1. Chapter 15: Protein Synthesis
Leaving Certificate Biology
Higher Level

2. Protein Synthesis: DNA versus RNA
Double-stranded
Deoxyribose
Nucleotides:
Adenine
Thymine
Guanine
Cytosine
Protein synthesis is carried out in three distinct stages: transcription; translation; and protein folding
RNA:
Single-stranded
Ribose
Nucleotides:
Adenine
Uracil
Guanine
Cytosine

3. 1: Transcription
Enzymes unwind the double helix and separate the two strands by breaking the hydrogen bonds between the nitrogenous bases only in the region where the gene to be transcribed is located
RNA polymerase synthesises messenger RNA (mRNA) using one of the strands of DNA as a template
RNA polymerase

4. 2: Translation
mRNA moves to cytoplasm and combines with a ribosome made up of two ribosomal RNA (rRNA) subunits
Each 3-base (triplet) sequence of the mRNA is a codon that specifies either a start codon, a particular amino acid, or a stop codon
Transfer RNA (tRNA) is another type of RNA – it is found free-floating in the cytoplasm and is responsible for carrying one amino acid
Remember amino acids are the building blocks of proteins
tRNA has a 3-base (triplet) sequence called the anti-codon that is complementary to a particular codon on the mRNA

5. Step 3: Translation (cont.)
Each tRNA in turn (with its own amino acid) lines up with the mRNA in the ribosome and an amino acids are joined together in a long polypeptide line which will form the protein
This process continues until the stop codon on the mRNA is reached at which point all the translation machinery separates and the protein is released and it folds up into its active form and goes and carries out its specific function

6. Translation

7. Translation (continued)

8. Transcription is the making of mRNA using a DNA template
Translation is the making of a protein using the code in mRNA