Cell Division & Inheritance

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  • Genes are found inside DNA which is found inside chromasomes which are found in the nucleus
  • Nucleus > Chromasome > DNA > Gene
  • DNA contains instructions to make amino acids which create proteins which fold to make a particular shape
  • The proteins control which characteristics someone has
  • DNA is read in the nucleus and the proteins are made in the cytoplasm at the ribosomes


  • Cells divide by mitosis so organisms can grow and replace damaged cells and tissues
  • One cell becomes two, so all the cells are identical
  • Happens in asexual reproduction 
  • First the chromasomes are copied, then the cell divides
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Cell Differentiation

  • When a fertilised egg begins to divide, the first cells are unspecialised
  • In animals, cells become specialised early on, but plant cells usually remain undifferentiated

Stem Cells:

  • Undifferentiated cells
  • Useful in medical research as they can develop into different types of cells, eg. brain cells, liver cells etc.
  • Doctors get stem cells from embryos, umbilical cords, bone marrow etc.
  • Can be used to treat previously incurable conditions, such as damaged organs and birth defects
  • However, new cells could be rejected by the body
  • Using cells from embryos results in ethical issues
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Meiosis // Gregor Mendel


  • Gametes are produced in the ovaries and testicles
  • The new cells only contain half the number of chromasomes and the combination of genes is different
  • Gametes fuse together so the fused cell haas the correct number of chromasomes


Gregor Mendel:

  • First person to continuously repeat an experiment and write down the results
  • He would choose flowers to be the parents and use a paintbrush to pollinate them
  • He was the first person to suggest seperatley inherited factors (genes)
  • However, no one believed him because he wasn't a scientist and because he had no way of explaining or providing evidence as genetics hadn't been explained yet
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Key words & Gender

  • Males are XY
  • Females are **
  • Females only have X chromasomes, so the sperm is the gamete which controls whether the resulting child will be male or female

Allele: Alternative forms of a gene- tall/ short, blue/ green/ brown eyes

Genotype: Alleles in a cell- RR or Rr

Phenotype: The characteristic- tongue roller, blue eyes

Dominant: Allele for the characteristic which is always shown if the allele is present

Recessive: Allele for the characteristic only shown if no dominant

Homozygous: 2 of the same allele- RR, rr

Heterozygous: Different alleles- Rr

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Genetic Fingerprinting // Mutations

  • Jeffereys used the variability of DNA to develop genetic fingerprinting
  • Certain parts of the DNA are variable amongst the population, but some are similar among related people
  • He used this to make genetic fingerprinting which can be used to show if a sample of DNA comes from the same person, related person, or a complete stranger
  • Forensics uses g.f. to find suspects
  • Maternity/ Paternity tests use g.f. to see if a man or woman and a child are related



  • When an organism develops a new characteristic that no other member of the species has had before
  • Mutations may be caused a chemical change or when the DNA isnt copied properly
  • They can be helpful because the can give the organism a survival advantage so the mutated organisms live on to become better at surviving
  • The mutated gene controls which enzymes are created which controls which characteristic is formed as the sequence of amino acids is changed
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Inherited Diseases


  • When someone has an extra finger/thumb
  • Caused by a dominant allele, so can be passed down even if only one parent has it

Cystic Fibrosis:

  • Thick, sticky mucus cannot be coughed up, leading to difficulties breathing and frequent lung infections
  • Cannot digest food properly as enzymes cannot pass into the gut, causing a shorter life span
  • Can be found using a sweat test as they have 2-5 the amount of salt in their sweat
  • No cure, but can be treated with physiotherapy, inhalors, antibiotics, gene therapy ( testing)
  • Recessive allele
  • A carrier of CF (has one CF allele but shown no signs) has a possibility of producing a child with CF with another carrier

Hungtington's Disease: Controlled by dominant allele

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Embryo Screening:

  • Parents can warned whether or not to have the child
  • Can select healthy embryos to keep
  • Could harm embryo/ cause miscarriage
  • Expensive
  • Unethical as all babies should have the right to life

Stem Cells:

  • Can find cures or treatments to previously untreatable diseases
  • Generally uses embryos which would have been destroyed anyway
  • Not stable
  • Unethical as all babies have the right to life
  • Expensive to develop- money could be spent elsewhere
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