Transcript Protein Synthesis - Building Directory

Protein Synthesis
Transcription & Translation
Protein Synthesis: An Overview
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“The DNA inherited by an organism leads to
specific traits by dictating the synthesis of
proteins.”
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Proteins are the link between genotype
(AAC) and phenotype (eye color)
Protein synthesis is the process of
transferring the information encoded in DNA
into proteins
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Step 1: Transcription
Step 2: Translation
Transcription
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Transcription is the synthesis of messenger RNA
(mRNA) from DNA
Occurs in the nucleus
DNA does not leave the nucleus

Analogy: blueprints do not get used at the job site
when constructing a house
DNA vs. RNA
DNA
RNA
Contains:
Adenine, guanine,
cytosine, thymine
Contains:
Adenine, guanine,
cytosine, uracil
5-carbon sugar is
deoxyribose
5-carbon sugar is
ribose
Double-stranded
Single-stranded
Transcription: Initiation
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Promoter = a specific
sequence of DNA (starting
point); consists of initiation
site and TATA box)
RNA polymerase binds to the
promoter on the DNA
molecule
Transcription factors
(proteins) bind to promoter
region to help RNA
polymerase find the starting
point.
RNA polymerase then
separates the 2 DNA strands
Transcription: Elongation
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As RNA polymerase
moves along the DNA, it
untwists the double helix
and separates the
strands
RNA polymerase adds
nucleotides to the 3’ end
of the mRNA molecule

Follows the base-pairing
rules
Transcription: Termination

Termination signal = sequence of bases in
DNA molecule that tell RNA polymerase to
stop transcription

“finish line” in a race
mRNA is released from the DNA
 Most common termination signal =
AATAAAA

RNA Modifications
In eukaryotes, mRNA is edited before
it’s sent out of the nucleus
 2 major types of modifications:

Alteration of mRNA ends
 RNA splicing

Alteration of mRNA Ends

5’ end receives a 5’ cap

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Modified form of a guanine nucleotide
3’ end receives a poly-A tail

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30 – 200 adenine nucleotides
Protects the ends from being degraded by
enzymes in the cytoplasm
Shoelace analogy
RNA Splicing

Genes have stretches of nucleotides that don’t code for anything
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Noncoding sequence = introns
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“junk DNA”
“intervening sequences”
Coding regions = exons
During RNA splicing, the introns are removed by enzymes and the
exons are joined together by SLICEOSOMES (made of snRNPs +
proteins)
Translation
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Big Picture:

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The building of a
polypeptide (protein) from
the information encoded
in mRNA
Occurs in the cytoplasm,
on a ribosome
tRNA
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Transfer RNA (tRNA):

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Cloverleaf shape
Contains anticodon
(triplet of bases,
complementary to
mRNA)
Carries specific amino
acid to ribosome during
translation
Ribosomes
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Made up of 2 subunits
composed of rRNA &
proteins:
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Large subunit
Small subunit
Have special binding sites:
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P site: holds tRNA with
amino acid chain
A site: holds “next” tRNA
• The “green room”
Translation
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Consists of 3 stages:
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Chain Initiation
Chain Elongation
Chain Termination
Translation: Chain Initiation
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mRNA and tRNA join

Codon +
complementary
anticodon
• mRNA is AUG (start
codon)
• tRNA is UAC
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Large and small
ribosomal subunits
join, forming a
functional ribosome
Translation: Chain Elongation
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Amino acids are added
one by one by the
following process:
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mRNA codon bonds with
anticodon on tRNA
Peptide bond is formed
between new amino acid
and the last one
tRNA moves over from A
site to P site
Translation: Chain Termination
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Elongation continues until a stop codon is
reached (UAA, UAG, UGA)
Water is added instead of an amino acid
Polypeptide (protein) is released from
ribosome
Point Mutations (aka gene
mutations)
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Just one change in
a nucleotide in a
single gene
Can seriously
affect phenotype 
genetic disorder
Sickle cell anemia
results from one
nucleotide
substitution.
Point Mutations
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Base-pair substitution
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Replacing one nucleotide
with another
Example: instead of G, it’s C
Can cause a noticeable
change in protein structure
Can be a silent mutation
with no actual change in
amino acid sequence.
Can change only one a. acid
Non sense mutations
change sequence to a stop
codon.
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Normal
Hemoglobin
sequence:
CTT
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GAA
Glutamic Acid
Sickle Cell
Sequence:
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CAT
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GUA
Valine
Point Mutations
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Base-pair
insertions or
deletions
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Adding or losing a
base-pair
Changes the
whole “reading
frame”
Where, within a
gene, would this
be the biggest
problem?
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Normal Sequence:
AAG GCG TAG 
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UUC CGC AUC 
Phenylalanine-arginineisolecine
Insertion:
AAG GGC GTA G 
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UUC CCG CAU C 
Phenylalanine-prolinehistidine
Frame Shift Mutations
Insertions- extra nucleotide is added;
moves reading frame up one
nucleotide & changes every amino
acid after that point.
 Deletions- one nucleotide is removed;
moves reading frame down one base.
 Both are more harmful if at the
beginning of a gene.
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Chromosomal Mutations
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A chunk of a chromosome is changed.
Addition or duplication: A piece of a
chromosome is added so genes appear
twice.
Deletion: part of the chromosome is lost
(genes lost)
Inversion: piece of the chromosome
breaks twice and broken piece inserts
upside down (reverses order of genes)
Translocation: two nonhomologous
chromosomes break & exchange genes.