1. Part III: Nucleic Acids
Biomacromolecules
Part III: Nucleic Acids

2. 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.

3. 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.

4. FUNCTIONAL LOCATION IN PROKARYOTIC CELL
NUCLEIC ACID
FUNCTIONAL 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
rRNA
Within ribosomes in the cytoplasm

5. What are nucleotides?
Nucleotides are the monomers that make up nucleic acids.
Each nucleotide is made up of three components:
A pentose sugar
A phosphate group with a negative charge
A nitrogen base

6. 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 b-D-ribose
The sugar used in DNA is b-D-deoxyribose
There is one less oxygen on the sugar used for DNA hence the name deoxyribonucleic acid.

7. 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.

8. 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.

9. Purines and Pyrimidines
MEMORY AID: PURLAG
PURines are Larger,
Adenine and Guanine

10. 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.

11. 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.

12. Ribonucleic Acid (RNA)
Multiple forms – all produced in the nucleus from a DNA template.
Messenger RNA (mRNA)
Transfer RNA (tRNA)
Ribosomal RNA (rRNA)

13. 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).

14. 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.

15. Ribosomal RNA (rRNA) Structural component of ribosomes
Works with ribosomal proteins to catalyse the assembly of amino acids into polypeptides.

16. 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.

17. 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.

18. 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