El diámetro de la hélice del DNA es de 2 nm y la altura por base es de 0.34 nm.

Las bases molecularesLas bases molecularesde la Vidade la Vida

El DNAEl DNA

Estructura y Síntesis del DNAEstructura y Síntesis del DNA

PURINAS

PIRIMIDINAS

Citosina Guanina

AdeninaTimina

Uracilo

Purine Bases

Pyrimidine Bases

                       

   

                          

Nucleosides

Structure of dA

La doble hélice del DNA

Features of the 5'-d(CGAAT)

•Alternating backbone of deoxyribose and phosphodiester groups •Chain has a direction (known as polarity), 5'- to 3'- from top to bottom •Oxygens (red atoms) of phosphates are polar and negatively charged •A, G, C, and T bases can extend away from chain, and stack atop each other •Bases are hydrophobic

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

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

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

A-DNA double helix 5'-d(GpApCpCpGpCpGpGpTpC)-3'

The major groove is very deep and the minor groove

is quite shallow

Z-DNA double helix

5'-d(CpGpCpGpCpG)-3',N-(2-aminoethyl)-1,4-diaminobutane

The basic building block consists of two nucleotides, each with different conformations. Z-DNA forms excellent crystals.

B-DNA double helix Compound name 5'-d(ApCpCpGpApCpGpTpCpGpGpT)-3'

Sequence in asymmetric unit

B-DNA is a right-handed double helix with a

wide and narrow groove. Tha bases are perpendicular

to the helix axis.

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

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