TERM• Chromosome – A structure made of DNA and histones• DNA (deoxyribonucleic acid) – A polynucleotide that contains the pentose sugar deoxyribose.• Chromatid – One of two identical parts of a chromosome held together by a centromere• Centromere – The centralize region joining two sister chromatids
• Genome – The complete complement of an organism’s genes – An organism’s genetic material• Chromatin – Content of the nucleus – Consisting of protein, DNA and RNA, which form threads• Euchromatin – Loosely packed chromatin• Heterochromatin – Densely packed chromatin• Cytokinesis – Division of the cytoplasm during cell division
• Karyokinesis – Division of the nucleus during cell division• Gene – Functional unit of heredity – Composed of DNA which carries information from one generation to the next• Gamete – A haploid cell, formed by meiotic cell division of a germ cell – Required for sexual reproduction
The concept of cell division• Basic characteristics of cell division• Characteristics of cell survival, DNA and chromosome
THE CONCEPT OF CELL DIVISION.• Modern cell theory states that ‘All new cells are derived from other cell’. – All cells which comprise a human are derived, through the cell division, from single zygote formed by the fusion of two gametes. – These gametes in turn were derived from the division of certain parental cell.
There are two basic types:1.Mitosis which results in all daughter cells having the same number of chromosomes as the parent.2. Meiosis which results in the daughter cells having only half the number of chromosomes found in the parent cell.
• Cell division – involves the distribution of identical genetic material (DNA) to two daughter cells.• A dividing cell – duplicates its DNA, allocates the two copies to opposite ends of the cell – then split into daughter cells.
The cell cycle• Description of the four stages in the cell cycle• General explanation events during the G1, S and G2 followed by events in the mitotic phase.
THE CELL CYCLE.1. The complete sequence of events in the life of an individual diploid cell.2. The four stages of the cell cycle; i. G1 • The first growth Phase ii. S Phase iii. G2 • The second growth phase iv. Mitotic Phase
i. G1 - The first growth phase• The longest phase• Volume of cytoplasm increase• Protein synthesis• Increase number of organelles
ii. S phase• DNA synthesis phase• The cell’s DNA replicates and non consist of two identical chromatids.iii. G2 - The second growth phase• Energy stores are increased.
iv. Mitosis• This process of nuclear division and followed by division of cytoplasm called cytokinesis.
Mitosis• Explain the mitotic cell division using diagrams and photographs• Show the position of the chromosomes at each stage• State the changes in the chromosomes• Describe briefly the cytokinesis process and list the differences between cell division in animal and plant cells• Significances of mitosis
MITOSIS2 phases : i. nucleus division ( karyokinesis ) ii. cytoplasm division ( cytokinesis )4 stages: i. prophase ii. metaphase iii. anaphase iv. telophase.
Prophase• Chromosomes – visible as long, thin tangled threads. – shorten and thicken – comprise two chromatids joined at the centromere.• Centrioles – migrate to opposite ends of poles of the cell (except for plant). – microtubules develop and form a star-shaped structure called an aster. – Some of these microtubule, called spindle fibers, span the cell from pole to pole.• The nucleolus disappears• Nuclear envelope disintegrates
Metaphase• The chromosomes arranged themselves at the centre of equator of the spindle, and become attached to certain spindle fibers at the centromere.• Contraction of these fibers draws the individual chromatids slightly apart.
Anaphase• The centromeres split and further shortening of the spindle fibers causes the two chromatids of each chromosome to separate and migrate to opposite poles.• The shorting of the spindle fibers is due to the progressive removal of the tubulin molecules of which they are made.• The energy for this process is provided by mitochondria which are observed to collect around the spindle fibers.
Telophase• The chromatids reach their respective poles and a new nuclear envelope forms around each group.• The chromatids uncoil and lengthen, thus becoming invisible again.• The spindle fibers disintegrate and nucleolus reforms in each new nucleus.
Cytokinesis – division of cytoplasmIn Animal Cells• Occur by a process known as cleavage.• The first sign of cleavage is the appearance of cleavage furrow. (Begins as a shallow growth in the cell surface)
Cytokinesis – division of cytoplasmIn Plant Cells• Have walls but no cleavage furrow.• During telophase, vesicles derived from Golgi apparatus move along microtubules to the middle of the cell producing a cell plate.• The cell plate enlarges until its surrounding membrane fuses with the plasma membrane along the perimeter of the cell.• Two daughter cells result, each with its own plasma membrane. A new cell wall arising from the contents of the cell plate has formed between the daughter cells.
Differences between mitosis in plant and animal cells Animal Cells Plant Cells 1. Involve aster or 1. Do not form spindle formation. centrioles and lack centrioles. 2. Cytokinesis occurs 2. Occurs by the by the constriction growth of a cell of microtubules – plate through the cleavage furrow. fusion of vesicles.
Significance of mitosisGenetic stability• Mitosis produce two nuclei which have the same number of chromosomes as the parent cell.• Daughter cells are genetically identical to the parent cell and no variation in genetic information can be introduced during mitosis.• This result in genetic stability within populations of cells derived from the same parental cells.
Significance of mitosisGrowth• The number of cell within organism increases by mitosis and this is the basis of growth in multicellular organisms.Cell replacement• Replacement of cells and tissues involves mitosis.Regeneration• Some animal are able to regenerate whole parts of the body, such as legs in crustacea and arms in star fish. Production of the new cells involve mitosis.Asexual reproduction• Mitosis is the basis of asexual reproduction, the production of new individuals of a species by one parent organism.