DNA Lesson 11

Success Criteria State the structure of DNA Explain how twisting of DNA molecule produces the double helix shape Outline how DNA replicates

Starter In pairs draw and label the structure of DNA

Task http://www.youtube.com/watch?v=8kK2zw jRV0M Watch the video and answer the questions

DNA Structure: Nucleotides

Antiparallel DNA Strands

Bases bond with Hydrogen Bonds Two types of bases Pyrimidine base Purine base A pyrimidine base (Thymine or Cytosine) always binds to a purine base (A or G) Bases are complementary to each other G & C bind with 3 H bonds A & T bind with 2 H bonds

DNA DNA molecules consist of 2 polynucleotide strands linked together The sequence of bases enables the DNA to store information Introns (junk DNA) are regions of DNA that enable several different proteins that share some sections in common to be produced from a single gene At some loci there are multiple repeats of certain triple letters e.g. CAGCAGCAG

Task Make a plasticene model of DNA in pairs and label

Task Ensure your homework is improved. Check you have included comparisons between RNA and DNA

Structure and Function What is the function of DNA? Information storage – in the form of codes to build proteins How does the structure ensure this function is performed? Long – Base-pairing rules – Double helix – Hydrogen bonds – large amount of information replication possible stability easy unzipping

DNA Replication

Semi Conservative DNA Replication DNA helix untwists Hydrogen bonds between bases break and the DNA ‘unzips’ Free nucleotides hydrogen bond to exposed bases Covalent bonds form between phosphate of one nucleotide and the sugar of the next Ends up with 2 identical DNA molecules

Semi conservative replication

Dna helicase Dna polymerase Breaks the hydrogen bonds connecting the bases Attaches free nucleotides to the exposed bases on each strand Dna polymerase http://www.youtube.com/watch?v=yqESR7E4b_8 Watch first 3 min of video: Chromosomes & Replication

A T C G G C C G A T C G this shows a double stranded length of DNA A T T A

helicase causes the 2 strands of DNA to separate G T G T C A helicase causes the 2 strands of DNA to separate And the separation of the strands

A C G T G T C A helicase causes the 2 strands of DNA to separate

A C G T G T C A

A C G T G T C A Free nucleotides bind to the exposed strands T A C G

A C G T G T C A free nucleotides are attracted to their complementary base pairs T A C G

A C G T G T C A free nucleotides are attracted to their complementary base pairs G A T A T C

A C G T G T C A G C T A G And the formation of the new strands C A T A T

A C G T G T C A T A G C C G G C A T G C A T A T

A C G T G T C A G T C A A C G T

A C G T G T C A G T C A A C G T finally condensation reactions join the nucleotides together…catalysed by DNA Polymerase

A C G T G T C A G T C A A C G T …forming 2 identical strands of DNA….

A C G T G T C A G T C A A C G T …because the strands each contain half of the original material…

…it is called the semi-conservative method. G T G T C A G T C A A C G T (The free nucleotides could be shown in a different colour to explain how radioactive markers were used to determine the semi-conservative method of replication) …it is called the semi-conservative method.

Task Draw a cartoon of DNA replication

Experimental work: Meselson and StaHl

Experimental work: Meselson and StaHl Grew bacteria in a medium containing heavy nitrogen (isotope 15N) Then put the bacteria in a growth medium containing light nitrogen (14N) The first generation of bacteria had DNA made from 15N, when the bacteria replicated the new half of the DNA strand was made from 14N

Copy and Complete

Answers

Evidence for semi conservative replication of dna

answers

Success Criteria State the structure of DNA Explain how twisting of DNA molecule produces the double helix shape Outline how DNA replicates

Plenary: Quick Quiz Define the term antiparallel A DNA strand have the base sequence ATTAGGCTAT, write the complementary strand A DNA molecule is 20% Thymine, what %age of each of the other types of bases would it contain? What type of diseases can result from DNA copying going wrong?

Plenary Answers Parallel, but with chains running in opposite directions TAATCCGATA 20% A, 30% C and 30% G Cancers may occur (when a mutation happens in a forming gamete genetic diseases can occur e.g. sickle cell anaemia)