IB Biology3 Chemistry of Life

3.4 DNA Replication

Jason de Nys

All syllabus statements ©IBO 2007All images CC or public domain or link to original material.

http://www.flickr.com/photos/sblackley/3572873378/

This is a helicase molecule from E. coli

The ‘-ase’ suffix should clue you in that this is an enzyme.

It’s job is to break the hydrogen bonds between the strands of DNA and unwind them

http://en.wikipedia.org/wiki/File:Helicase.png

3.4.1 Explain DNA replication in terms of unwinding the double helix and separating the strands by helicase, followed by the formation of the new complementary strands by DNA polymerase

Meet polymerase. This is DNA polymerase from a human.

It’s job is to help the right bases attach to the separated

strands of DNA and join the bases together to make the DNA double-stranded again

What sort of reaction is the polymerisation reaction?

The polymerisation reaction is a

condensation reaction

http://commons.wikimedia.org/wiki/File:DNA_replication_split_horizontal.svg

DNA Polymerase adds bases from the surrounding “soup” to the separated strands

Helicase

DNA Polymerase

http://commons.wikimedia.org/wiki/File:DNA_replication_split_horizontal.svg

You’ll notice that there is more than one DNA polymerase at work

Helicase

DNA Polymerase

http://commons.wikimedia.org/wiki/File:DNA_replication_split_horizontal.svg

You’ll notice that there is more than one DNA polymerase at work.This is explained in 7.2 and is not required at SL

Helicase

DNA Polymerase

3.4.2 Explain the significance of complementary base pairing in the conservation of the base sequence of DNA

http://commons.wikimedia.org/wiki/File:Base_pair_AT.svghttp://commons.wikimedia.org/wiki/File:Base_pair_GC.svg

A fits T and C fits G, they are complementary

The way the molecules fit together makes it very unlikely that they will bond with the wrong partner.So the genetic code is faithfully copied during replication

When things do go wrong, we have a point mutation. (See 4.1.3 and 4.1.4)

2 H bonds

3 H bonds

3.4.3 State that DNA replication is semi-conservative.

Semi-conservative?

http://commons.wikimedia.org/wiki/File:Mitt_Romney_2012_CPAC.jpg

Semi-conservative?Means: half

In this case, means: conserved or saved

There are three ways that DNA could be copied:

http://en.wikipedia.org/wiki/File:DNAreplicationModes.png

Each time DNA is copied, the new double stranded molecule consists of one old template strand plus a new complementary strand made

from previously free bases

http://www.flickr.com/photos/pagedooley/5283257753/

Further information:

Three of the best sites for IB-specific Biology information. The top link takes you to the PPT by Stephen Taylor