DNA

• Cells -> Nucleus -> Chromosome -> DNA -> Gene
• Nucleotide structure:

  A) Phosphate

  B) Deoxyribose sugar

  C) Base

  
• DNA (deoxyribonucleic acid): A double helix made of nucleotides, found in the nucleus of eukaryotic cells; the molecule that determines the genetic characteristics of most living things.
• The ‘rungs’ of the DNA ladder are made out of the (four) nitrogen-rich bases. 
• Strong bonds between the sugar of one nucleotide and the phosphate of the next nucleotide are called phosphodiester bonds and form the sugar/phosphate sides.
• The four nitrogen-rich bases: 

  • Adenine 
  • Thymine 
  • Guanine 
  • Cytosine 
• Complementary base pairing rule: 

  The bases are joined together by weak hydrogen bonds. 

  • A pairs with T (2 bonds)
  • G pairs with C (3 bonds) 
• • DNA is located in the nucleus of eukaryotic cells.
  • Chromosome: Incredibly long threads of DNA tightly coiled around each other and proteins (histones). 
  • Segments of DNA are called genes.
• Gene: A section of DNA that carries the genetic code for a particular characteristic / A section of DNA arranged along the chromosome that codes for a specific protein or trait. 

  • These proteins create structures and perform actions needed for cells to survive, grow, and function. 

  For example these proteins may be:

  Structural, eg., collagen 

  Enzymes, eg., pepsin

  Regulatory, eg., insulin
• Chromosomes in humans: 

  • 46 chromosomes (23 pairs) in every cell

  -> 22 regular pairs, 1 sex pair 

  
  

  Gametes (egg cells in females, sperm cells in males) have half the number of chromosomes:

  • 22 chromosomes, 1 sex chromosome
• Homologous chromosomes: Chromosome pairs that have the same gene order. 

  • Same position of genes
  • Same length
  • Same position of the centromere. 
• Types of chromosomes:

  Autosomes - Chromosomes that carry genes for general characteristics. No differences in gender. Humans have 44 (22 pairs). 

  Sex chromosomes - Genes for sex are located here. Humans have 2, XX for female, XY for male. 
• Karyotype: a picture of someone’s chromosomes. 

  1. Chromosomes are dyed
  2. Chromosomes are arranged in order in homologous pairs 
• Karyotyping is used to determine: 

  • Gender
  • Chromosomal abnormalities