Download CH. 12.3 : DNA, RNA, and Protein

Genes and Proteins
• The sequence of nucleotides in DNA contains
information.
• This information is put to work through the
production of proteins.
• Proteins fold into complex, three-dimensional shapes
to become key cell structures and regulators of cell
functions.
• Thus, by encoding the instructions for making
proteins, DNA controls cells.
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Genes and Proteins
• You learned earlier that proteins are polymers of
amino acids.
• The sequence of nucleotides in each gene contains
information for assembling the string of amino acids
that make up a single protein.
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DNA  Proteins  Cells  Bodies
 DNA has the information to build proteins

genes
proteins
cells
bodies
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DNA gets all the glory,
Proteins do all the work
Cell organization
• DNA
– DNA is in the nucleus
• genes = instructions for making proteins
– want to keep it there = protected
• “locked in the vault”
nucleus
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cytoplasm
Cell organization
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• Proteins
– chains of amino acids
– made by a “protein factory” in cytoplasm
– protein factory = ribosome
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cytoplasm
nucleus
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build
proteins
ribosome
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Passing DNA instructions: need RNA
• RNA like DNA, is a nucleic
acid.
• RNA structure differs from
DNA structure in three ways.
• Has ribose sugar instead of
deoxyribose (DNA).
• Replaces thymine (T) with
uracil (U).
• Single stranded as opposed
to double stranded DNA.
Phosphate
group
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Nitrogenous
base
(A, G, C, or U)
Uracil (U)
Sugar
(ribose)
Ribonucleic Acid (RNA)
• RNA has a different function than DNA.
• Whereas DNA provides the instructions for protein
synthesis, RNA does the actual work of protein
synthesis.
• RNA takes from DNA the instructions on how the
protein should be assembled, then—amino acid by
amino acid—RNAs assemble the protein.
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Ribonucleic Acid (RNA)
• 3 types of RNA
– 1. Messenger RNA (mRNA), single, uncoiled strand
which brings instructions from DNA in the nucleus
to the site of protein synthesis.
– 2. Ribosomal RNA (rRNA), globular form, makes up
the ribosome –the construction site of proteins
binds (site of protein synthesis); binds to the mRNA
and uses the instructions to assemble the amino
acids in the correct order.
– 3. Transfer RNA (tRNA) single, folded strand that
delivers the proper amino acid to the site at the
right time.
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Passing DNA instructions
• Need to get DNA gene instructions
from nucleus to cytoplasm.
– need a copy of DNA
– messenger RNA
cytoplasm
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build
proteins
mRNA
nucleus
ribosome
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Protein Synthesis
Step #1-Transcription: DNA  mRNA
• In the nucleus, enzymes make an RNA copy of a
portion of a DNA strand.
The main difference between transcription and DNA
replication is that transcription results in the
formation of one single-stranded RNA molecule
rather than a double-stranded DNA molecule.
Step #2- Translation: mRNA  Protein
Process of converting the information in a sequence of
nitrogenous bases in mRNA into a sequence of amino
acids in protein.
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From nucleus to cytoplasm
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nucleus
transcription
DNA
mRNA
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protein
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translation
cytoplasm
trait
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Transcription
• Making mRNA from DNA
• DNA strand is the
template (pattern).
– match bases
•U:A
•G:C
• Enzyme
– RNA polymerase
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Matching bases of DNA & RNA
• Double stranded DNA intact.
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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Matching bases of DNA & RNA
A
• Match RNA bases to DNA bases
on one of the DNA strands.
G
C
G
U
C
A
C
A C C
RNA
polymerase
A
U
A
G
U
AG
U
C
G
A
U
A
RNA base ‘bank’
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
Matching bases of DNA & RNA
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• U instead of T is matched to A.
DNA
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TACGCACATTTACGTACGC aa
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mRNA
AUGCGTGUAAAUGCAUGCG
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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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RNA Processing
Not all the nucleotides in the DNA of eukaryotic
cells carry instructions—or code—for making
proteins.
Genes usually contain many long noncoding
nucleotide sequences, called introns, which are
scattered among the coding sequences.
Regions that contain coding nucleotide sequences
are called exons because they are expressed.
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RNA Processing
When mRNA is transcribed from DNA, both introns
and exons are copied.
The introns must be removed from the mRNA before
it can function to make a protein.
Enzymes in the nucleus cut out the intron segments
and paste the mRNA back together.
The mRNA then leaves the nucleus and travels to the
ribosome.
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RNA Processing: simplified
• Noncoding segments called introns are spliced
out ( coding segment = exon).
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Translated Sequences
• Transfer of DNA to mRNA uses “language” of
nucleotides.
– Letters: nitrogen bases of nucleotides (A,T,G,C)
– Words: codons ≡ triplets of bases
( ex. AGC)
– Sentences: polypeptide chain
– The codons in a gene specify the amino acid
sequence of a polypeptide.
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The Genetic Code
The nucleotide sequence
transcribed from DNA to
a strand of messenger
RNA acts as a genetic
message, the complete
information for the
building of a protein.
Virtually all organisms
share the same genetic
code.
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Translation: From mRNA to Protein
• Translation occurs in the ribosomes in the
cytoplasm.
Translation involves 3 types of RNA:
1. Messenger RNA (mRNA) = carries the
blueprint for construction of a protein.
2. Ribosomal RNA (rRNA) = the construction site
where the protein is made.
3. Transfer RNA (tRNA) = the truck delivering the
proper amino acid to the site at the right time.
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Transfer RNA are interpreters
• In the cytoplasm, a ribosome
attaches to the mRNA and
translates its message into a
polypeptide (protein).
• The process is aided by
transfer RNAs.
• Each tRNA molecule has a
triplet anticodon on one end
and an amino acid
attachment site on the other.
• Anticodon base pairs with
codon of mRNA.
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nucleus
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cytoplasm
protein aa
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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
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How does mRNA code for proteins?
TACGCACATTTACGTACGCG
DNA
ribosome
AUGCGUGUAAAUGCAUGCGC
mRNA
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Met Arg Val Asn Ala Cys Ala
protein
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How can you code for 20 amino acids with
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only 4Website
DNA
bases (A,U,G,C)?
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mRNA codes for proteins in triplets
DNA
TACGCACATTTACGTACGCG
codon
mRNA
AUGCGUGUAAAUGCAUGCGC
AUGCGUGUAAAUGCAUGC ribosome
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protein
Met Arg Val Asn Ala Cys A
 Codon = block of 3 mRNA bases that code for each amino acid
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The Genetic code
• For ALL life!
– strongest support for a
common origin for all
life.
• Code has duplicates
– several codons for each
amino acid.
– mutation insurance!
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The Genetic code
• For ALL life!
• Start codon
• AUG  methionine
• Stop codons
• UGA, UAA, UAG
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Codons for Amino Acids
DNA
mRNA
TACGCACATTTACGTACGCGG
AUGCGUGUAAAUGCAUGCGCC
UAC
tRNA
amino
acid
Met
codon
GCA
Arg
CAU
anti-codon
Val
Websitebases
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 Anti-codon = block of 3 tRNA
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
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U A C
tRNA
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A G
tRNA
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C
U AG
tRNA
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From gene to protein
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nucleus
transcription
DNA
cytoplasm
translation
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protein
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mRNA
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ribosome
A C CA U GU C G A U C A GU A GC A U GGC A
tRNA
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trait
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protein
transcription
nucleus
cytoplasm
translation
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trait
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From gene to protein
protein
transcription
translation
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