DNA

• DNA

  Deoxyribonucleic acid
• DNA

  • The blueprint for controlling the characteristics of organisms
  • Contains all the instructions for the cell and the entire organism
  • Usually a very thin, long, string-like structure that is almost invisible
  • Has a double helix shape, often referred to as a 'twisted ladder'
• DNA structure

  1. Nucleotides contain a nitrogenous base, a deoxyribose sugar, and a phosphate

  2. The sugar and phosphate make up the sides of the DNA ladder, also known as the sugar-phosphate backbone

  3. The bases of the two strands join together using complementary base pairing to make up the rungs of the ladder
• Nucleotides

  • Arranged in a way that makes the DNA a double helix (twisted rope ladder)
  • The uprights of the ladder are made of alternating phosphate and sugar groups
  • The rungs are formed by the pairing of nitrogen-rich bases (bases)
  • Chemical bonds called hydrogen bonds hold the two strands of DNA together by bonding the complementary bases
• DNA bases

  • Adenine (A)
  • Guanine (G)
  • Cytosine (C)
  • Thymine (T)
• Complementary bases

  • Adenine will only ever bond with Thymine (A-T)
  • Guanine will only ever bond with Cytosine (G-C)
• DNA is usually a very thin, long, string-like structure that is almost invisible
• DNA supercoiling

  1. When the cell needs to divide, the DNA winds itself up (supercoils)

  2. Supercoiled DNA takes the form of chromosomes, which are visible with a microscope
• Chromosomes

  • Thread-like structures that DNA can be compacted into
  • Contain the genetic code (DNA)
  • Human body cells generally contain 46 chromosomes (in 23 pairs)
  • The only exceptions are sperm, egg, and red blood cells
• Human gametes (sperm and egg cells) only have 23 chromosomes, as opposed to 46
• Not all organisms have the same number of chromosome pairs
• What each human cell (excluding gametes and red blood cells) contains

  • 46 chromosomes (23 pairs of chromosomes)
  • Around 2 metres of DNA
  • Around 3 million DNA bases (A, T, G, C)
  • Approximately 32,000 genes
• Genes

  • Sections of DNA that code for a protein
  • Each chromosome can have over 1000 genes
  • Each gene codes for a specific protein
  • Proteins control many characteristics or functions in the body
• Difference between one gene and the next

  • The order of bases along the DNA strand, as well as the length of the DNA strand
  • The order of the bases along the DNA strand is the genetic code
  • The genetic code determines specific characteristics or traits, such as eye and hair colour
• Chromosomes contain the specific instructions for different characteristics or traits (e.g. eye colour) or protein functions (e.g. making enzymes)


  This code is divided into units called genes
• Homologous pairs

  • Each chromosome has a partner that is the same size and shape (apart from the male sex chromosomes)
  • One of the pair came from mum (maternal chromosome), and one came from dad (paternal chromosome)
  • They are not identical though, as the sequence of bases on each can be slightly different
• Alleles

  The different forms of a specific gene
• Diploid number


  The number of chromosomes in your body cells (2n)
• Haploid number

  The number of chromosomes in your gametes (n)
• Sex chromosomes

  • Two of the 46 chromosomes in human cells
  • Determine whether male or female
  • The other 44 chromosomes are autosomes
• Female sex chromosomes

  A pair of X chromosomes (XX)
• Male sex chromosomes

  One X and one Y chromosome (XY)
• The nucleus is the control centre of the cell.
• DNA stands for deoxyribonucleic acid
• Chromosomes are made up of DNA, which contains genes that determine characteristics such as eye colour or height.