1. Cellular Metabolism Chapter 4
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2. Genetic Code
Specification of the correct sequence of amino acids in a polypeptide chain
Each amino acid is represented by a triplet code of DNA bases
The base sequence of a gene then determines the amino acid sequence in a polypeptide
Since DNA stays in the nucleus, and proteins are made in the cytoplasm, DNA’s code must be copied and carried to the cytoplasm. RNA molecules accomplish this transfer of the genetic code.

3. RNA Molecules Single strand of nucleotides
Each nucleotide contains: ribose, phosphate, base (A, G, C, and Uracil instead of Thymine)
Shorter than DNA
Different types: mRNA (carries code from DNA to ribosome), tRNA (brings amino acids to ribosome), rRNA (a component of ribosomes)

4. Protein Synthesis
Protein Synthesis involves 2 steps, each requiring RNA and enzymes
Transcription occurs in the nucleus and is the process of copying DNA information into an RNA sequence
Translation occurs at the ribosomes in the cytoplasm as the code is transferred to a growing chain of amino acids

5. Animation: Stages of Transcription
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6. Transcription of Messenger RNA (mRNA)
Occurs in nucleus
A section of DNA opens up (just where the gene coding for the particular protein is)
mRNA nucleotides pair up with the DNA bases on one side---uracil is used instead of thymine
mRNA moves away and DNA closes up
Controlled by RNA polymerase
Post-Transcription
Once the mRNA has gone through splicing, capping, and tailing it goes through nuclear pore and attaches to a ribosome in the cytoplasm.
DNA
RNA S P A U P S S P T A P S S P
Direction of “reading” code G C P S S P C G P S S P G C P S

7. Animation: How Translation Works
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8. Translation Occurs in cytoplasm Ribosome attaches to mRNA
Each tRNA molecule has an attachment point for a specific amino acid
Each tRNA also has a region of 3 bases called an anticodon which is attracted to complementary mRNA codons
Once a ribosome finds the start codon (AUG) it moves down the mRNA strand, tRNA’s bringing their amino acids are attracted to the mRNA codons and a peptide bond is formed and the tRNA is released
Process continues until a stop codon (UAA, UAG, UGA) is reached and then the polypeptide is released

9. Post -Translation
Modifications to create a mature protein with the proper conformation and function
Phosphorylation – addition of phosphate groups
Cleavage – removal of certain amino acids
Glycosylation- the addition of carbohydrate chains on amino acids
Lipidation – the attachment of lipid molecules on amino acids (especially for proteins destined for cell membranes)
Formation of disulfide bridges from cysteine residues

10. Translation

11. Protein Synthesis Cytoplasm 3 Translation begins as tRNA anticodons
recognize complementary mRNA codons,
thus bringing the correct amino acids into
position on the growing polypeptide chain
DNA
double
helix
Amino acids
attached to tRNA
Nucleus T A T A 6
tRNA molecules
can pick up another
molecule of the
same amino acid
and be reused G C G C 2
mRNA leaves
the nucleus
and attaches
to a ribosome A T A T
Polypeptide
chain
Messenger
RNA G C
DNA
strands
pulled
apart A T A T T U A C G G G C T A G G C G G C C C G 5
At the end of the mRNA,
the ribosome releases
the new protein C G T U A T A C C G G C C C G A T G C G C C G A A T G C A A T
Nuclear
pore 4
As the ribosome
moves along the
mRNA, more amino
acids are added
Amino acids
represented A T C C G C G G G C T A G G C 1
DNA
information
is copied, or
transcribed,
into mRNA
following
complementary
base pairing A C C G U
Codon 1
Methionine A A T C G G G C G T A G G C G C C G G
Codon 2
Glycine G C A T T U A C C C G C C G U G C A A T C
Codon 3
Serine A T T U A C C G G G C
Messenger
RNA
DNA
strand G T A A A T C C G C
Codon 4
Alanine G C A C G G C A T A C G C
Codon 5
Threonine G C A T G
Transcription
(in nucleus) G C
Translation
(in cytoplasm) G G C C
Codon 6
Alanine C G A U A G C G G
Codon 7
Glycine C

12. Protein Synthesis 1 2 1 The transfer RNA molecule
for the last amino acid added
holds the growing polypeptide
chain and is attached to its
complementary codon on mRNA.
Growing
polypeptide
chain 3 4
Next amino acid 5 6
Transfer
RNA
Anticodon U G C C G U A U G G G C U C C G C A A C G G C A G G C A A G C G U 1 2 3 4 5 6 7
Codons 1
Peptide bond 2 2
A second tRNA binds
complementarily to the
next codon, and in doing
so brings the next amino
acid into position on the ribosome.
A peptide bond forms, linking
the new amino acid to the
growing polypeptide chain.
Growing
polypeptide
chain 3 4
Next amino acid 5 6
Transfer
RNA
Anticodon U G C C G U A U G G G C U C C G C A A C G G C A G G C A A G C G U
Messenger
RNA 1 2 3 4 5 6 7
Codons 1 2
Next
amino acid 3
The tRNA molecule that
brought the last amino acid
to the ribosome is released
to the cytoplasm, and will be
used again. The ribosome
moves to a new position at
the next codon on mRNA. A 3 4 5 7 6
Transfer
RNA U G C C C G C G U A U G G G C U C C G C A A C G G C A G G C A A G C G U
Messenger
RNA 1 2 3 4 5 6 7
Ribosome 1 2 4
A new tRNA complementary to
the next codon on mRNA brings
the next amino acid to be added
to the growing polypeptide chain. 3 4 5
Next
amino acid 6 7
Transfer
RNA C G U C C G A U G G G C U C C G C A A C G G C A G G C A A G C G U
Messenger
RNA 1 2 3 4 5 6 7

13. Animation: Protein Synthesis
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14. Changes in Genetic Information
Only about 1/10th of one percent of the human genome differs from person to person
Nature of Mutations
Mutations – change in genetic information
Result when:
Extra bases are added or deleted
Bases are changed
Code for
glutamic
acid
Mutation
Code for
valine P P T T S S
Direction of “reading” code P P T A S S
May or may not change the protein P P C C S S
(a)
(b)

15. Protection Against Mutation
Repair enzymes correct the mutations
Inborn Errors of Metabolism
Occurs from inheriting a mutation that then alters an enzyme
This creates a block in an otherwise normal biochemical pathway