Transcript Chapter 15: Protein Synthesis

Chapter 15: Protein Synthesis
Leaving Certificate Biology
Higher Level
Protein Synthesis:
DNA versus RNA
• DNA:
– Double-stranded
– Deoxyribose
– Nucleotides:
• Adenine
• Thymine
• Guanine
• Cytosine
• RNA:
– Single-stranded
– Ribose
– Nucleotides:
• Adenine
• Uracil
• Guanine
• Cytosine
• Protein synthesis is carried out in three distinct
stages: transcription; translation; and protein
folding
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
RNA polymerase
a template
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 anticodon that is complementary to a particular codon on
the mRNA
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
Translation
Translation
(continued)
• Transcription is
the making of
mRNA using a
DNA template
• Translation is the
making of a
protein using the
code in mRNA