Biology

• DNA


  Deoxyribonucleic acid
• DNA


  • Contains deoxyribose (a 5-carbon sugar)
  • DNA is a nucleic acid (molecule made up of nucleotides)
• Nucleotide


  Consists of a sugar molecule attached to a nitrogen-containing base and a phosphate group
• Parts of a nucleotide

  • Sugar
  • Phosphate
  • Nitrogen Base
• Nitrogen bases


  • Adenine
  • Guanine
  • Cytosine
  • Thymine
• DNA


  • Double stranded molecule - the two strands are connected by the nitrogen bases
  • Adenine can only pair with thymine (and vice versa)
  • Guanine can only pair with cytosine (and vice versa)
• Categories of nitrogen bases

  • Purines - double ring structures (adenine and guanine)
  • Pyrimidines - single ring structures (thymine and cytosine)
• Purines always bond with pyrimidines (as per the base pairing rules)
• The players


  • James Watson
  • Francis Crick
  • Maurice Wilkins
  • Rosalind Franklin
• Watson and Crick's discovery

  • The structure of DNA
  • How DNA can carry information and be copied
• Double helix


  The shape of the DNA molecule described by Watson and Crick
• Hydrogen bonds


  The weakest type of bond that connects the nitrogen bases in DNA
• DNA comes apart during DNA Replication (the copying of DNA)
• DNA


  Holds the "recipe" for making proteins
• Your entire body is made out of proteins
• DNA is your personalized instruction manual and yours is unique to you (though everyone in this room shares about 99% of the same DNA, that's what makes us human!)
• Differences between DNA and RNA

  • RNA contains the base Uracil instead of Thymine
  • RNA is a single stranded molecule unlike DNA, which is a double strand (helix)
  • RNA contains the 5-carbon sugar ribose (DNA contains deoxyribose)
• RNA can move out of the nucleus into the cytoplasm, DNA is always in the nucleus
• Gene expression


  The precise way in which the genetic information in a gene is decoded in the cell and used to make a protein
• Requirements for protein synthesis

  • A supply of amino acids - cytoplasm
  • Instructions as how to join the amino acids together - genetic code
  • An assembly line - ribosomes
  • A messenger to carry information from the DNA to the ribosomes
• Protein synthesis


  1. Initiation

  2. Transcription

  3. Translation
• DNA makes RNA makes Protein
• RNA


  • Ribose: smaller sugar than deoxyribose of DNA
  • Phosphate
  • 4 Nitrogenous Bases A,G,U,C
  • Single stranded and thus smaller & able to leave the nucleus of the cell
• Stages of transcription


  • Enzymes in nucleus start to unwind the DNA double helix at the site of the gene that is going to produce a protein
  • Only 1 DNA strand serves as the template
  • Complementary RNA bases join to the exposed DNA strand to form mRNA.
• Codon


  A sequence of three bases of DNA or RNA
• Each codon will eventually cause one amino acid to become part of the protein being made
• What an mRNA molecule carries

  • A start codon (e.g. AUG)
  • A series of codons to specify particular amino acids
  • A stop codon (e.g. UAA)
• Translation


  1. mRNA moves from the nucleus to the cytoplasm

  2. Ribosomes are made up of ribosomal RNA (rRNA) and protein

  3. The mRNA strand forms weak bonds with the rRNA in a ribosome. This will be the site of protein synthesis

  4. The cytoplasm contains a supply of transfer RNA (tRNA) molecules

  5. Each tRNA carries a special triplet (anticodon) and a particular amino acid

  6. tRNA molecules are attracted to the mRNA that is in the ribosome. Each anticodon on a tRNA is complementary to a codon on the mRNA. The tRNA molecules enter the ribosome

  7. The first tRNA molecule will attach to the mRNA just after the start codon. In doing this it brings a particular amino acid to the ribosome

  8. The adjacent amino acids are detached from their tRNA molecules and are bonded together by the ribosome to form part of the new protein

  9. The tRNA molecules leave the ribosome without any attached amino acids. In doing so they pull the mRNA strand through the ribosome

  10. A peptide bond joins the amino acids to form a polypeptide chain

  11. The polypeptide chain gets longer

  12. This continues until a stop codon is reached. The polypeptide is then complete
• Once the protein is formed it folds to allow it to have the correct shape
• Types of RNA


  • Messenger RNA (mRNA)
  • Transfer RNA (tRNA)
  • Ribosome RNA (rRNA)
• All types of RNA are produced in the nucleus
• Anticodon


  A special triplet on a tRNA molecule that is complementary to a codon on the mRNA
• The anticodon UAC belongs to a tRNA that recognizes and binds to a particular amino acid


  The DNA base code for this amino acid is TAC
• When pollen grains have matured the walls of the anther dry and split. Pollen grains are then exposed and are ready for pollination.
• After pollination


  1. The generative nucleus (n) divides by mitosis to form two sperm nuclei (n)

  2. These will each play a role in double fertilisation
• Pollen Grain


  • Exine = Outer Protective Layer
  • Intine = Inner Protective Layer
• Ovule


  • Each ovary contains one or more ovules
  • An ovary has two walls called integuments
  • Between the integuments is a small opening (micropyle) through which a pollen tube can enter
  • The nucellus provides nutrients for growth in the ovule
• Embryo sac development


  1. One cell in the ovule, is called the megaspore mother cell (2n). It divides by meiosis to form four haploid cells

  2. Three of these cells disintegrate

  3. The remaining cell is called the embryo sac (n)
• Embryo sac development


  1. The embryo sac (n) then divides by mitosis three times forming eight haploid nuclei

  2. Five of these nuclei will later die

  3. The remaining three are the gametes - two polar nuclei (n) and one egg cell (n)
• Pollination


  Transfer of pollen from the anther to the stigma of a flower of the same species