Vsauce Science Friday http://www.youtube.com/watch?v=2lR7s1Y6Zig &list=UU6nSFpj9HTCZ5t-N3Rm3-HA KWL What do you know about DNA? What do you want to know about DNA? What have I learned about DNA? Bell-Ringer What do we call the monomer that makes up DNA? What two substances make up the DNA Backbone? What are the names of the four bases that make up DNA? DNA’s Discovery and Structure Important Scientists in the “Race for the Double Helix” James Watson & Francis Crick (1953) – – Created the first accurate model of DNA Won the Nobel Prize DNA’s Structure is a double helix – this makes DNA look like a “twisted ladder”. Each molecule of DNA is composed of a “backbone” on the outside and bases that point towards the middle of the molecule. The monomer for DNA is the nucleotide DNA Nitrogenous Bases DNA is made of nitrogenous bases. These four bases are: adenine (A) – cytosine (C) – guanine (G) – thymine (T) – DNA Bases Purines – double-ringed molecule – Ex. Adenine and Guanine Pyrimidines – single-ringed molecule – Ex. Cytosine and Thymine • Complementary Base-Pairing Rules DNA is built following the complementary basepairing rules. These state that: – – – – A always pairs with T T always pairs with A C always pairs with G G always pairs with C In RNA, adenine (A) pairs with uracil (U). We will look at this in a later lesson. Sugar-Phosphate Backbone Crashcourse DNA Video http://www.youtube.com/watch?v=8kK2zwjRV0 M DNA Replication Replication = Duplication – – Happens in the S phase of Interphase Must replicate before mitosis or meiosis I Part of the double helix is unwound Replication in long and small pieces (Okazaki fragments) Enzyme stitches pieces together later Semi-Conservative Replication One-half of each new molecule of DNA is old (template strand) One-half of new molecule of DNA is new (complementary strand) DNA Replication • • 14 Enzyme Helicase unwinds and separates the 2 DNA strands by breaking the weak hydrogen bonds DNA polymerase can then add the new nucleotides copyright cmassengale 15 copyright cmassengale DNA Replication Animations 16 http://www.youtube.com/watch?v=5_vzY7H9G Yk – Simple Animation http://www.youtube.com/watch?v=teV62zrm2P 0 – Detailed Animation copyright cmassengale DNA Replication • • • Begins at Origins of Replication Two strands open forming Replication Forks (Y-shaped region) New strands grow at the forks 5’ Parental DNA Molecule 3’ 17 copyright cmassengale 3’ Replication Fork 5’ DNA Replication • • DNA polymerase can only add nucleotides to the 3’ end of the DNA This causes the NEW strand to be built in a 5’ to 3’ direction 5’ 3’ Nucleotide 18 DNA Polymerase copyright cmassengale RNA Primer 5’ Synthesis of the New DNA Strands • 5’ The Leading Strand is synthesized as a single strand from the point of origin toward the opening replication fork Nucleotides 19 DNA Polymerase copyright cmassengale RNA Primer 3’ 5’ Synthesis of the New DNA Strands • • 20 The Lagging Strand is synthesized discontinuously against overall direction of replication This strand is made in MANY short segments It is replicated from the replication fork toward the origin copyright cmassengale Lagging Strand Segments • • Okazaki Fragments - series of short segments on the lagging strand Must be joined together by an enzyme DNA Polymerase Okazaki Fragment RNA Primer 5’ 3’ 21 Lagging Strand 3’ 5’ copyright cmassengale What is a Gene? A length of DNA on a chromosome May be in several parts – – Exon – the expressed parts of the DNA sequence Intron – the intervening, “junk DNA”, not expressed Parts are cut, put together and then used to make RNA and proteins Odd Types of Genes Multigene families Transposons (Barbara McClintock) Protective Genes Viruses may give us new DNA that can be kept over time to cause new evolutionary changes!