GCSE B2 Chapter 5

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Mitosis - cell division

  • Mitosis is the process of two identical cells being produced from the original cell
  • A copy of each chromosome is made - the chromosomes contain the genes which are passed on to each new cell
  • The cell then splits into two
  • Each new cell gets one of each chromosome
  • The two cells produced are identical to each other 
  • Mitosis is used for the growth of an organism or for the repair of damaged tissue
  • In asexual reproduction mitosis is used to produce the cells of offspring
  • In animals most cells diffferentiate early on in development of the organism
  • Stem cells are unspecialised cells which are found in the human embryo and the adult bone marrow
  • So in animals mitosis is only used for growth or repair of tissues or replacement
  • Plant cells differentiate throughout the life of the plant as it continues to grow
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Meiosis - cell division in sexual reproduction

  • Meiosis is the process of producing 4 daughter cells with half the number of chromosomes as the original and which are all genetically different from each other and the original cell
  • Gametes are produced by meiosis
  • This occurs in the reproductive organs (human gametes are the sperm and ova)
  • Each gamete has only one chromosome from each pair of chromosomes of orginal cell
  • A copy of each chromosome is made
  • The cell splits into two in the same way as in mitosis
  • The two cells produced are identical to the parent cell
  • The two cells then divide straight away without prducing copies of chromosomes
  • Each daughter cell only recieves one chromosome from each pair of the original
  • All cells are gentically different
  • The four cells produced are gametes and are used in sexual reproduction
  • When the male gamete and the female gamete fuse the chromosomes join together and one body cell is produced with the correct number of chromosomes
  • The cell then develops into an individual by dividing repeatedly by mitosis
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Stem cells

  • Stem cells are unspecialised cells
  • They are found in the human embryo and in the adult bone marrow
  • In the adult bone marrow the stem cells can differentiate into any type of cell the body needs
  • Research is being done into using stem cells to cure different conditions
  • It is hoped that stem cells can be made to diffeentiate into specific types of cells needed to treat different disorders - paralysis can be treated by differentiating stem cells into nerve cells
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  • Gregor Mendel was a monk who worked out how characteristics were inherited
  • He suggested the idea of seperately inherited "factors"
  • Mendel's factors are now called genes which control individual characterisitcs of individuals
  • It took a long time for his ideas to be accepted by other scientists because at the time no one knew about DNA and chromosomes
  • Genes are short sections of DNA which are found on chromosomes
  • Every individual except for identical twins has different DNA
  • Each gene codes for a specific combination of amino acids which make a paticular protein
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  • Humans have 23 pair of chromosomes
  • One pair is the sex chromosomes (humans - female ** - male XY)
  • Alleles are different versions of the same gene so control the same characteristics
  • Alleles for different characteristics are either dominant (mask the effect of others) or recessive (are masked by others)
  • Genotype = The genetic make up of a characteristic - the genes the offspring inherits
  • Phenotype = The physical appearance of the characteristic - dimples or no dimples
  • Hetrozygous = Two different alleles for the same characteristic were inherited
  • Homozygous = Two of the same alleles for a characteristic were inherited by offspring
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Inherited conditions in humans

  • There are many genetic disorders 
  • They can be caused by dominant or recessive alleles
  • Polydactly is caused by a dominant allele and therefore only one parent needs to have the disorder for the offspring to have a chance in inheriting it
  • It results in offspring being born with extra fingers or toes
  • These can be surgically removed
  • Cystic fybrosis is caused by a recessive allele and therefore both parents must be at least carriers of the disease for the offspring to have a chance in inheriting the disorder
  • It affects cell membranes and causes the build up of thick and sticky mucus affecting several organs including the lungs and pancreas
  • Genetic diagrams can be used to how a disorder has been inherited or predict the likelyhood of the offspring inheriting the disorder
  • Punnet squares can be used to show this
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science and ethics of stem cells and embryos

  • Stem cells found in the adult bone marrow can be made to differentiate into specialised cells to treat conditions such as leukaemia - by differentiating stem cells into blood cells to treat blood cancer
  • Most recently the possibility of using embryonic stem cells for differentiation is being researched
  • The stem cells can be used to grow new tissues or organs for transplants
  • The embyonic stem cells can be taken from spare IVF embryos or the umbillical cords of babies
  • There are many ethical issues surrounding the use of embryos because embryos are potential life wich is destroyed, the embryo cannot give permission, research is expensive
  • Embryo screening can be done to diagnose disorders before the baby is born
  • The couple then have the option to have an abortion if the embryo has inherited genetic disorders or they can prepare for th delivery of an affected baby
  • Many people believe this is unethical
  • In IVF the embryos are screened and any which have genetical faults are destroyed which is also considered unethical
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