Higher Human Biology Sub topic 2b DNA Replication Higher Human Biology Sub topic 2b

Describe the stages of DNA replication Learning Intention Describe the stages of DNA replication

DNA can replicate to produce an identical copy of itself. DNA Replication DNA can replicate to produce an identical copy of itself.

3 proposed theories of how DNA replicates.... THE CONSERVATIVE MODEL

3 proposed theories of how DNA replicates.... THE DISPERSIVE MODEL

3 proposed theories of how DNA replicates 3 proposed theories of how DNA replicates.... THE SEMI - CONSERVATIVE MODEL

strands begin to separate pool of free nucleotides Very basic theory of DNA replication double-stranded DNA template strand strands begin to separate pool of free nucleotides two identical double-stranded DNA molecules produced complementary base pairing between free nucleotides and template strands backbone forms between paired nucleotides

DNA REPLICATION Requirements:- DNA to act as a template Primer Enzymes: DNA polymerase and ligase Free nucleotides ATP for energy A C T G T G A C

DNA Replication DNA replication occurs in the nucleus and involves a DNA molecule unwinding. Once the DNA has unwound, it then unzips to expose its bases. These bases then act as a template for a new DNA strand to be made.

DNA Replication As the Hydrogen bonds (between the bases) break the DNA becomes unzipped forming a replication fork.

DNA Replication A complimentary RNA PRIMER (small sequence of RNA nucleotides) binds to the start of the DNA sequence being copied. The primer joins at the 3’ end of the DNA.

DNA Replication Leading Strand Once the primer is in place the enzyme DNA polymerase can now continue the new strand by adding complimentary DNA nucleotides in a 5’ to 3’ direction. THIS IS CALLED CONTINUSOUS REPLICATION. IT FORMS THE LEADING STRAND.

DNA nucleotides pair with template strand unpaired bases elongation primer attachment replication fork enzyme not shown DNA nucleotides pair with template strand unpaired bases Base pairing with RNA primer RNA nucleotides complementary to DNA bases

DNA Replication Once the nucleotides are lined up, they join to their partner by hydrogen bonds. The DNA strand then forms a sugar phospate backbone.

DNA Replication Lagging Strand A new DNA strand can only be made 5’ to 3’. The other DNA strand has to be made in fragments – discontinuous replication. A primer must join before each fragment along the lagging strand. The enzyme LIGASE then joins the fragments together.

Double stranded DNA ready to replicate (enzyme not shown) 5’ 3’ Double stranded DNA ready to replicate (enzyme not shown) 3’ 5’ RNA primer forms Lagging strand: discontinuous replication by fragments from multiple RNA primers Leading strand: continuous replication 5’ to 3’ 3’ Short fragment forms 5’ to 3’ RNA primer forms 5’ primers removed then gaps sealed by DNA ligase Short fragment forms 5’ to 3’ 3’ 3’ RNA primer forms 3’ 5’ 3’ 5’ DNA Replication

Copied DNA 2 Daughter DNA strands Original DNA

DNA Replication The DNA strands then wind back up to form 2 identical double helix. Each double helix contains one original strand and one ‘new’ strand – SEMI-CONSERVATIVE

A RECAP OF REPLICATION. 6. Daughter DNA 5. Sugar- phosphate bonds form molecules form 5. Sugar- phosphate bonds form 4. Weak hydrogen bonds form between bases 2. Hydrogen bonds break and DNA ‘unzips’ 3. . Free nucleotides from nucleoplasm find complementary bases 1. Parent DNA unwinds A RECAP OF REPLICATION.

When copying a long chromosome many replication forks operate simultaneously to speed up the replication process.

Importance of DNA DNA replication makes an exact copy. This ensures that the correct genetic information is passed on from cell to cell and generation to generation. It is essential for the continuation of life.

Video Summary Summary (includes structure of RNA) Video

Give an account of the replication of DNA. (maximum of 7 marks): DNA is unwound and unzipped… …by the enzyme helicase… …to form two template strands. DNA polymerase is the enzyme which adds nucleotides to the new DNA strand. DNA polymerase needs a primer to start replication. DNA polymerase can only add complementary nucleotides to the deoxyribose / 3  end of the DNA strand. This results in one strand / the leading strand being continuously replicated, and the other strand / the lagging strand being replicated in fragments, which are joined together (by the enzyme ligase).