OCR 21st Century Science A B1 B2 B3 Revision Notes

Here are some notes given to me by a friend which have been quite helpful. 

HideShow resource information
  • Created by: hajtastic
  • Created on: 13-05-13 22:37
Preview of OCR 21st Century Science A B1 B2 B3 Revision Notes

First 346 words of the document:

Science B1,2,3
Biology 1: Genes
Unit 1.1: What your genes do (Pg. 4)
Genes carry instructions that control how you develop and function ­ They are long molecules of a chemical
called DNA.
Genotype: A person's genetic makeup. (Dimples)
Phenotype: Observable physical features ­ Can be affected by the environment.
Identical twins have the same set of genotype.
Separated twins have shown that environmental factors affect one's genotype.
Continuous variation shows when some characteristics are controlled by several genes working together, eye
colours and heights.
Functional Protein: Enables the body to function. Eg: Enzymes and Antibodies.
Structural Protein: Gives the body structure, rigidity and strength. E.g. Keratin, Skin, Hair.
We have 20,000 ­ 25,000 genes on out chromosomes.
Cell ­ Nucleus ­ Chromosomes ­ DNA ­ Gene ­ Base.
Unit 1.2: Genes working together and variation (Pg. 5)
We all have 46 chromosomes, 23 pairs.
Sex cells: Boy ­ XY Girl ­ XX
Chromosome 23 determines our sex.
Chromosome 21 means that someone has Down's syndrome.
Changes in our DNA are called mutations.
Chromosome mutation results in having an extra chromosome.
Everyone's chromosomes are different.
Homozygous: Two alleles of a gene are identical
Heterozygous: Two alleles of a gene are different
Capital letters ­ Dominant allele
Lowercase ­ Recessive allele
Unit 1.3: Genetic crosses and sex determination (Pg. 6)
Genes of a particular trait will be found in the exact same place as it is on the other chromosome pair.
The presence of a Y gene on the Y chromosome determines whether the embryo is male or not, it triggers
the development of testes in the embryo. And therefore, vice versa, in the absence of the Y chromosome,
ovaries develop.
There are parts of the X chromosome which can have no matching allele on the Y chromosome, due to the
shape of the sex cell. This can result in a sexlinked disorder.
Sex linked diseases, such as haemophilia and regreen colour blindness, are far more likely to be present in

Other pages in this set

Page 2

Preview of page 2

Here's a taster:

Unit 1.4: Gene disorders, carriers and genetic testing. (Pg. 7)
Some disorders are caused by faulty or defective alleles.
Huntington's disease is a dominant disorder, the presence of just one dominant allele can cause the disease. It
occurs in middle age.
Symptoms include:
Uncontrollable shaking
Memory loss
Inability to concentrate
Mood swings
Cystic fibrosis is an example of a recessive disorder, both alleles on the chromosome pair must be recessive.…read more

Page 3

Preview of page 3

Here's a taster:

Advantages: Disadvantages:
Successful No genetic variation, if
characteristics shown living conditions change,
the population could be
wiped out.
Useful when organism
lives in isolation
Embryonic stem cells can change into any cell doing a different job.
Adult stem cells stay the same.
Because embryonic stem cells can develop into other cell types they have huge potential.…read more

Page 4

Preview of page 4

Here's a taster:

The body's external defences include skin, saliva and tears, and acid in the stomach.
The body's internal defence is our immune system.
The immune system uses white blood cells to defend the body. White blood cells:
are made in the bone marrow
are found as several types: some engulf and digest invading microorganisms (T cells) while others
produce antibodies, which recognise and destroy invading microorganisms (B cells)
Antigens are chemicals which recognise foreign antibodies on pathogens.…read more

Page 5

Preview of page 5

Here's a taster:

Links the veins and arteries
One cell thick transfers substances to and from cells
Collects blood and returns it to the heart
Thin, elastic and muscular
Transfers blood under low pressure
The coronary arteries can become blocked by fatty deposits, this prevents the heart from receiving the blood
it needs leading to a heart attack.
The main lifestyle factors that increase a person's risk of Coronary heart disease (CHD):
1. Smoking
2. Poor diet (High in fat and salt)
3.…read more

Page 6

Preview of page 6

Here's a taster:

The kidneys help to balance levels of water, urea, salts and other chemicals in the blood.
The kidneys respond to changes in the blood plasma by changing the concentration of urine that is excreted
from the body.
Alcohol causes the kidneys to produce a large volume of dilute urine and the body becomes dehydrated
Ecstasy causes the kidneys to produce very small volumes of concentrated urine.…read more

Page 7

Preview of page 7

Here's a taster:

Carbon is key element of the chemicals that make up all living things, it is continuously recycled through the
carbon cycle.
Nitrogen is an essential component of living things. It is recycled through the nitrogen cycle.
Environmental change can be measured using:
Nonliving indicators, e.g. carbon dioxide levels, temperature and nitrate levels
Living indicators, e.g. phytoplankton, lichens and aquatic organisms such as mayfly larvae.
Unit 1.3: Fossils and DNA
Organisms are classified by putting them into groups.…read more

Page 8

Preview of page 8

Here's a taster:

To ensure sustainability, we need to maintain biodiversity. The loss of a single species removes a food
supply and can have a big impact on the whole ecosystem.
Intensive monoculture crop production maximises crop yields but it not sustainable reduces biodiversity.…read more



Hello Hajrah!

Thanks for these resources, they're really good! I don't suppose you have B456, C456 and P456?

Thanks again, Isa.

Similar Biology resources:

See all Biology resources »See all resources »