Biology Unit 2 Notes

Unit 2 notes for edexcel specification:

Topic 3: Voice of the genome

Topic 4: Biodiversity and natural resources

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  • Created by: DimaK.
  • Created on: 24-04-11 14:02
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Biology Unit 2 Notes
Topic 3: the voice of the genome
Cells and Organelles
Prokaryotic cells
o Small and simple cells.
o No nucleus and no membrane bound organelles.
o DNA is circular and free floating in the cytoplasm
o Always have a cell wall
o Contain plasmids (rings of DNA) and a flagellum
(used for movement to propel the cell)
o Include bacteria cells like E-coli.
Eukaryotic cells
o Complex cells, which include animal and plant cells.
o Surrounded by a nuclear envelope
(double membrane), which contains
o Contains chromatin ­ genetic material
which controls the cells activities
o Pores allow substances like RNA to
move between the nucleus and
o The nucleolus makes RNA and
o The site where proteins are made
o Consist of small and large subunits.
o Very small organelle that is either attached to rough endoplasmic reticulum or floats free in
o Has a double membrane
o It is the site of respiration where ATP is produced
o Inner membrane is folded to form structures called cristae
o Matrix contains enzymes involved in respiration

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Found in large numbers in cells that are active and require a lot of energy
Golgi apparatus:
o Group of flattened sacs
o Vesicles often seen at the edges
o The golgi processes and packages substances made by the cell, mainly lipids and proteins
o Also makes lysosomes
o Round organelle surrounded by a membrane
o Contains digestive enzymes that are kept separate from cytoplasm
o Can be used to digest invading cells or to break down worn out components of the cell.…read more

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Protein is chemically modified, processed and `finished off'
6. Vesicles are budded off the Golgi and move to the cell surface membrane, along
7. Vesicles fuse with the membrane and the contents are releases. This is called secretion or
Cell organisation:
Multicellular organisms like humans are made of many different types of cells. Cells need to be
organised into groups to work together.
Similar cells organised into tissues: - one or more similar cells are organised together, and carry
out a particular function.…read more

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S phase: Synthesis of DNA ­ replicated.
o G2: gap phase. Period after DNA duplication and cell prepares for division.
o Chromatin condenses, getting shorter
and fatter, and form chromosomes with
each chromosome having two chromatids
joined by a centromere.
o The nucleolus breaks down
o Centrioles start moving to opposite poles
of the cell and begin to form the spindle
fibres across it.
Metaphase :
o The spindles made of microtubules have
been fully formed by the centrioles.…read more

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Add 10 drops of stain (e.g. acetic orcein) so that the chromosomes darken and can
be seen under microscope ­ ratio of stain to hydrochloric acid should be 10:1
4. Warm the watch glass on a hotplate for 5 minutes
5. Place the root tip on a microscope slide and use a needle to break it open and
spread the cells out thinly
6. Add a few more drops of acid
7.…read more

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Crossing over produces new combinations of alleles
o This increases the genetic variation
Independent assortment of chromosomes:
o Happens during meiosis 1
o The homologous pairs line up randomly
o Maternal and paternal chromosomes from parents are therefore randomly distributed into
gametes.…read more

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Fertilisation in plants:
1. Male gamete: Pollen grain, female gamete: inside ovule of ovary
2. Pollen grain lands on stigma of flower, and begins to germinate (The pollen grain
must be from the same species)
3. A pollen tube grows out of the pollen grain and moves down the style
4. There are three nuclei in the pollen tube ­ two male gamete nuclei and one tube
nucleus at the tubes tip.…read more

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There are two main types of stem cells: embryonic and adult stem cells.
o Potency refers to the differentiation potential (the potential to differentiate into different
cell types) of the stem cell. The three types are:
1. Totipotency: The ability of a stem cell to produce all cell types, this includes all
specialised cells in an organism and extra-embryonic cells (cells of the umbilical cord
and placenta). Their potential is `Total'. A fertilised egg is totipotent.
2.…read more

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Can improve the quality of life for many people e.g. replacing damaged cells in the eyes of
people who are blind.
Arguments against the use of stem cells:
o Obtaining stem cells from embryos by IVF raises ethical issues ­ viable embryos are
destructed and could have been a potential human life.
o Many people believe that life begins at conception, and it is immoral and wrong to destroy
and embryo, even to reduce suffering in existing human life.…read more

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Under the right conditions, some genes are activated and other genes are
o mRNA is only transcribed from the active genes
o This is then translated into proteins
o The proteins modify the cell ­ they determine the cell structure and control cell
processes (including activation of more genes, which produces more proteins)
o Changes to the cell produced by these proteins cause the cell to differentiate and
become specialised.…read more



Brilliant! Thank you!

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