syndrome  Non-separation of chromosomes  Non-separation of chromosomes occurs in first meiotic cell division occurs in first meiotic cell division in mother during formation of ova in mother during formation of ova  The formed ovum will contain two  The formed ovum will contain no Def X chromosomes (XX). X chromosomes (0).  If this ovum is fertilized by sperm  If this ovum is fertilized by sperm with Y chromosome, the resulting with X chromosome, the resulting zygote will contain XX Y zygote will contain X0 chromosomes. chromosomes. Karyotype  47,XXY  45 (XO) DR. El-Sawy 8  Genetics Karyotyping & Chromosome Down’s syndrome (Mongolism or Trisomy 21)  Most common  The addition of a chromosome to a pair is called trisomy  Mongols (Down's syndrome) with trisomy 21 Def DR. El-Sawy 9  Genetics Karyotyping & Chromosome  Detachment and loss of portions from chromosomal arms. ① Cri-du-chat syndrome: Deletion of short arm on chromosome number 5 DELETION Characterized by cat-like cry made by cri-du-chat babies. ② Wolf syndrome: Deletion of short arm on chromosome number 4.  Exchange of material ( ) non-homologous chromosomes  Philadelphia chromosomes: Translocation Trans location between chromosomes 9 &22 Occur in Chronic myeloid leukemia DR. El-Sawy 10