Genes

• A gene is A section of DNA that codes for a specific allele.
• What are chromosomes ?

  DNA that is tightly woven around protein
• An allele is what defines a certain feature of an organism.
• What do nucleotides consist of?

  Pentose sugar, phosphate, nitrogen bases
• What are the four nitrogen bases?

  adenine (A), thymine (T),

  guanine (G) and cytosine (C).

  note - GC-AT!
• The nucleotides are organised in a way that makes DNA a double helix. The shape of a double helix is like that of a twisted rope ladder, the uprights of the ladder are made of alternating phosphate and sugar groups.
• The sequence of these bases along the length of the DNA molecule determines the genetic code of an individual.
• Each base can only bond with its complementary partner. This means that adenine always pairs with thymine, while guanine always pairs with cytosine.
• Inside the rungs of the ladder are the nitrogenous bases which pair up with one another to form hydrogen bonds between them.
• Inside the cell, DNA is found as long strands coiled into structures called chromosomes.
• Each base pairs with another specific base to form a pair called a base pair. Adenine always pairs with Thymine and Guanine always pairs with Cytosine.
• James Watson and Francis Crick discovered the structure of DNA in 1953
• Dominant vs Recessive Alleles

  Dominant - variation of gene that always shows

  Recessive - variation of gene that only shows when two parents have that gene
• Outside the rings of the DNA double helix are alternating phosphate and sugar groups
• meiosis

  
• DNA replication

  
• There are two types of mutations:

  1. Pure chance (spontaneous)
  2. Mutagens (factors that trigger mutations) - can cause uncontrollable cell division, cancerous cells(eg. gamma rays, x-rays)

  • There are beneficial/detrimental mutations
  • positive, eg. one pair of sickle cells mutations in malaria-prone countries
  • negative eg. two pairs of sickle cell mutations cause sickle cell disease = death
• Mutations


  an error in the DNA sequence during replication