20.1 DNA STRUCTURE AND

REPLICATION

What are the 4 nitrogenous bases?

Which bases bond together?

REMEMBER

• Complementary base pairing – the pairing of bases from one strand of DNA with its sister strand

• Adenine = Thymine• Guanine = Cytosine• There are enough hydrogen bonds to

make the double-stranded DNA molecule very stable

DNA STRUCTURE

• The sequence of DNA strands will always be complementary

5’AATGCTAGC3’3’TTACGATCG5’

• The strands are called antiparallel because they align in opposite directions

• The 5’ carbon always attaches to the phosphate group• The 3’ end will have the hydroxyl group of the sugar at

its end• We always talk in order of 5’ to 3’

THE 2 STRANDS

Copy down this segment and draw the complementary strand for the

segment. Show directionality

5’ – TAGGGCTTATG – 3’

QUICK PRACTICE

• DNA replication occurs through similar mechanisms in both prokaryotic and eukaryotic cells

• DNA replication is when DNA replicates itself exactly

• DNA replication is semi-conservative• Semi-conservative – a parent strand is

separated and a daughter strand is created from each. Therefore each new strand has one strand of the parent DNA and one strand of the daughter DNA

DNA REPLICATION

1)The double-stranded DNA is “unzipped”

2)Each strand is copied – the parent strands are called the templates

3)Errors are repaired

BASIC STEPS OF DNA REPLICATION

NOTE: any molecule ending in ase is an enzyme working on a specific thing

a) DNA helicase attached to the DNA molecule and unwinds it by breaking the hydrogen bonds

b) Proteins bind to the exposed DNA to keep the strands separate

• Where the DNA opens it is called the replication fork

• At the replication fork one strand of DNA runs from 5’ to 3’ while the other runs from 3’ to 5’

1) SEPARATING THE DNA

a) A short strand of RNA, a primer, attaches to the template strand

b)DNA polymerase III attaches to the primer (acts as the starting point) and builds a new strand of DNA by linking together free nucleotides that “match” the parent nucleotides

• DNA polymerase III can only build in a 5’ 3’ direction

2) COPYING THE STRANDS

• The leading strand of the DNA is the one that is built from 5’ to 3’ in a continual strand going “into” the replication fork

• The lagging strand needs to be built in small pieces – called Okazaki fragments going away from the fork

• The Okazaki fragments are made starting with a RNA primer where the polymerase can attach, and the polymerase creates until it runs into the next RNA primer – here it detaches and starts all over on the next Okazaki fragment

c) After the leading and lagging strands are copied (or pieces of them), DNA Polymerase 1 replaces the RNA primers with DNAd) DNA ligase – “glues” the fragments together to make an unbroken strand

Check out figure 5 on page 665http://www.youtube.com/watch?v=OKBVDCpAipU&feature=related

• Both DNA polymerase III and I check the bonding of the newly synthesized strand

• If there is a mistake the polymerase can backtrack, cut it out, and then continue adding from there

• The repair should be made asap• There are more repair mechanisms

done later on

3) DNA REPAIR

p. 666 #2-7 – hand in

TO DO: