Biology Unit 2 Revision Notes

These notes cover the edexcel syllabus and contain diagrams and explanations. Also includes core practicals. 

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  • Created on: 21-05-12 10:37
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Biology Unit 2 Revision
Topic 3
Ultrastructure: The structure within a cell which can only be seen with an n electron microscope
Meiosis= sexual reproduction
Meiosis is the process where the number of chromosomes in a cell is halved as homologous
chromosomes in a diploid cell separate.
1. DNA replication two identical copies of each
2. DNA condenses to form chromosome made of 2 sister
3. Chromosomes arrange into homologous pairs, ones
with matching chromosomes
4. Meiosis 1: homologous pairs separate and
chromosome number halved
5. Meiosis 2: pairs of sister chromatids are separated
6. Four new gametes produced who are genetically
How does independent assortment lead to genetic variation?
Randomly aligning chromosomes
Always NEW combinations of parental alleles.
How does crossing over lead to genetic variation?
Breaking and re-joining of chromatids
On same chromosome pair
Recombines genes/ alleles
Mitosis = asexual reproduction

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1. G1: Gap phase where new proteins and organelles are made and there is cell growth.
2. S: DNA synthesises and replicated
3. G2: Gap phase after DNA replication where the cell prepares for division
1. Chromatin condenses forming visible chromosomes, which are formed of two chromatids joined by
a centromere.
2. The nuclear envelope breaks down
3. Nucleolus breaks down
4. Centrioles start moving to opposite poles of the cell and begin forming spindle fibres across the
1.…read more

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Sperm contains less cytoplasm
How do cells become specialised?
1. Some genes are active/ some are inactivated
2. mRNA is only copied from active genes
3. Active mRNA is translated to proteins and synthesised
4. Proteins modify cell
5. Changes are permanent and difficult to reverse
6. Called differentiation
Role of rER and Golgi apparatus in cells (May2011)
1. Proteins produced by ribosomes
2. Ribosomes are attached to rER
3. Proteins transported within rER
4. Proteins assume their 3D shape/ fold within rER.
5.…read more

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Uses of stem cells in medicine
Therapeutic cloning: diploid cell removed from patient, the nucleus from this cell is then fused with
an empty ovum. Also known as somatic nuclear transfer.
Produce organs: As the cells are undifferentiated, a future possible use would be to give a signal
which would trigger their growth into a specific organ.
Research: The embryo grows to form a blastocyst.…read more

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When Pollen tube has
grown into embryonic
sac, the 2 male gamete
nuclei enter the sac.
One nucleus fuses with
the egg cell diploid
zygote. This will divide to
form the embryo.
Other nucleus fuses with
the two polar nuclei
triploid cell. This divides
to form the endosperm,
stored food for the seed.
Therefore, plants have a
double fertilisation(1=
egg cell fertilisation, 2=
polar body fertilisation)
Why does the male nucleus fuse with the egg cell nucleus in plant fertilisation?
1.…read more

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What does the triploid nucleus (3n) become?
The endosperm
What happens to the female gamete after the acrosome releases its enzymes (Jan 09)
1. The Zone Pellucida is hydrolysed
2. Sperm enters egg cell
3. Meiosis is completed as the nuclei fuse
4. Cortical granules released from (egg) cell surface membrane
5. Enzymes are released (by lysosomes in ovum)
6. Zone pellucida hardens
7. To prevent other sperm entering (polyspermy)
8. Egg nucleus envelope breaks down
9.…read more

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Genotypes and gene expression
Phenotype: observable effect (characteristics) of cell due to interaction of the genotype and the
Genotype: combination of 2 alleles for that gene, genetic make up
Monohybrid inheritance: each locus is responsible for a different heritable feature
Polygenic inheritance: When a number of genes are involved in the inheritance of a characteristic.
Multifactorial: where several genetic and one or more environmental factors are involved.
Environmental risks: Radiation.
Melanin: dark pigment in skin and hair.…read more

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Oncogenes: genes coding for the proteins that stimulate the transition between stages of the cell cycle.
Tumour suppressor genes: genes producing proteins that stop the cell cycle.
Mutations in these genes mean that the cell cycle is not stopped and there is uncontrolled division of cells. A
type of tumour suppressor gene is p53 which stops the cycle by inhibiting enzymes at the G1/ S transition
which prevents the cell from copying DNA.…read more

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Behavioural Change in behaviour increasing its Some cold blooded animals orientated
chances of survival themselves towards the sun to
maximise their absorption of heat until
they reach active temperature.
Seed banks
1. Seeds collected
2. Seeds dried and stored in cool conditions around -20 degrees.
3. Seed germination tested at regular intervals
4. If germination falls under 75%
5. Seedlings grown into mature plants
6. New seedlings can be collected to be stored.…read more

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Inbreeding leads to reduction in genetic diversity, reduces chance of animals adapting to
environmental change
Increased risk of genetic conditions due to increased number of recessive alleles.
Techniques reducing inbreeding
Isolate partners so they don't continuously breed with the same partners
Select partners, IVF or inter-zoo swapping
Keep record of individuals in captivity who have bred(stud books)
Taxonomy method: binomial system
Genus + ...= e.g. Plasmodium falciparum
Five-kingdom classification of organisms:
1. Prokaryote= Prokaryotes
2. Protoctists= Protoctists
3. Fungi
4. Plantae= Plants
5.…read more


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