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12.3 RNA and Protein Synthesis
The Structure of RNA
• single stranded
• working copy of one
gene
– disposable
The Structure of RNA
• one gene from DNA
can produce many
strands of RNA
• instead of thymine (T),
uracil (U)
• RNA has A, U, C, G
Differences between RNA and DNA
• DNA is double stranded, RNA is single
stranded
• DNA contains deoxyribose sugar, RNA
contains ribose sugar
• DNA has thymine; RNA has uracil
Types of RNA
• three types
– messenger RNA
(mRNA)
– ribosomal RNA (rRNA)
– transfer RNA (tRNA)
Types of RNA
• Messenger RNA
– Also known as mRNA
– Carry instructions for making amino acids into proteins
– Messenger from DNA to the rest of the cell
Types of RNA
• Ribosomal RNA
– Also known as rRNA
– Make up part of the ribosome (protein making organelle)
Types of RNA
• Transfer RNA
– Also known as tRNA
– Transfers each amino acid to the ribosome
– mRNA tells tRNA where to take the amino acid
Transcription
• copying part of the
DNA into RNA
Transcription
• requires enzyme RNA
polymerase
• RNA polymerase binds to
DNA and separates the
strands
– Similar to DNA replication
Transcription
• RNA polymerase
uses one strand of
DNA as a template to
make RNA
• Begins transcription
at promoters
– Promoter sequences on
the DNA shows RNA
polymerase where to
start making RNA
• Makes tRNA, mRNA
and rRNA
RNA Editing
• DNA in eukaryotes
contain introns and
exons
– Introns do not code for
proteins
– Exons code for proteins
• Expressed
RNA Editing
• RNA will copy both
introns and exons from
the DNA
– Introns need to be cut out
to make a working piece
of RNA
• Remaining pieces of
exons will be spliced
together to form the
final mRNA sequence
The Genetic Code
• Genetic code, or language of mRNA
instructions, is read three letters at a time
The Genetic Code
• Each three letter sequence, or “word”, codes
for one amino acid
• these three letter words are known as codons
• Codons are found on the mRNA
• The protein is determined by the order of each
amino acid in the sequence
The Genetic Code
• There are 64 possible
codons
• One codon, AUG, is known
as the “start” codon for
protein synthesis
• There are also three possible
sequences that are called
“stop” codons
– Show where polypeptide
(protein) ends
– UGA, UAA, UAG
Translation
• Making a protein from a
strand of mRNA
• before translation
occurs, mRNA is
transcribed from DNA
Steps of Translation
1. mRNA attaches to a
ribosome
– Each codon moves
through the
ribosome
– Codons are “read”
by the ribosome
Steps of Translation
2. tRNA brings the correct amino acid to
the ribosome
(amino acid matches the codon on the
mRNA sequence)
More about tRNA
• each tRNA carries only
one type of amino acid
– each tRNA has an
anticodon
– the anticodon sequence
is complimentary to
(matches) the codon (on
the mRNA)
Translation Continues…
3. Ribosome forms a
peptide bond between
the amino acids
(forming a protein)
Translation
4. The tRNA from the first
amino acid is then
released from the
ribosome
5. Ribosome then moves
to the next codon and a
new tRNA comes in
Translation Continues!
6. Polypeptide chain
(protein) grows until the
ribosome reaches a
“stop” codon
Translation Complete!
7. Once a stop codon is
reached, the ribosome
releases it’s polypeptide
chain and the mRNA
Label each Lettered Object
The Roles of RNA and DNA
• DNA is like the master plan of the cell
• If DNA is damaged, the whole cell could die
• RNA is like copies of this master plan that can
be taken all around the cell to be made into
product or proteins
• If RNA is damaged, it’s okay, more can be
made
Genes and Proteins
• genes code for proteins which are what carry
out expression of these genes
• proteins code for enzymes which cause certain
reactions to take place
– these reactions are what cause traits!