Transcript Protein Synthesis

Gene- a series of nucleotides found on a strand of DNA that
codes for a particular protein
Protein Synthesis- when the info stored on a gene is “read” and
then used to make a protein
3 Parts to Protein Synthesis:
1. Transcription- creating RNA from DNA (occurs in the
nucleus)
2. Processing- When RNA is trimmed of its non-proteincoding nucleotides, made small enough that it can exit the
nucleus
3. Translation- The RNA assembles a specific sequence of
amino acids to make the desired protein (occurs on the
ribosome)
The Big Picture!!
• There are 3 macromolecules
involved in Protein Synthesis
• DNA makes RNA
• RNA codes for proteins
• Proteins carry out ALL of the
cell’s important functions
and activities
Recall: There are Two Types of
Nucleic Acids

DNA – Deoxyribonucleic
Acid
RNA – Ribonucleic Acid
Similarities:


1.
2.
3.
4.
Nucleic acids
Store genetic material
Made up of strands of
nitrogen bases
Follow some type of Base
Pairing Rules
Differences Between RNA/DNA
 DNA:
 Double-stranded
 Cytosine-Guanine
 Adenine- Thymine
 5-carbon sugar =
deoxyribose
 Never leaves Nucleus
 RNA:
 Single-Stranded
 Cytosine –Guanine
 Adenine- Uracil
 5-carbon sugar = ribose
 Can leave nucleus
RNA types

Three major types: (to be discussed further)
1.
2.
3.
Messenger (mRNA)
Ribosomal (rRNA)
Transfer (tRNA)
mRNA
 Temporary copy of a gene that encodes for
a protein;
 Provides the pattern that determines the
order and types of amino acids making
that protein.
 Very unstable, cell WILL try to break it
down unless it is processed
 Contain codons (3-base sequence)
 Precursor mRNA – exists in nucleus
 Mature mRNA – genes to be copied by
ribosome (has been processed) exists in
cytoplasm
rRNA
 Makes up 80% of RNA in cells
 Used to make portions of ribosomes
in cells in combination with
proteins.
 Can be free floating
 Or attached to Endoplasmic
reticulum (RER)
tRNA
 Free floating in cell
 Enzyme attaches ONE tRNA
to ONE amino acid(charged
tRNA)
 tRNA ‘carries’ the amino acid
to ribosome to add to the
growing polypeptide
(protein) chain
 Contains an anti-codon
sequence (3-bases as bottom
of tRNA), which is
complementary to each
codon
The Genetic Code• The nucleotides serve as the four “letters” of the DNA
“alphabet” (A,C,G, & T)
• 3 nucleotides make a Codon (ex. AAG)
• Codons code for an Amino Acid (AAG codes for lysine)
• Amino Acids are the building blocks for proteins
• Since there are 4 nucleotides, when three are grouped
together, there are 64 possible triplet combinations (43 = 64)
• However, there are only 20 amino acids so some amino acids
have more than one codon (ex. GGA, GGC, and GGG all code
for glycine)
Summary:
 DNA = permanent copy (like your hard drive)
 RNA = Temporary copy (similar to floppy copy)
 Leads up to two of the most important processes:
Transcription and Translation
 animation
Part 1: Transcription
The building of mRNA, tRNA, and rRNA (takes place in
the NUCLEUS)
STEPS:
 Initiation: RNA polymerase binds to the gene’s promoter on the
template strand of DNA(a specific sequence of the DNA that acts as a
“start signal” for transcription)
 RNA polymerase unwinds and separates the two strands of the DNA.
 Elongation: RNA polymerase adds and then links complementary
RNA nucleotides as it “reads” the gene”.
 Termination: A “stop” signal on the DNA tells the RNA polymerase to
detach from the DNA and release the RNA molecule.
 Tutorial/Quiz
Part 2: RNA Processing
Before leaving the nucleus……..
1. A cap (methyl-guanine or mG) is added by
enzymes to the starting end of the mRNA
molecule
2. A poly-A tail is added to the end of the
mRNA
3. The molecule is spliced. Introns are
removed (non-coding nucleotides) and
exons remain.
RNA Processing Animation
Part 3: Translation
Tutorial
The assembly of a protein (occurs
on the ribosome in the cytoplasm)
1. mRNA leaves the nucleus
through the nuclear pore and
forms a functional ribosome with
two ribosomal subunits, and a
tRNA
2. Initiation: The mRNA “start”
codon AUG is oriented in a
region of the ribosome called the
P site where the tRNA molecule
carrying methionine can bind to
the start codon.
Translation- cont. (another animation)
3. The codon in the area of the ribosome
called the A site is ready to receive the
next tRNA.
4. Elongation: Both the A site and the P site
are holding tRNA molecules- each carrying
a specific amino acid. A peptide bond
forms between the adjacent amino acids
5. The tRNA in the P site detaches and
leaves its amino acid behind
6. The tRNA in the A site moves to the P
site. The tRNA carrying the amino acid
specified by the codon in the A site arrives.
7. Termination: Steps 4-6 are repeated
until a stop codon is reached.
Translation animation
One more Translation animation
If a segment of DNA is:
TAC AAA GTA ACT
The mRNA strand would be:
AUG UUU CAU UGA
This would code for the following amino acids:
Met
Phe
His
Stop
Types of Mutations
Point Mutation- a change of one or just a few
nucleotides in a gene. 3 Main Types of Point
Mutations:
•Substitutionone nucleotide is replaced by another.
•Insertion –
an extra nucleotide is added.
•Deletiona nucleotide is omitted.
Mutations can be “Silent”
When a substitution mutation in a base pair does
not result in a change in the sequence of amino
acids in a protein
Mutations can be “Neutral”
A change in a base pair results in an amino acid
change, but the new amino acid has the same chemical
properties as the old amino acid. (ie hydrophobic to
hydrophobic; acidic to acidic)
Mutations can be
“Missense”
a mutation results a change in an amino acid where
the new amino acids has a different property than the
old amino acid. The protein with the new primary
structure may have reduced or no activity.
Mutations can be “Nonsense”
A mutation results in a new “stop” translation
condon formed before the naturally occurring one.
Translation is stopped prematurely and a shortened
protein is made.
Mutations can result in
“Frameshifts”
A deletion or insertion of one base results in a
change in the translational reading frame