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Vsauce Science Friday
http://www.youtube.com/watch?v=2lR7s1Y6Zig&list=UU6nSFpj9HTCZ5t-N3Rm3-HA
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
DNA 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
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 base-pairing 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=8kK2zwjRV0M
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 ReplicationDNA Replication
• Enzyme Enzyme HelicaseHelicase unwinds unwinds and separates the 2 DNA and separates the 2 DNA strands by breaking the strands by breaking the weak hydrogen bondsweak hydrogen bonds
• DNA polymerase can then add the new nucleotides
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DNA Replication Animations
http://www.youtube.com/watch?v=5_vzY7H9GYk – Simple Animation
http://www.youtube.com/watch?v=teV62zrm2P0 – Detailed Animation
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DNA ReplicationDNA Replication
• Begins atBegins at Origins of ReplicationOrigins of Replication• Two strands open forming Two strands open forming Replication Replication
Forks (Y-shaped region)Forks (Y-shaped region)• New strands grow at the forksNew strands grow at the forks
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ReplicationReplicationForkFork
Parental DNA MoleculeParental DNA Molecule
3’
5’
3’
5’
DNA ReplicationDNA Replication
• DNA polymeraseDNA polymerase can only add can only add nucleotides to the nucleotides to the 3’ end3’ end of the DNA of the DNA
• This causes the This causes the NEWNEW strand to be built in strand to be built in a a 5’ to 3’ direction5’ to 3’ direction
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RNARNAPrimerPrimerDNA PolymeraseDNA Polymerase
NucleotideNucleotide
5’
5’ 3’
Synthesis of the New DNA Synthesis of the New DNA StrandsStrands
• The The Leading StrandLeading Strand is synthesized as a single single strandstrand from the point of origin toward the opening replication fork
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RNARNAPrimerPrimerDNA PolymeraseDNA PolymeraseNucleotidesNucleotides
3’5’
5’
Synthesis of the New DNA Synthesis of the New DNA StrandsStrands
• The The Lagging StrandLagging Strand is is synthesized discontinuouslydiscontinuously against overall direction of replication
• This strand is made in MANY short segments It is replicated from the replication fork toward the origin
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Lagging Strand SegmentsLagging Strand Segments
• Okazaki FragmentsOkazaki Fragments - - series of short segments on the lagging strandlagging strand
• Must be joined together by an Must be joined together by an enzymeenzyme
copyright cmassengale20 Lagging Strand
RNARNAPrimerPrimer
DNADNAPolymerasePolymerase
3’
3’
5’
5’
Okazaki FragmentOkazaki Fragment
What is a Gene?
A length of DNA on a chromosomeMay 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 familiesTransposons (Barbara McClintock)Protective Genes
Viruses may give us new DNA that can be kept over time to cause new evolutionary changes!
