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RNA and Protein
Synthesis
12-3 RNA and Protein Synthesis
12–3 RNA and Protein
Synthesis
Genes are coded DNA instructions that control
the production of proteins.
Genetic messages can be decoded by copying
part of the nucleotide sequence from DNA into
RNA.
RNA contains coded information for making
proteins.
The Structure of RNA
The Structure of RNA
There are four main differences between RNA and DNA:
• The sugar in RNA is ribose instead of deoxyribose.
• RNA is single-stranded. DNA is double-stranded.
• RNA contains uracil in place of thymine.
• DNA stays in the nucleus, but RNA can leave the nucleus and
go into the cytoplasm.
Types of RNA
Types of RNA
There are three main types of RNA:
• Messenger RNA (mRNA)
• Ribosomal RNA (rRNA)
• Transfer
RNA (tRNA)
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Types of RNA
Messenger RNA (mRNA) carries copies of instructions
for assembling amino acids into proteins.
Ribosome
Types of RNA
Ribosomal RNA
Ribosomes are made up of proteins and ribosomal
RNA (rRNA).
Amino acid
Types of RNA
Transfer RNA
During protein construction, transfer RNA (tRNA)
transfers each amino acid to the ribosome.
Transcription
Transcription (in nucleus)
Protein synthesis begins in
the nucleus with a process
called transcription.
DNA is copied in the form of a single strand of
RNA
The process begins at a section of DNA called a
promoter.
Benefits of Transcription
• Transcribed copies of the DNA (in the form of
RNA) are used instead of the original DNA.
• In eukaryotes, DNA is broken down in the
cytoplasm, but RNA is not. RNA remains intact.
How does Transcription Work?
1. DNA double helix must be separated by breaking
the hydrogen bonds between nitrogen bases.
2. Only one DNA strand is “read” by the enzyme
RNA polymerase.
3. RNA polymerase constructs an RNA polymer.
• An enzyme, RNA Polymerase, unzips DNA
molecule.
• It chemically tells RNA nucleotides to come
and base pair with the open DNA molecule
according to the base-pairing rules:
• Adenine-Uracil (A-U) Always Up
• Guanine-Cytosine (G-C) Going Camping
In Nucleus
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Building an RNA Polymer
DNA  (codes for) RNA
A
-------  U
T
-------  A
C
-------  G
G
-------  C
Transcription
RNA
RNA polymerase
DNA
What happens to RNA
once it is created?
• In prokaryotes, the RNA
is immediately
translated.
• In eukaryotes, the RNA
is processed.
– Introns removed
– Exons joined together
RNA Processing
• Introns - segments
of useless genes.
They are removed.
• Exons – are the
expressed genes
that remain.
RNA Editing
Some DNA within a gene is not needed to produce
a protein.
Remember, DNA stays in the nucleus, but the
edited mRNA leaves the nucleus and goes into
the cytoplasm to the ribosome
The Genetic Code
The Genetic Code
The genetic code is the “language” of mRNA instructions.
The code is written using four “letters” (the bases: A, U,
C, and G).
Adenine, Uracil, Cytosine, Guanine
These letters are arranged into “words”
consisting of 3 letters  codon
The Genetic Code
A codon consists of three consecutive
nucleotides on mRNA that specify a particular
amino acid.
Protein Synthesis
DNA
molecule
DNA strand
(template)
5
3
TRANSCRIPTION
mRNA
5
3
Codon
TRANSLATION
Protein
Amino acid
The Genetic Code
Ribosomes use this decoding system to determine how to
build the appropriate protein.
“The dictionary”
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Translation
Translation
Translation is the decoding of a mRNA message into
a polypeptide chain (protein).
Translation takes place on ribosomes.
During translation, the cell uses information
from messenger RNA to produce proteins.
Nucleus
mRNA
How Does the Decoding Work?
RNA: AUGCGAGGGAGAUUAUAGGAC
Ribosomes read:
AUG – CGA – GGG – AGA – UUA – UAG – GAC
Each 3 nucleotide “word” is called a codon.
Decoding the Genetic Code
Let’s try it!
Try to decode:
AUG CGA GGG AGA UUA UAG GAC
Met – Arg – Gly – Arg – Leu - stop
tRNA anticodons to mRNA codons ( which codes for
specific amino acid)
according to base pairing rules: A-U C-G
Phenylalanine
Methionine
Ribosome
mRNA
Start codon
Lysine
tRNA
As the amino acids are brought close together,
they are joined by peptide bonds to make a
protein.
mRNA
Ribosome
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The process continues until the ribosome
reaches a stop codon.
Polypeptide
Ribosome
tRNA
mRNA
Codon
Genes and Proteins
Codon Codon
DNA
Single strand of DNA
Codon Codon Codon
mRNA
mRNA
Protein
Alanine Arginine Leucine
Amino acids within
a polypeptide
Protein Synthesis
DNA
molecule
DNA strand
(template)
5
3
TRANSCRIPTION
mRNA
5
3
Codon
TRANSLATION
Protein
Amino acid
The role of a master plan in a building is similar to
the role of which molecule?
•
•
•
•
messenger RNA
DNA
transfer RNA
ribosomal RNA
A base that is present in RNA but NOT in DNA is
•
•
•
•
thymine.
uracil.
cytosine.
adenine.
The nucleic acid responsible for bringing individual
amino acids to the ribosome is
•
•
•
•
transfer RNA.
DNA.
messenger RNA.
ribosomal RNA.
A codon typically carries sufficient information to
specify a(an)
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single base pair in RNA.
single amino acid.
entire protein.
single base pair in DNA.