Manchester Cell Division PDF 2024-2025

Prof. Dr. Shehab Hafez Prof. Dr. Shehab Hafez Professor of Histology & Cell Biology. Head of Histology Department Faculty of Medicine, Mansoura University 2024-2025  Prof. Dr. Shehab Hafez Introduction Microtubule organizin...

Prof. Dr. Shehab Hafez Prof. Dr. Shehab Hafez Professor of Histology & Cell Biology. Head of Histology Department Faculty of Medicine, Mansoura University 2024-2025  Prof. Dr. Shehab Hafez Introduction Microtubule organizing centers (MTOCs) Centers in the cytoplasm that direct the formation of microtubules from tubulin protein. Importance: play an important role in formation of mitotic spindle. Include: 1. Centrosome (the area of cytoplasm around the Centrioles). 2. Basal bodies of cilia. 3. Centromeres of the chromosomes.  Prof. Dr. Shehab Hafez Cell cycle Definition The regular sequence of events that result in the production of new cells. Phases of the cell cycle 1. Mitosis. 2. G1 stage of interphase. 3. S stage of interphase. 4. G2 stage of interphase. Mitosis includes ▪ Karyokinesis: nuclear division. ▪ Cytokinesis: cytoplasmic division.  Prof. Dr. Shehab Hafez Mitosis Occurs in somatic cells. Duration: short; less than 2 hours. Definition Result: 2 identical daughter cells, each one has the same diploid number of chromosomes (46 s-chromosomes). Phases of mitosis 1. Prophase. 2. Prometaphase. 3. Metaphase. 4. Anaphase. 5. Telophase.  Prof. Dr. Shehab Hafez In G2 stage of interphase (the last stage of interphase), The cells prepare for mitosis by:- 46 s-Chromosome Each identical pair of S-chromosomes forms one d-chromosome → The Chromosomal content: 46 d – chromosomes. Duplication of centrioles → 2 pairs of centrioles.  Prof. Dr. Shehab Hafez 1. Prophase 1. Most organelles, microfilaments and microtubules: disassemble. 3. Formation of mitotic spindle: ▪ The centrosome (the area of the cytoplasm around the centrioles) is one of the MTOCs. ▪ The two pairs of centrioles migrate towards opposite poles of the cell ▪ A spindle of microtubules is formed between the 2 poles (interpolar microtubules). 4. Then, the chromosomes become condensed and short, and the nucleolus disappears.  Prof. Dr. Shehab Hafez 2. Prometaphase ▪ Begins with Disassembly of the nuclear envelope. ▪ Each chromosome becomes attached at the kinetochore to another group of microtubules of the mitotic spindle (kinetochore or chromosome microtubules). 3. Metaphase ▪ The chromosomes are much more condensed (ideal for karyotyping). ▪ All the chromosomes are attached to the mitotic spindle and lined up at the cell equator.  Prof. Dr. Shehab Hafez 4. Anaphase ▪ Chromosomes split longitudinally: 46 d - chromosomes → 2 identical sets, each set: 46 s-chromosomes. ▪ Each set of s -chromosomes moves to the opposite pole by the mitotic spindle. 5. Telophase ▪ Mitosis is complete. Each daughter cell has an identical set of diploid s-chromosomes (46 s-chromosomes). ▪ The mitotic spindle disassembles. ▪ The nuclear envelope reforms. ▪ The nucleolus reappears.  Prof. Dr. Shehab Hafez Cytokinesis ▪ Actin filaments encircles the cell equator (contractile ring). ▪ Its contraction leads to separation of the two daughter cells. Net Result Production of 2 daughter cells Each having 46 s- chromosome and the cells enter the G1 stage of interphase  Prof. Dr. Shehab Hafez Phases of mitosis  Prof. Dr. Shehab Hafez Steps of mitosis in Lymphocytes that are stimulated to divide  Prof. Dr. Shehab Hafez Interphase It is the period between two successive cell divisions. Stages of interphase 46 s-Chromosome G1 (1st gap) S (synthesis) G2 (2nd gap) stage stage stage  Prof. Dr. Shehab Hafez G1 (1st gap) stage Definition: The gap between the end of mitosis & the beginning of (s) stage. Duration: the longest: 8 – 25 hours Events: ▪ A period of active RNA and protein synthesis. ▪ The daughter cells restore the size of the mother cell. ▪ During the G1 phase, cells differentiate and perform their specialised functions. Chromosomal content: 46 s-chromosomes.  Prof. Dr. Shehab Hafez S (synthesis) stage Definition: The stage of synthesis (DNA replication). 46 s-Chromosome Duration: about 8 hours The Chromosomal content: 92: (46 identical pairs of S – chromosomes). G2 (2nd gap) stage Definition: The gap between (s) stage and the beginning of next mitosis. Duration: very short (½-1 hour). Each identical pair of S-chromosomes form one d-chromosome → The Chromosomal content: 46 d – chromosomes. Duplication of centrioles.  Prof. Dr. Shehab Hafez Turnover (renewal) of Differentiated (specialised) Cells Tissue renewal occurs either by: Mitosis of the differentiated cells OR Mitosis of reserve (stem) cells. Tissues are classified according to their ability of renewal into: 1 Terminally differentiated cells 2 Continuously cycling cells 3 Facultative dividers  Prof. Dr. Shehab Hafez 1. Terminally differentiated cells Enter G0 phase: the cells perform their normal functions, with no division. No replacement. When the tissue is injured; the number of cells decreases. Example: 1. Nerve cells. 2. Cardiac muscle cells. 2. Continuously cycling cells Turnover by division of stem cells. The number of cells is kept constant. Example: 1. Skeletal muscle cells. 2. Intestinal epithelium. 3. Epidermis of skin.  Prof. Dr. Shehab Hafez Stem cell divides & give 2 daughter cells. One cell remains as stem cell. The other (progenitor cell) will undergo division & differentiation. 3. Facultative dividers Cells enter the G0 phase but retain the capacity to re-enter the G1 stage of the cell cycle when stimulated. Example: Liver cells. After liver injury, the cells are stimulated to re-enter the G1 stage of interphase.  Prof. Dr. Shehab Hafez Meiosis Definition ▪ Definition: (meiosis from a Greek First meiotic division word means lessening) is the process 2 divisions Second meiotic division by which the amount of genetic material is reduced to haploid Time of meiosis amount. ▪ In male starts at puberty. ▪ Occurs in germ cells of the testis and ▪ In female: ovary. The oocytes start prophase I in the 3rd month intrauterine ▪ Also called Gametogenesis. & remain suspended in prophase till the age of puberty. ▪ Gives 4 gametes, each contains half 2nd meiosis starts after ovulation. the number of chromosomes; 23 s - 2ry oocyte completes 2nd meiosis only if fertilization chromosomes (haploid number). occurs. If no fertilization within 24 hours, 2ry oocyte will degenerate.  Prof. Dr. Shehab Hafez First Meiosis Prophase I Prophase I is the longest phase of meiosis. Condensation of chromosomes. The homologous d-chromosomes (one is maternal, and the other is paternal) are arranged in pairs. Crossing over occurs between the non-sister chromatids of the homologous pair. This allows for exchange of genetic information. At the end of prophase I, nuclear membrane & the nucleolus disappear, and the spindle is formed.  Prof. Dr. Shehab Hafez Metaphase I ▪ Maximum condensation of the chromosomes. ▪ The homologous chromosomes line up in pairs in the cell equator. Anaphase I ▪ The homologous chromosomes separate (disjunction). ▪ Each half of the homologous chromosomes is pulled to each pole of the cell. Telophase I & Cytokinesis ▪ Each daughter cell contains haploid number (23d– chromosomes). ▪ A new nuclear membrane surrounds each haploid set. ▪ Cytokinesis.  Prof. Dr. Shehab Hafez Second Meiosis Interphase : no S-stage ( no duplication of DNA). Prophase II Disappearance of the nucleolus and nuclear membrane. Spindle is formed. Chromosomes condense & become attached to the spindle fibers. Metaphase II : 23 d – chromosome arrange singly at the cell equator. Anaphase II: each d- chromosome splits into 2 s-chromosomes and each set: 23 s – chromosomes moves to one pole of the cell. Telophase II Nuclear envelope & nucleolus reform. Cytokinesis resulting in two cells, each cell has haploid number: (23 s – chromosomes).  Prof. Dr. Shehab Hafez Meiosis Result of Meiosis In male: one cell gives 4 viable spermatids; each has a haploid number (23 s-chromosome). In female: one cell gives one viable Ovum and 3 non- viable polar bodies, each with haploid (23 s-chromosomes).  Prof. Dr. Shehab Hafez Chromosomal abnormalities A deviation from the normal number or from the Definition normal morphology (shape) of the chromosomes. 1. Numerical chromosomal Types abnormalities 2. Structural chromosomal abnormalities Causes 1. Radiation. 2. Infection with German measles. 3. Pregnancy in old age. 4. Auto-immune diseases. 5. Family history of chromosomal abnormalities.  Prof. Dr. Shehab Hafez Numerical abnormalities of Sex chromosomes A. In males Klinefelter's syndrome ▪ One of the most common anomalies of X chromosome. ▪ It is a condition in which a male has an extra X chromosome (XXY) in his cells. Causes ▪ Nondisjunction (failure of separation) of X chromosome at one of the meiotic divisions → an ovum with XX. When this ovum is fertilized by a sperm carrying Y → a baby with XXY. Karyotype 47, XXY  Prof. Dr. Shehab Hafez B. In females Turner's syndrome ▪ A condition in which a female has only one X instead of XX in her cells → XO (monosomy of X chromosome). Causes ▪ Nondisjunction of X chromosomes during the formation of ova → an ovum with no X chromosome. When this ovum is fertilized by a sperm with X chromosome → a baby with XO. Karyotype 45, XO  Prof. Dr. Shehab Hafez Numerical abnormalities of autosomes Down's syndrome (Mongolism or Trisomy 21) ▪ The most common chromosomal abnormality. ▪ The risk increases with the age of the pregnant mother. ▪ Karyotyping shows an extra chromosome 21 (trisomy 21). Causes ▪ Nondisjunction i.e. failure of chromosome 21 to separate during the first meiotic division → an ovum with 24 chromosomes. ▪ When this ovum is fertilized by a sperm carrying 23 chromosome → a fertilized ovum with 47 chromosomes. Male Mongol: 47, XY+21 Karyotype Female Mongol: 47,XX+21  Resources ▪ Junqueira LC, Carneiro J: Junqueira’s Basic Histology. Text and Atlas, thirteenth edition (2013), LANGE Mc Graw Hill. ▪ Gartner LP and Hiatt JL: Color Textbook of Histology. Third edition (2007). Saunders Elsevier. Prof. Dr. Shehab Hafez