GCSE AQA Biology 2 DNA & Cell Division

1. DNA = deoxyribonucleic acid
   1. Contains instructions to put organism together & make it work

1. Gene: Section of DNA
2. Contains instructions to make specific protein
   1. Cell make proteins by putting amino acids together in particular order
      1. Only 20 amino acids used - make 1000s of different proteins
         1. Genes tell cells the order
3. DNA determines what proteins cell produces
   1. Which determines cell type

Nota:

• Apart from identical twins & clones

1. DNA fingerprinting

   Nota:

   • * Way of cutting up person's DNA into small sections & separating * Everyone's pattern is different - can tell people apart
   1. Used in...
      1. Forensic science
         1. DNA from crime scene compared with DNA sample from suspect
      2. Paternity testing
         1. To see if man is father of particular child
   2. Some people think there should be national genetic database
      1. Others think it's an invasion of privacy
      2. There are scientific problems - false positives can occur

1. Makes new cells for growth or repair
   1. Body cells normally have 2 copies of each chromosomes - 23 pairs in total
      1. When body cell divides it needs to make identical new cells (with same no. of chromosomes) - MITOSIS
2. Used in asexual reproduction
   1. Offspring have same genes as parent - no variation
3. Mitosis: when a cell reproduces itself by splitting to form 2 identical offspring
   1. In cell that's not dividing - DNA spread out in long strings
      1. 1) Signal to divide = duplicate DNA (forms X-shaped chromosomes - each arm has same DNA)
         1. 2) Chromosomes line up at centre & cell fibres pull them apart - 2 arms go to opposite ends of cell
            1. 3) Membranes form around each set & cytoplasm divides
               1. 2 cells contain exactly the same DNA

1. Gametes

   Nota:

   • During sexual reproduction, gametes combine
   1. Have 1 copy of each chromosome - can combine to end up with right number in body cells
      1. New individual has mixture of chromosomes - produces variation
2. Meiosis: Produces cells which have half the normal number of chromosomes
   1. 1) Signal to divide = duplicate DNA (forms X-shaped chromosomes - each arm has same DNA)
      1. 2) In 1st division - chromosome pairs line up in centre
         1. 3) Pairs pulled apart - each new cell has 1 copy of each chromosome
            1. 4) In 2nd division - chromosomes line up in centre - arms of chromosomes pulled apart
               1. 4 gametes each have single set of chromosomes
                  1. After fertilisation - cell grows by mitosis
   2. In humans - only happens in reproductive organs