DNA, RNA, and Protein
Synthesis
Function of DNA as “master” program
• DNA codes for the primary structure of a protein which impacts the
tertiary structure, which determines the function of the protein
• DNA doesn’t control protein synthesis directly…instead
DNA VS RNA
Differences Between DNA and RNA
DNA
double-stranded
RNA
single-stranded
sugar = deoxyribose
sugar = ribose
bases = A,T,C,G
bases = A,U,C,G
(uracil takes the place of thymine)
Types of RNA
Messenger RNA (mRNA) – temporary copy of a gene
that carries information from nucleus to ribosome
Ribosomal RNA (rRNA) – a type of RNA that makes up
ribosomes (where protein synthesis takes place)
Transfer RNA (tRNA) – a type of RNA that attaches to
the amino acids used to make the protein
mRNA
tRNA
rRNA
How is DNA like the English language?
• Language analogy – 26 letters in the
English language that can combine to
make millions words
• Genetic code – DNA is made of 4 letters
(Nitrogen bases), and 3 letters form a
word (amino acid); but there are only 20
possible “words”
*So how many DNA nucleotides are needed
to code for the 20 possible amino acids?
Why are Proteins important?
• Material that makes up cell structures or tissues
(keratin, collagen, myosin, etc.)
• Enzymes that make the chemical reactions of living
systems happen quickly
• Proteins bond to other molecules (ex. hemoglobin &
oxygen in blood)
• Key role in communication; hormones – chemical
signals given off by cells that regulate behavior of cells
in different area
Protein Synthesis requires 2 processes:
1. Transcription – the process of making mRNA
from DNA
- occurs in nucleus
2. Translation – the process in which amino acids
(coded for in the mRNA) are added to a protein
being made = PROTEIN SYNTHESIS
- occurs in cytoplasm
RNA Synthesis (Transcription)
• Produces mRNA
• One strand of DNA acts as the
template for RNA synthesis
(C ↔ G; A → U; T → A)
• Main enzyme = RNA polymerase,
which adds the RNA nucleotides to
pair with the DNA
• Occurs in 3 stages
Transcribe the following DNA:
Transcription – 3 Stages
1. Initiation
- RNA polymerase binds to the DNA at the
promoter region. It is directed to this
region by proteins called initiation factors.
2. Elongation
- RNA polymerase moves along the coding
strand of DNA away from the promoter,
building the RNA strand.
3. Termination
- RNA polymerase reaches the terminator
region of the DNA & is released from the DNA
with the primary transcript.
RNA Processing
• All 3 types of RNA (mRNA, tRNA, &
rRNA) are processed in the nucleus
before they leave it.
• This can involve adding, chemically
changing, and/or removing
nucleotides. (proofreading)
RNA Splicing
All RNA transcripts undergo processing called splicing.
• The removal of meaningless segments of RNA, called introns, and fusing
together the remaining, meaningful sections, called exons.
• Requires precise recognition of the site to be cut by a restriction enzyme
Protein Synthesis Step 2- Translation
• mRNA is translated into the amino acid
sequence of a protein.
• It happens on ribosomes in the cytoplasm.
• ATP provides the energy to form the bonds.
• Divided into same 3 stages as Transcription:
initiation, elongation, & termination.
• tRNA acts as a carrier molecule that transfers amino acids to the
ribosome.
• A tRNA anticodon matches with an mRNA codon. (3 nitrogen bases)
Practice:
mRNA: AUG CGA AAC UCA
tRNA:
tRNA and Amino Acids
-There are 64 possible
codon combinations (3
nitrogen bases) BUT
only 20 amino acids.
- SO – some codon
“words” code for the
same amino acid.
- We can interpret or
“read” this code using a
codon table:
Let’s try “AUG”
Now try:
• CCU
• CCG
• UGA
• GGA
What do you notice?