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RNA & Protein synthesis
DNA vs. RNA
• DNA can be compared
to the “master plan”
of a builder
• It stays safely in the
nucleus (office)
• Double stranded
• Contains thymine
• Contains deoxyribose
• RNA can be compared to
the “disposable
copies/blueprints”
• It goes to the proteinbuilding ribosome site in
the cytoplasm (job site)
• RNA is made from the
DNA
• Single stranded
• Contains uracil
• Contains ribose
How are genes expressed?
• Genes are coded DNA instructions that control
the production of proteins within the cell.
• The 1st step in decoding the DNA is to copy
part of the nucleotide sequence into RNA
• RNA = ribonucleic acid
• The RNA’s make proteins
What is RNA?
• RNA consists of a long chain of nucleotides
(like DNA)
– A sugar called ribose
– A phosphate group
– A nitrogenous base
Adenine (A)
Uracil (U) … not Thymine (T)
Cytosine (C)
Guanine (G)
• RNA is single-stranded
RNA Nucleotide
Phosphate
Group
Nitrogenous
Base
Ribose
Sugar
RNA Structure
Nitrogenous
Base
Ribose Sugar
Types of RNA
• RNA’s assemble amino acids into proteins
• There are 3 types of RNA
– Messenger RNA (mRNA)
– Ribosomal RNA (rRNA)
– Transfer RNA (tRNA)
Messenger RNA
• Copies instructions in genes
• Serves as a “messenger” from the DNA to the
cell
Ribosomal RNA
• Ribosomes make proteins
• Ribosomes are made of proteins and contain
their own RNA
Transfer RNA
• Transfer amino acids to ribosomes based on
specified codes in mRNA
Section 12-3
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Section:
How is RNA made?
• RNA is made by transcription: DNA to RNA
• Transcription uses an enzyme RNA polymerase
• During transcription, RNA polymerase binds to DNA and
separates the DNA strands, RNA polymerase then uses one
strand of DNA as a template (stencil) from which nucleotides
are assembled into a strand of RNA
– For example:
DNA:
RNA:
ACTGTGGACCT
UGACACCUGGA
http://www.johnkyrk.com/DNAtranscription.html
TRANSCRIPTION
The Genetic Code
• Remember: Proteins are chains of amino acids
called polypeptides
• The order of the amino acids determines the
properties of the protein
• The instructions for making different amino
acids are in the mRNA = the genetic code
• The genetic codes is read 3 letters at a time,
so each “word” is 3 bases long = codon
How are proteins made?
• Proteins are made by translation: RNA to protein
• Translation occurs in the cytoplasm
• During translation, the cell uses information from
mRNA to make proteins.
• mRNA instructs amino acids on tRNA to join together
in the ribosome containing rRNA
http://www.johnkyrk.com/DNAtranslation.html
What is tRNA again?
• Transfer RNA brings amino acids to the
ribosome.
• One end of the tRNA has an amino acid, the
other end has 3 bases called the anticodon,
that are complementary to one of the mRNA
codons
Amino acid
Anticodon
Codons
• Three consecutive nucleotides that
specify a single amino acid that is to
be added to the polypeptide
– For example:
RNA:
Codons:
UCGCACGGU
UCG-CAC-GGU
Amino acids: Serine-Histidine-Glycine
There are 64 possible codon
combinations, but there are only
20 amino acids!
*More than one codon can code
for an amino acid
When does it begin or end?
• There is 1 “start” codon that
initiates protein synthesis if
it is the 1st codon (otherwise
it codes for an amino acid)
– AUG
• There are 3 “stop” codons
that do not code for any
amino acid and signify the
end of a polypeptide.
– UAA
– UAG
– UGA
Let’s try to make a protein!
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•
•
•
•
DNA:
CCAGATAGGTTT
mRNA:
GGUCUAUCCAAA
codons:
GGU-CUA-UCC-AAA
tRNA:
CCA-GAU-AGG-UUU
Amino acids for the mRNA codons
on tRNA’s:
Glycine-Leucine-Serine-Lysine