3.24c Mitosis 3

1. Looking down a microscope, at a cell, we can see the nucleus, a spherical structure. However, the chromosomes would not be seen.  
2. It is during the interphase where the DNA replication takes place. 
3. The first sign of mitosis, we see when we look down a microscope, is when the nuclear membrane breaks down and the chromatids (pairs of chromosomes) become visible. This is a phase known as the prophase. 
4. Now that the nucleus is gone, inside the cell there is a network of protein molecules known as the spindle, each made out of a fibre. These extend from one pole of the cell to the other. During late prophase, the pair of chromatids will move towards the spindle and will join onto one of the spindle/fibre at the centromere. 
5. The next stage is known as the metaphase: 
• The pair of chromatids is attached to the spindle firbe by the centromere.
• The characteristicof this stage is that the chromosomes are in the middle, arranged across the equator of the cell.
6. Anaphase: 
• The spindle/fibre shortens, pulling one chromatid upwards and the other downwards. 
• The pair of chromatids are moving apart to the poles. the chromatids are being separated. 
7. Telophase: 
• The chromosomes are at opposite ends of the cell. 
• The nucleus begins to reform around the chromosomes at either end of a cell. 
• These will be the new nuclei of the new cells 
• In the telophase we see the formation of two nuclei 
8. Cytokinesis: 
• In this process, the cell splits into two. 
• This is not a part of mitosis. 
• We see the nucleus that had reformed in the anaphase. 
• The cell begins to divide its cytoplasm in half 
• The membrane of the cell fuses across the equator to form the two cells
• The two new cells will contain 1 chromosome just like the parental cells do. 
9. In human cells we won't see one pair of chromosomes separating, but 23 separating at the same time. 

3.24b Mitosis 2

1. The cell which will undergo the process of mitosis will have to copy its chromosomes. 
2. The process of copying chromosomes is called DNA replication. In this process each chromosome undergoes a copying process to form an identical copy of itself, with the same genes. 
3. These pairs of copies are held together through a bonding in the centre region called the centromere. 
4. These pairs are referred to as a pair of chromotids 
5. The process of DNA replication takes place in the nucleus while its still intact. We cannot see the process. 
6. This is known as the interphase of the cell cycle. It is during this resting stage of the cycle during which the chromosomes are copied. 

3.24a Mitosis

1. Mitosis is a type of cell division which causes growth which occurs via the increasing numbers of cells. 
2. The number of chromosomes in a nucleus is known as a diploid number (abbreviated as 2n). 
3. The diploid number in a human cell is 46. 2n = 46
4. for cats 2n = 38
5. In the process of mitosis a particular cell divides into two cells, each with a nucleus. When we look inside the nucleus of these cells, we find that each of these cells have a diploid nucleus. 
6. These cells are identical: 
• They have the same number of chromosomes 
• Same set of chromosomes- both cells have identical chromosomes, to each other. 

3.16 DNA and Genetic Information

3.15 Genes

1. A gene is a section of a molecule of a DNA. 
2. Genes carry information which form the characteristic (e.g. blood group) of the organism. 
3. The genes are located in the nucleus. 
4. The information is passed to the cytoplasm, where the genetic information is transformed into a protein. 
5. This protein controls the production of the characteristic. 
6. Basically, the information flows from the gene (nucleus) to the protein (cytoplasm). 

3.14 chromosomes

1. The nucleus of a cell contains chromosomes on which genes are located. 
2. Chromosomes are the genetic information within cells 
3. The chromosome is composed of a molecule known as DNA
4. This forms a shape known as a double helix.
5. Sections of these molecules are called genes. 
6. one chromosome will possibly have thousands of genes. 
7. Each gene carries the information for the structure of the protein. 
8. The protein gives us the characteristics associated with the gene e.g. Blood Group
9. Different organisms have different no. of chromosomes e.g. cat=38, chicken=76, chimpanzee=42, human=46 per cell. 
10. The chromosomes work in pairs known as homologous pairs 
11. The homologous nature is based on the length of the chromosomes. 
12. Genes are located in the same places within the homologous pair, in the gene locii. 
13. We have 2 versions of each gene for one characteristic. These versions are called alleles. 

3.1 Sexual and Asexual reproduction

Sexual: 

1. Sexual reproduction involves sexes which are identified as males and females. 
2. Organisms, who reproduce sexually, produce cells known as gametes. This takes the form of a sperm cell in the male and an egg cell in the female. 
3. The type of cell division that produces gametes are known as meiosis. 
4. Meiosis have two effects. One of which is to half the number of adult chromosomes in a gamete cell. 
5. The total number of chromosomes in an adult is 46 per cell whereas the total number of chromosomes in a gamete cell is 23. 
6. Going from 46 to 23 is the process of cell division known as meiosis. 
7. We have the process of fertilization in which the sperm cell fuses with the egg cell. 
8. We find variation in the sexually producing population. This variation (differences) is broad. We find many differences in an individual of the sexually reproducing population. 

Asexual:

1. No gametes 
2. No sexes. 
3. Process of cell division is known as mitosis in eucaryotic cells and binary fission in procaryotic cells which exist in bacteria. 
4. In this process the number of chromosomes is maintained constantly. E.G. a cell with 20 chromosomes would divide to produce 20 chromosomes in 2 cells. These two cells would be identical. 
5. Small amount of variation due to mutation. They are usually identical. This is known as a clone. 
6. No fertilization.