Cells + DNA

• • Throughout the Cytoplasm, there is a network of Endoplasmic Reticulum (ER) which are covered with tiny granules called Ribosomes
  • The spaces between the membranes of the Endoplasmic Reticulum act as a transportation system, sending protein to the part of the cell where it is needed

  
• Each chromosome contains one DNA molecule

  • The DNA is folded and coiled so that it can be packed into a small space
  • The DNA is coiled around proteins called Histones (the balls)

  A) Histones

  B) DNA

  
• A molecule of DNA is made from two strands of molecular groups called Nucleotides

  • Each Nucleotide contains a sugar called Deoxyribose, a Phosphate group, AND a nitrogen-containing group called a Base

  A) Phosphate

  B) sugar

  C) Nitrogenous

  
• The Four bases in a Nucleotide are:

  • (A) Adenine
  • (T) Thymine
  • (G) Guanine
  • (C) Cytosine
• A close - up Nucleotide:

  A) Thymine

  B) Cytosine

  C) Hydrogen

  D) Phosphate

  
• The two Base strands are held together by Hydrogen Bonds between the complementary base pairs.

  • These are weak bonds between Hydrogen atoms on one base and Oxygen or Nitrogen atoms on another base.
  • They can be easily broken, allowing the chains to separate, which is used when DNA makes a copy of itself
• DNA Replication - This happens when the DNA is copied to make two identical copies of the original DNA

  • As a result, each ‘daughter cell’ that is formed receives exactly the same amount and type of DNA
  • New strands of DNA are assembled from Nucleotides under the control of an enzyme called DNA polymerase
• How DNA Replicates itself:

  1. The Polynucleotide strands of DNA separate
  2. Each strand acts as a template for the forming of a new strand of DNA
  3. DNA polymerase assembles nucleotides into two new strands according to the base-pairing rule, using the templates
  4. Two identical DNA molecules are formed, where each contains a strand from the parent DNA and a new complementary strand

  
• Only ONE of the strands of a DNA molecule actually codes for the making of Proteins in a cell. This strand is called the Template Strand.

  (The other strand is called the Non-template strand).

  
• Many of the proteins made by the Template Strand in DNA are Enzymes, which control certain processes within the cell

  • Some proteins are structural, for example, Keratin in the skin or Myosin in muscles
  • Other proteins have particular functions, such as Haemoglobin and some (H) Hormones
• Proteins are made of chains of Amino Acids

  • A sequence of THREE bases in the template strand of the DNA codes for ONE amino acid
  • Because three bases are needed to code for one amino acid, the DNA code is a Triplet code.
  • The sequence of bases that codes for all the Amino Acids in a protein is called a (G) Gene

  A) Gene

  B) Protein

  
• Before Protein Synthesis:

  • DNA stays in the Nucleus but Protein Synthesis takes place in the Cytoplasm
  • So before proteins can be made, the genetic code must be copied and transferred out from the Nucleus to the Cytoplasm (which is called Transcription)
  • This is carried out by a different kind of nucleic acid called Ribonucleic Acid (RNA)
• There are three differences between DNA and RNA:

  • DNA is a double helix, RNA is a single strand
  • DNA contains the sugar Deoxyribose, RNA just contains Ribose
  • RNA contains the base Uracil (U) instead of Thymine (T)
• A Mutation is a random change in the DNA of a cell
• The 4 types of Gene Mutations are:

  • Duplication
  • Deletion
  • Substitution
  • Inversion
• DNA Mutations:

  • Duplication - The Nucleotide is inserted twice, instead of once. The entire base sequence is altered, and will code for an entirely different protein
  • Deletion - A Nucleotide has been deleted, which again, alters the entire base sequence
• More DNA Mutations:

  • Substitution - Here, a different Nucleotide is used. The triplet of bases will then change, and code for a different Amino Acid. Therefore, the STRUCTURE of the protein will be different
  • Inversion - The sequence of bases in a triplet is reversed. Only one triplet is affected, and it may or may NOT result in a different Amino Acid