3.4 & 7.2 DNA Replication

Pp 16 – 19 & 60 - 61

Pp 58 – 60 & 197 -200

How it works Prior to division: cell has to

duplicate its DNA DNA replication occurs during S

phase of interphase in preparation for mitosis cell division

DNA replication is semi – conservative i.e. 50% of daughter DNA molecule is new strand while 50% is old (original) strand

Helicase splits double strand DNA polymerase binds

nucleotides to create strand 2 new strands are formed Strands are identical to original

due to complementary base pairing

http://highered.mcgraw-hill.com/olc/dl/120076/micro04.swf

http://highered.mcgraw-hill.com/olc/dl/120076/bio23.swf

significance of complementary base pairing Adenine forms

hydrogen bonds with Thymine and Guanine form hydrogen bonds with Cytosine

complementary base pairing ensures proper bases are incorporated into the new DNA strand

This ensures conservation of the base sequence by making identical copies of the DNA strand

Semi-conservative DNA Replication

http://bcs.whfreeman.com/thelifewire/content/chp11/1102002.html

Direction of DNA ReplicationDNA replication occurs in a 5’ to 3’ direction!

Enzymes drive replicationHelicase:- uncoils DNA & splits

the complimentary strandsDNA polymerase III:- adds

nucleotides in 5’ → 3’ direction (leading strand) – movement towards helicase (replication fork)

RNA primase:- adds short RNA primer strand to DNA (primer = allows DNA polymerase to start on lagging strand)

DNA polymerase I:- removes RNA primer & replaces it with DNA leaving gaps between Okazaki fragments (i.e. short lengths of DNA formed between RNA primers)

DNA ligase:- seals up gap (creates sugar-P bond) between Okazaki fragments

Okazaki fragmentsIn eukaryotes,

replication is initiated at many points concurrently.

On lagging strand, DNA replication occurs away from the replication fork, forming Okazaki fragments (i.e. short lengths of DNA formed between RNA primers)

Process of DNA replication DNA replication occurs during S

phase of interphase in preparation cell division

DNA replication is semi-conservative i.e. the new DNA molecule has one new strand & one old strand

Enzyme helicase unwinds the double helix & separates the strands by breaking the hydrogen bonds between the two complimentary strands

each strand of parent DNA is used as template for synthesis of a new strand

synthesis of a new strand is continuous on leading strand but not continuous on lagging strand

Okazaki fragments (small strands of DNA) are formed on lagging strand

synthesis of new DNA strand occurs in 5’ → 3’ direction i.e. new nucleotides are added to 3’ of the growing strand

RNA primer synthesized on parent DNA using RNA primase to act as the starting point of synthesing a new strand

DNA polymerase III adds the nucleotides to the 3 end according to complementary base pairing

adenine pairs with thymine and cytosine pairs with guanine

DNA polymerase I removes the RNA primers and replaces them with DNA

DNA ligase joins Okazaki fragments

as deoxynucleoside triphosphate joins with growing DNA chain, two phosphates are broken off releasing energy required for bond formation

http://www.mcb.harvard.edu/losick/images/trombonefinald.swf

Revision QuestionsExplain the significance

of complementary base pairing during DNA replication. [3]

State a role for each of five different named enzymes in DNA replication. [5]

Explain the process of DNA replication. [8]