Biology B1

Revision notes on B1 for OCR biology

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You and Your Genes- Nuclei and Chromosones

  • The Nucleus contains Chromosomes
  • Chromosomes are made from long strands of DNA
  • These are made from Base Molecules with the base pairs A=T C=G
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A Gene

  • A GENE = A section of DNA which contains the genetic code for a particular protein.
  • These proteins can be: Structural or Enzymes

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A human body has 23 pairs of chromosomes.

The sex cells of both males and females carry half the information, 23 individual chromosomes.

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  • Each chromosome has genes in the same places. But different versions of the genes called alleles
  • Genes can either be dominant or recessive alleles

R+R = R



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  • Variation are differences
  • identical twins are not the same even though they carry the same genetic

information, this is because of ENVIRONMENTAL factors.

  • examples are: Climate, diet, physical accidents, lifestyle and culture.
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Genetic Testing

Antenatal - Analyzing the babies DNA before birth. This can't detect all disorders.

Prenatal - Testing for couples with an increased risk of the child having a disorder. prenatal for downs syndrome is offered to all women.

Neonatal - Testing the baby just after birth for disorders which can be treated early. All babies are screened for 3

Carrier testing - offered to individuals with a family history of disorders for example cystic fibrosis which is caused by a recessive allele. If a couple are both carriers they have an increased risk of having a baby with the disorder

predictive testing- Looks for progressive disorders which develop later on in life. This is offered to people who have conditions

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Limits to genetic Testing

Tests are not currently available for all disorders

The testing is not completely safe

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Gene Therapy

What this involves:

1) A normal allele is taken and cut out

2) Copies are made of it

3) They are inserted into the chromosomes of the patients cell.

Stage 3 causes difficulty because the information/cells may not work. The cells may naturally be replaced. In this case nose sprays, viruses and injections can be used to get it into the body

Gene therapy does not remove the faulty allele, and therefore it can still be passed onto offspring.

It is currently illegal to use gene therapy to alter sex cells.

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Asexual Reproduction

This requires only one parent

There is no fusion of cells or mixing of genetic information, which means a clone of the parent is produced

Plants do this by producing side branches, runners or underground food storage or organs which develop into the next generation

This is unusual in animals, but starfish and anemones reproduce this way.

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Clones are genetically identical individuals

Twins are natural clones

Embryo transplants and fusion cell cloning produce clones

These processes allow the good features of an individual to be selected and duplicated in offspring

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Stem cells

These are unspecialized cells that are taken from embryos.

Therapeutic cloning:

The cells that are needed are taken from stem cells, but if they do not contain the patients genetic information then the body will reject them. So a clone of the patients own cell is made, this is used to produce an embryo, the growth of which is stimulated in the lab. Stem cells can be taken from the embryo to treat many diseases.

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