Download Regents Biology How does mRNA code for

From gene to protein:
protein
transcription
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translation
Transcription
 Making mRNA from DNA
 DNA strand is the
template (pattern)

match bases
U:A
G:C
 Enzyme

RNA polymerase
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Transcription
 Occurs in the nucleus
 3 stages
Initiation
 Elongation
 termination

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Transcription
 Initiation
DNA is unwound and unzipped in the
area of the gene to be transcribed by
RNA polymerase which binds to the
promoter region “upstream” of gene
(promoter region signals which DNA
strand is to be copied)
 Region has a high concentration of A’s
& T’s (called the TATA box)

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Matching bases of DNA & RNA
 Double stranded DNA unzips
T G G T A C A G C T A G T C A T CG T A C CG T
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Matching bases of DNA & RNA
 Double stranded DNA unzips
T G G T A C A G C T A G T C A T CG T A C CG T
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Transcription
 Elongation
With 1 DNA strand serving as the
“template strand”
 RNA polymerase attaches
ribonucleotides in 5’ to 3’ direction
 By complementary base pairing

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Matching bases of DNA & RNA
 Match RNA bases to DNA
C
G
one of the
bases on
DNA strands
U
A
G
G
U
U
C
A
AG
A
C
G
A
U
A
C
A C C
RNA
polymerase
A
U
G
T G G T A C A G C T A G T C A T CG T A C CG T
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U
C
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Matching bases of DNA & RNA
 U instead of T is matched to A
aa
aa
aa
DNA
mRNA
TACGCACATTTACGTACG
aa
aa
aa
AUGCGUGUAAAUGCAUGC aa
aa
aa
aa
ribosome
A C C A U G U C G A U C A G U A G C A U G G C A
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Transcription
 Termination
RNA polymerase transcribes the DNA
sequence up to the end of the gene
where a “terminator sequence” is
encountered mRNA transcript is
released from DNA Strand
 Prokaryotes & eukaryotes have
different terminator sequences

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mRNA modification
 In eukaryotes, mRNA is modified prior
to leaving the nucleus (ensuring it
remains intact)
1.
Capping
 A 5’ cap is added to the start of the
primary transcript (protects mRNA from
digestion as it exits the nucleus and
enters the cytoplasm)
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mRNA modification
 In eukaryotes, mRNA is modified prior
to leaving the nucleus (ensuring it
remains intact)
1.
2.
Capping
Tailing
 Approximately 200 adenine
ribonucleotides are added to the 3’ end
 Called a poly A tail
3.
Introns
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mRNA modification
 In eukaryotes, mRNA is modified prior
to leaving the nucleus (ensuring it
remains intact)
1.
2.
3.
Capping
Tailing
Introns
 Non coding regions
 Removed before translation by
spliceosomes so that only exons (coding
regions) remain
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mRNA modification
 The modified mRNA strand now =
mRNA transcript
 Post transcriptional Editing Video
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cytoplasm
aa
aa
aa
aa
aa
aa
proteinaa
aa
aa
aa
nucleus
ribosome
A C C A U G U C G A U C A G U A G C A U G G C A
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trait
How does mRNA code for proteins
 mRNA leaves nucleus
 mRNA goes to ribosomes in cytoplasm
 Proteins built from instructions on mRNA
How?
mRNA
A C C A U G U C G A U C A GU A GC A U G GC A
aa
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aa
aa
aa
aa
aa
aa
aa
Translation
 Occurs in the cytoplasm
 3 stages
1.
2.
3.
Initiation
Elongation
Termination
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Translation
1.
Initiation
 An initiation complex is formed when the first
tRNA is positioned on the ribosomal surface
 Must be accurate or the reading frame will be
inaccurate
 This complex then binds to mRNA at the
beginning of the gene
 2 ribosomal subunits recognize the 5’ cap
2.
3.
Elongation
Termination
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Translation
 tRNA
A single stranded, Clover leaf shaped,
Nucleic acid
 Contains the anticodon (sequence of 3
bases) that recognizes the codon of mRNA
 Each kind of tRNA carries a corresponding
amino acid at its 3’ end

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Translation
1.
2.
Initiation
Elongation
 Code on mRNA is read in triplets (codons)
 AUG (methionine) is the start codon in eukaryotes
(fMET in prokaryotes)
 Ribosomes have 2 sites for tRNA attachment A
(acceptor) site and P (peptide) site
 AUG enters the P site and is paired with the
correct tRNA anticodon
3.
Termination
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Translation
1.
2.
Initiation
Elongation
 Then the A site is filled with 2nd codon and its




complimentary tRNA anticodon
When their, aa are bonded, the ribosome moves
along the mRNA strand in the 5’ to 3’ direction
The 2nd codon and its tRNA moves to the P site
and the 3rd codon moves into the A site and is
paired with its complementary tRNA, and so on
AUG is the only codon that begins in the P site
Released tRNA reunite with fresh aa from the
cytoplasm
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3.
Termination
Translation
1.
2.
3.
Initiation
Elongation
Termination
 3 stop codons UGA, UAG, UAA have no





complementary tRNA
Release factor (a protein) releases the polypeptide
chain from the ribosome
Ribosomal subunits separate
Translation ends
Protein is folded and modified as necessary
Video (20 seconds)
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How does mRNA code for proteins?
TACGCACATTTACGTACGCGG
DNA
ribosome
AUGCGUGUAAAUGCAUGCGCC
mRNA
?
Met Arg Val Asn Ala Cys Ala
protein
aa
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aa
aa
aa
aa
aa
aa
How can you code for 20 amino acids with
only 4 DNA bases (A,U,G,C)?
aa
Codon
 Sequence of 3 adjacent nucleotides that
code for 1 amino acid
 Basic unit of genetic code
mRNA
A C C A U G U C G A U C A GU A GC A U G GC A
aa
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aa
aa
aa
aa
aa
aa
aa
mRNA codes for proteins in triplets
DNA
TACGCACATTTACGTACGCGG
codon
mRNA
ribosome
AUGCGUGUAAAUGCAUGCGCC
?
protein Met
Arg
Val
Asn
 Codon = block of 3 mRNA bases
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Ala
The mRNA code
 For ALL life!

strongest support
for a common origin
for all life
 Code has duplicates


several codons for
each amino acid
mutation insurance!
 Start codon


AUG
methionine
 Stop codons
UGA,
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Biology
UAA, UAG
How are the codons matched to
amino acids?
TACGCACATTTACGTACGCGG
DNA
mRNA
AUGCGUGUAAAUGCAUGCGCC
codon
tRNA
amino
acid
UAC
Met
GCA
Arg
CAU
anti-codon
Val
Translation Video
 Anti-codon = block of 3 tRNA bases
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mRNA to protein = Translation
 The working instructions  mRNA
 The reader  ribosome
 The transporter  transfer RNA (tRNA)
ribosome
mRNA
A C C A U G U C G A U C A GU A GC A U G GC A
U GG
tRNA
aa
aa
aa
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U A C
tRNA
aa
A G
C
tRNA U A G
aa
tRNA
aa
From gene to protein
Video (3min)
aa
aa
transcription
DNA
aa
translation
protein
aa
mRNA
aa
aa
aa
ribosome
A C C A U G U C G A U C A GU A GC A U GGC A
nucleus
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tRNA
cytoplasm
aa
trait
Whoops!
See what happens when
your genes don’t work right!
Any Questions??
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Websites
 Series of MANY protein synthesis
Videos:
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