Take me to NZQA Documents relating to this standard AS Demonstrate understanding of life processes at the cellular level The Replication of DNA

Contents Why replicate? The basics The enzymes Okazaki fragments Fixing errors Review Questions

What we need to know! Describe the structure of DNA Double helix Deoxyribonucleotides A,G,C,T Anti-parallel structure Use the base pairing rule to write a complementary strand of DNA Define mitosis/ meiosis and state whey they are important Describe the process of DNA replication Okazaki Fragments Semi- conservative replication Leading strand and lagging strand

DNA Structure – the basics 3D animation

Name the bits

Why replicate? DNA replicates before it forms a chromatid. I.e. when it still looks like… Over an organisms life the DNA in the zygote is copied trillions of times with minimal error. Error rates are generally 1 in 50 million base pairs (bp). Stuff that any respectful Bio student really should know… Mitosis and Meiosis What are they and where they happen! Meiosis Revision exercise When we copy this 35,000,000,000,000 times it turns into this

Why replicate? Each new cell must have a copy of the entire DNA genome. Some of the DNA released from a single human chromosome

DNA Replication - the very basics The basics of DNA REPLICATION Unwind – Unzip – Add nucleotides – wind it all back up. If only it could be that simple Two things make it a little more fiddly DNA is a VERY LONG double helix chemical molecule It has a anti-parallel structure SciPAD – Page 64

Anti-parallel structure of DNA The molecule of DNA in a single human chromosome ranges in size from 50,000,000 bp (1.7 cm) up to 250,000,000 bp (8.5 cm). Ever tried unraveling a long bit of string? Check the DNA length calculator Check the DNA length calculator It’s a long Double Helix

DNA replication forks Because the DNA chromosome is soooooooooo long it needs multiple replication forks working at the same time. How this is coordinated and how strands are placed back together is beyond the scope of L3 Bio (so don’t worry bout it)

DNA Replication The detail. 4 different enzymes (see next slide) DNA polymerase moves along template strand in 3’ to 5’ direction (on the parent strand). McGraw Animation You tube - Replication song. You tube - Replication song.

The Enzymes HelicaseUnwinds the DNA DNA PolymeraseAdds new nucleotides to the template strand RNA PrimaseAdds a RNA primer DNA LigaseRemoves primer and joins okazaki fragments together

Anti-parallel structure (DNA) This diagram shows a lot of stuff not yet discussed. What you need to understand at this point When an enzyme (DNA polymerase) travels along a strand of DNA it travels in the 3’ to 5’ direction of the original strand DNA Polymerase travels 3’ to 5’ on original strand

Okazaki Fragments An Okazaki fragment is a relatively short fragment of DNA created on the lagging strand. The lengths of Okazaki fragments are between 1,000 to 2,000 nucleotides long in E. coli and are generally between 100 to 200 nucleotides long in eukaryotes. Each Okazaki fragment is joined together by DNA ligase after the primers have been removed. Replication animation crash course in DNA (summary)

Semi-conservative replication A simple idea really… S.C.R is simply the formation of two double helix molecules where each molecule contains one of the original strands and one new strand of nucleotides (daughter strand). This helps to minimise the errors made in replication as each molecule contains a copy of the original nucleotide sequence.

Key words: Nucleotide Okazaki Fragments Semi conservative replication Leading and lagging strands Summary

Review Question What will be the sequence of bases on the new strand? Original strandNew strand A T C T G A T A G C T

Review Question When DNA is replicated, there is a lagging strand and a leading strand. Explain these two terms and why they are nessasary in DNA replication? You may use a labelled diagram to support your answer.

Review Question Add the 3’ and 5’ labels to the diagram Click for answers

Review Question