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Protein Synthesis
DNA  RNA  Protein
2 Major Steps
1. Transcription – DNA is transcribed (copied!)
into single stranded mRNA (DNA code
transcribed into RNA code)
2. Translation – mRNA is translated into protein
(amino acids)
Transcription
• Occurs in nucleus (where the DNA is located!)
• Generally divided into three steps – initiation,
elongation, termination
A. Initiation
• RNA polymerase (enzyme) attaches to promoter
regions of DNA at TATA box
• RNAP requires TATA box and other transcription
factors to be present
B. Elongation
• RNAP can only add nucleotides to the 3’ of the
growing mRNA strand; thus mRNA is synthesized
in the 5’ 3’ direction
C. Termination
• Transcription ends
when mRNA
polymerase reaches a
specific STOP sequence:
– ATT
– ATC
– ACT
mRNA processing
• After transcription, mRNA must be modified so
that it can exit the nucleus
1. GTP cap added to 5’ end of mRNA (stability &
attachment point for ribosome down the road)
2. Poly-A tail added to 3’ end of mRNA (stability &
guidance so mRNA goes from nucleus to ribosome)
3. Removal of introns
Introns vs. Exons
• Exons – sequences that contain the code for a
polypeptide (protein); exons are expressed
• Introns – non-coding sequences of mRNA
• Original, unprocessed mRNA contains both
introns and exons
• Before mRNA exits nucleus for translation at
ribosome, snRNPs (small nuclear ribonucleoproteins)
remove introns and splice exons together
Translation
• Occurs in cytoplasm where processed mRNA
meets ribosome and is translated into protein
• Steps:
– Initiation
– Elongation
– Termination
For translation to occur, you need…
1. Activated tRNA
-tRNA in cytoplasm
finds the correct amino
acid
-Attachment (activation)
requires 1 ATP and
enzyme
2. Ribosomal subunits
• Ribosomes consist of 2 subunits (small and large)
– Each subunit is made of rRNA and protein
– Each is built separately in nucleolus
• The two subunits exit nucleolus and form the
ribosome in the cytoplasm when translation starts
The Genetic Code
• Codon – triplets of bases
• mRNA makes a template strand (from DNA) that
is translated into protein via a triplet code
– Every three base pairs codes for a particular aa
1. Initiation
• Brings together mRNA (from nucleus), tRNA (in
cytoplasm), and ribosomal subunits (from
nucleolus)
1. Small ribosomal subunit binds to mRNA and a
specific tRNA (methionine)
2. Small subunit scans mRNA until it finds AUG
(start codon)
3. Large subunit attaches the complex
2. Elongation
• Begins with the
next tRNA arriving
at the P site of the
ribosome
Polyribosome
3. Termination
• Occurs when the ribosome encounters one of
three STOP codons located on mRNA
• At this point, polypeptide synthesis is
complete and the ribosome detaches from the
polypeptide
Point Mutations
• Chemical changes in just one base pair of a
gene
• 2 general types:
– Base pair substitutions
– Base pair insertions or deletions
1. Base pair substitutions
• Substitution of one base pair with another
• Often silent mutations because they don’t have
an effect on encoded protein due to redundancy
of genetic code:
– DNA:
– mRNA:
– AA:
CCG  CCA
GGC  GGU
gly  gly
Example: Sickle Cell Anemia
• Results from a single point
mutation in a gene that
codes for one of the
polypeptide chains that
form hemoglobin
• Fatigue, paleness, rapid
heart rate, shortness of
breath, jaundice
2. Insertions & Deletions
• Additions or deletions of nucleotide base pairs
• BAD NEWS… these are often called frameshift
mutations because they literally shift the reading
frame of the mRNA  protein
Example: Tay-Sachs Disease
• Inherited frameshift
mutation on chromosome
15
• Mutation results in
malformation of
hexoaminidase A, a protein
that breaks down a
particular chemical in
gangliosides (nerve tissue)
• Deafness, blindness,
dementia, paralysis, slow
growth, mental retardation
etc
Mutagens
• Spontaneous mutations – errors that result from
cellular machinery malfunction
• Environmental mutations – damage to DNA
caused by environment
– Mutagens – chemical agents that interact with DNA
and cause problems
• X-rays, UV light, various drugs (seriously)