The Cell Cycle, Chromosomes and Mitosis

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1 SIM uition he ell ycle, hromosomes and Mitosis Biology S he DN in a diploid human cell is nearly 2 m long. he diameter of a cell is only m. With this in mind, it is obvious, that eukaryotic DN must be tightly packaged. During interphase, when cells are not dividing, DN is packaged into chromatin. Further condensation during mitosis produces chromosomes. he lowest level of packaging is the nucleosome, which consists of two and a half helical turns of DN wrapped around histone proteins. DN Replication the Semi-conservative theory - double strand of DN / helix - unzips/unwinds - H bonds between the organic bases are broken - by helicase as a result, the strands are separated - each strand acts as a template for the formation of a new strand - new organic bases / nucleotides - pair up with exposed bases - complementary / specific base pairing o (i.e. with ) o ( with ) - DN polymerase catalyses the joining of the separate nucleotides o all in one go so complete a new strand for the 5-3 strand - for the 3 5 strand DN, polymerase produces short segments of strand which have to be joined by DN ligase to make the completed new strand - result 2 identical strands his is called the semi-conservative theory. - each new double helix contains ONE strand of the DN and ONE new strand See diagram on next page. Page 1

2 SIM uition Biology S base pairs sugar phosphate backbone One original molecule of DN he enzyme DN helicase breaks the hydrogen bonds between the cases causing the strands to separate (to unzip) from one end. he exposed bases attract the complementary bases of free DN nucleotides. (e.g. will only attract ) omplementary bases join with hydrogen bonds and free nucleotides link up to for a new strand original (old) DN strand wo exact copies of the original DN molecule have been made new strand he sequence of the bases is kept exactly the same due to specific base pairing Page 2

3 SIM uition Mitosis Biology S his is the name given to the form of cell division which occurs in normal body cells. he result of this division is: - 2 genetically identical daughter cells - the chromosome number is maintained his type of cell division is used for growth, repair and replacement. Once the process of cell division has started, it is a continuous process and 4 distinct stages have been identified by humans: P M Interphase - this is often termed the resting phase (but it is not really lots does occur) - no chromosomes are visible - DN replication occurs in this phase. Prophase - chromosomes appear; longitudinally split (i.e. double stranded) - each strand = a chromatid (i.e. a molecule of DN) - a chromosome is made of two chromatids - the 2 chromatids making up a chromosome are genetically identical centromere - the centriole: DN chromatid o a small body just outside the nucleus o this divides and each half migrates to the opposite side of the nucleus o as the centrioles move apart, they give rise to spindle fibres o the spindle fibres become attached to the centromere of the chromosomes when the chromosomes are lined up at the equator of the cell Page 3

4 SIM uition Biology S Metaphase - the double membrane of the nucleus disappears - the chromosomes become aligned along the equator of the cell - the chromosomes are attached to the spindle fibres by their centromere centriole centromere spindle fibre naphase - a short, rapid phase - the centromere splits in half and each half repels each other - at the same time, the spindle fibres shorten elophase - this covers the period in which the two groups of chromosomes arrive at the poles and become surrounded by a nuclear membrane - the spindle fibres disappear and the cell cleavage appears - the cytoplasm divides into two by means of formation of a cell membrane (or lamella in plants) between the two sets of chromosomes - the telophase may be either a transitory period short enough only for DN replication to occur before the cells divide again or long : permanent interphase may follow. Separation of the chromatids due to (a) the centromeres split and each half repels each other half the spindle fibres shorten Page 4

5 SIM uition Role of spindle fibres (a) attach the chromosomes / chromatids by the centromere shorten to separate chromatids (i.e. movement of chromosomes) Biology S Energy for process from P (i.e. respiration) Is interphase a resting phase? No (a) DN replication occurs (i.e. each chromatid manufactures a genetically identical partner see DN stages later on) a build up of P for the process of DN replication and cell division Outcome (a) (c) 2 daughter cells same chromosome number as each other daughter / parent cell genetically identical to each other and parent cell (i.e. no variation) onsequence of mitosis (a) from fertilised egg: - repeated cell divisions give rise to a population of cells which later become tissues and organs - all cells have the same genetic complement each individual of a species has a characteristic number of chromosomes e.g. humans: 46 (i.e. 23 pairs) (c) (d) (e) enetic information carried on chromosomes varies Some cells lose their genetic material once they are developed (e.g. red blood cells, xylem cells) mitosis is used for growth, repair and replacement N.B. here is a difference between plant and animal mitosis (see next page) Plant no asters (protein tubules radiating from centrioles) no centrioles cell wall cell plate and middle lamella nimal asters centrioles no cell wall no cell plate Page 5

6 SIM uition Biology S ell cycle Mitosis is part of the cell cycle. - i.e. where the existing cell divides into two new cells (daughter cells) - does not result in a change in chromosome number - daughter cells are identical to each other and the parent cell 1 - ap 1 - cell grows and develops S - Synthesis - DN replication so that each chromatid manufactures an identical partner chromosome 2 - ap 2 - chromosomes begin to condense (get ready to appear) M - Mitosis - nuclear division - cytokinesis - the division of the cytoplasm and separation of daughter cells - distinct from nuclear division. - It involves the constriction that divides the cell into two - It is usually well underway by the end of the elophase and does not involve the formation of the cell plate S 1 2 M SYLLBUS HEKLIS - Unit 1.4 he cell cycle understand the sequence of division of a nucleus following the replication of DN during interphase; hromosome structure understand that chromosomes consist of DN and histones in the nucleus of eukaryotic cells; recall the replication of DN; understand the role of the enzymes involved; understand that a leaf palisade cell and a liver cell have a diploid chromosome number and have been produced by nuclear division followed by differentiation. Mitosis recall the structure of a chromosome; understand the behaviour of chromosomes during the stages of the mitotic cell cycle; describe the events of prophase, metaphase, anaphase and telophase; understand the significance of mitosis in growth and replacement; understand the significance of daughter nuclei with chromosomes identical in number and type; understand that the production of new individuals involves the transfer of genetic information from parent to offspring; understand that inherited information in the offspring is identical to that of the parent; understand the significance of mitosis in achieving this; understand the nature of natural and artificial cloning in plants and animals. Practical work to include preparation and staining of root tip squashes to recognise and study stages in mitosis using a light microscope. Page 6