Mitosis and Meiosis Introduction PDF

Introduction; Mitosis and Meiosis Davit Kalmakhelidze KWIU 2024 Genetics The science that studies heredity and how traits and diseases are passed from one generation to the next. The founder of the modern human genetics can be considered Austrian monk Gregor Mendel who,...

Introduction; Mitosis and Meiosis Davit Kalmakhelidze KWIU 2024 Genetics The science that studies heredity and how traits and diseases are passed from one generation to the next. The founder of the modern human genetics can be considered Austrian monk Gregor Mendel who, in 1865, presented the results of his breeding experiments on garden peas. It can be considered that medical genetics started with the recognition of Mendel's laws of inheritance. It was proven that genes exist and that they transmit hereditary information. Mendel’s research has served as the foundation of genetics.  In 1903, Walter Sutton, an American medical student, and Theodor Boveri, a German biologist, independently proposed that chromosomes carry the hereditary factors, or genes. James Watson and Francis Crick were awarded with Nobel Prize for the discovery of the double-helix structure of DNA in 1953. But it was only 3 years after the discovery of the double helix structure of DNA, that the correct number of 46 chromosomes in humans was determined. After the discovery of double helix structure of DNA, the relationship between the sequence of bases in DNA and the sequence of amino acids in protein (i.e., the genetic code) was discovered. Main Concepts & Keywords Genetics - is the science that studies genes, heredity and variation in living organisms. Gene - the fundamental unit of heredity - made of DNA, which is composed of a polymer of chemical subunits called nucleotides. Allele – one of the alternative forms of a gene or DNA sequence at a given locus. Locus – position of a gene on a chromosome. Chromosomes - are threadlike structures found in the nucleus of human cells and other higher organisms which contain chromosomes.  There are four different nucleotides in DNA. Combinations of these four nucleotides define which amino acids will be used to make specific proteins in the cell. Genes are comprised of sequences of nucleotides contained on a double- stranded helical DNA molecule Chromosomes Telomere – The ends of the chromosome. Centromere – The primary constriction of the chromosome. The centromere also divides the chromosome into a short arm (p) and a long arm (q) “p” stands for petit (fr), small Chromatid – A single molecule of DNA Chromosome classification by size and position of centromere Metacentric – centromere in the middle of the chromosome Submetacentric – centromere divides the chromosome into 1/3 and 2/3 Acrocentric – centromere is near the end of the chromosome Telocentric – chromosome is like a straight rod with the centromere in terminal position  1-22 chromosomes – autosomes X and Y chromosomes – sex chromosomes A karyotype is the complete number and appearance of metaphase chromosomes in the nucleus of an eukaryotic cell, seen under the microscope  In most eukaryotes the members of each species have a characteristic number of chromosomes called diploid number (2n). Chromosomes in diploid cells exist in pairs, called homologous chromosomes. Haploid (n) – is a cell or organism with one member of each pair of homologous chromosomes. Normal human body cells (“somatic” cells) are DIPLOID: 23 pairs of chromosomes: – Numbers 1-22 (autosomes) – X and Y (sex chromosomes) XX in females, XY in males, germ cells (egg, sperm) have 23 chromosomes - HAPLOID There are two types of cells divisions: Mitosis - is the stage of the cell cycle, by which a diploid cell duplicates into two genetically identical daughter cells. Meiosis - is a special type of cell division, in which daughter cells receive only one chromosome from the chromosome pair. Mitosis results in identical diploid cells, Meiosis results in haploid gametes.  Genotype is the genetic makeup of a cell, an organism, an individual or a species OR a set of alleles for a given trait carried by an organism. Phenotype is the the physical, clinical, cellular, or biochemical manifestation of the genotype as a certain characteristic or a trait. Not all organisms with the same genotype look or act the same way (because appearance and behavior are modified by environmental and developmental conditions) Not all organisms that look alike necessarily have the same genotype. Mitosis and Meiosis Overview Mitosis is ordinary somatic cell division by which the body grows, differentiates, and effects tissue regeneration. It results in two daughter cells, each with chromosomes and genes identical to those of the parent cell. Meiosis occurs only in cells of the germline It results in the formation of reproductive cells (gametes), each of which has only 23 chromosomes— one of each kind of autosome and either an X or a Y. The Cell Cycle A human being begins life as a fertilized ovum (zygote) Mitosis is obviously crucial for growth and differentiation, but it takes up only a small part of the life cycle of a cell. The period between two successive mitoses is called interphase, the state in which most of the life of a cell is spent. Cells undergo a cycle that include interphase, consisting of: G1 (organelle duplication begins) S (DNA replication) G2 (synthesis of various proteins) and M (Mitosis followed by cytokinesis)  G1 phase – the cells grow, accumulate enzymes and molecules required for DNA replication, the cellular organelles duplicate S phase – cell’s chromosomal DNA is replicated and duplicated chromosomes are produced. G2 phase – cell continues to grow and prepare for division, synthesis of various enzymes and other types of proteins. M – Mitosis: Prophase, Prometaphase, Metaphase, Anaphase, Telophase and G0 Phase Some cell types do not divide at all and are permanently arrested G0 phase. During G0, the cell remains metabolically active but does not grow or divide. neurons red blood cells cardiomyocytes (heart muscle cells) liver cells Cell Cycle Checkpoints The cell cycle is governed by checkpoints that determine the timing of each step of cell division Checkpoints monitor and control the accuracy of DNA synthesis and chromosome movement. If damage is detected, these mitotic checkpoints arrest cell cycle progression until repairs are made or until the cell is instructed to die by programmed cell death (apoptosis). Mitosis Process of cell division in somatic cells 1 diploid cell → 2 diploid cells Goes through several defined stages Chromosomes are passed on as exact copies without recombination (usually) “Segregation” is the process where one copy of each replicated chromosome goes to each daughter cell 1.Prophase Chromosome condensation Centrosome separation: The centrosome is the point of origin of the mitotic spindle. It consists of two centrioles and a surrounding matrix, from which the microtubules emerge. Formation of the mitotic spindle 2.Prometaphase Degradation of the nuclear membrane into small vesicles and storage of intercellular vesicle Completion of the mitotic spindle formation 3.Metaphase: maximal condensation of the chromosomes, which are aligned along the equatorial plane of the cell 4.Anaphase Separation of sister chromatids due to cohesion dissolution at the centromere by the enzyme separase Cell elongation 5.Telophase Decondensation of the chromosomes Disintegration of the mitotic spindle Formation of a new nuclear membrane Cell bodies division at the equatorial plane Cytokinesis The division of a cell's cytoplasm into two new cell bodies Characteristics Usually begins at the end of anaphase Cytoplasm and cell organelles are distributed between the two new cells. Each of the new cells contains one copy of the sister chromatids. Meiosis Prophase I Longest and most complex & most crucial phase During this phase, each pair of chromatids pair up with their homologous pair and fasten together (synapsis) in a group of four called a tetrad. That is when crossing over can occur Crossing Over is the exchange of segments between homologous chromosomes during synapsis. Homologous Chromosomes: Pair of chromosomes (maternal and paternal) that are similar in shape and size. Homologous pairs (tetrads) carry genes controlling the same inherited traits. Each locus is in the same position on homologues.  Crossing over may occur between nonsister chromatids at the chiasmata. Segments of nonsister chromatids break and reattach to the other chromatid. Chiasmata (chiasma) are the sites of crossing over. Metaphase I Shortest phase Tetrads align on the metaphase plate. INDEPENDENT ASSORTMENT OCCURS Anaphase I Homologous chromosomes separate and move towards the poles. Sister chromatids remain attached at their centromeres. Telophase I Each pole now has haploid set of chromosomes. Cytokinesis occurs and two haploid daughter cells are formed. Cytokinesis: Occurs simultaneously with telophase I, Forms 2 daughter cells Meiosis I separates homologous chromosomes – reduction division Meiosis II Meiosis II is like mitosis, except that the chromosome number is 23 instead of 46. The chromatids of each of the 23 chromosomes separate, and one chromatid of each chromosome passes to each daughter cell. However, because of crossing over in meiosis I, the chromosomes of the resulting gametes are not identical. Prophase II - same as prophase in mitosis Metaphase II - same as metaphase in mitosis Anaphase II - same as anaphase in mitosis, sister chromatids separate Telophase II - Same as telophase in mitosis. Nuclei form. Cytokinesis occurs. Nondisjunction The failure of a chromosome pair to separate (disjoin) during meiosis where normally one chromosome goes to each daughter cell. It may happen in meiosis I, meiosis II & mitosis In humans this often occurs with the 21st pair – producing a child with Downs Syndrome Gametogenesis Oogenesis Oogenesis is the development of an immature oocyte into a secondary oocyte or mature ovum (if fertilization occurs). Begins in utero during fetal growth and ends with menopause. At birth, a normal ovary contains about 1– 2 million primary oocytes. There is a continuous decline in