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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…

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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…

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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


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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…

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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…

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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…

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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…

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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…

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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…

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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…


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