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Protein Synthesis
Flow of Genetic Information
• Flow of genetic information from DNA to
RNA to protein
• The DNA genetic code (genotype) is
expressed as proteins which provide the
physical traits (phenotype) of an
organism
GCTGCTAACGTCAGCTAGCTCGTAGC
GCTAGCGCTTGCGTAGCTAAAGTCGA
GCTCGCTTGCGTAGCTAAAGTCGAGC
TGCTGCTAACGTCAGCTAGCTCGTAG
AGCGCTTGCGTAGCTAAAGTCGAGCT
AGCGCTTGCGTAGCTAAAGTCGAGCT
GCTGCTAACGTCAGCTAGCTCGTAGC
AGCGCTTGCGTAGCTAAAGTCGAGCT
AGCGCTTGCGTAGCTAAAGTCGAGCT
GCTGCTAACGTCAGCTAGCTCGTAGC
AGCGCTTGCGTAGCTAAAGTCGAGCT
AGCGCTTGCGTAGCTAAAGTCGAGCT
GCTGCTAACGTCAGCTAGCTCGTAGC
AGCGCTTGCGTAGCTAAAGTCGAGCT
GCTGCTAACGTCAGCTAGCTCGTAGC
AGCGCTTGCGTAGCTAAAGTCGAGC,
cont.
RNA
Proteins
Different DNA Sequence….
GCTGTAATTACGTAACTAGCTCGTAG
CCTAGCGCTTGCGTAGCTAAAGTCGA
GCTCGGCTGTAATTACGTAAGTCGAG
CTGCTGCTAACGTCAGCTAGCTCGTA
GGCTGTAATTACGTAAAGCGCTTGCG
TAGCTAAAGTCGAGCTGCTGTAATTA
CGTAAAGCGCTTGCGTAGCTAAAGTC
GAGCTGCTGCTAACGTCAGCTAGCTC
GTAGCGCTGTAATTACGTAAAGCGCT
TGCGTAGCTAAAGTCGAGCTGCTGTA
ATTACGTAAAGCGCTTGCGTAGCTAA
AGTCGAGCTGCTGTAATTACGTAA
GCTGCTAACGTCAGCTAGCTCGTAGC
GCTGTAATTACGTAAGCTGTAATTAC
GTAAGCTGTAATTACGTAAGCTGTAA
TTACGTAAGCTGTAATTACGTAAGCT
GTAATTACGTAAGCTGTAATTACGTA
AGCTGTAATTACGTAA, cont.
RNA
Proteins
Protein Synthesis
• Transcription
 Process in which a
molecule of DNA is
copied into a
complementary strand
of RNA
• Translation
 Process in which the
message in RNA is
made into a protein
Forms of RNA
3 Main Types of RNA
1) mRNA (messenger RNA) – RNA that decodes
DNA in nucleusbrings DNA message out of
nucleus to the cytoplasm
Each 3 bases on mRNA is a “codon”
2) tRNA (transfer RNA) – RNA that has the
“anticodon” for mRNA’s codon The anticodon
matches with the codon from mRNA to
determine which amino acid joins the protein
chain
3) rRNA (ribosomal RNA) – make up the
ribosomes—RNA that lines up tRNA molecules
with mRNA molecules
Transcription produces genetic messages
in the form of RNA
RNA
polymerase
RNA nucleotide
Direction of
transcription
Template
strand of DNA
Newly made RNA
Figure 10.9A
Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings
Transcription
1. Initiation:
• RNA polymerase (enzyme)
attaches to DNA at the
promoter and “unzips” the
two strands of DNA
2. Elongation:
• RNA polymerase then
“reads” the bases of DNA
and builds a single strand of
complementary RNA called
messenger RNA (mRNA)
3. Termination:
• When the enzyme reaches
the terminator sequence,
the RNA polymerase
detaches from the RNA
molecule and the gene
Transcription
The code on DNA tells how mRNA is put together.
Example: DNAACCGTAACG
mRNAUGGCAUUGC
• Each set of 3 bases is called a triplet or codon
(in mRNA)
UGG CAU UGC
•
Eukaryotic RNA is processed before
Noncoding leaving the nucleus
segments called
introns are spliced
out
• Coding segments
called exons are
bonded together
• A 5’cap and a 3’
poly-A tail are
added to the ends
Exon Intron
Exon
Intron
Exon
DNA
Cap
RNA
transcript
with cap
and tail
Transcription
Addition of cap and tail
Introns removed
Tail
Exons spliced together
mRNA
Coding sequence
NUCLEUS
CYTOPLASM
Figure 10.10
Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings
Protein Synthesis
• Transcription
• Translation
Process in which RNA is used to make a protein
Transfer RNA molecules serve as
interpreters during translation
Amino acid attachment site
• In the cytoplasm, a
ribosome attaches
to the mRNA and
translates its
message into a
polypeptide
• The process is aided
by tRNAs
Hydrogen bond
RNA polynucleotide chain
Anticodon
Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings
Figure 10.11A
Ribosomes build polypeptides
Copyright © 2003 Pearson Education, Inc. publishing Benjamin Cummings
Elongation adds amino acids to the
polypeptide chain until a stop
codon terminates translation
• The mRNA moves a codon at a time relative to
the ribosome
– A tRNA pairs with each codon, adding an amino acid
to the growing polypeptide
1. Initiation:
–
–
–
Translation
mRNA molecule binds to the small ribosomal subunit
Initiator tRNA binds to the start codon (AUG—
Methionine) in the P-site of the ribosome
The large ribosomal subunit binds to the small one so
that the initiator tRNA is in the P-site to create a
functional ribosome
2.
Elongation:
–
–
–
Translation
Codon recognition: anticodon
of incoming tRNA molecule,
carrying its amino acid, pairs
with the mRNA codon in the Asite of the ribosome
Peptide formation: polypeptide
separates from the tRNA in the
P site and attaches by a
peptide bond to the amino
acid carried by the tRNA in the
A site
Translocation:
the tRNA in the P-site now
leaves the ribosome, and the
ribosome moves along the
mRNA so that the tRNA in the
A-site, carrying the growing
polypeptide, is now in the Psite. Another tRNA is brought
into the A-site
Translation
3.
Termination:
–
–
Elongation continues until a
stop codon is reached—UAA,
UAG, or UGA
The completed polypeptide
is released, the ribosome
splits into its subunits
• An exercise in translating the genetic code
DNA
RNA
Start
codon
Polypeptide
Stop
codon
Mutations
•
•
Mutagenesis—creation of mutations
Can result from Spontaneous Mutations
•
•
Errors in DNA replication or recombination
Mutagens—physical or chemical agents
– High-energy radiation (X-rays, UV light)
Types of Mutations
• Mutations within a gene
– Can be divided into two general categories.
• Base substitution
• Base deletion (or insertion)
– Can result in changes in the amino acids in proteins.
Normal hemoglobin DNA
mRNA
Mutant hemoglobin DNA
Sickle-Cell
Disease
mRNA
Normal hemoglobin
Glu
Sickle-cell hemoglobin
Val
Substitution Mutations
• Missense mutation:
altered codon still codes
for an amino acid,
although maybe not the
right one
• Nonsense mutation:
altered codon is a stop
codon and translation is
terminated prematurely
– Leads to nonfunctional
proteins
Insertions and Deletions
• Frameshift mutation:
addition or loss of one
or more nucleotide
pairs in a gene shifts the
reading frame for
translation and
incorrect protein is
made
Are all Mutations Bad?
• Although mutations are often harmful,
– They are the source of the rich diversity of
genes in the living world.
– They contribute to the process of evolution by
natural selection.