Nucleic Acid

The structure of DNA and RNA

5C sugar

The structure of a nucleotide

P

Organic base

DNA and RNA are polymers of nucleotides

There are two types of bases

Purines

are double ring structures

Adenine and

Guanine

are both purines

The bases in nucleotides can differ

A or G

Purines and pyrimidines

Pyrimidines are single ring structures

CTU

Cytosine, thymine and uracil are pyrimidines

More simply shown

A

C

T

G

Nucleotides join together to form a polymer

The bases stick out to the side

The nucleotides can be in any order

The phosphate group of one nucleotide joins to the pentose sugar of the next

The structure of DNA

DNA is made up of four types of DNA nucleotides

DNA nucleotides contain deoxyribose sugar

The DNA nucleotides contain the basesadenine

thymine

cytosine

guanine

C

GT

A

DNA has two chains of nucleotides

The two chains are joined by the bases

The base pairing is specific and complementaryAdenine always joins with thymine

Guanine always joins with cytosine

Purines always join with pyrimidinesThis makes the molecule the same width along its length

Bases are joined by H bonds

If two pyrimidines pair the molecule would be thinner

The two backbone strands of sugar phosphate are the same distance apart

If two purines pair the molecule would be thicker

Complementary base pairing

A T

G C

A

G

T

C

The bases are joined by hydrogen bonds

2 H bonds between adenine and thymine

A

T

G

C

3 H bonds between guanine and cytosine

To pair one of the nucleotides must flip over

T

A

A C

G

T

G

A

C

T

G

C5

3

3

5

The two strands run anti-parallel

The two strands are twisted into a double helix

The helical shape and the H bonds help to keep DNA stable

Subject to copyright clearance a suitable image showing a double helix could be inserted here.

e.g. one similar to that found at:

http://library.thinkquest.org/3659/orgchem/dna.gif

Sugar–phosphate backbone

The two strands are joined by the bases

The double helix

Subject to copyright clearance a suitable image showing a double helix could be inserted here.

e.g. one similar to that found at:

http://library.thinkquest.org/3659/orgchem/dna.gif

The structure of RNA

Messenger RNATransfer RNA

Ribosomal RNA

Structure of RNA

RNA nucleotides

•contain the bases

adenine

uracil

cytosine

guanine

C

GU

A

The thymine in DNA is replaced by uracil in RNA

•form single strand polymers

•Contain ribose sugar

Messenger RNA (mRNA)

•is made in the nucleus from RNA nucleotides

•is involved in protein synthesis

•is relatively short lived

Transfer RNA (tRNA)

• Is a single strand folded into a cloverleaf shape• Has a specific site to which specific amino

acids can attach• There is a different type of tRNA for each type

of amino acid• Each type of tRNA has three bases which are

specific to the type of amino acid which will attach

• These three bases are called the anticodon

GCA

tRNA Specific amino acid attaches

Anticodon – specific to the amino acid which attaches to this tRNA molecule

More simply drawn

Draw up a table of differences between DNA and RNAClick here to go to AS Guru and build a molecule of DNA

Activities

End