Nucleic Acids

Jon Wilson

Cell Physiology

Structure of DNA

DNA is a double helix made of complimentary antiparallel strands

The backbone is constructed of a five carbon sugar and a phosphate group

The rungs consist of the different bases

Structure of Nucleic Acids

DNA is a five carbon sugar with two hydrogen’s on the second carbon

RNA is a five carbon sugar with a hydrogen and hydroxyl group on the second carbon

Structure of the DNA bases

Two classes of bases: Purines and Pyrimidines

Purines consist of Adenine (A) and Guanine (G)

Pyrimidines consist of Thymine (T) and Cytosine (C)

Bases are the same in RNA except that Uracil (U) is substituted for Thymine

Backbone linkage Any nucleotide can be

connected with a phosphodiester bond

Nucleotides are triphosphated

The 5’ ends with a phosphate and the 3’ ends with a hydroxyl

The phosphate is attached to the 5’ carbon and the 3’ carbon of the sugar

The bases are attached to the first carbon

DNA Linkage

Helix is anti-parallel and complimentary

Left side is from 5’ end to 3’ end

Right side is from 3’ end to 5’ end

A matches with TStabilized by two hydrogen

bonds C matches with GStabilized by three hydrogen

bonds

DNA Replication

Replication is semi-conservative

Each strand is a template for another strand

New DNA strands contain one new strand and one parental strand

Bases on strands are complimentary (A w/ T & C w/ G)

DNA Replication DNA synthesizes in one direction, from the 5’ end

to the 3’ OH end Many enzymes contribute in the replication of DNAHelicases- unwind the DNATopoisomerases- releases the tensionSingle Stranded Binding Proteins- maintain the

single strands after unwindingDNA polymerases- add nucleotides and reads the

template strand. There are five polymerases for mammals. Must have a 3’ OH end

DNA Replication

DNA polymerase cannot initiate the synthesis of new DNA, it can only extend a chain

RNA polymerase do not require a 3’ OH end A RNA polymerase called primase synthesis

a piece of RNA on the DNA template This hybrid of RNA-DNA is called the primer The primer produces the 3’ OH end needed

for DNA polymerase

Types of DNA Structures B DNAThe usual form of DNAA right handed helixHas a helical turn every 10 base

pairsHas a Major groove and a Minor

groove Z DNAHas a zig-zag appearanceHas more bases per turn than B

DNACaused by a high salt

concentration and certain proteins

Types of DNA Structures

A DNAHas 11 base pairs per turnContains a central holeAdopted by RNA-DNA &

RNA-RNA helices Triple-Helix DNAAlso called H DNACan occur in stretches

where all purines in one strand are paired up with all pyrimidines in the other strand

Summary DNA is a double helix with complementary anti-

parallel strands DNA consists of sugar, phosphate and bases There are two classes of bases: Purines and

Pyrimidines Nucleotides are connected by a phosphodiester

bond Replication is semi-conservative from the 5’ end to

the 3’ end and involves many enzymes There are four known types of DNA structures: A

DNA, B DNA, H DNA and Z DNA