There are 2 types of nucleic acid: DNA (Deoxyribonucleic acid) & RNA (Ribonucleic acid).
- basic units of nucleic acids are nucleotides
- A nucleotide is made up of:
1. a pentose sugar (deoxyribose for DNA, ribose for RNA)
2. a nitrogenous base - 3. a phosphate group
What is DNA molecule?
- DNA molecules can be found inside the nucleus, the mitochondria & chloroplast organelles.
- The sequence of nitrogenous base in a DNA molecule is called gene.
- A DNA molecule contains thousand of genes which code for the synthesis of specific proteins. (eg: enzyme)
- A DNA molecule consists of two polynucleotide strands coiled together/twisted around each other to form a DOUBLE HELIX. (above diagram)
How are the nucleotides linked?
Each nucleotide is joined to the next nucleotide through the phosphate group to form a long polynucleotide strand.
Diagram shows 2 polynucleotide strands arranged in anti-parallel. 
In 1953 James D. Watson and Francis Crick suggested what is now accepted as the first correct double-helix model of DNA structure.
What is RNA?
- can be found in the nucleus, ribosomes & cytoplasm.
- there are 3 types of RNA (mRNA, rRNA, tRNA) - this is not covered under spm syllabus.
- messenger RNA (mRNA) carries DNA's genetic code into the cytoplasm & direct the synthesis of proteins.
What are the differences between DNA & RNA?
1. DNA consists of 2 strands of polynucleotide (that coiled together to form a double helix) while RNA consists of only one strand of polynucleotide.
2. The pentose sugar in DNA is deoxyribose while in RNA it is ribose.
3. types of nitrogenous bases - you will learn in form 5.
