3.24a-c)- Mitosis

a) Understand that division of a diploid cell by Mitosis produces two cells which contain identical sets of chromosomes

The diploid number is the number of chromosomes in the nucleus. This can be abbreviated as 2n. Humans have 46 chromosomes so 2n=46.In the process known as mitosis a cell is divided to two cells - each with a nucleus. If we look at the nucleus we will find that each one will have a diploid nucleus.We often say that these cells are 

identical sometimes described as daughter cells. (same no. of chromosomes, same set of chromosomes)

b) Mitosis is performed through DNA replication, in this each chromosome replicates itself. The two copies are held together by a structure around the center region known as the centromere. They are known as a pair of chromatids. DNA replication takes place in the nucleus while its still intact. The process of DNA replication takes place inside the nucleus while its still inact. This is known as the interphase.

 

 

c) The stages of mitosis

Observing a cell down a microscope, we would normally see a nucleus has a spherical structure and would be unable to observe the chromosomes. It is during the interphase that the process of DNA replication occurs.

When the membrane breaks down and the chromosomes become visible as a pair of chromatids= Prophase.

 

The nucleus is gone, inside the cell, there is a network of protein molecules called spindle. These extend from one pole of a cell to the other. The pair of chromatids will join on one of the spindle fibres at the centromere.

Metaphase is when the pair of chromatids are attached to a spindle fibre by the centromere, characteristic of the metaphase is that the chromatid is in the middle across the equator of the cell.

In the Anaphase the fibre shortens pulling the chromatids apart and they move to the poles of the cell. The pair of chromatids are seperated. 

The last phase is called Telophase, in which the nucleus begins to reform around the chromosomes at either end of the cell. We see the formation of two nuclei at opposite ends of the cell.

Cytokinesis is where the cell splits into two, this is not part of mitosis. Here we see the nucleus that has reformed in the anaphase, the cell is beginning to move inwards, dividing the cytoplasm in half and the 

membrane fuses across the equator to form two cells. Both of these cells contain a chromosome. 

3.16- DNA and Genetic information

Describe a DNA molecule as two strands coiled to form a double helix, the strands being linked by a series of paired bases: adenine (A) with thvmine (T) and cvtosine (C) with guanin (G)

If we look at a section of the double helix we will see bases that are close together and there are four different types of bases: adenine (A) with thvmine (T) and cvtosine (C) with guanin (G).

In the molecule, these bases are holding together the two helixes and are held together by pairing between A with T on one side and C with G on the other. These are known as the base pairs.

If we look on one side, we can see the order is ACTGAACCAG. This is the order of the bases and this is called the gene.

Gene can be defined as the order of the bases and the number of the bases. This is for the construction of the protein in the cytoplasm and this gives us the characteristic.

3.15- Genes

Understand that a gene is a section of a molecule of DNA.

A section of a molecule of DNA is called a gene. This gene carries the gene which forms the characteristic of an organism. This could be a blood group or petal colour. This information is passed to the cytoplasm and in there it   is transformed into a protein and it is this protein that controls the production of the characteristic.

Gene (nucleus)-------------->Protein (cytoplasm)

3.14- Chromosomes

Recall that the nucleus of a cell contains chromosomes on which genes are located

Chromosomes:

• Contains genetic information within a cell, in a nucleus of a cell. 
• Chromosomes is composed of DNA and this forms a shape known as a double helix. Sections of this molecule are called genes. 
• One chromosome will have many genes. Each gene carries the information for the construction of a protein.
• This gives the characteristic associated with a gene. For instance- blood group.
• Different organisms have a different amount of chromosomes: 
• Cat- 38
• Chicken- 78
• Chimpanzee-42
• Human-46
• Chromosomes operate in homologous pairs and the nature depends upon the length of the chromosomes.

If we look at a gene, the position is known as a gene loci. If we go to the same position on the homologous we find the same gene loci. Therefore we have 2 versions of each gene. This version is called alleles. 

3.1- Sexual and Asexual reproduction

Describe the differences between sexual and asexual reproduction.

1. Organisms that show sexual reproduction show sexes( male/ female), no such sexes exist in asexual reproduction.
2. Sexually reproducing organisms produce cells called GAMETES (sperm/egg). In plants the sperm cell is known as the pollen grain and the egg cell is know the ovule. Asexually reproducing populations have no gametes.
3. The type of cell division that produces gametes is known as MEIOSIS. It has a number of effects and one of them is half the chromosomes in the gamete cell. In humans, the totals number of chromosomes per cell is 46, whereas, in a gamete cell there are 26 per cell (through MEIOSIS). In asexually reproducing population there is MITOSIS in Eukaryotic cells and binary fission in prokaryotic cells. In this process the number of chromosomes are maintained constantly. 
4. In sexually reproducing population, we find the process of fertilisation, in which the male gamete fuses with the female gamete. In asexual reproduction, there is no cell fusion.
5. If we examine the population of a sexually reproducing organisms we will find broad variation and in asexually reproducing populations there will be small variations due to mutations. By enlarge they are identical, this is known as a clone. 

Question: What are eukaryotic and prokaryotic cells?