DNA structure and Function

• DNA


  Deoxyribonucleic acid, a molecule that contains the genetic code and makes up genes and chromosomes
• Gene


  A segment of DNA that carries the genetic information to code for the production of a particular protein which determines a particular trait
• Chromosome


  A long strand of coiled up DNA found in the nucleus of cells, each chromosome carries the code for a couple of thousand genes
• Humans have 46 chromosomes (23 pairs of chromosomes) per body cell and 23 chromosomes per sex cell (i.e. egg cell and sperm cell)
• The first 22 pairs are called autosomes, the 23rd pair are called the sex chromosomes
• Females have two matching sex chromosomes (XX), whereas males have two mismatched sex chromosomes (XY)
• DNA Structure


  DNA consists of two molecules that are arranged into a ladder-like structure called a Double Helix
• Nucleotide


  A subunit of DNA, consisting of a phosphate group, a sugar (deoxyribose), and a nitrogenous base
• Nitrogenous bases in DNA


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


  Adenine (A) pairs with Thymine (T), Cytosine (C) pairs with Guanine (G)
• DNA Replication


  1. Unwinding of DNA molecule (double helix) to leave two exposed strands of bases

  2. Making new DNA strands as free bases from the nucleus join up to complementary bases on each of the two exposed strands

  3. Rewinding the DNA molecule to leave two identical molecules of DNA
• Sister chromatids


  After DNA replication, each chromosome consists of two identical copies
• Mistakes in the replication of DNA are called mutations
• Protein


  A long molecule made from chemical units called amino acids
• Genes


  Carry the instructions for inherited characteristics
• Cells


  Have different characteristics because they make different types of proteins
• Genetic code


  The unique sequence of bases that make up a gene code for the production of a unique protein
• Different combinations of amino acids make different proteins
• How genes make proteins


  1. Genes are made of DNA and have a unique sequence of bases

  2. This sequence of bases acts as a code which instructs the cell on which proteins to make

  3. Only one strand of the DNA molecule forming the gene is 'read'

  4. Each amino acid is coded for by its own special sequence of three bases called a triplet

  5. The amino acids join together to form a protein molecule
• Each gene contains the sequence of bases for one protein
• The order of triplets in a gene determines the sequence of amino acids