Nucleic Acids

• Polymer

• Individual unit called a (mono)nucleotide

• DNA is a type of nucleic acid

– deoxyribonucleic acid

DNA is a polymer made up of

individual units called nucleotides

Nucleotide Structure3 components:

– Pentose sugar (ribose in DNA)

– Phosphoric acid– Organic base (always contains nitrogen)

Phosphate

sugarbaseStay the same

ChangesContains nitrogen & carbon

Pentose sugar (5 Carbon atoms)

4 Bases• Purines –

– Double ringed structure

• Pyrimidines – – Single ringed structure

Bondings

• The base and sugar join with a:

glycosidic bond

• The phosphate and sugar join with an:

ester bond

Both require a condensation reaction to occur

H

H

OH

OH

Phosphate

Sugar Base

2 condensation reactions occur in the formation of a nucleotide:

Ester bond

Glycosidic bond

Tell me…4 names of bases in DNA3 components of a nucleotide2 ends of a strand are called1 place where a condensation

reaction occurs

2 types of bonds in a nucleotide1 difference between purine and

pyrimidine bases

Compl mentary Base Pairing

*Purines always pair with pyrimidines*

A joins to T (2 hydrogen bonds)C joins to G (3 hydrogen bonds)

Therefore, when discussing the proportions of a particular base, you will always find A=T and C=G

or! A+C = G+T

see

Type of Base Purine Pyrimidine

Structure

Bonding relationships

Adenine

Guanine

Thymine

Cytosine

= hydrogen bond

C

A

G

T

2 hydrogen bonds

3 hydrogen bonds5’

3’

3’

5’

0.34nm

2nm

Phosphodiester Bonds:the sugar-phosphate-backbone

C

A

G

T

5’

3’

3’

5’

0.34nm

2nm

A T

C G

5C

5C

5C

3C

3C

5C

3C

5C

5C

5C

5C

3C

3C

3C

C

A

G

T

2 hydrogen bonds

3 hydrogen bonds

5’

3’

3’

5’

0.34nm

2nm

A T

C G

Strands run anti parallel

Points to remember:• DNA stands for deoxyribonucleic acid

• It is a polymer of mono

• Each nucleotide has three components:• pentose sugar (deoxyribose)• phosphate group• organic base (4 types - purines/pyrimidines)

• 2 condensation reactions occur when the• base and sugar form a glycosidic bond• phosphate and sugar form an ester bond

• Phosphodiester bonds build a sugar phosphaphate backbone on each strand of DNA, with the bases all facing inwards

• The 2 strands of the DNA molecule form anti-parallel to one another; one strand runs 5’ to 3’ and the other strand runs 3’ to 5’

• The strands wind around one another into a double helix (like a twisted ladder)

4 Bases• Purines –

– Double ringed structure

• Pyrimidines – – Single ringed structure

DNA Replication – why?

• DNA carries the genetic code in the order of its bases (more info to come)

• To pass this information on the molecule needs to replicate itself

• DNA replication always occurs just before mitosis. This way the cell can make an exact copy of itself

•When might this occur in the cell?

Movie time!http://207.207.4.198/pub/flash/24/menu.swf

Key words to look out for:– Replication fork– Template – Complementary base pairing– Leading strand– Lagging strand– Okazaki fragments– Enzymes:

– helicase – dna polymerase– ligase

Summarise into 3 stages

Semi-conservative Replication

½ of the old strand of DNA is conserved in each new strand and

the 2 new strands are identical