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Protein Synthesis
Transcription & Translation
Protein Synthesis: An Overview

“The DNA inherited by an organism leads to
specific traits by dictating the synthesis of
proteins.”


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


Step 1: Transcription
Step 2: Translation
Transcription



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
Structure of RNA

Made of nucleotides
 5-carbon sugar (ribose)
 Phosphate group
 Nitrogenous base (A,G,C,U)

Usually single stranded
 Can fold on itself… “hairpins”
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: 3 Steps
1)
2)
Initiation - RNA polymerase
(an enzyme) binds to a
specific sequence of DNA
called the promoter
Elongation - RNA
polymerase uses one strand
of DNA as a template to
assemble a strand of mRNA

3)
Follows the base-pairing
rules
Termination – a specific
sequence of DNA acts as a
“termination signal” for
transcription to stop, and the
mRNA is released from the
DNA
Elongation – A Closer Look


As RNA polymerase
moves along the DNA, it
untwists the double helix
and separates the
strands
RNA polymerase adds
nucleotides to the mRNA
molecule

Follows the base-pairing
rules
The template strand of a given
gene includes the sequence
GCCACGTATCAG
 A.
What is the sequence of the
complementary strand?
 B. What is the mRNA sequence made?
RNA Editing

In eukaryotes, mRNA is edited before
it’s sent out of the nucleus

RNA splicing
• Introns are cut out and exons are spliced
together
• Allows for multiple products to be made from
the same gene
The Genetic Code

mRNA is “read” in threeletter “words” called
codons


Consists of three
consecutive nucleotides
that specify a single
amino acid
Proteins are made by
joining amino acids
together into long chains
called polypeptides
Translation

Big Picture:


The building of a
polypeptide (protein) from
the information encoded
in mRNA
Occurs in the cytoplasm,
on a ribosome
tRNA

Transfer RNA (tRNA):



Cloverleaf shape
Contains anticodon
(triplet of bases,
complementary to the
codons found in the
mRNA)
Carries specific amino
acid to ribosome during
translation
Translation

Consists of 3 stages:



Chain Initiation
Chain Elongation
Chain Termination
Translation: Chain Initiation

mRNA and tRNA join

Codon +
complementary
anticodon
• mRNA is AUG (start
codon)
• tRNA is UAC
Translation: Chain Elongation

Amino acids are added
one by one by the
following process:


mRNA codon bonds with
anticodon on tRNA
Peptide bond is formed
between new amino acid
and the last one
Translation: Chain Termination



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
DNA
transcription
amino
acids
mRNA
Put it all
together
ribosome
protein
tRNA
translation