AQA unit 2 biology notes

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  • Created on: 12-04-13 16:20
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3.2.1- 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 has
equal chance of being selected) of a population
This is important so the sample is representative of the population, and so that there is no
investigator bias
Standard deviation measures how far spread the results are from the mean-for example in
height, very few people are giants or dwarves
The data is represented by a normal distribution curve- as SD increases, the curve becomes
flatter and vice versa
On average, 68% of all measurements fall into 1SD, and 95% falls into 2SD
Variation can result from both environmental and genetic factors
Discontinuous variation is clear-cut distinctions in phenotype (physical characteristics, e.g.
blood group) this is caused by alleles of one gene
Continuous variation is unclear distinctions in phenotype. E.g. mass- caused by several genes
Homozygous recessive- a characteristic that only appears when two recessive alleles are
3.2.2- DNA is an information carrying molecule. Its sequence of bases
determines the structure of proteins, including enzymes
DNA has a double helix structure, meaning large amounts of information can be stored in a
small volume, can withstand high temperatures and it is stable, so information is not
DNA and RNA are nucleic acids- polymers of nucleotides
The pentose sugar is ribose in RNA
There are 4 nitrogenous bases- Adenine, Cytosine, Guanine and Thymine (or Urasil for RNA)
The base pairing rule means that only C pairs with G, and A pairs with T/U
C+T (U) make a pyrimidine base, and A+G make a purine base
The phosphate group and pentose sugar create a sugar-phosphate backbone
Each DNA strand is made of two nucleotides in each `step', two sugar-phosphate backbones,
and two nitrogenous bases joined by hydrogen bonds, they are on opposite strands, and are
Hydrogen bonds are used as they are strong (as many are used) and can be easily broken
when needed for protein synthesis and cell division'
Each nucleotide is held by a strong phosphodiester bond, as a result of a condensation
On a particular strand of DNA, each gene has a fixed position (locus)
The loci of genes that code for the same polypeptide is homologous (the same positions)

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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 as alleles, meaning the polypeptides
they code for may also differ
A specific amino acid is coded for by a codon (3 bases) this also
determines the sequence in a polypeptide
In prokaryotes, the DNA molecules are…read more

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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 from one chromatid to another
This means that alleles of genes that occur on the same chromosome can
be combined in new ways
3.2.…read more

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Starch is a polysaccharide made of -glucose, and are joined by glycosidic bonds-it is made
of amylose and amlopectin
Starch and glycogen are used as a storage molecules, as they are compact and insoluble, and
so osmosis is prevented
Although both starch and glycogen are branched, glycogen has far more branches
Starch is used in plants, whereas glycogen is found in animals
This is the structure of a palisade cell,
containing many chloroplasts for
photosynthesis, as well as being long and
thin for an increased…read more

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Translation then begins-the start codon attaches to a ribosome, which reads the sequence
until the end codon
tRNA have complementary bases on one end, and an amino acid on the other, which join to
form an amino acid chain, which forms a primary then tertiary structure
Mitosis is used for growth and repair, as it produces clones or copies of the original cell (they
are genetically identical)
At the end of the process, two cells are produced
There are 5 stages-interphase (G1, S and G2),…read more

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In complex multicellular organisms, cells are organised into tissues,
then organs, then systems
when cells become specialised for different functions it is called cell differentiation
Unspecialised cells tissue organ system
3.2.…read more

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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 close the stoma when they are not
turgid (and have therefore lost water), which
prevents water loss and during times when carbon
dioxide cannot be used for photosynthesis (at night)
The mesophyll layer has many air spaces, to allow
gases to circulate to and from cells by diffusion
As well as allowing carbon dioxide to enter…read more

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In a plant, the water first enters via osmosis, due to the root hair cell, which increase the
surface area for osmosis of water
The water enters, as mineral ions are taken into the cell via active transport, which lowers
the water potential, meaning water moves in by osmosis
For the water to reach the xylem thorough the cortex, it can either pass through using the
symplast or apoplast route
The symplast route involves water moving through the cytoplasm and plasmodesmata
(between the cells), which…read more

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As water moves out, it is replaced by the water in the xylem vessels, as there is a cohesive
force between water molecules, as water leaves the plant, a column of water is drawn up as
a result, due to the tension the column of water is under
The xylem vessels are narrow enough for this column not to be broken, and lignin (a water
proof substance) supports the structure to stop it from collapsing, as it thickens the xylem
The rate of transpiration is…read more

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Adaptation and selection are major components of evolution and
make a significant contribution to the diversity of living organisms
Antibiotics are used to treat bacterial infections (not viral)
One way in which antibiotics work is to cause osmotic lysis, by preventing cell walls from
They can also work by interfering with replication and transcription of prokaryotic DNA by
binding to RNA polymerase-the bacteria therefore becomes more permeable, and leads to
cell death
Narrow-spectrum antibiotics are only effective against a few types of bacteria (used…read more


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