Edexcel Biology; Topic 3 - Voice of the genome

Prokaryotic cells, meaning 'before nucleus':

- Are very small in size (typically 0.5 - 5 um).

- Have no true nucleus.

- Have circular DNA which lies free in the cytoplasm.

- No chromosomes.

- Cell walls are always present.

- No nucleus or other membrane bound organelles.

Eukaryotic cells, meaning 'true nucleus':

- Are larger than Prokaryotic cells.

- Contain membrane bound organelles (e.g. mitochondria).

- Have a nucleus.

- Contains chromosomes.

- Cell walls are sometimes present (e.g. in plants).

Centrioles: These are nine microtubules that have come together to form a cyclinder centriole. Centrioles are involved in the formation of the spindle in cell division (mitosis), and also in transportation within the cell cytoplasm.

Mitochondria: This is made up of two membranes, the inner of the two membranes folds to form finger like projections which are known as Cristae. The mitochondria is the site of the later stages of respriation (ATP energy).

Nucleus: The nucleus is inclosed within a nuclear envelope that has pores. It contains chromosomes and a nucleolus. The nucleolus is where ribosomes are formed.

Rough endoplasmic reticulum: Ribosomes are found attached to the outer surface of the rough endoplasmic reticulum. The proteins made by the ribosomes travel through the rough endoplasmic reticulum and consequently the protein changes structure (3D shape), it is then transported to other parts of the cell.

Smooth endoplasmic reticulum: This is very similar to the Rough Endoplasmic reticulum, except the Smooth endoplamis reticulum has no ribosomes attached. Smooth ER makes lipids and steriods.

Ribosomes: Made of RNA and proteins, ribosomes are often found attached to the Rough Endoplasmic reticulum or found free in the cytoplasm. Ribosomes are the site of protein-synthesis.

Lysosomes: Sacs containing digestive enzymes. Lysosomes break down unwanted structures and also whole cells when they are old.

Golgi Apparatus: These modify proteins and package them into vesicles for transportation.

Cell Surface Membrane: Made up of a phospholipid bilayer.

- DNA is transcripted to messenger RNA.

- The messenger RNA than leaves the nucleus through pores in the nuclear envelope.

- Protein synthesis occurs on the ribosomes that are attached to the Rough Endoplasmic reticulum.

- The proteins move through the Rough Endoplasmic reticulum and consequently change their structure to take on a 3D shape.

- Vesicles are pinched off the Rough Endoplasmic reticulum and fuse with the Golgi Apparatus.

- Within the Golgi Apparatus the porteins are modified and are pinched off into vesicles to fuse with the cell surface membrane releasing the protein.

The sperm:

- Is much smaller than the ovum.

- It is motile (meaning can move), this is achieved by a tail, called the flagellum, that is powered by energy released by the mitochondria.

- Also has an acrosome, a special type of lysosome. The acrosome releases digestive enzymes which break down the Zona Pellucida of the ovum.

- Haploid nucleus.

The ovum:

- The ovum is alot larger than the spem and is unable to move.

- The cytoplasm of the ovum provides protein and lipid food for a developing embryo.

- Haploid nucleus.

- It releases chemicals to attract the sperm.

- After a sperm has fused with the egg chemicals are realeased from the ovum which thicken the Zona Pellucida preventing any more sperm from entering the egg.  

- The two haploid nucleus fuse together to produce a fertilised egg with a diploid nucleus.

Sometimes pairs of chromosomes come into contact with one another and effectively swap their alleles at the point where they have come into contact.This gives rises to large genetic variation.

Interphase is often the longest proccess in mitosis because during interphase new cell organelles are synthesised and DNA replication occurs. By the end of interphase the cells contain enough cell contents to produce two new cells.

During prophase chromosomes condense, becoming shorter and thicker. Microtubules form a 3D structure called the spindle, and centrioles position themselves at the opposite sides of the cells, these form the two poles.

The chromosome's centromeres attach to the spindle fibres at the equator of the cell.

The centromeres split, and the spindle fibres shorted pulling the centromeres to the opposit poles of the cells. By the end of Anaphase the spindle fibres have completely broken down.

The chromosomes unravel and the nuclear envelope reforms so that two sets of genetic information become enclosed within seperate nuclei.

In late telophase (the starting of cytokinesis) the cell membrane constricts around the centre of the cell until the cell is divided into two new cells. 

The two gametes involved in the ferilisation of plants are the pollen grain and the ovules.

- The pollen grain lands on the surface of the stigma (a short anther recieves its pollen by insects, and a longer anther recieves its pollen by the wind).

- Molecules on the pollen grain decide whether or not the pollen grain will fertilise, it also has the ability to recognise if the pollen is from the same plant.

- The pollen tube is formed down the style, and grows through the ovary to the ovules.

- Once the pollen grain has travelled through the pollen tube into the ovules it goes through mitosis to form two grain male gamete nucleus.

- One male gamete fertilises with the egg nucleus (dipolid nucleus).

- And the other male gamete nucleus fertilises with the polar nuclei to form an endosperm with a triploid nucleus (which is used in the storage of food).