3.24

3.24 understand that division of a diploid cell by mitosis produces two cells which contain identical sets of chromosomes

Mitosis is cell division that causes growth, an increase in number of cells. A normal cell has diploid number of chromosomes (2n). For humans, this would be 2n = 46. Through mitosis, a cell will divide into two. Each will contain a diploid nucleus. These cells are identical daughter cells. We know that they have the same number of chromosomes and same set of chromosome (i.e. if you choose a random chromosome, you will find an identical version within the cell).

Phases

1)     Interphase – this is when DNA replication takes place. During this process, each chromosome undergoes a copying process to form an identical copy of itself. The pair of chromosomes (exact copies) are held together by centromere. We refer to the copied pair as a pair of chromatids. The replication process takes place inside the nucleus while the nucleus is still intact.

2)     Prophase – we can see the breakdown of the nuclear membrane. Chromosomes are now a pair of visible chromatids.

3)     Late prophase – network of protein molecules begin to form (spindle). These spindle fibres extend from one pole to the other. The chromosome pair moves towards the spindle and join at the centromere.

4)     Metaphase – the pair reaches the middle of the spindle fibre. The characteristics of this phase are that the chromosome is in the middle and the spindle fibre is arranged across the equator of the cell.

5)     Anaphase – the spindle fibre shortens. This pulls one chromatid in one direction and the second one in the other direction. The characteristics of this phase are that the pair move apart, move towards the poles of the cell and they separate.

6)     Telophase – the nucleus begins to reform around the chromosomes at either ends of the cell. Here two nuclei are formed and two sets of chromosomes remain at opposite ends of the cell. 

7)     Cytokinesis – cells split into two. This is not part of mitosis! The reformed nucleus form into two cells when the cytoplasm divides in half. The membrane fuses across the equator forming two new cells, each containing ONE chromosome. In humans, 23 pairs separate all at the same time.

3.16

3.16 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 thymine (T), and cytosine (C) with guanine (G)

There are thousands of genesà expand: HELIX à ATGC holding two strands together

The two sugar phosphate backbones are parallel. What hold the two strands together are the bases (adenine, thymine, cytosine and guanine). They hold the helices together by pairing between A & T and G & C.

Looking at the picture on the very right, we can see that the order is ACTGAACCAG. This is the order of bases.

Genes are determined by the order and number of bases, which lead to construction of protein in the cytoplasm into determining characteristics. 

3.15

3.15 understand that a gene is a section of a molecule of DNA

Genetic information is passed on from the gene (nucleus) to the protein (cytoplasm). Characteristics are controlled by genes transforming into proteins. 

3.14

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

Chromosomes contain genetic information within a cell. In a typical cell, there is a nucleus with a number of chromosomes. They are composed of DNA and have the shape of double helix. Sections of the DNA are known as genes that contain information for construction of protein e.g. blood group, petal colour. Different organisms have different number of chromosomes e.g. Chicken 78, Humans 46, Cats 38 etc. The chromosomes operate in homologous pairs and the characteristics are based on the lengths of chromosomes. If we locate a gene (gene position: gene loci) and find the same position on the homologous pair, we find the same gene. There are two versions of each gene for one characteristics and these genes are called alleles.

3.1

Sexual Reproduction	Asexual reproduction
Male and female	No gender
Produce cells called gametes (male sperm + female egg)	No gametes
Meiosis (2n à n chromosomes) (Note: Mitosis also occurs in sexually reproducing animals, but not in the reproduction process)	Mitosis (2n à 2n) & Binary fission (bacterial cells)
Fertilisation(cell fusion)	No fertilisation
Variation – broad (differences)	Small variation due to mutation, but they are still identical (clone)

What is binary fission?