REVISION: DNA STRUCTURE & REPLICATION Topic 3 IB Biology Miss Werba

DNA STRUCTURE

3.3.1 NUCLEOTIDE STRUCTURE Nucleotides are the building blocks of RNA and DNA Each nucleotide consists of : a nitrogenous base (either Adenine, Thymine, Cytosine, Guanine or Uracil) a pentose sugar (either deoxyribose or ribose sugar) a phosphate group J WERBA – IB BIOLOGY 3

NUCLEOTIDE STRUCTURE Purines = 2 ring bases = A and G 3.3.1 NUCLEOTIDE STRUCTURE Covalent bonds link each part of the nucleotide to the other Covalent bonds also link each nucleotide to one another Purines = 2 ring bases = A and G Pyrimidines = 1 ring bases = C, T and U J WERBA – IB BIOLOGY 4

COMPLEMENTARY BASE PAIRING 3.3.2 3.3.4 COMPLEMENTARY BASE PAIRING Base pairing rule across the strands is: Adenine always joins with Thymine (A=T) Guanine always joins with Cytosine (C≡G) This is due to the length of the bases and the number of hydrogen bonding sites Uracil replaces thymine in RNA only!!!!!!!!!!!!!!! DNA contains A, T, G & C RNA contains A, U, G & C J WERBA – IB BIOLOGY 5

DOUBLE HELIX STRUCTURE 3.3.3 3.3.4 3.3.5 DOUBLE HELIX STRUCTURE Nucleotides form long chains or strands 2 strands form a twisted ladder  double helix J WERBA – IB BIOLOGY 6

DOUBLE HELIX STRUCTURE 3.3.3 3.3.4 3.3.5 DOUBLE HELIX STRUCTURE Sugar-phosphate backbone runs along the sides of the ladder: Made of alternating sugar and phosphate molecules Held together by strong covalent bonds Nitrogen base pairs runs along the centre: Follow base pairing rule Held together by weak hydrogen bonds J WERBA – IB BIOLOGY 7

DNA REPLICATION

PROCESS DNA double helix “unwinds” Two strands of DNA “unzip” 3.4.1 PROCESS DNA double helix “unwinds” Two strands of DNA “unzip” helicase enzyme catalyses this step helicase breaks the hydrogen bonds between the bases J WERBA – IB BIOLOGY 9

PROCESS Free DNA nucleotides pair up with exposed bases 3.4.1 PROCESS Free DNA nucleotides pair up with exposed bases nucleotides float freely within the nucleus form complementary pairs with the nucleotides of both strands of DNA A new complementary strand forms nucleotides react with each other & form a strand DNA polymerase enzyme catalyses this step J WERBA – IB BIOLOGY 10

3.4.1 3.4.3 RESULT The result of DNA replication are two identical DNA double helices, each consisting of: one “old” strand and one “new” strand DNA replication is therefore referred to as being semi- conservative (b/c part is conserved, part is new) J WERBA – IB BIOLOGY 11

CONSERVATION OF THE CODE 3.4.2 CONSERVATION OF THE CODE Complementary base pairing means that the new strand of DNA will match one strand of the parent DNA molecule. Means that exact copies of the DNA can be made in preparation for cell division. Mistakes resulting from errors in replication are called mutations. J WERBA – IB BIOLOGY 12

Ref: Biology for the IB Diploma, Allott

Ref: IB Biology, OSC