1. National 5 Biology Course Notes Part 4 : DNA and production of
Unit 1 : Cell Biology
Part 4 : DNA and production of
proteins

2. Structure of a DNA molecule
A DNA molecule is made up of many smaller DNA nucleotide molecules.
Each DNA nucleotide molecule has 3 parts
Deoxyribose sugar
Phosphate
A base
This diagram represents a DNA nucleotide
Nucleotide molecules are joined by bonds between deoxyribose and phosphate to form nucleotide strands:
Phosphate
Base
Deoxyribose
There are 4 different bases in DNA, these are:
Adenine
Thymine
Guanine
Cytosine
So there are 4 different DNA nucleotides – the difference between them is that each has a different base G A C T

3. A DNA molecule has two strands of nucleotides.
They run in opposite directions
They are held together by hydrogen bonds between complementary bases
Adenine bonds with thymine
Guanine bonds with cytosine
The two DNA strands are twisted to form a double helix
Dotted lines represent hydrogen bonds

4. DNA is the chemical of which genes are made.
Genetic code
DNA is the chemical of which genes are made.
Each person’s DNA is different because it has it’s own unique base sequence.
The sequence of bases in DNA is the genetic code.
The role of a gene is to control the production of a particular protein.
The base sequence of the gene’s DNA controls the amino acid sequence of the protein.
Messenger RNA (mRNA)
The base sequence of DNA controls the amino acid sequence of a protein but DNA is found in the nucleus while amino acids are assembled into a protein on the ribosomes.
The DNA base sequence initially acts as a set of instructions to make messenger RNA whose base sequence therefore depends on the base sequence of DNA.
Production of mRNA occurs in the nucleus.
mRNA then goes from the nucleus to the ribosomes where it’s sequence of bases acts as a set of instructions to control the amino acid sequence of the protein.
Base sequence of DNA
Base sequence of mRNA depends on the base sequence of DNA
Amino acid sequence of protein depends on mRNA base sequence
Amino acid sequence of the protein therefore depends on the base sequence of DNA