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Meiosis and genetic variation 8.4
- Meiosis brings about this genetic variation in the following two ways:
1. Independent segregation of homologous chromosomes,
- This occurs during meiosis 1.
- The homologous pairs line up randomly, and are pulled into each daughter cell at
- The combination of chromosomes in the daughter cells are random.
- This is called Independent segregation.
- These different combinations can produce many different offspring, therefore
variation will occur.
2. Recombination of homologous chromosomes by crossing over.
- The chromatids of each pair become twisted around one another.
- During this twisting process, tensions are created and portions of the chromatids
break off.
- These broken portions then re-join with the chromatids of its homologous partner.
This is called recombination.
- Usually it is the equivalent portions of homologous chromosomes that are exchanged.
- This causes variation.…read more

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Starch, glycogen and cellulose 10.3
- Starch is a polysaccharide that is found in many plants in small grains.
- It is a major energy source in most diets.
- They are made of alpha glucose monosaccharide's linked by glycosidic bonds formed
through condensation reactions.
- Starch is insoluble so does not affect water potential, also it does not easily diffuse out
of cells,
- It is compact, so a lot of it can be stored in a small space,
- When hydrolysed it forms a-glucose, which is both easily transported and readily used
in respiration.…read more

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Starch, glycogen and cellulose 10.3
- Glycogen is a major carbohydrate storage in animals.
- It is found mainly in small granules in the liver and muscles.
- Glycogen has a similar structure to starch but is shorter and has more
- Because it is a smaller chain, it is more readily hydrolysed to a-glucose.…read more

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Starch, glycogen and cellulose 10.3
- Cellulose is made up of - glucose.
- Cellulose has a straight unbranched chain which run parallel to each other.
- This means hydrogen bonds can form between them.
- Cellulose molecules are grouped together to form microfibrils.
- It makes up most of a pant cell wall because it is very strong.
- It stops the cell bursting or shrivelling up by exerting an inward pressure that stops
any further influx of water.…read more

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Replication of DNA 11.1
Replicating DNA
- Nuclear division is the process by which
the nucleus divides. There are two
types of nuclear division: mitosis and
- Cell division follows nuclear division and
is the process by which the whole cell
- Semi conservative replication:
- The enzyme DNA helicase breaks the hydrogen bonds linking the base pairs of
- As a result the double helix separates into its two strands and unwinds.
- Each exposed polynucleotide strand then acts as a template to which
complementary nucleotides are attracted.
- Energy is used to activate these nucleotides.
- The activated nucleotides are joined together by the enzyme DNA polymerase to
form the `missing' polynucleotide strand on each two original polynucleotide
strands of DNA.
- Each of the new DNA molecules contains one of the original DNA strands. The
process is therefore called Semi-conservative replication.…read more

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