6 - Inheritance

• What type of human cell has chromosomes that are not paired? Give an example of these cells in a human?

  
  

  How many chromosomes do these cells have?

  - gametes (sex cells)

  e.g. sperm cell or egg cell

  
  

  - both contain 23 single chromosomes
• What type of cell division produces gametes?

  Why is this type of cell division differrent to mitosis?

  - meiosis

  
  

  - produces non-identical cells, i.e. every sperm cell and every egg cell is different
• What are the gametes produced via meiosis in flowering plants?

  - pollen

  - egg cells
• What are the differences between sexual and asexual reproduction?

  - sexual reproduction involves the fusion of male and female gametes (fertilisation)

  - sexual reproduction involves the mixing of genetic information, i.e. the offspring receives genetic information from both the parents

  - sexual reproduction leads to genetic variation in offspring

  
  

  - asexual reproduction only involves one parent

  - produces genetically identical offspring (clones)

  - only involves mitosis
• In humans, what are the reproductive organs where meiosis takes place?

  - testes in males

  - ovaries in females
• What are the stages of meiosis?

  1) All of the chromosomes are copied (cell initially has 23 pairs but now has 46 pairs)

  2) Cell divides into two (each new cell with 23 pairs)

  3) These two cells divide again, forming four genetically different gametes (each with 23 single chromosomes)
• How does fertilisation restore the original number of chromosomes?

  - gamete (male) and gamete (female) join/fuse together

  => fertilised cell now has 23 + 23 = 46 chromosomes (23 PAIRS)
• What happens to the new, fertilised egg cell after fertilisation?

  - fertilised egg cell divides by mitosis, producing an embryo (ball of identical cells)

  
  

  - cells in the embryo then differentiate to form different cell types, e.g. neurones and muscle cells
• What are the advantages of sexual reproduction?

  - produces genetic variation in the offspring (some of the offspring have a survival advantage by natural selection if the environment changes)

  - gives a species a greater chance of survival if conditions become challenging

  - natural selection can be sped up by humans in selective breeding to increase food production
• What are the advantages of asexual reproduction?

  - only one parent needed

  - more time and energy effecient as do not need to find a mate

  - faster than sexual reproduction

  - many identical offspring can be produced when conditions are favourable
• What is one key disadvantage of asexual reproduction?

  - risk that all the offspring could die if conditions become unfavourable (all are genetically identical)
• Give 3 examples where some organisms reproduce both sexually and asexually.

  - malarial parasites reproduce asexually in the human host, but sexually in the mosquito vector

  
  

  - many fungi reproduce asexually by spores but also reproduce sexually to give genetic variation

  
  

  - many plants produce seeds sexually, but also reproduce asexually by runners such as strawberry plants, or bulb division such as daffodils
• What molecule do chromosomes contain?

  Why is this molecule a genetical material?

  DNA (deoxyribonucleic acid)

  - determines our inherited features
• What is the structure of DNA?

  - two polymer strands which wrap around each other to form a DOUBLE HELIX

  
• What is a gene?


  - a small section of DNA found on a chromosome that codes for a particular sequence of amino acids, to make a specific protein
• What is a genome?

  - the entire genetic material of an organism
• What benefits does understanding the human genome provide?

  - helps us SEARCH FOR GENES THAT ARE LINKED TO A DISEASE, e.g. genes that increase the risk of developing cancer or Alzheimer's disease

  
  

  - helps us UNDERTSTAND AND TREAT INHERITED DISORDERS, e.g. cystic fibrosis

  
  

  - we can use the human genome to TRACE HUMAN MIGRATION PATTERNS FROM THE PAST => discover ancestry
• What is the polymer DNA made of?

  - nucleotides
• What are the 3 main parts of a nucleotide?

  - phosphate group

  - sugar molecule

  - base

  
• What never changes with a nucleotide?

  - phosphate group and sugar molecule never change in DNA
• What are the 4 bases of a nucleotide?

  (A)denine

  (T)hymine

  (C)ytosine

  (T)hymine
• What does it mean if DNA strands are complementary?


  - the same bases always pair on the opposite strands

  - C is always linked to G

  - A is always linked to T
• How is the shape of a protein determined?


  - specific order of amino acids
• How does the cell read the DNA sequence?

  - as triplets of bases (each group of 3 bases codes for a specific amino acid in the protein)
• What happens in the first stage of protein synthesis?

  Transcription:

  
  

  - base sequence of the gene is copied into a complementary template molecule (messenger RNA / mRNA, a single stranded molecule)

  
  

  - mRNA now passes out of the nucleus, into the cytoplasm
• What happens in the second stage of protein synthesis?

  Translation:

  
  

  - mRNA molecule attaches to a ribosome

  - amino acids are now brough to the ribosome on carrier molecules (transfer RNA / tRNA)

  
  

  - ribosome now reads the triplets of bases (letters) on the mRNA, and uses this to join together the correct amino acids into the correct order

  
  

  - once the protein chain is complete, it now folds into its unique shape (shape enables the protein to do its job)
• What is a change to a base called?

  mutation
• How can a mutation affect an enzyme?

  - protein now has a different amino acid

  - active site of the enzyme has changed shape so it can no longer attach to the specific substrate
• How can a mutation affect a structural protein?

  - mutation can change the shape of collagen => loses strength
• How can a mutation affect a non-coding region?


  - these regions switch on and off (tell genes when to produce proteins)

  
  

  - mutation may cause a gene to turn on when it should be turned off (cell would produce a protein that it is not meant to have at that time) => significant effect on a cell, e.g. uncontrolled mitosis leading to cancer
• How many pairs of chromosomes determine your gender?

  uno (one chromosome in the pair from each parent)
• Give 3 examples of where a characteristic is only controlled by a single gene.

  - fur colour in mice

  - red-green colour blindness in humans

  - ear wax
• What are alleles?

  - different forms of a gene
• What tells us the alleles present in a person?

  genotype
• What does it mean if someone if homozygous?


  - when they have two copies of the same allele
• What is the phenotype of a person?


  - the physical characteristics caused by a person's alleles
• What does it mean if someone if heterozygous?

  - they have two different alleles
• What is the different between a dominant and a recessive allele?

  - a dominant allele will show in the phenotype even if there is only one copy present (capital letter)

  - a recessive allele will only show if two copies present (lower-case letter)
• What is cystic fibrosis?

  - an inherited disorder of cell membranes

  - recessive (person has to inherit a defective allele from each parent)
• What is the genotype for a carrier for cystic fibrosis?

  Cc (heterozygous)

  - does not have cystic fibrosis but can still pass the allele onto offspring