Mitosis & Meiosis PDF

MITOSIS GENERAL BIOLOGY – in this phase, cells are not actively preparing to divide (resting phase) MITOSIS – the cell...

MITOSIS GENERAL BIOLOGY – in this phase, cells are not actively preparing to divide (resting phase) MITOSIS – the cell is in a quiescent stage (inactive) that – is a process where a single cell divides into occurs when cells exit the cycle two identical daughter cells (cell division) – some cells enter G0 temporarily until an WHY DO CELLS DIVIDE? external signal trigger the onset of G1 – the major purpose of mitosis is for growth – other cells that never or rarely divide such as and to replace worn out cells mature cardiac muscle and nerve cells, remain – if not corrected in time, mistakes made G0 permanently during mitosis can result in changes in the. G1 PHASE. DNA that can potentially lead to genetic – is called the first gap because from a disorder microscopic aspect, little change is visible CELL CYCLE – the cell is accumulating the building blocks of chromosomal DNA and the associated – is an ordered series of events involving cell proteins as well as accumulating sufficient growth and cell division that produces two energy reserves to complete the task of new daughter cells replicating each chromosome in the nucleus – cells on the path to cell division regulated – decides if the cell will proceed to the next stages of growth. DNA replication and division stage of the cell cycle that produces two identical (cone cells).. S PHASE. – the cell cycle has two major phases: – throughout interphase, nuclear DNA Interphase and Mitotic phase remains in a semi-condensed chromatin - During interphase, the cell grows and configuration DNA is replicated. – In the S phase, DNA replication can proceed - During mitotic phase, the replicated through the mechanisms that result in the DNA and cytoplasmic contents are formation of sister chromatids that are firmly separated, and the cell divides attached to the centromeric region INTERPHASE (“between”) – centrosome is duplicated in this phase – cell undergoes normal growth processes – 2 centrosomes will give rise to the mitotic while preparing for cell division spindle: the apparatus that orchestrates the – THREE STAGES: G1, S, and G2 movement of chromosomes during mitosis. G0 PHASE. – At the center of each animal cell, – not all cells adhere to the classic cycle centrosomes are associated with centrioles (1 pattern that immediately enters interphase pair) which are at right angles to each other. – centrioles help organize cell division  (golgi complex and endoplasmic reticulum) fragment and disperse toward the periphery of the cell – nucleolus disappears (disperses) – centrosomes begin to move to opposite poles of the cell – microtubules that will form the mitotic spindle extend between the centrosomes, chromatin fiber: composed of DNA and pushing them farther apart as the microtubule protein that produce ribosome fibres lengthen 8 histones = 1 nucleosome – the sister chromatids begin to coil more. G2 PHASE. tightly with the aid of condensin proteins and – the cell replenishes its energy stores and become visible under a light microscope. synthesizes the proteins necessary for. PROMETAPHASE. chromosome manipulation – The First Change Phase – some cell organelles are duplicated – the remnants of the nuclear envelope – cytoskeleton is dismantles to provide fragment. The mitotic spindle continues to resources for the mitotic phase develop as more microtubules assemble and – there may be additional cell growth stretch across the length of the former nuclear – final preparations should be made before area entering the first stage of mitosis – chromosomes become more condensed and THE MITOTIC PHASE discrete. Each sister chromatid develops a – is a multistep process during which the protein structure called a kinetochore in the duplicated chromosomes aligned, separated, centromeric region and move into two new identical daughter – the proteins of the kinetochore attract and cells bind mitotic spindle microtubules – first portion is called karyokinesis (nuclear – as the spindle microtubules extend from the division), the second is called cytokinesis is the centrosomes, some of these come into contact physical separation of the cytoplasmic with and firmly bind to the kinetochores components into the two daughter cells – once a mitotic fiber attaches to a chromosome, the chromosome will be KARYOKINESIS orientated until the kinetochores of sister – also known as mitosis, is divided into a series chromatids face the opposite poles of phases — prophase, prometaphase, metaphase, anaphase, and telophase — that result in the division of the cell nucleus. PROPHASE. – The First Phase (Pro= advance) – the nuclear envelope starts to dissociate into small vesicles. the membranous organelles – eventually, all the sister chromatids will be – the cell becomes visibly elongated (oval attached via their kinetochore to microtubules shaped) as the polar microtubules slide against from opposing poles each other at the metaphase plate where they – Polar Microtubules: spindle microtubules overlap that do not engage the chromosomes. These microtubules overlap each other midway between the two poles and contribute to cell elongation – Kinetochore Microtubules: are those that attach to chromosomes at the kinetochore to facilitate movement to opposite poles before cytokinesis – Astral Microtubules: aid in spindle. TELOPHASE. orientation and are required for the regulation – The Distance Phase (Telo = final or distant) of mitosis – the chromosomes reach the opposite poles and begin to decondense (unravel) relaxing into a chromatin configuration – the mitotic spindle are depolymerized (turned into smaller units) into tubulin monomers that will be used to assemble cytoskeletal components for each daughter cell – nuclear envelopes form around the chromosomes, and nucleosomes appear within the nuclear area. METAPHASE. – The Change Phase (Meta = along with) – all chromosomes are aligned in the metaphase plate or the equatorial plane midway between the two poles of the cell – sister chromatids are still tightly attached to each other by cohesin proteins – At this time, the chromosomes are maximally condensed. ANAPHASE. – The Upward Phase (Ana = up or apart) CYTOKINESIS – cohesin proteins degrade and sister – “cell motion” chromatids separate at the centromere – second main stage of mitotic phase during – each chromatid, now called chromosome, is which cell division is completed via the pulled rapidly towards the centrosome to which its microtubule is attached physical separation of the cytoplasmic components into two daughter cells REGULATION OF – division is not complete until the cell INTERNAL CHECKPOINTS components have been apportioned and completely separated into two daughter cells. G1 CHECKPOINT. – although the stages of mitosis are similar to – restriction point most eukaryotes, cytokinesis is quite different – the cell irreversibly commits to the cell – In animal cells, cytokinesis follows the onset division process of anaphase – check for genomic DNA damage – a contractile ring composed of actin. G2 CHECKPOINT. filaments forms just inside plasma membrane – cell size and protein reserved are assessed to at the former metaphase plate ensure that all chromosomes have been – Actin Filament: pulls the equator of a cell replicated and DNA isn’t damaged inward, forming a fissure (crack). M CHECKPOINT. – Animal cell: cleavage furrow – near the end of metaphase stage of – Plant cell: cell plate karyokinesis – the furrow deepens as the actin ring – spindle checkpoint contracts, eventually the membrane is cleaved into two ERRORS IN MITOSIS – due to nondisjunction which may result to chromosomal mutation Mosaicism Hemophilia Marfan Syndrome MEIOSIS GENERAL BIOLOGY MEIOSIS INTERPHASE – a source of distinction – similar to mitosis interphase – The form of cell division by which – chromosomes (DNA) replicate in the S phase GAMETES, with HALF the number of – Each duplicated chromosome consists of two chromosomes are produced identical sister chromatids attached at their – Diploid (2n) Haploid (n) centromeres – Meiosis is SEXUAL reproduction – centriole pairs also replicate – Two divisions (Meiosis I and II) – Sex cells divide to produce GAMETES (sperm and egg) – Gametes have HALF the number of chromosomes. Occurs only in GONADS (testes or ovaries). SPERMATOGENESIS. – production of sperm – nucleus and nucleolus visible MEIOSIS I. OOGENESIS. – cell division that reduces the chromosome – production of egg or ova number by one-half – consists of 5 phases: Prophase I, Prometaphase I, Metaphase I, Anaphase I, and Telophase I. PROPHASE. – longest and most complex (90%) – homologous chromosomes form a synapse – Synapsis occurs – Homologous chromosomes come together to form a tetrad – Tetrad: two chromosomes or four chromatids (sister and non-sister chromatids) – nucleus and nucleolus disappears – spindle forms – chromosomes coil and synapsis occurs – tetrads form and crossing over occurs 5 DISTINCT SUB-STAGES (PROPHASE I) SYNAPSIS 1. Leptotene: chromosomes begin to condense – tight pairing of homologous chromosomes 2. Zygotene: synapsis begins with a that form a tetrad synaptonemal complex forming between – In synapsis, the genes on the chromatids of homologous chromosomes the homologous chromosomes are aligned 3. Pachytene: crossing over of genetic material precisely with each other. occurs between non-sister chromatids 4. Diplotene: synapsis ends with HOMOLOGOUS CHROMOSOMES disappearance of synaptonemal complex; – pair of chromosomes that are similar in homologous pairs remain attached to the shape and size chiasmata – carry genes controlling the same 5. Diakinesis: chromosomes become fully inherited traits condensed and nuclear membrane – each locus is in the same position disintegrates prior to metaphase I – Humans: 23 pairs of chromosome a. Autosomes: first 22 pairs PROMETAPHASE I b. Sex Chromosomes: 1 pair – attachment of the spindle fibre microtubules to the kinetochore proteins at centromeres CROSSING OVER – Kinetochore proteins: are multiprotein – may occur between non-sister chromatids at complexes that bind the centromeres of a sites called chiasmata chromosomes to the microtubules of the – segments of non-sister chromatids break and mitotic spindle reattach to the other chromatid – At the end of prometaphase I, each tetrad is – Chiasmata are where chromosomes touch attached to microtubules from both poles, with each other and exchange genes one homologous chromosome facing each – causes genetic recombination pole. – Chiasmata: site of crossing-over – physical exchange of regions of the – homologues are still held by chiasmata chromatids – In addition, the nuclear membrane has broken down entirely SYNAPTONEMAL COMPLEX METAPHASE I – a lattice of proteins between homologous chromosomes forms at specific locations and – shortest phase then spreads to cover the entire length of the – tetrads align on the equator with the chromosomes kinetochore facing opposite poles – tetrads line up at the midway point between PROMETAPHASE II the two poles of cell to form metaphase plate – same as prophase in mitosis – Independent assortment: chromosomes – nucleus and nucleolus are broken down separate randomly causing genetic – spindle fibres are fully formed recombination – kinetochore formation ANAPHASE I METAPHASE II – microtubules pull the linked chromosomes – sister chromatids are maximally condensed apart. Homologous chromosomes separate and move towards the poles and aligned at the equator of the cell – sister chromatids remain tightly bound ANAPHASE II together at the centromere – sister chromatids are pulled apart by the – chiasmata are broken in anaphase I as the kinetochore microtubules. Non-kinetochore microtubules attached to the fused microtubules elongate the cell kinetochores pull the homologous TELOPHASE II chromosomes apart – nucleus and nucleolus reform TELOPHASE I – spindle fibres disappears – separated chromosomes at opposite poles – chromosomes decondense – each pole now has haploid (1n) set of – nuclear envelopes form chromosomes – cytokinesis separates 2 cells into 4 unique – cytokinesis occurs and two haploid haploid cells called GAMETES (eggs/sperm) daughter cells are formed KARYOTYPE – In plants, cell plate is formed – In most animals and some fungi, a cleavage – an organised picture of the chromosomes of a human arranged in pairs by size (largest to furrow is formed smallest) –In some organism, chromosomes decondense FERTILIZATION MEIOSIS II – the fusion of a sperm and egg to form a – no interphase II and is very short zygote (fertilized egg) – no DNA replication NON-DISJUNCTION – similar to mitosis – is the failure of homologous chromosomes, PROPHASE II or sister chromatids, to separate in meiosis – same as prophase in mitosis – results with the production of zygotes with – chromosomes condense again abnormal chromosome number, damaging the offspring – if nuclear envelopes are formed, they Down’s Syndrome (Trisomy 21) fragment into vesicles Patau Syndrome (Trisomy 13) – centrosomes duplicated during interkinesis Turner’s Syndrome (Monosomy 23 or X0) move away from each other Klinefelter’s Syndrome (Trisomy 23 or XXY) – new spindle fibres are formed Edward’s Syndrome (Trisomy 18) Jacob’s Syndrome ( Trisomy 23 or XXY MITOSIS (SUMMARY MEIOSIS (SUMMARY) MITOSIS vs. MEIOSIS MITOSIS vs. MEIOSIS