GCSE AQA Biology 2 DNA & Cell Division

GCSE AQA Biology 2 DNA & Cell Division

1. DNA is found in nucleus of animal & plant cells - in really long molecules (chromosomes)

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

2. Genes

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

3. Everyone has unique DNA

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

4. Mitosis

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

5. Meiosis

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