B1, B2, B3 Revision Notes

21st Century Science

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  • Created on: 13-05-16 05:59
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B1 ­ You and your genes
B1.1 What are genes and how do they affect the way that organisms develop?
A GENE is a short section of DNA. Genes carry instructions that control how you develop and function ­ they are
long molecules of a molecule called DNA. Each gene codes for a specific protein by specifying the order in which
AMINO ACIDS must be joined together.
These proteins can be: STRUCTURAL PROTEIN: Gives the body structure, rigidity and strength JJJJJJJJJ
E.g. Skin, Hair, Muscles etc
FUNCTIONAL PROTEIN: Enables the body to function E.g. Enzymes, ffffff
Antibodies etc.
The differences between individuals of the same species are described as VARIATIONS.
Variations may be due to:
GENOTYPE ­ The genetic makeup of an organism. The different characteristics that an individual inherits, E.g.
whether you have dimples or not.
PHENOTYPE ­ The observable characteristics the organism has. How the environment changes an individual,
E.g. cutting the skin may cause a scar.
IDENTICAL TWINS have the same set of genotype however any differences between them is because of
CONTINOUS VARIATION shows when some characteristics are controlled by several genes working together e.g.
eye colours and height. For instance it was originally believed that eye colour was due to a single gene. It is now
known that there are a number of genes coding for different pigments in the iris, mainly on chromosome 15 in
humans. This means that there is enormous variation in eye colour.
B1.2 Why can people look like their parents, brothers and sisters but not identical to them?

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A a
a Aa aa
A Punnett square diagram can also be used to represent how sex is determined. This is because one of the 23
pairs of chromosomes in a human cell is the sex chromosome. In females the sex chromosomes are the same ­
they are both X chromosomes. In males they are different ­ there is an X chromosome and a Y chromosome.…read more

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HUNTINGTON'S DISEASE is another genetic that affects the central nervous system. However is caused by a
dominant allele ­ the presence of just one dominant allele can cause the disease. You only need to inherit one
copy of the faulty allele to have Huntington's disorder, unlike cystic fibrosis, where you need to inherit both
copies. You can inherit the disorder if one or both of your parents carry the faulty allele, because it is DOMINANT.…read more

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However testing adults and foetuses for alleles that cause genetic disorders has implications that need to be
considered, including:
Risk of miscarriage as a result of cell sampling for the genetic test
Using results that may not be accurate , including false positives and false negatives
Outcome Test Result Reality
True Positive Subject has the disorder Subject has the disorder
True Negative Subject does not have the result Subject does not have the result
False Positive Subject has the disorder Subject does not have the…read more

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Plants asexual reproduction in plants can take a number of forms:
Some plants such as strawberries produce shoots called RUNNERS. These eventually break off and become new
strawberry plants, clones of the original.
Other plants grow BULBS. When bulbs are planted they grow into genetically identical plants. Again the
environment will alter them.…read more

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EMBRYONIC STEM CELLS are unspecialised cells that can develop into ANY type of cell, including more embryonic
stem cells.
As a result of being unspecialised, stem cells from embryos and adults offer the potential to treat some
For example ­ skin can grow as a treatment for serious burns and sight can now be restored to people who are
blind due to damage of their corneas.
The majority of cells of multicellular organisms become specialised during the early development of the organism.…read more

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B2 ­ Keeping Healthy
B2.1 How do our bodies resist infection?
MICROORGANISMS are organisms that are too small to see with the naked eye. They include BACTERIA , VIRUSES
and FUNGI . They can be beneficial to us (e.g. the bacteria that live in our intestines can produce certain vitamins)
or they can cause us harm (e.g. bacteria that cause food poisoning).
PATHOGENS are microorganisms that cause infectious diseases. BACTERIA and VIRUSES are the main
PATHOGENS.…read more

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When microorganisms enter the body, they release toxins. The toxins damage cells to cause the symptoms of the
disease. The body's first line of defence is its NATURAL BARRIERS which include:
Chemicals in tears
Chemicals in sweat
Stomach acid
The body's first line of defence is called PASSIVE IMMUNITY , which means preventing the PATHOGEN from
entering in the first place. If a pathogen manages to get into the body, the second line of defence takes over
which is called ACTIVE IMMUNITY .…read more


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