Biology Unit 2

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  • Created by: Scarlett
  • Created on: 27-05-13 13:32

Types of Cell


  • Bacteria, Viruses etc.
  • Small
  • no organelles except ribosomes and nucleus.
  • Always has a cell wall


  • Contain organelles
  • Dont all have cell walls
  • Larger

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Stem Cells

Totipotent - 8 identical cells that can develop into a complete being.

Blastocyst - 6 days after conception, a ball of cells is present. Outer cell layer forms the placenta. (Used in stem cell research).

Pluripotent - Ball of 50 or so cells. Each can be most types of cells.

Multipotent - Adult cells - Specialised.

Therapeutic Cloning - A diploid cell is removed and its nucleus is fused with an empty ovum. It`s stimulated to divide and left for blastocyst to develop. Genetically identical to patient. Eliminates risk of rejection. Can use animal/human cells where the supply is not limited. 

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Centrioles- Every cell has one pair. Made of 9 protien microtubules and form the spindle fibres in mitosis. 

Lysosome- Contain digestive enzymes that help break down unwanted strutures and bound by a single membrane. (The acrosome in a sperm is a lysosome). 

Golgi Apparatus - Stacks of flattened vesicles. Modifies proteins and packages them in vesicles for transport.

Smooth Endoplasmic Reticulum - Make steroids and lipids.

Rough Endoplasmic Reticulum - Membrane bound flattened sacs with ribosomes attached. Protien made and transported through the ER to other parts of the cell. 

Mitochondria - Two mebranes and cristae. Site of aerobic respiration.

Nucleolus - Site where ribosomes are made.

Nucleus - Contains the nucleolus and enclosed by an envelope perforated with pores.

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Cell Organelles Diagram


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Interphase - Intense preparation. New organelles are made and DNA is replicated. (Interphase -Intense)

Prophase - Chromatids shorten and fatten and nucleur envelope breaks down. Centrioles move to oppostie ends of the cells to form spindles.

Metaphase - Centrometers align along middle of spindles. (Metaphase - Middle) 

Anaphase - Spindle fibres shorten, pulling the 2 halves of the chromosomes to opposite ends. Spindle break down. (Anaphase - Apart)

Telophase - Chromosomes unravel and nucleur envelope reforms. (Telophase - Untie)

Cytoplasmic Divison - Protien filaments bound to the inside surfaceof cell contract until cell seperate.  

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1- Sperm reaches ovum and cells around ovum release chemicals triggering acrosome reaction.

2- Acrosome swells and fuses with sperm cell surface membrane.

3- Digestive enzymes in acrosome released and digest through follicle cells and zona pellucida.

4- Fuses with ovum membrane and enters ovum.

5- Enzymes released from lysosomes and thicken zona pellucida.

6- Nuclei fuse.


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                     Egg cell - Ovum                                              Sperm cell

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  • Chromosomes replicate so each chromosome is made of 2 chromatids.

  • Homologus (same) chromosomes pair up and seperate.

  • Chromatids seperate and gametes re formed. Each with half the number of chromosomes.

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Independent Assortment

Causes genetic variation allowing evolution and natural selection.Chromosomes cross over (Chiasma). This causes a mix in the gentic information.

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Flower Stucture

 Sepal - A   Petal - A&B   Stamen- B&C  Carpel- C

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Flower Reproduction

1- Pollen grain germinates on the style and the pollen tube grows through it towards the ovary. Growth controlled by the tube nucleus. 

2 - Pollen grain contains 2 nuclei, tube nucleus and a general nucleus. The generative nucleus divides to form 2 haploid gamete nuclei. These move down the pollen tube.

3- Pollen tube grows through a pore into the emryo sac. The 2 male gamete nuclei enter.

4- One nuclei fuses with the egg cell and forms a zygote. The second fuses with 2 nuclei in the emryo sac called polar nuclei to form a triploid cell. Zygote cell forms emryo. The triploid cell forms the storage tissue (Endosperm).  

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Protein Production

  • DNA is transcripted to mRNA, this leaves the nucleus.
  • Protein made on ribosomes entern rough endoplasmic reticulum.
  • Protein moves through RER assuming 3D shape.
  • Vesicles pinched off RER contain protein.
  • Vesicles fuse to form flattened sacs of Golgi Appartus.
  • Proteins are modified.
  • Vesicles pinched off contain modified protein.
  • Vesicle fuses with cell membrane releasing protein. 
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Switching Genes On

B galactosidase breaks down lactoseinto glucose and galactose but only when enough lactose is present. When there isnt enough lactose, a suppresor molecule binds to the section of DNA needed for the transcription of B galactosidase. If lactose is present, supressor no longer binds to the DNA and protein can be exprssed by RNA ploymerase. Ribosomes translate the expressed mRNA into proteins while 2nd protien digests the lactose so it enters the cell. Lactose molecules on repressor release and repressor binds to controllling region again.


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Cells have recognition proteins (adehesion molecules) on their cell membranes which help cells stick together. The exposed section of recogniton protein binds to a complementary protein on other cells.

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Melanin is made of melanocytes. These are activated by melanocyte-activating hormone (MSH). There are receptors for this on the surface of melancyte cells which place melanin into melansomes. The melanosomes are transferred to skin and hair cells where they collect around the nucleus protecting the DNA from harmful UV  light. In hair, UV light causes damage to melanin and proteins in hair cells and hair lightens. 

Tyrosinase- An enzyme used to convert tyrosine into melanin. Some animals have mutations of tyrosinease causing it to be unstable and unactive at body temperature making the tips (tail, paws and ears) darker.

Melanin - Made of Melanocytes

MSH - Hormone that activates Melanocytes.

Melanosomes - Carries Melaninto skin and hair cells.

Skin darkens as UV light encourages more melanin to collect around skin cells. 

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1/3 suffer from cancer and 1/4 will die from it. Cancer occurs when cell multiplcation rate is greater than cell death which causes tumour growth. Often happens in tissues with high mitosis rates (lungs, bowels, gut and bone marrow). This happens when DNA is damaged by UV light or carcinogens.

Oncogens - Code for the proteins that stimulate the transition of one stage in the cell cylce to the next. A mutation in this can lead to the cycle being continuously active creating excessive cell divison. 

Tumour Suppresive Genes - Produce suppresor proeins that stop the cycle. Mutations mean there is no break in cell cycle leading to excess cells.


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