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AQA Biology Unit 2
Revision Booklet

By Jack Richardson

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2.1- Living organisms vary, and this variation is influenced by genetic and
environmental factors
· Variation exists between members of a species (a group of organisms which can interbreed
and produce fertile offspring
· It is important to take a random sample (each individual is chosen entirely by chance, and…

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· A gene is a chemical code that contains the instructions for making a
complete protein or polypeptide/a section of DNA that contain coded
information as a specific sequence of bases, the code for polypeptides
that determine the nature and development of organisms
· Different forms of genes are known…

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join with any male gamete
· Crossing over takes place during first prophase division-two of the
chromatids on a homologous pair of chromosomes can cross over the
point at which they cross is known as the chiasma
· When the chromosomes separate during anaphase, parts of the
chromatids are swapped…

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· Cellulose allows plant cells to be turgid and stops them from bursting
· As cellulose molecules are long and straight, they are perfect
for forming structural fibres
· Starch is a polysaccharide made of -glucose, and are joined
by glycosidic bonds-it is made of amylose and amylopectin
· Starch…

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· For protein synthesis, mRNA is used-uracil is used
instead of thymine as a base
· Bases are attached to the coding strand of DNA using
RNA polymerase (transcription)
· mRNA then leaves the DNA as a single strand, and
the DNA zips up again due to RNA polymerase

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· This uncontrolled cell division results in the formation of a
tumour, and therefore cancer-benign tumours stay where they
are, whereas malignant tumours travel in the blood to the other
parts of the body to become new tumours (metastasis)

2.6- In complex multicellular organisms, cells are
organised into tissues, then…

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· In plants, the exchange surface changes
depending on the sort of climate they grow
· The cuticle prevents gas exchange, the
stomata (mainly on the underside of the
leaf) provides an opening for gases to enter
and escape, controlled by guard cells
· Guard cells Xylem brings water…

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· Arterioles are able to contract, and divert the blood, such as to the muscles rather than the
stomach during exercise for example
· Veins have the same layers as arteries, but are much thinner, as the blood has a lower
pressure compared to the arteries, and the lumen is…

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· This is slow, as the cytoplasm and membrane provide a lot of resistance, and so as there is
less resistance in the cellulose fibres, water mostly passes via the apoplast route (through
the cell walls)
· Just before the xylem, there is an epidermis cell and the casparian strip…




Brilliant, Thank you so much!! This is amazing 

tilda mascall


Fantastic resorce, thanks :)

Miss Meera J


Humza Productions!!!!

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