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Protein Synthesis:
Transcription
How is mRNA created from
DNA?
Chromosome
DNA
Gene
• Protein Synthesis is when DNA’s code is used
by the cell to make proteins. This is also
known as gene expression.
• The genes “express themselves” when the
proteins they code for are made.
• IMPORTANT VOCABULARY:
– GENE
– BASE TRIPLET
– CODON
– ANTICODON
– AMINO ACID
– PROTEIN
Gene Expression
GENE: sequence of DNA that codes for a
protein and thus determines a trait
BASE TRIPLET: nucleotide triplet on DNA
CODON: nucleotide triplet on mRNA that
codes for a specific amino acid
ANTICODON: nucleotide triplet on tRNA
that pairs with a codon
AMINO ACIDS: amino acids make up
proteins
PROTEIN: biomolecule made up of a chain
of amino acids
Sections within a gene
Start Triplet
Codes for protein
TAC
Stop Triplet
ATT, ATC, or ACT
Base Triplets
Examples ATC CCG
Overall Process of Protein Synthesis
(Gene Expression)
Transcription
DNA
Translation
mRNA
Protein
Transcription
Translation
Protein
DNA
contains
made
the
bycode
mRNA
forand
all
traits/
tRNA
mRNA
isismade
by
copying
DNA’s
code
for
proteins
a proteinis “deciphered” to build a
Information
protein.
Information in DNA is “rewritten” as a
molecule of mRNA
TRANSCRIPTION
TRANSCRIPTION: mRNA is made
copying DNA’s code
Occurs in the nucleus
Makes mRNA from DNA template
click to play animation
TRANSCRIPTION
G
G
C
A
T
C
C
G
T
A
1. The enzyme
helicase break the
hydrogen bonds
between the
nitrogen bases,
unzipping a gene
portion of the DNA.
A
TRANSCRIPTION
G
G
G
C
A
T
C
C
A
T
T
1. (continued) a
gene portion of
the DNA strand
separates
A
G
G
G
C
A
T
C
C
A
T
T
A
TRANSCRIPTION
T
A
G
C
A
T
C
G
G
C
A
T
2. mRNA is
made from the
DNA template
The enzyme
RNA polymerase
brings in mRNA
nucleotides and
matches them
with their DNA
complement
Base Pairing Rule
mRNA nucleotides match by the
base pairing rule.
U
A
A
G
G
T
C
A
G
G
C
A
T
C
G
C
A - T (RNA has U)
C-G
T
A
A
A
G
G
C
T
C
C
A
A
G
G
T
U
T
A
U
T
A
A
A
T
G
G
C
mRNA
DNA
C
A
A
G
G
T
C
C
G
U
T
A
A
editing)
leaves the
nucleus
G
G
C
A
T
C
G
A
G
C
3. mRNA
separates
from DNA
and (after
C
T
G
A
A
A
G
G
C
C
A
A
G
G
T
C
T
G
C
T
A
U
T
A
A
C
C
4. The two DNA
strands move
back together
and…..
G
T
G
G
C
A
T
G
G
C
A
T
C
C
G
T
A
4. …an enzyme
helps the DNA
strands rezip.
T
A
REVIEWING TRANSCRIPTION
TRANSCRIPTION: mRNA is made
copying DNA’s code
1. DNA unzips
2. mRNA is made from the DNA
template
3. mRNA separates from DNA and
(after editing) leaves the nucleus
4. The DNA molecule rezips (closes
back up.)
click to play animation
mRNA Editing

Pre-mRNA is processed to remove noncoding segments
introns
come out!
exons = coding segments
 introns = non-coding segments


inbetween sequence (junk)
intron = noncoding (inbetween) sequence
eukaryotic DNA
exon = coding (expressed) sequence
RNA splicing enzymes
snRNPs
snRNA
intron
exon

Spliceosome

5'
Enzyme that
recognizes introns
and removes them

exon
3'
spliceosome
5'
3'
cut & paste enzyme
lariat
5'
mature mRNA
exon
5'
3'
exon
3'
excised
intron
More mRNA editing

Need to protect mRNA on its trip from
nucleus to cytoplasm

enzymes in cytoplasm attack mRNA so we
need to protect the ends of the molecule
add 5 GTP cap
add poly-A tail

longer tail, mRNA lasts longer
3'
A
mRNA
5'
G
P
P
P
Now mRNA is ready to
leave the nucleus!
3'
A
mRNA
5'
G
P
P
P
TRY THIS!
Transcribe this segment of DNA:
TACAACCGATGA
Predict the corresponding amino acids for
each codon. AUG UUG GCU ACU