Cell Division, Cell Diversity and Cellular Organisation
(a) state that mitosis occupies only a small percentage of the cell cycle and that the remaining percentage includes the copying and checking of genetic information
(b) describe, with the aid of diagrams and photographs , the main stages of mitosis (behaviour of the chromosomes, nuclear envelope, cell membrane and centrioles)
(c) explain the meaning of the term homologous pair of chromosomes
One chromosome in each pair comes from each parent and theyare the same size and have the same genes, although they could have different versions of those genes (called alleles) – these are homologous pairs
(d) explain the significance of mitosis for growth, repair and asexual reproduction in plants and animals
(e) outline, with the aid of diagrams and photographs, the process of cell division by budding in yeast
- A budforms/bulges at the surface of the cell
- The cell undergoes interphase – the DNA and organelles are replicated ready for the cell to divide
- The cell begins to undergo mitosis
- Nuclear division is complete – the budding cell contains a nucleus that has an identicalcopy of the parentcell’sDNA
- Finally the budseparates/pinchesoff from the parent cell, producing a new, geneticallyidentical yeast cell (daughter cell)
(f) state that cells produced as a result of meiosis are not genetically identical (details of meiosis are not required)
Meiosis is a type of cell division that happens in the reproductive organs to produce gametes. Cells that are formed from meiosis have half the number of chromosomes as the parent cell. They are NOT genetically identical because each new cell ends up with a different combination of chromosomes.
During cell division by meiosis, homologous chromosomes pair up and one member of each pair goes into each daughter cell. Most eukaryotes have pairs of homologous chromosomes because one is inherited from each parent. They carry the same genes but may carry different alleles of each.
(g) define the term stem cell
Only SUM cells are stem cells in adult organisms, for example:
- in bone marrow of human adults
- in meristems (e.g. root and shoot tips) of plants
Stem cells are:
- able to Specialiseinto any type of cell
- Undifferentiated
- capable of Mitosis
(h) define the term differentiation, with reference to the production of erthrocyes (red blood cells) and neutrophils derived from stem cells in bone marrow, and the production of xylem and phloem sieve tubes from cambium
Differentiation is the process of becoming specialised.
In animals, adult stem cells are used to replace damaged cells e.g. to make new skin or blood cells. Plants are always growing, so stem cells are needed to make new shoots and roots throughout their lives. Stem cells in plants can differentiate into various plant tissues including xylem and phloem.
Being specialised means being adapted for a specific function. Organisms can be specialised by:
- having a specific size/shape of cell e.g. root hair cell
- having a certain number of organelleswithin the cell e.g. sperm cells have a large number of mitochondria for movement and energy
- having specific contents in the cell e.g. phagocytes have enzymes to break down substances
(i) describe and explain, with the aid of diagrams and photographs, how cells of multicellular organisms are specialised for particular functions, with reference to eythrocytes (red blood cells), neutrophils, epithelial cells, sperm cells, palisade cells, root hair cells and guard cells
(j) explain the meaning of the terms tissue, organ and organ system
(k) explain, with the aid of diagrams and photographs, how cells are organised into tissues, using squamous and ciliated epithelia, xylem and phloem as examples
(l) discuss the importance of cooperation between cells, tissues, organs and organ systems
Multicellular organisms work efficiently because they have different cells specialised for different functions. It’s advantageous because each different cell type can carry out its specialised function more effectively than an unspecialised cell could. Specialised cells can’t do everything on their own. Each cell type depends on other cells for the functions it can’t carry out. This means the cells, tissues and organs within multicellular organisms must cooperate with each other to keep the organism alive and running.
For example:
- In plants, a palisade cell is good at photosynthesising, but it’s not good at absorbing water and minerals from the soil, so it depends on root hair cells to do this.
- In humans, muscle cells need oxygen but depend on erythrocytes (red blood cells) to carry oxygen to them from the lungs.