Cells are the building blocks of all living things. All cell contain organelles; these are the different parts of a cell's structure.
For example in a plant cell, the organelles are:
- Nucleus - controls what happens in the cell and holds genetic information
- Cell Membrane - this controls what moves in and out of the cell
- Cell Wall - made from cellulose, strengthens the cell
- Vacuole - made from cell sap (sugars), supports the cell
- Ribosomes - where protein synthesis (making the protein) takes place
- Cytoplasm - where chemical reactions take place
- Chloroplasts - contain Clorophyll which absorbs light energy for photosynthesis
Mitosis is the growth and division of cells to produce cells for repair, growth and replacing old tissue.
How does it work?
1. You begin with a parental cell that has 23 pairs of chromosomes.
2. Each chromosome replicates itself, so you now have 46 pairs of chromosomes, double the usual. (growth)
3. The chromosomes then line up in the middle of the cell.
4. The chromosome copies are then pulled to opposite ends of the cell.
5. And the cell splits into two separate daughter cells (division)
6. The two daughter cells produced have 23 pairs of chromosomes and are identical to the parental cell.
Meiosis however, is the growth and division of cells for reproduction to produce gametes - sex cells i.e. sperm and egg cell; these contain half the number of chromosomes of the parent cell. This only takes place within the sex organs only.
1. Cell contains 23 chromosomes.
2. Each chromosome replicates itself; you now have 46 chromosomes. (growth)
3. The chromosomes separate to opposite sides of the cell with their 'copies'
4. Cell divides for the first time. (division)
5. Copies then separate again for the second cell division. (division)
6. You now have 4 gametes produced, each with half the number of chromosomes of the original cell. These gametes are not identical and show genetic variation.
When these gametes fuse with another gamete i.e. in fertilization, they produce a zygote - a single body cell with a whole set of chromosomes.
Genes and Growth and Development
Genes determine characteristics e.g. the ability to roll your tongue, or the colour of your hair etc. They do this by providing the instructions for the production of proteins and therefore the control of the development of the whole organism.
The instructions for making proteins comes in the form of a code, made up of four bases:
- Adenine (A)
- Thymine (T)
- Guanine (G)
- Cytosine (C)
These bases are always found in pairs and they hold together the DNA strands. They always pair up in the same way, A with T and G with C.
So these bases within the DNA code for how to make proteins, however this information is within the nucleus and proteins are made outside of the nucleus in the ribosomes in the cytoplasm.
The DNA molecules are too large to be able to pass outside of the nucleus to send the information and so there must be some other form of messenger that carries the information there. This is called the mRNA or messengerRNA, which has molecules small enough to move between the nucleus and cytoplasm.
1. The bases have weak bonds between each other that split and create 2 strands.
2. The mRNA bases bond with the free DNA bases to create two strands, identical to the original - a copy has been made.
3. The mRNA strand leaves the nucleus and goes to the ribosomes where protein synthesis occurs and the ribosome forms chains of amino acids (proteins)