L10 Morphology of chromosomes and Karyotyping .pdf

Morphology of Chromosomes and Karyotyping Dr. Preetha J Shetty February 27, 2024 www.gmu.ac.ae LEARNING OBJECTIVES Describe the morphology & nomenclature of chromosomes. Define a karyotype COLLEGE OF MEDICINE Chromosomes Chromosomes are the rod-shaped, filamentous bodies present in the nucleus, whic...

Morphology of Chromosomes and Karyotyping Dr. Preetha J Shetty February 27, 2024 www.gmu.ac.ae LEARNING OBJECTIVES Describe the morphology & nomenclature of chromosomes. Define a karyotype COLLEGE OF MEDICINE Chromosomes Chromosomes are the rod-shaped, filamentous bodies present in the nucleus, which become visible during cell division. They are the carriers of the gene or unit of heredity. Chromosome are not visible in active nucleus due to their high-water content but are clearly seen during cell division. Numbers of chromosomes Constant for each cell in the body (except sex cells which only have half sets). Constant throughout the life of an individual (you don’t lose or gain chromosomes) Constant for all members of a species Chromosome Size The size of chromosomes varies depending upon the stages of cell division. Interphase: longest & thinnest Prophase: progressive decrease in length and increase in thickness Anaphase: chromosomes are smallest. Metaphase: easily observed, very thick, quite short and well spread in the cell. Therefore, chromosomes measurements are generally taken during mitotic metaphase. IDENTIFYING CHROMOSOMES Chromosomes can be identified by: Their size Their shape (the position of the centromere) Banding patterns produced by specific stains (Giemsa) Chromosomes are analysed by organising them into a KARYOTYPE CHROMOSOME CLASSIFICATION Based on the position of centromere Chromosome are classified into :  Metacentric chromosome  Submetacentric chromosome  Acrocentric chromosome (In some species, e.g. rodents, there may be telocentric chromosomes as well). February 27, 2024 www.gmu.ac.ae COLLEGE OF MEDICINE Chromosome arms are designated as “p” for the short arm (p for petit) and “q” for the long arm (q for quinine) METACENTRIC CHROMOSOME Metacentric chromosomes have the centromere placed at or near the middle of the Chromosome. Both arms of the chromosome are of about equal length. The centromere is sometimes called the “primary constriction”. February 27, 2024 www.gmu.ac.ae COLLEGE OF MEDICINE SUB METACENTRIC CHROMOSOME Submetacentric chromosomes have their centromere located closer to one end of the Chromosome than the other. Another way to look at sub metacentric chromosomes is that one chromosome arms is longer than the other, but there are still clearly two chromosome arms ACROCENTRIC CHROMOSOME Acrocentric chromosomes have their centromere close to one end. There is a secondary constriction, thin strands of chromatin of variable length tipped with non coding DNA often referred to as chromosomal satellites. The secondary constriction contains the loci for the ribosomal RNA (rRNA) genes, (the Nucleolar Organizing Region). Human chromosomes are divided into 7 groups & sex chromosomes Group A 1-3 Largest metacentric and sub metacentric chromosomes Group B 4&5 Large submetacentric Group C 6-12 & X Very similar, Medium sized, (arranged in order of decreasing size sub metacentric) Group D 13-15 Medium-sized acrocentric plus satellites Group E 16-18 Smaller , metacentric and submetacentric Group F 19-20 Even smaller, metacentric and submetacentric Group G 21,22 & Y Smallest acrocentric with satellites. Y no satellites Euchromatin and Heterochromatin Chromosomes may be identified by regions that stain in a particular manner when treated with various chemicals. Several different chemical techniques are used to identify certain chromosomal regions by staining then so that they form chromosomal bands. The dark-stained regions are heterochromatic region or heterochromatin. Light stained are euchromatic region or euchromatin. Heterochromatin is classified into two groups: (i) Constitutive heterochromatin remains permanently in the heterochromatic stage, i.e., it does not revert to the euchromatic stage (ii) Facultative heterochromatin: In contrast, facultative heterochromatin consists of euchromatin that takes on the staining and compactness characteristics of heterochromatin during some phase of development. Centromeres and Telomeres Centromeres and telomeres are two essential features of all eukaryotic chromosomes. Each provide a unique function i.e., absolutely necessary for the stability of the chromosome. Centromeres are required for the segregation of the chromatids during meiosis and mitosis. Telomeres provide terminal stability to the chromosome and ensure its survival Normal karyotype Karyotype : a standard arrangement of chromosome pairs in order of decreasing length. Chromosomes of a stained metaphase spread is photographed or imaged , and individual chromosomes are cut and arranged in pairs. Karyotyping is the study of chromosome Levitsky first defined the karyotype as the phenotypic appearance of the somatic chromosomes. Idiotype The karyotype of a species may be represented diagrammatically, showing all the morphological features of the chromosome; such a diagram is known as Idiotype. Normal male karyotype with Gbanding Normal female karyotype with G-banding Karyotyping G banding (from “ Giemsa stain” ) helps to identify each chromosome by the pattern of alternating light and dark bands. It is the standard analytical method in cytogenetics Light bands (euchormatin), rich in guanine and cytosine bases; contain more active genes, replicates in early S phase of cell cycle. Dark bands ( heterochromatin) rich in adenine and thymine bases, (has high affinity for Giemsa stain) and contain few active genes Patau's Syndrome (Trisomy 13) KLINEFELTER’S SYNDROME Have 47 chromosomes (XXY) & a sex chromatin Barr body or 48(XXXY); more the number of X more the chances of mental impairment Cause: Nondisjunction of XX homologue Found only in males, detected at puberty Incidence ---1 in 500 males S/S Sterility, testicular atrophy, hyalinization of seminiferous tubules, gynecomastia.