Modern cell theory states that ' all new cells are derived from other cells'. Human body (10 14 cells) A B parental cells ancestral cell where A = B = Mitosis: give daughters cells having the number of chromosomes as parent Meiosis: give daughter cells having only number of chromosomes as parent 1 8.1 Chromosomes 8.1.1 Chromosome Structure - Chromosome consists of DNA, proteins and RNA - only visible during cell division; as invisible in resting cells - each chromosome consists of 2 threads (chromatids) joined at a point (centromere) - chromosomes vary in shape and size, both within and between species 8.1.2 Chromosome Number - chromosome number varies from one species to another but is always the same for normal individuals of the same species - majority of organisms have 10 to 40 chromosomes in their cells 8.2 Mitosis cell cycle: a regular pattern of events taking place in a dividing cell Interphase: nucleus is mechanically inactive although chemically very active Mitosis: when the nucleus is mechanically active Interphase or (resting phase): chromosomes are not visible but DNA content doubles; duplication of organelles takes place Prophase: initially chromosomes occur as long, thin tangled thread; then shorten & thicken to form two chromatids joined at the centromere; centrioles* migrate to opposite poles of cell, developing an aster and microtubules to form the spindle; nucleolus disappears & nuclear envelope disintegrates * not occurring in higher plants Metaphase: chromosomes arrange themselves at equator of spindle, become attached to spindle fibres at centromeres Anaphase: shortening of spindle fibres causes sister chromatids to separate and move to opposite poles, using energy from mitochondria Telophase: chromatids reach their respective poles;
new nuclear envelope forms around each group; chromatids uncoil, lengthen and become invisible again; spindle fibres disintegrate and nucleoli form in each new nucleus 2 8.2.1 Differences between Mitosis in Plants and Animal Cells 1. No centrioles thus no aster in higher plants 2. Animal cells: cell division by constriction of centre of parent cell Plant cells: cell wall formation across equator of parent cell 3. Most animal cells are capable of mitosis Plant cells only carry out mitosis in meristematic cells
8.3 Meiosis (Reduction Division) - 1 division of the chromosomes followed by 2 divisions of nucleus and cell - a diploid (2n) parent cell give rise to 4 haploid (n) daughter cells - forms gametes (sperms and ova) or spores in some plants 1st meiotic division: similar to mitosis except for a highly modified prophase 2nd meiotic division: a typically mitotic division 3 Prophase I - chromosomes become visible, shorten and fatten - homologous chromosomes pair together (synapsis) to form a bivalent - sister chromatids join at chiasmata where crossing over can occur - nucleolus disappear, nuclear membrane breaks down - centrioles migrate to poles to form a spindle Metaphase I - bivalents arrange themselves on equator with each of a pair of homologous chromosomes orientated to opposite poles randomly
Anaphase I - spindle fibres attaching to the centromeres contract and pull sister chromosome of homologous chromosomes to opposite poles Telophase I - chromosomes reach opposite poles; nuclear envelopes form; - spindle fibres disappear; chromatids uncoil; cell divides into two; - nucleus may enter into interphase but no DNA replication OR cell enters prophase II directly Prophase II - nucleolus disappears; nuclear envelope breaks down; - centrioles divide and move to opposite poles; spindle develops at right angle to spindle of 1st meiotic division Metaphase II - chromosomes arrange themselves on equator of spindle with spindle fibres attached to centromeres of each chromosome Anaphase II - centromeres divide and pulled by spindle fibres to opposite poles, carrying the chromatids with them Telophase II - upon reaching poles, chromatids unwind; nuclear envelope and nucleolus reform - spindle disappears, 2 cells divide to give 4 cells (a tetrad) 4 8.4 Comparison of Mitosis and Meiosis Table 8.2 Differences between mitosis and meiosis Mitosis Meiosis
Differences between Nuclear Division and Cell Division Nuclear Division Cell Division 5 similar in plants and animals involves duplication of chromosomes daughter nuclei may be similar (mitosis) or dissimilar (meiosis) spindle formation occurs often, but not always followed by cell division different in plants and animals involves duplication of organelles daughter cells are always similar no spindle formation always preceded by nuclear division 8.5 The Significance of Cell Division 8.5.1 Significance of Mitosis Mitosis produce daughter cells which are exact copies of the parental cell for: a) Growth b) Repair c) Asexual reproduction 8.5.2 Significance of Meiosis The long-term survival of a species depends on its ability to Therefore it is necessary for offspring to be different from their parents as well as different from each other. There are 3 ways variation is brought about: a) Production and fusion of haploid gametes - meiosis ensures that chromosome number is not doubled at each generation b) Random distribution of chromosomes during metaphase I and consequent independent assortment c) Crossing over between homologous chromosomes MEIOSIS VARIATION EVOLUTION of the fittest! In the absence of control, the tissue grows, invading and crowding out other tissues, sometimes destroy them. Other causes of cancer: 1. mutation by chemicals or radiations 2. viral infections
Exercise: 1. Compare and contrast the process of mitosis and meiosis. Discuss the roles and significance of mitosis and meiosis in the life of flowering plants and mammals, illustrating your answer with examples where appropriate. (20 marks) 96-II-6 6
7 2. Describe and explain which stage of mitosis is most suitable for determining chromosome number. (2 marks) 94-I-4 3. (a) Distinguish between (i) codon and anticodon, (ii) continuous and discontinuous variation (4 marks) 93-II-1(a)