Cell Division and Chromososmes

The Cell Cycle
1) The cell cycle is the sequence of events which occurs between one cell division and another and has three main stages
a) Interphase
– period of synthesis and growth
– the cell produces many materials required for its own growth and for carrying out all its functions
– DNA replication occurs during this stage
b) Mitosis this is the process of nuclear division
c) Cell division / Cytokinesis this is the process of division of the cytoplasm to form two daughter cells
2) The length of the cycle depends on the type of cell and external factors such as temperature, food and oxygen supplies eg:
- bacteria 20 mins.
- Epithelial cells of intestinal wall 8-10 hours
- Onion root tips up to 20 hours
3) In humans normal cells cannot continue dividing forever often 50 divisions after which they stop dividing and start to die
4) Some cells do not divide at all once specialized eg once a neurone is formed it never enters the cell cycle or divides again

Chromosomes
1) Chromosomes are most important cell structure during cell division as they transmit the hereditary information from one generation of cells to the next
2) Each species has a characteristic number of chromosomes in each cell (most species have between 12 and 50 per cell)
- humans 46
- fruit flies 8
- cats 38
- dogs 78
3) Genes are the units of inheritance and are short pieces of the chromosome and in humans approx 100,000
4) Chromosomes are found in pairs in the nucleus 23 homologous pairs (one of the pair from the mother and the other from the father)
5) Only one odd pair; the sex chromosomes XY male XX female
6) Majority of animal species and half the plant species are diploid (2n) with two sets of chromosomes per nucleus or cell. A few species are haploid (n) and some (including many plants) are polyploid
7) Diploid is advantageous since:-
- genetic variation is increased, since an individual will have a mixture of characteristics from each parent
- if a gene on one chromosome of a pair is faulty, the second chromosome may provide a healthy backup
8) Structure
a) made of DNA (deoxyribonucleic acid) and protein with small amounts of chromosomal RNA (ribonucleic acid)
b) each chromosome contains one molecule of DNA
c) each individual chromosome is normally a very loosely coiled, long, thin thread and cannot be seen clearly in the nucleus
d) DNA has negative charges along its length and positively charged protein molecules called histones are bonded to it
e) DNA – protein complex is called chromatin
f) Human cell contains about 2.2m of DNA distributed among the 46 chromosomes about 4.8cm of DNA per chromosome
g) To maintain a high degree of organization, the DNA is folded around the histone proteins which form a ‘scaffolding’ for DNA
h) The DNA helix combines with groups of eight histone molecules to form nucleosomes beads on a string
i) The nucleosomes and DNA are then coiled into a helix with six nucleosomes per turn solenoid fibre (30nm diameter)
j) The solenoids themselves are further folded and coiled to pack the DNA in even tighter exact details are not known.

Mitosis
1) Mitosis is the process in which a cell nucleus divides to produce two daughter nuclei containing identical sets of chromosomes to the parent cell
2) This is followed by division of the whole cell to form two daughter cells cell division or cytokinesis
3) Mitosis plus cell division results in an increase in cell numbers and is the method by which growth, replacement and repair of cells occurs in eukaryotes
4) Just before mitosis, the chromosomes coil up into more compact shorter, thicker structures made up of two chromatids held together at a point called the centromere
5) Mitosis is a continuous process but can be divided up into four stages in an animal cell
a) Prophase
b) Metaphase
c) Anaphase
d) Telophase
e) And back into Interphase
6) The centrioles are thought to be involved in orientating the spindle in nuclear division. Therefore they help determine in which plane the cell divides
7) The centrosome is responsible for making the spindle (microtubules). The centrosome also ‘reel in’ the spindle fibres by removing tubulin subunits and cause the movement of the chromosomes and chromatids during nuclear division
Cytokinesis
1) This stage follows telophase, and cell organelles become evenly distributed between the two poles of the cell along with the chromosomes
2) In animal cells the cell surface membrane begins to invaginate to form a furrow due to the action of microtubules until the membranes join up and completely separate the two cells
3) In plant cells some spindle microfilaments remain in the centre of the cell and guide Golgi vesicles to the equator of the cell. These vesicles enlarge and fuse to form a cell plate where cellulose forms on either side to form two new cell walls.

Significance of Mitosis
1) Genetic stability – two nuclei with same number of chromosomes as parent cell are produced and so will carry the same hereditary information on their genes. Also daughter cells are identical to parent and no genetic variation occurs resulting in genetic stability within populations of cells derived from the same parental cell
2) Growth – cell numbers in an organism grow and so mitosis is the basis of growth in a multicellular organism
3) Cell replacement – cells are constantly dying and being replaced and so mitosis replaces cells and tissues eg skin
4) Regeneration – some animals can regenerate whole body parts and so mitosis is involved in this
5) Asexual reproduction – basis of asexual reproduction by many species

Natural and Artificial Cloning in Plants and Animals
Natural Cloning
1) Asexual reproduction is an example of natural cloning
a) genetic material from a single individual
b) no genetic mixing or variation
c) no production of gametes
d) offspring produced from a single individual belong to a clone
e) all members of a clone share the same identical genetic constitution (unless mutation occurs)
2) Examples of asexual reproduction include:
a) Fission
- binary fission, where the organism divides into two or more equal parts eg prokaryotes
- multiple fission, where the nucleus divides repeatedly eg scizogony eg parasitic protoctists such as malarial parasite.
b) Spore formation eg prokaryotes, protoctists, fungi and many plants.
c) Budding eg Hydra
d) Fragmentation eg sponges and certain worms
e) Vegetative reproduction eg in many different plants