Transcript Protein Synthesis

Protein Synthesis
Traits are determined by proteins
(often enzymes)
*Protein – 1 or more polypeptide
chains
*Polypeptide – chain of amino acids
linked by peptide bonds
*One gene – one polypeptide (sort of)
The “Central Dogma” of Biology
DNA
(replication)

RNA

Protein
(transcription)
(translation)
*the concept is basically right but modern biology is leading to an
understanding of much more complexity
RNA
Differs from DNA:
1. Sugar is ribose (has an OH off the 2’ carbon)
2. Uracil replaces thymine
3. Single stranded (usually)
Types of RNA
in protein synthesis
1. Messenger RNA (mRNA)
-triplet codons, long strand
2. Transfer RNA (tRNA)
-small (70-80 nucleotides)
-about 45 different ones
-carry amino acids to ribosome
on board
3. Ribosomal RNA (rRNA)
-together with proteins it makes up
ribosomes
-assembled in nucleolus
-large and small subunits come together
 ribosomes
Transcription
1. Initiation – RNA polymerase attaches to
DNA at promoter region
(beginning of gene – 3’ end)
- unzips DNA strands
2. Elongation – RNA polymerase links RNA
nucleotides
-mRNA strand made 5’3’
3. Termination – mRNA strand breaks off
from DNA template
RNA Splicing
• mRNA is processed before it leaves
nucleus
-portions not necessary for polypeptide
production are removed
Intron – portion of gene that does not code
for anything
-removed by RNA splicing
Exon – segments that contain the info
The Genetic Code
• Almost universal –strong evidence for
common ancestry
• Must be triplet:
– 20 amino acids  need at least 20 base
combos
– 4 bases (A,U,G,C)
• 42 = 16 (No good!)
• 43 = 64 combos (sufficient, and also allows for
redundancy)
Translation
* mRNA is read, code is translated into
polypeptide
1. Initiation – all of the below come together
at 5’ end of mRNA at “start codon” (AUG)
– small ribosome subunit
– Large ribosome subunit
– tRNA carrying methionine
2. Elongation
-ribosome moves down mRNA a codon
at a time
-at each mRNA codon a tRNA (with
complementary anticodon):
-temporarily binds
-leaves its amino acid behind
-breaks off
-the amino acids are linked as a
polypeptide chain (by peptide bonds)
3. Termination
-upon reaching a stop codon
-ribosome units, mRNA, polypeptide
separate