Transcript Central Dogma of Cell Biology

CENTRAL DOGMA
OF BIOLOGY
Transcription & Translation
How do we make sense of the DNA message?
Genotype to Phenotype
Central Dogma of Cell Biology
• DNA codes for DNA = REPLICATION
• DNA codes for RNA = TRANSCRIPTION
• RNA codes for protein = TRANSLATION
Replication vs. Transcription
• DNA-DNA
• Starts at replication
origins
• Unwinds with Helicase
• DNA polymerase
• Proofreads
• Start with 1 DNA
End with 2 DNA:
½ new, ½ old
• DNA-RNA
• Starts at promoter
regions
• Does not need Helicase
to unwind
• RNA polymerase
• No proofreading
• Start with 1 DNA
End with same DNA and
1 RNA
Transcription
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Process of converting DNA to mRNA
Takes place in the nucleus
3 main steps:
INITIATION  RNA pol binds to the DNA
ELONGATION  nucleotide chain is built,
complementary to the DNA message
• TERMINATION  RNA pol stops transcribing
How do we know what to transcribe?
• Promoter
– Characteristic region of DNA that signals the start
of a gene.
– A sequence of letters that signals “gene ahead!”
– “TATA box” and enhancers
How do we know what to transcribe?
• Start and stop codons
– What are codons?
Each 3 bases form a
codon that codes for a
particular amino acid
AUG  methionine amino
acid, but also START
UAA, UGA, UAG  STOP
Transcription - Step One:
RNA
Polymerase
unwinds
and unzips
the DNA
double
helix.
Transcription: Step Two:
RNA polymerase adds on
free-floating nucleotides
A, G, C, U
What does each bind with?
Stops at STOP codon
Releases RNA polymerase
releases mRNA
mRNA
How do we know what to translate?
• Before the mRNA message leaves the nucleus,
RNA editing & modifications occur
• DNA & RNA contain sequences that do not
code for proteins = INTRONS
– INTRONS = IN (BETWEEN) RONS
• Sequences that code for proteins are
expressed = EXONS
– EXONS = EX (PRESSED SEQUENCE) ONS
RNA Editing
While still in the nucleus:
• INTRONS are cut out, and
• EXONS are spliced together
before the RNA sequence is sent to the
cytoplasm for translation
http://wwwclass.unl.edu/biochem/gp2/m_biology/animation/gene/gene_a2.html
• Transcription animation
Transcription  Translation
Translation
• The mRNA leaves the nucleus  cytoplasm
• Message is read at the ribosome
• 1 Codon (3 letter message) is translated into 1
amino acid
• tRNA molecule has one end (anticodon) that
matches the mRNA . Each anticodon specifies
an amino acid.
• The amino acids are bonded together as
peptide chains…which fold into proteins
The Genetic Code:
3 letters = 1 codon  1 amino acid
http://wwwclass.unl.edu/biochem/gp2/m_biology/animation/gene/gene_a3.html
• Animation of translation
Practice with this sequence
• DNA: TCGATGTTCCGCCGTACGTCGTAACCG
AGCTACAAGGCGGCATGCAGCATTGGC
Use the bottom strand as the complement to the mRNA.
What’s that mean?
Hint: Look for where it starts. How do you know?
Once you’ve found the “reading frame”, write in triplets
mRNA 
Use your genetic code wheel to write the amino acid
sequence. How will you know when to stop?
Try again without help
DNA: CCGTCATGTTCGCGCTACAAATGAAATGA
GGCAGTACAAGCGCGATGTTTACTTTACT
mRNA:
Polypeptide: