Cell Division

Cell Division

CHROMOSOMES

• Chromosomes are structures located in the nucleus of the cell. 
• They are made of DNA and protein. 
• Chromosomes are long thin threads called chromatin until cell division occurs. 
• Then they become visible as rod-like chromosomes. 
• Chromosomes are composed of genes. 
• All the genes of an organism make up the organism s genome. 
• Genes control the physical characteristics of a species. 
• All organisms of the same species contain the same number of chromosomes in their nuclei.

HAPLOID AND DIPLOID CELLS

HAPLOID AND DIPLOID CELLS

• Haploid cells contain 1 set of chromosomes in their nuclei. 
• Diploid cells contain 2 sets of chromosomes in their nuclei. 
• The human species contain 46 chromosomes in their nuclei. 
• This is the diploid (2n) number. 
• The sex cells of the species have 23 chromosomes in their nuclei. 
• This is the haploid (n) number. 
• When fertilisation takes place the 23 chromosomes (n) from the father (called Paternal chromosomes) and the 23 chromosomes (n) from the mother (called the Maternal chromosomes) combine to form the diploid (2n=46) number of chromosomes in the fertilised egg cell.

CELL CONTINUITY

CELL CONTINUITY

• All cells develop from existing cells. 
• This is called cell continuity. 
• Cell continuity enables organisms to grow, replace dead cells, and reproduce. 
• In single celled organisms cell division results in reproduction
• In multicellular organisms cell division results in growth, repair and replacement.
• A cells life can be described with the cell cycle. 
• There are three stages of the cell cycle: interphase, division of the nucleus (mitosis or meiosis) and Cytokinesis.

INTERPHASE

INTERPHASE

• This is the part of the cell s life process when it does not divide. 
• In this phase the cell grows, proteins and enzymes are made, and it increases the number of cytoplasmic organelles. 
• Near the end of interphase chromosome duplication occurs. 
• When this occurs the single strand chromosome becomes a double strand. 
• Each strand has identical genes.

MITOSIS

MITOSIS-DIVISION OF THE NUCLEUS

• Mitosis is the division of the nucleus. 
• Two daughter nuclei, genetically identical the original nucleus, are formed. 
• The 2 cells formed by mitosis are called daughter cells.
• There are four stages of mitosis. They are prophase, metaphase, anaphase, and telophase.

Prophase

• The chromatin condenses forming chromosomes. 
• Each chromosome is composed of two identical sister chromatids connected at the centromere. 
• At this stage the nucleolus and the nuclear membrane break down and the spindle fibres form.

Metaphase

• The chromosomes line up along the middle of the cell.
• Each chromosome is connected to both sides of the cell by spindle fibres attached to the centromeres.

Anaphase

• The spindle fibres shorten (contract) and split the pair of chromosomes at the centromeres. 
• The 2 sets of chromosomes are pulled to the opposite sides (poles) of the cell.

Telophase

• Each chromosome group becomes a nucleus when a nuclear membrane is formed around it.
• The chromosomes uncoil to become chromatin and the nucleolus reforms. 

CYTOKINESIS

• Cytokinesis results in 2 daughter cells each with identical chromatin. 
• The process differs in animal and plant cells.
  

ANIMAL CELL CYTOKINESIS

• Animal cell cytokinesis occurs by a process called cleavage. 
• A cleavage furrow appears, this furrow becomes deeper until, eventually, it breaks the 2 halves apart to form 2 daughter cells.

PLANT CELL CYTOKINESIS

• Sacs called vesicles form around the centre of the cell. 
• These sacs contain cellulose which will form a cell plate. 
• 2 new cell walls develop along the middle lamella. 
• The cells then break apart forming 2 daughter cells.

Animal CELL CYTOKINESIS

PLANT CELL CYTOKINESIS

Cancer

• A group of disorders in which certain cells lose their ability to control both the rate of mitosis and the number of times mitosis takes place.
• Causes: Carcinogens, Radiation, Virus,

Meiosis

• The nucleus now divides to form two daughter nuclei, each with a mixture of paternal and maternal chromosomes but with half the full complement of genetic material (and no pairs at all). 
• Finally the two daughter nuclei themselves divide to form gametes. 
• The chromosomes (really pairs of chromatids) split apart to form the genetic material of the four new cells. 
• The end result is four sex cells each with a complete but single set of 23 chromosomes.
  

Comparison

                        Mitosis

• Produces 2 new daughter cells

• The daughter cells have the same amount of chromosomes as the parent. 

• The daughter cells are genetically identical

                              Meiosis

• Produces four cells

• The daughter cells have half the amount of chromosomes as the parent

• The daughter cells are genetically different