1. Chromatin (Unit 1&4)
    1. Inside the nucleus each molecule of DNA is spooled around histone proteins. This combination of DNA and proteins is called chromatin.
  2. Chromosome: unduplicated (Units 1&4)
    1. A chromosome represents the recognisable structure formed after chromatin has condensed.
  3. Genome Mapping
  4. DNA Replication (Unit 4)
    1. DNA replication involves * the "unzipping" of the complementary strands by an enzyme called helicase at a site called the replication fork * the adding of free nucleotides to the parental template strands by DNA polymerase * Two double helices will be created- each bearing 1 parental strand and 1 newly replicated strand. This is known as the semi-conservative model of DNA replication. The process begins at regular intervals along the DNA, forming 'bubbles' of replication along the strand. * As each section meets DNA ligase seals them together to make up a continuous strand. Remembering that complementary strands run antiparallel to each other consider the following. Template strand 1: 3' --------------------5' Replicating strand: 5' ------------>3' One new strand will be continuously synthesised in the 5' > 3' direction. This is the leading strand. BUT remember that our opposing parent strand was antiparallel to the other so..... Template strand 2: 5' -------------------------- 3' Replicating strand 2: 3' <------5' 3' <------5' ? The other new strand needs to be synthesised in the opposite direction. DNA polymerase achieves this by doing it in many short fragments. This is the lagging strand. The online activity linked here is excellent but also very detailed. For Unit 4 Biology we are interested in knowing only in the terms I've mentioned here.
  5. Heredity
  6. Karyotype (Units 1&4)
    1. Chromosomes are easily recognised throughout cell division following interphase & prophase. A karyotype is a chromosome map of homologous pairs for an organism. It is usually made by photographing stained DNA during mitosis. Because karyotypes are photographed after the synthesis stage of mitosis they show double the normal quantity of DNA (2 x 2n). Two different chromosomes are considered homologous, or homologues of each other, where their DNA share the same group of genes but different alleles. They are usually the same size and shape except in the case of sex chromosomes in some organisms. A karyotype matches homologous chromosomes; essentially creating a genetic map in which chromosomal abnormalities can be identified. In the picture shown here the subject can be seen to have 3 copies of chromosome 21. This leads to a condition known as Trisomy 21, or Down Syndrome.
  7. DNA Strand Structure (Units 1,3&4)
    1. The DNA double helix is a molecule composed of 2 complementary strands of deoxyribonucleic acid. These strands are held together by the formation of hydrogen bonds between complementary nitrogenous bases Adenine - Thymine Guanine - Cytosine
  8. DNA Nucleotide Structure (Unit 3&4)
    1. These monomers have 1. a 5 carbon sugar (deoxyribose here) 2. a negatively charged phosphate group 3. an organic, nitrogen containing base that is either Adenine, Thymine, Guanine or Cytosine Online activity: http://www.dnai.org/a/index.html Select Code > Finding the Structure > Putting it Together
  9. Direction & Prime (Units 3&4)
    1. Biochemists use a carbon numbering system called prime (eg. 3') in order to refer to the position of different functional groups on an organic molecule. For nucleotides: * The 1' carbon links the nitrogenous base * Next door is the 2' carbon * 3' carbon has a hydroxyl (OH) group * Next door is the 4' carbon * The 5' carbon links off the 4' carbon and holds the phosphate group. Nucleotides link together in a 5' to 3' direction. * The phosphate group of a 5' carbon undergoes a condensation reaction * with the hydroxyl group forms a phosphodiester bond with a hydroxyl group of a 3' carbon. * This covalent bond is called a phosphodiester linkage. Thus nucleic acids are built in a 5' to 3' direction. Because nucleic acids both have a directionality and are complementary to each other the two strands of a DNA molecule run antiparallel to each other. 3' ------------------- 5' 5' ------------------- 3' This feature is significant for DNA replication because nucleic acids are synthesised in a 5' > 3' direction.
  10. Chromosome: duplicated (Units 1&4)
    1. DNA replication leads to the production of two identical copies of the DNA double helix; both wound around the same histone proteins that made up the structure of chromatin. Despite there being 2 identical copies we still refer to the overall structure as a chromosome and will continue to do so as long as they remain bound at the centromere. We call these identical copies sister chromatids.
  11. Crossing Over (Unit 4)
  12. Mitosis & Meiosis (Unit 1)