Inheritance & Variation & Evolution

• whats is the difference between meiosis and mitosis

  • mitosis forms 2 identical cells from 1 cell
  • meiosis forms 4 non identical cells from 1 cell
• what is sexual reprodruction

  the joining of male + female gametes, each containing genetic information from mom or dad

  • gametes are formed by meiosis (non identical)
  • a normal cell has 46 chromosomes + 2 sets of chromosomes in 23 pairs from mom + dad
  • each gamete has 23 chromosomes that fuse in fertilisation
  • both parents genetic information mixes
  • this produces variation in offspring
• what is asexual reproduction

  one parents with no gametes join

  • mitosis occurs as 2 identical cells form from 1 cell
  • no genetic information is mixed
  • this makes the offspring clones who are genetically identical to each other + parents
• what is meiosis


  the formation of 4 non identical cells from 1 cell

  • cells make copies of its chromosomes, to provide double the genetic information
  • the cell divides into 2 cells with half amounts of chromosomes (46)
  • the cell divides again to produce 4 cells with quarter amounts of chromosomes (23)
  • these cells are called gametes + are genetically different as chromosomes shuffle during the process
• what do gametes from miosis do at fertilisation

  all 4 gametes with 23 chromosomes join to produce a cell with 46 chromosomes

  • the cell divides through mitosis to produce many copies
  • more cells are produced + an embryo forms
  • the cell begins to differentiate now
• what are the advantages of sexual reproduction

  provides variation

  • even if environments change, an organism in the species will have a characteristic allowing them to survive (a survival advantage)
  • decreases chances of a whole species becoming extinct

  selective breeding

  • allows organisms with desirable characteristics to breed offspring with even more desirable characteristics
  • speeds up natural selection
• what are the advantages of asexual reproduction

  • only one parent is needed
  • uses less energy + is faster as organisms dont require a mate
  • in favourable conditions, lots of identical offspring can be produced
• what is DNA

  a polymer made up of 2 strands of a double helix

  • contained in the nucleus in chromosomes
  • between the 2 strands are the 4 nitrogenous bases liined up in rows
• what is a gene

  a small section of DNA on a chromosome

  • a triplet of bases that code for a specific protein
  • they each code for a particular sequence of amino acids + a chain of the amino acids produces a protein
• what is a genome

  all genes coding for all of the proteins within an organism

  • understanding genomes can help links genes with diseases they could cause
• what are chromosomes

  structures made up of long molecules of DNA found in the nucleus of a cell
• how is DNA structured

  it is made up of many small parts (nucleotides)

  • each nucleotide is made up of a sugar molecule + a phosphate molecule + a singular type of organic bases (A + T + C + G)
  • each DNA molecule contains 2 twisted DNA strands
  • each base connects to another base in the other strand through complementary base pairing
  • each 3 bases code for an amino acid to join to make proteins -> there are 20 types of amino acid
  • some DNA parts control genes being switched on/off rather than coding for proteins
• what does each nucleotide contain

  • a sugar molecule
  • a phosphate molecule (forms the backbone)
  • 1 in 4 of the organic bases
• what is protein synthesis

  the process of producing a protein from DNA

  • if a gene is coded to make a protein it has been expressed
• what are the steps for protein synthesis

  1. DNA contains the genetic code for protein but it is too big to move out of the nucleus

  2. The 2 strands pull apart + mRNA nucleotides match to their complementary base on the strand

  3. The mRNA nucleotides joined to create a new strand that is a template of the original DNA

  4. The mRNA then leaves the nucleus to the cytoplasm + onto ribosomes

  5. The bases on the mRNA read in 3s to code for an amino acid

  6. The corresponding amino acids are brought to the ribosomes by carrier molecules

  7. The amino acids connect together to form a protein
• what are mRNA nucleotides

  single stranded molecules carrying genetic information from DNA -> ribosomes

  • this serves as a template for protein synthesis
• what can proteins be used for

  • enzymes -> biological catalysts speeding up the rate of reactions
  • hormones -> chemical messengers that send signals around the body
  • structural protein -> strong proteins needed to form structures like collagen
• what three ways can mutations change the sequence of bases in DNA

  • a base is inserted into the code
  • a base is deleted from the code
  • a base is substituted
• how does a base being inserted into the code change the sequence of bases in DNA

  • as they are read in threes it changes the way it is read
  • it may change all the amino acids coded for after this insertion
• how does a base being deleted from the code change the sequence of bases in DNA

  • as they are read in threes this changes the way it is read
  • it may change all the amino acids coded for after this deletion
• how does a base being substituted change the sequence of bases in DNA

  • this can only change one amino acid in the sequence
  • it may not change the amino acid as the new sequence can sometimes still code for the same amino acid
• what does a change in the type or sequence of amino acids do

  affect the way it folds + is structured

  • most mutations do not alter the protein or do so slightly
  • there can sometimes be side effects but this can also be in the non coding parts of DNA controlling whether genes are expressed
• what are the side effects of DNA mutations

  changes to the shape

  • a structural protein may lose its shape
  • the substrate will not fit into the active site so it cannot act as a protein
• why has variation occurred between two organisms within DNA

  • the coding DNA that determines the proteins + their activity
  • the non coding DNA that determines which genes are expressed
• what is a gamete

  an organisms reproductive cells

  • contains half the no. of chromosomes (23)
  • egg in females + sperm in males
• what are alleles

  different forms of the genes

  • humans have 2 alleles for each gene
  • we inherit one from each parent
• what is the dominant allele

  • only requires 1 in 2 of the alleles for it to be expressed
  • only requires 1 in 2 of the alleles for the corresponding phenotype to be observed
• what is the recessive allele

  • requires both alleles for it to be expressed
  • requires both alleles for the corresponding phenotype to be observed
• what does homozygous mean

  inherited alleles are the same from each parent

  • either 2 dominant alleles OR 2 recessive alleles
• what does heterozygous mean

  inherited alleles are different from each parent

  • one allele is dominant + one allele is recessive
• what is a genotype

  the combination of alleles an individual has

  • e.g. Aa
• what is a phenotype


  the physical characteristics observed due to genotypes

  • e.g. eye colour
• how can a punnet square diagram show the likelihood of a phenotype being passed down

  parents phenotype = short x tall (tall is dominant allele here)

  parents genotype = tt x TT -> TT as its homozygous/ Tt if hetrozygous

  | t | t |

  --------------------

  Tt | Tt | T |

  --------------------

  Tt | Tt | T |

  --------------------

  offspring genotype = 100% Tt

  offspring phenotype = 100% tall
• what is polydactyly

  a genetic condition leading to babies having extra fingers or toes

  • caused by a dominant allele
  • can be passed down by one allele with the allele
• what is cystic fibrosis

  a disorder of cell membranes leading to thick mucus in airways + pancreas

  • caused by a recessive allele
  • both parents have to be carriers or one has CF + one is a carrier
• what is embryonic screening

  scientists observe whether a child will have a genetic condition

  • if the embryos developed in a lab cells can be taken from it + analysed
  • DNA from embryos in the womb can be extracted
• what are arguments for embryonic screening

  • ethical -> reduces the no. of people suffering
  • economic -> treating disorders is very expensive
  • social -> there are regulations to prevent embryonic screening being excessive
• what are arguments against embryonic screening

  ethical + economic

  • encourages people to choose characteristics for their children -> creating designer babies
  • it is expensive to carry out screening

  social

  • promotes prejudice as it suggests those with genetic disorders are unwanted + will live subservient lives
  • decisions about terminating pregnancies have to be made
  • procedure can lead to a miscarrige
• how is a babies sex genetically determined

  • human body cells have 23 pairs of chromosomes + 22 control characteristics and contain similar looking chromosomes
  • the 23rd pair carries sex determining genes + the chromosomes look different (y chromosomes are smaller than x chromosomes)
• what are the possibilities of chromosomes in the 23rd pair of chromosomes

  X chromosomes + Y chromosomes

  • cells undergo meiosis to form a gamete so only one sex chromosome goes into each gamete