1. The Cell & Cell Division
Leaving Certificate Agricultural Science
The Cell & Cell Division

2. Introduction All living things have cells.
Cells contain DNA, the genetic chemical.
DNA is found within structures called CHROMOSOMES, located in the nucleus of a cell.
Humans have 46 chromosomes in each of their cells, 23 HOMOLOGOUS PAIRS obtained from both parents.
These cells are called diploid.
Sperm and egg cells contain 23 chromosomes, one set. These cells are called haploid.
Each chromosome contains a series of genes which control our characteristics.
We have two genes for each characteristic.

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7. Plant & Animal Cells Compared
Plant Cells
No Cell Wall
Have a Cell Wall
Have NO Chloroplasts
Have Chloroplasts
Have small vacuoles
Have LargeVacuoles (mostly containing water)

8. Cell Organelles
Cells contain tiny specialised organelles, each designed to carry out a particular function.
In plant cells, the cell wall give structure and support to the plant.
The cell membrane controls what exits and enters the cell.
Chloroplasts use light energy to make food during photosynthesis.
The mitochondria are involved in respiration, breaking down sugars into energy.
The cytoplasm is a jelly – like materials where all organelles are suspended.

9. Cell Division
Cell division is required for growth and repair of body tissues.
Cell division is an ongoing process – we produce millions of cell every second.
Cell division involves the copying of the DNA in the chromosomes and all the organelles.
There are two types: mitosis and meiosis.
Mitosis is normal cell division, when one cell produces two identical cells.
Meiosis is called reduction division and is only used in the production of sex cells or gametes e.g. Sperm and Eggs.

12. Mitosis and Meiosis – A comparision
A single division
A Double division
The number of chromosomes remains the same.
The number of chromosomes is halved.
Daughter cells are identical to parent cell
Daughter cells are genetically different from parent cell
Two daughter cells are formed
Four daughter cells are formed.
Mitosis is normal cell division
Meiosis is cell division involved in sex cell or gamete formation

13. Mitosis in Detail
Mitosis takes about 5 – 10% of the total cell cycle time.
The division of the nucleus or cell during mitosis occurs in a number of distinct stages.
Interphase
Prophase
Metaphase
Anaphase
Telophase
Cytokinesis

14. Interphase Chromosomes are not visible but DNA is being replicated.
In this state the DNA is in the form Chromatin, indistinguishable chromsomes.
Organelles are being replicated.
Just before division, each chromosome has an exact copy of itself (i.e. there are four genes for each trait)

15. Prophase Chromatin contracts to form visible chromosomes
The visible chromosomes are double stranded structures, with each strand called a chromatid and the point of attachment known as the centromere.
The nucleolus disappears
The centri0les (spindles) separate forming spindle fibres

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18. Metaphase The nuclear membrane breaks down
A spindle fibre from each end of the cells attaches to the centromere of the chromatid pairs (sisters)
The chromosomes line up along the equator of the cell.

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23. Telophase The chromosomes lengthen and become invisible again
A nuclear membrane forms around the chromatin at each of the two ends (poles) of the cell
The spindle fibres break down
The nucleoli reform.
At this stage nuclear division is complete as the nucleus of one cell has become two, however there is still only one cell.

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26. Cytokinesis
This process completes the cell division and is where a new membrane pinches from either end and forms two new daughter cells.
In plant cells, a new cell wall forms from a cell plate.