Download Nervous System

Protein Synthesis
From Gene to Polypeptide
What is a gene?
 Recall that a gene is a specific section
of DNA along the length of a
chromosome.
 It has a beginning (the “promoter”) and
an end (the “termination signal”).
 A gene holds the information (“recipe”)
for making a specific polypeptide
(protein).
Gene 1 and Gene 2
have different N-base
sequences.
Insulin and the receptor
protein differ in the #,
types, and sequence of
amino acids.
Gene
Nucleotide
DNA Structure
An overview of the path from DNA to protein
DNA vs. RNA
 Similarities
 Nucleic acids
 Made of nucleotides with a sugar,
nitrogen base, and phosphate groups
 Differences
 RNA has ribose sugar (instead of
deoxyribose)
 RNA has uracil instead of thymine
 RNA is single-stranded
Base-pairing between DNA & RNA
during transcription
Videos:
 http://www.teachersdomain.org/reso
urce/tdc02.sci.life.gen.proteinsynth/
 http://www.californiastreaming.org/pl
ayer.asp?mediaID=12495&segmentID
=20846
mRNA
 Location: nucleus and cytoplasm
 Structure: single, unfolded chain of
nucleotides. Contains a series of
codons.
 3 bases = codon = code for an a.a.
 Function: to carry the “recipe” for
the protein from the nucleus to the
ribosomes.
tRNA
 Location: cytoplasm
 Structure: single chain folded into
“cloverleaf” shape. Has an anticodon
on one end and binds an amino acid
on the opposite end.
 Function: to transfer (deliver) the
correct amino acids to the ribosome.
 There are many different tRNAs; each
carries a different a.a.
rRNA/Ribosome
 Location: cytoplasm (“free” or on the
rough ER)
 Structure: rRNA, combined with
proteins. Has two subunits.
 Function: coordinates the building of
a protein. Binds mRNA and tRNA;
catalyzes peptide bond formation.
Transcription: What
is it?
 Creating the “messenger”
 Making a strand of mRNA using DNA as a
template.
 mRNA = copy of gene
 Allows for transfer of info from DNA to
site of protein synthesis (cytoplasm)
Translation: What it is
Making a polypeptide (chain of
amino acids) using an mRNA
template.
 mRNA = copy of gene
 Every 3 letters on mRNA = codon = codes for a
specific amino acid (a.a.).
e.g.: the codon AUG codes for methionine (“met”)
 Amino acids will be bonded together in a specific
sequence. Correct a.a. sequence is critical to the
protein being functional (having the right shape).
Steps of Transcription:
1. RNA polymerase binds to a
promoter on specific gene in DNA

Promoter marks beginning of info to be
transcribed
2. DNA molecule in that region
“unzips”

Hydrogen bonds are broken and strands
move apart. Only one of the DNA strands
is used as template for transcription
Transcription, cont’d…
3. RNA nucleotides are paired to
complementary bases on the DNA
template strand
 Complementary base-pairing: C-G, G-C, TA, A-U
4. Covalent bonds (S-P) connect the RNA
nucleotides into a single strand.
 RNA Polymerase helps position the
nucleotides and catalyzes the formation of the
covalent bonds.
Transcription, cont’d…
5. mRNA strand lengthens until RNA
polymerase reaches a “termination
signal”
6. mRNA strand separates from the
DNA template.
 DNA re-twists into double-helix.
mRNA Carries Message from
Nucleus to Cytoplasm
http://nobelprize.org
/educational/medicin
e/dna/b/transcriptio
n/transcription_ani.h
tml
http://www.teachersd
omain.org/resource/ls
ps07.sci.life.stru.celltr
ans/
Problem Solving
 If you know the DNA sequence, you
can determine the mRNA sequence
More Practice
 Use the DNA sequence below to
determine the mRNA sequence:
DNA: T-A-C-G-A-T-A-T-C
mRNA: A-U-G-C-U-A-U-A-G
mRNA-tRNA connection
 mRNA contains the “codes” for the
amino acids.
 tRNA delivers the amino acids.
 tRNA brings the right a.a. to the right
place in the protein by binding to the
mRNA.
 Codon and anticodon are
complementary.
The “Genetic Code”
 The amino acid
brought by tRNA is
specified by the
mRNA codon, NOT
the anticodon.
http://www.dnalc.org/view/155
13-How-many-bases-code-foran-amino-acid-3D-animationwith-basic-narration-.html
http://www.dnalc.org/view/164
94-Animation-22-DNA-wordsare-three-letters-long-.html
mRNA, tRNA, and a ribosome work
together in constructing a protein
 mRNA = “messenger”; it contains the
message that is being translated.
 tRNA = “transfer”; it transfers (delivers) the
right a.a. to the right codon. It is the
“interpreter”.
 When you translate an mRNA sequence, you are
the “interpreter”. You determine the correct a.a.
for each codon by looking at the Genetic Code
table.
 rRNA = “ribosomal”; as part of the structure
of the ribosome, it helps bond amino acids
together.
At the ribosome: Steps of
Translation
1. mRNA attaches to a ribosome in the
cytoplasm.
2. A tRNA, carrying the amino acid
“met”, binds to the start codon (AUG)
on mRNA. A second tRNA w/an a.a.
binds to the ribosome.
 H-bonds between complementary base
pairs allows the tRNA to bind with mRNA
(anticodon to codon).
3. A peptide bond is formed between the
a.a.
Translation, cont’d
4. The first tRNA leaves and the ribosome
moves over to a new codon.
5. The process of a.a. delivery and peptide
bond formation continues until the chain is
100+ amino acids long.
6. When a stop codon is reached, translation
ends and the polypeptide is released.
7. The polypeptide’s a.a. sequence will determine
how the protein folds into a 3-D functional
shape.
http://nobelprize.org/educational/medicine/dna/b/translation/translation_ani.html
http://www.teachersdomain.org/resource/lsps07.sci.life.stru.celltrans/
Mutations
 Changes in the N-base sequence of
DNA.
 Typically 1 letter: substitution, deletion, or
insertion
 The change in DNA results in one or more different mRNA
codons.
 The different codon(s) may result in a different amino
acid(s).
 A change in the amino acid sequence of a protein will
likely change the protein’s shape. (Remember-shape
determines function!)
Substitution Mutation
“Sickle” shape of the red blood
cell is due to the hemoglobin
proteins having an abnormal
shape. Due to the difference of
one amino acid (val vs glu), the
protein doesn’t fold correctly.
Beginning of
Gene is located.
DNA unwinds &
transcription
begins.
As RNA polymerase
moves down the DNA
template, more RNA
nucleotides are added,
growing the RNA strand.
Transcription stops
at the end of the
gene.
The amino acid “Tyr” is
carried by a tRNA, but it is
coded for by the mRNA
codon (UAC)