Chromosome Structure Lecture 2024 PDF

Lecture I presentation 2024/2025 Chromosomal structure  Learning Objectives 1- Identify the normal chromosomal structure at interphase and metaphase. 2- Identify the normal chromosomal number. 3- Describe Barr’s body. 4- Classify different types of chromosomes. 5- Enumera...

Lecture I presentation 2024/2025 Chromosomal structure  Learning Objectives 1- Identify the normal chromosomal structure at interphase and metaphase. 2- Identify the normal chromosomal number. 3- Describe Barr’s body. 4- Classify different types of chromosomes. 5- Enumerate different methods of chromosomal studying. 6- What is the G banding technique? 7- Describe different methods of chromosomal study.  chemical structure of chromosomes: -The nucleus is the organelle that houses chromosomes. -Chromosomes consist of DNA, which contains heredity information and instructions for cell growth, development, and reproduction. -Chromosomes are present in the form of strings of double stranded DNA and histones (protein molecules) called chromatin. -The repeated units of chromatin granules called nucleosomes. -When a cell is “resting” i.e. not dividing, the chromosomes are organized into long entangled structures called chromatin.  Normal morphology of the interphase (non-dividing) chromosomes The chromosome is a long thread-like structure. Along its length, portions are extended and expose its genes to be transcribed; active genes. The extended chromatin can not be seen by the light microscope (Euchromatin). Some portions of the chromosomes are coiled, with inactive genes, they appear as electron-dense masses of chromatin and are called (Heterochromatin). The chromosome of interphase at the G1 stage is formed of single-stranded DNA, single-threaded (s- s-chromosome or chromatid). N.B: chromatin pattern is a guide of cell activity - Nucleus with euchromatin= active nucleus=active protein synthesis= lightly stained - Nucleus with heterochromatin= inactive nucleus=inactive protein synthesis= deeply stained Normal morphology of metaphase chromosomes in dividing cells - The chromosome appears as a cross- shaped structure due to the union of its 2 chromatids (S-chromosomes) at the central region known as the centromere. -Centromers: areas that contain kinetochore proteins of the mitotic spindle. -It has 4 arms radiating from the point of the centromere. ( long arm →q and short arm → p ). -Telomeres: specialized repetitive sequences that ‘cap’ the ends of chromosomes.  Normal number of chromosomes * Somatic cells (present in the whole tissues of the body) contain 46 chromosomes ( present as 23 pairs) (diploid number = 2N). Each pair is derived from one of the parents, one maternal and one paternal). Each of them has the same genes at the same loci. - 22 pairs of identical chromosomes (autosomes) and -one pair of sex chromosomes (XX or XY) * Gametes (sperms or ova) contain 23 chromosomes only (haploid number =1N). -22 autosomes plus X chromosome in females and -plus either X or Y chromosome in males.  Classification of chromosomes A) According to the position of the centromere: The chromosome may be: -Metacentric chromosomes: Centromere lies near the middle of the chromosome such as chromosomes number 1, 3, 16, 19, and 20, where the p arm is equal to the q arm. -Sub-metacentric chromosome: Centromere is present midway between the center of chromatids and their upper ends such as chromosomes number 2, 4 to 12, 17, 18, and X chromosome, where the p arm is shorter than q. -Acrocentric chromosomes: Centromere is present near one end thus the short arm p is very small such as chromosomes number 13, 14, 15, 21, 22, and Y chromosome. -Telocentric chromosomes: Centromere is present at the terminal end (not in humans). B) Denver classification: The chromosome set can be classified into 7 groups (A to G) according to the chromosome length, the position of the centromere, and the presence of specific landmarks. Group A: - chromosomes 1,2 and 3. -Large with approximately median centromeres. - chromosome 1 is the largest metacentric chromosome -Chromosome 2 is submetacentric Group B: Chromosomes number 4-5, are large and submetacentric, chromosome number 4 is longer. Group C: Chromosomes number 6-12 and X chromosome, they are medium- sized and submetacentric. The chromosomes are arranged according to a decreasing size pattern. Secondary constriction is found in chromosome 9. Chromosome 12 has the smallest short arms in the group - Group D: Chromosome 13, 14 and 15. Median-sized acrocentric. and have satellite. Group E: Chromosomes number 16 to 18, are smaller than group D chromosome 16 is metacentric and has secondary constriction and chromosomes 17 and 18 are submetacentric. Group F: Chromosomes number 19 and 20, they are short and metacentric. Group G: Chromosomes number 21, 22, and Y, they are very short acrocentric chromosomes. Chromosomes 21 and 22 are the smallest and contain satellites. Y does not contain a satellite. *Within these groups, chromosomes are arranged in descending order of length with their centromere on the same line. *The X chromosome is long, while the Y chromosome is short. in the female, one of the two XX chromosomes is inactive and remains highly condensed and stains darkly during interphase forming a Barr Body. X chromosome: It is one of the longest two or three chromosomes in the group C. In the female, one X is stained slightly darker than the other. Y chromosome: *Longer than 21 & 22. *Long arms are less divergent. *No secondary constrictions on the short, thus no satellites. *Centric constriction is indistinct. Y *Secondary constriction is seen in the long arm. *The terminal region of the long arm is poorly x defined. *Usually found toward the periphery of the metaphase preparation.  Barr body Darkly stained mass of chromatin, lying on the inner surface of the nuclear envelope of 60% of the epithelial cells nuclei of female cells. It appears as a drumstick-like mass attached to the nucleus in 3-5% of the blood leukocytes (neutrophils). Barr’s body can be demonstrated in: ❑ in the epithelial cells by simple scrap of buccal mucosa, stained with basic stain, or ❑ in neutrophils by staining with Leishman stain. Barr body represented one of the two X chromosomes present in the female nuclei, which remain coiled (condensed) and inactive, as each somatic cell must have at least one X chromosome to be active. It carries other genes rather than sex-determining genes. Barr body appears in female cells and male cells with an extra chromosome (XXY, Klinefelter syndrome). Barr’s body does not appear in normal male cells or female cells with only one chromosome (XO, Turner syndrome). No Barr body Apparent Barr body Methods of chromosomal Identification 1-Morphological approach: -Arms long or short - Centromeric position - Specific morphological feature 2-Autoradiography: - Permits the localization of radioactive substances in tissue  3. Chromosomal banding: -It is a part of chromosome that is clearly distinguishable from its adjacent segments by G-positive appearing darker or lighter as a result of new staining. Giemsa -bands (G-bands); *The most frequently used type of banding. *This name is derived from the Giemsa stain. G-negative Giemsa-bands (G-bands) When they are stained, the mitotic chromosomes have a banded structure that identifies each chromosome of a karyotype. metaphase chromosomes exhibit light bands (active gene rich area) and dark bands (gene poor area) under appropriate staining conditions. The short arm, p, and the long arm, q, are divided into regions then bands which are then subdivided into sub- bands moving outwards from the centromere, -band 1 being the nearest to the centromere. Centromeres and telomeres are not numbered.  4. Fluorescence In Situ Hybridization (FISH) Used to visualize karyotypes using different combinations of DNA probes that bind to specific DNA sequences for each chromosome. Time for an easy quiz  (1) A chromosome with a very short arm and a very long arm is referred to as: a) Metacentric b) deleted chromosome c) Acrocentric d) Sub-metacentric (2) In which stage of the cell cycle chromosomes are best visualized and studied? a) Anaphase. b) Interphase. c) Metaphase. d) Prophase.  (3) Regarding X- chromosome: a) it is an acrocentric chromosome. b) it is present in group B in the Denver classification of chromosomes. c) it is a submetacentric chromosome. d) it is shorter than Y chromosome. (4) According to Denver's classification of chromosomes, the acrocentric chromosomes are involved in…….. a) groups A and B b) groups G and D. c) group E. d) group A. (5) The number of autosomes in human somatic cells is a- 22 chromosomes. b- 23 pairs of identical chromosomes c- 46 pairs of chromosomes. d- 22 pairs of identical chromosomes. 6. Which of the following statements is true about the ends of the chromosome? (a) The ends of the chromosome are called Telomeres (b) The ends of the chromosome are called Centromeres (c) The ends of the chromosome are called Satellites (d) The ends of the chromosome are called Kinetochore  References JUNQUEIRA,S Basic Histology: Atlas and Text by Anthony L. Mescher https://drive.google.com/file/d/1EYyTFKo15GnLxRASgZS6-kt- EMYW8ssD/view?usp=sharing Textbook of Histology by Leslie P Gartner https://drive.google.com/file/d/1Co74V1K35z77LHgS-NAtIwBj4OZF7Py- /view?usp=sharing Histology: a text and atlas: with correlated cell and molecular biology/Michael H. Ross, Wojciech Pawlina https://drive.google.com/file/d/1q1lpkOWNJNLXpkRcmIHvBvm_YYHHIynY/view?usp= sharing THANK YOU