BiomacromoleculesPt III: Nucleic Acids

Nucleic acids• Linear polymers made up of

monomers called nucleotides.• They are of critical importance to

the cell because of their roles in the storage, transmission and expression of genetic information.

• They are essentially information molecules.

Two types of nucleic acid• There are two major types of

nucleic acid:– Deoxyribonucleic acid (DNA)– Ribonucleic acid (RNA)

• They differ in their chemical components and the role they play in the cell.

NUCLEIC ACIDFUNCTIONAL LOCATION IN PROKARYOTIC

CELL

FUNCTIONAL LOCATION IN

EUKARYOTIC CELL

DNA On a single chromosome in the cytoplasm.As circular structures called plasmids.

On linear structures called chromosomes in the nucleus.In mitochondria.In chloroplasts (plant cells)

RNA mRNA Cytoplasm Nucleus and cytoplasm

tRNA Cytoplasm Cytoplasm

rRNA Within ribosomes in the cytoplasm

Within ribosomes in the cytoplasm

What are nucleotides?

• Each nucleotide is made up of three components:– A pentose sugar – A phosphate group

with a negative charge

– A nitrogen base

• Nucleotides are the monomers that make up nucleic acids.

Pentose sugar• Both types of nucleic acid (DNA and RNA)

contain a pentose sugar.• Pentose sugars contain 5 carbons.

– The sugar used in RNA is -D-ribose– The sugar used in DNA is -D-deoxyribose

• There is one less oxygen on the sugar used for DNA hence the name deoxyribonucleic acid.

Phosphate group• Located on C5 of the

pentose sugar.• The phosphate group on C5

of one sugar reacts with the OH group on C3 on the adjacent sugar to link monomers together.

• This process is known as condensation polymerisation as a H2O molecule is eliminated.

• The resulting bond linking the two nucleotides is called a phosphodiester bond.

Nitrogen bases• Two families of nitrogen bases: the

purines and pyrimidines.

• Purines – two ring structure– adenine (A) & guanine (G)

• Pyrimidines – single ring structure– cytosine (C), thymine (T) and uracil (U)– Uracil only found in RNA, it replaces thymine

which is only found in DNA.

Purines and Pyrimidines

MEMORY AID: PURLAGPURines are Larger,

Adenine and Guanine

Pairing between nitrogen bases• Bases contain many nitrogen

atoms and an oxygen functional group that are capable of forming hydrogen bonds between bases.

• Purines and pyrimidines have a complementary relationship – a purine always pairs with a pyrimidine.– A forms two hydrogen bonds with T

(or U)– G forms three hydrogen bonds with

C• This base pairing is a fundamental

property of nucleic acids and provides the mechanism for the coding of genetic information.

Ribonucleic Acid (RNA)• Single-stranded polynucleotide.• Contains uracil instead of thymine.• Nucleotides are added in a 5 to 3

direction– ie the phosphate group on the 5’

carbon reacts with the OH group on the 3’ carbon of the previous nucleotide to form a phosphodiester bond between the nucleotides.

Ribonucleic Acid (RNA)• Multiple forms – all produced in the

nucleus from a DNA template.– Messenger RNA (mRNA)– Transfer RNA (tRNA)– Ribosomal RNA (rRNA)

Messenger RNA (mRNA)• Formed in nucleus from DNA

template stand.• Complementary copy of DNA

nucleotide sequence.• mRNA carries the genetic

information in a series of three nitrogen base code “words” to a ribosome in the cytoplasm.

• Once it reaches the ribosome the coded information is translated into specific amino acids that form a polypeptide (protein).

Transfer RNA (tRNA)• Smallest of all RNA

molecules.• Make up about 10-

20% of total cellular RNA.

• Position amino acids in their correct order on the ribosome for condensation polymerisation into a polypeptide.

• Each different amino acid has a specific tRNA molecule.

Ribosomal RNA (rRNA)• Structural component of

ribosomes• Works with ribosomal

proteins to catalyse the assembly of amino acids into polypeptides.

Deoxyribonucleic acid (DNA)• Double stranded

polynucleotides• Two strands with

complementary nitrogen base sequences pair to form a double helical structure.

• Base pairs are held together by hydrogen bonding.

• For base pairing to occur, the two strands run in opposite directions. We say the strands are antiparallel.

Deoxyribonucleic acid (DNA)• The two complementary

strands twist around a common axis to form a right-handed helical structure.

• The alternating sugar phosphate groups form a ‘backbone’ and are highly polar.

• They are on the outside of the DNA molecule where their interactions with water can be maximised.

• If we know the order of bases on one strand we also know the order of bases on the complementary strand.

Memory Aids for DNA• Atoms in DNA - PONCH

– Phosphorous , Oxygen, Nitrogen, Carbon, Hydrogen

• Nucleotide - t is for three parts– Base, sugar and phosphate.

• Directionality of DNA molecule – (POH)– 5’ is phosphate (P) end, 3’ is

hydroxyl (OH)– 3 comes before 5 & hydroxyl

before phosphate– New nucleotides always

added to 3’ end so POH –POH-POH

• Difference between purines and pyrimidines – PURLAG– PURines are Larger, Adenine

and Guanine

• To remember pyrimidines – CUT– Smaller – have only one ring

so they have been CUT– Cytosine, uracil, thymine

• Number of hydrogen bonds between bases – GCAT32– G & C have 3– A & T have 2