What you need to know:  How is DNA replication preserve the sequence of bases?  What is semi-conservative replication?  How is DNA different from RNA? DNA Replication

Theories of DNA replication Conservative Original DNA double helix acts as a template for producing a new template The original double helix is conserved

Destructive Nucleotides from the original strands end up in all daughter strands The original double helix is destroyed

Semi Conservative... the strands are separated and each daughter double helix inherits one original strand The original DNA strands are conserved, but...

adenine guanine cytosine tyrosine How do we know which theory is correct? Note: all bases contain nitrogen The Meselson-Stahl Experiment

 Bacteria grown on a source of heavy nitrogen ( 15 N) produces heavy DNA  Bacteria grown on a source of light nitrogen ( 14 N) produces light DNA  We can measure the weight of the DNA in a cell by extracting it and then centrifuging it in a caesium chloride gradient

DNA sample CsCl gradient Low CsCl conc. High CsCl conc. Centrifuge:spin very fast When centrifuged molecules migrate until they reach a layer of the same density

Bacteria fed 14 NBacteria fed 15 NBacteria fed 15 N then one generation on 14 N Intermediate band implies that DNA replication is not conservative Light band Heavy band

Bacteria fed 15 N then two generations on 14 N Further generations on 14 N  Light band gets bigger  Intermediate band remains the same size  Heavy band does not reappear

DNA replication is semi conservative The two original strands act as templates for the synthesis of two new strands ACT TAG ACT TAG ACT TAG ACT TAG ACT TAG Free nucleotides pair with complementary bases Template strand Pairing of complementary bases preserves the sequence of bases when DNA replicates

A C T TAG A C T TAG A C T TAG A C T TAG A C T TAG The enzyme Helicase: - unwinds the double helix - breaks the hydrogen bonds between complementary bases - separates the two strands The Details:

A C TTAGA C TTAG A C TA C TA C T TAGTAGTAG Free nucleotides pair up with complementary bases, and are held in place, by hydrogen bonding Helicase

A C TTAGA C TTAG A C TA C TA C T TAGTAGTAG DNA Polymerase creates two new strands of DNA by creating covalent bond between adjacent nucleotides TAG TAG T A G A C TA C TA C T

A C TTAGA C TTAG A C TA C TA C T TAGTAGTAG DNA Polymerase TAG TAG T A G A C TA C TA C T New DNA strand New covalent bonds

RNA – Ribose Nucleic Acid Phosphate Ribose (Pentose sugar) Base cytosine (C) guanine (G) adenine (A) uracil (U) RNA is single stranded

 Function of RNA – to form a copy of a gene which can be used by ribosomes as a template for synthesising a polypeptide  Each gene is a template for the production of one polypeptide  The copying of a gene from DNA to RNA is called transcription  The ‘reading’ of a RNA copy of a gene and synthesis of a polypeptide chain by the ribosome is called translation

Homework:  Outline the process of DNA transcription  Explain what a codon is