AS Biology - Unit 2 (AQA)

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Biology - Unit 2
Chapter 8 ­ DNA and Meiosis
DNA is made up of nucleotides.
A nucleotide is made up of: -
A sugar called deoxyribose
A phosphate group
An organic nitrogen containing base:-
Single-ring bases ­ Cytosine and Thymine
Double-ring bases ­ Adenine and Guanine
Adenine pairs with Thymine with 2 hydrogen bonds
Guanine pairs with Cytosine with 3 hydrogen bonds
How DNA is adapted to carry out its functions: -
It's very stable, and can pass from generation to generation without change.
Its two separate strands are joined only with hydrogen bonds, which allow them to separate
during DNA replication and protein synthesis.
Extremely large molecule so can carry a lot of genetic information.
The base pairs are within the deoxyribose-phosphate backbone, so the genetic information
is protected from being corrupted by outside chemical and physical forces.
Genes are sections of DNA that contain the coded information for making polypeptides. The coded
information is in the form of a specific sequence of bases along the DNA molecule.
Chromosomes appear as two threads, joined at a single point (centromere). Each thread is called a
The DNA in chromosomes is held in position by proteins.
The total number of chromosomes is the diploid number.
Mitosis produces 2 daughter nuclei with the same number of chromosomes as the parent cell and as
each other.
Meiosis produces 4 daughter nuclei, each with half the number of chromosomes as the parent cell.

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Meiosis: -
First division (Meiosis 1) ­ The homologous chromosomes pair up and their chromatids
wrap around each other. Equivalent portions of these chromatids may be exchanged in
crossing over. The homologous pairs separate with one chromosome from each pair going
into one of the two daughter cells.
Second meiotic division (Meiosis 2) ­ The chromatids move apart. At the end 4 cells have
been formed with half the chromosome number.…read more

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Second growth (G) phase ­ Organelles grow and divide and energy stores are
Nuclear division ­ Nucleus divides into either 2 (mitosis) or 4 (meiosis).
Cell division ­ The whole cell divides into 2 (mitosis) or 4 (meiosis)
Chapter 13 ­ Exchange and Transport
The larger the organism, the smaller the surface area to volume ratio, so diffusion takes longer.…read more

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Blood that has less oxygen meets water which has had most of its oxygen removed. So there
is diffusion of oxygen from the water to the blood.
If flow of water and blood was in the same direction (parallel flow), the diffusion gradient would
only be maintained across part of the length of the gill lamellae and only 50% of available oxygen
would be absorbed.
In plants most gaseous exchange occurs in the leaves.…read more

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Tissue fluid ­ Watery liquid that contains glucose, amino acids, fatty acids, salts and oxygen. It
supplies this stuff to the tissues and receives CO and other waste materials from the tissues. Tissue
fluid is the means by which materials are exchanged between blood and cells.
When blood reaches the narrow capillaries it creates a pressure called hydrostatic pressure at the
arterial end of the capillaries. This hydrostatic pressure forces tissue fluid out of the blood plasma.…read more

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Root hairs are surrounded by a soil solution which is mostly water and therefore has a very high
water potential so water moves by osmosis from the soil solution into the root-hair cells down this
water potential gradient.
The apoplastic pathway ­ As water is drawn into endodermal cells, it pulls more water along behind
it, due to the cohesive properties of water molecules, creating a tension that draws water along the
cell walls of the cells of the root cortex.…read more

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Factors affecting transpiration
Light ­ Stomata open when the carbon dioxide needed for photosynthesis diffuses in.
photosynthesis occurs during the day so stomata open in the light and close in the dark. An
increase in light intensity causes an increase in the rate of transpiration.
Temperature ­ An increase in temperature increases the kinetic energy and so increases the
speed of movement of water molecules. This increases the rate of evaporation of water, so
transpiration increases.…read more

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Selective breeding (Artificial Selection) ­ Identify individuals with the desired characteristics and
use them to parent the next generation. Variety of alleles in the population is restricted. Leads to
reduced genetic diversity.
Founder effect ­ When a few individuals from a population colonise a new region, they give rise to a
population with less genetic diversity than the population they came from. This is because these few
individuals only carried a small fraction of the alleles of the population as a whole.…read more

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When hydrolysed it forms alpha glucose which is easily transported and readily used in
Glycogen is similar to starch but has shorter chains and is more highly branched. It's found in
animal cells. It's stored in the muscles and liver.
Cellulose is made up of monomers of beta glucose. It has straight unbranched chains. These run
parallel to one another, allowing hydrogen bonds to form cross-linkages between adjacent
Cellulose provides rigidity to the plant cell.…read more

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Each cell is specially adapted to their own particular function so performs it more effectively. So the
whole organism functions efficiently.
Chapter 14 ­ Classification
Species ­ They are similar to one another but different from members of other species. They occupy
the same ecological niche. They are capable of breeding to produce living, fertile offspring. They
belong to the same gene pool.
Organisms are identified by two names. This is known as the binominal system. It's based on Latin or
Greek names.…read more



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