Transcript File - Mr. Doyle SUIS Science

9.5 Translation: RNA to Protein
• Translation converts genetic information carried by an mRNA
into a new polypeptide chain
• The order of the codons in the mRNA determines the order of
the amino acids in the polypeptide chain
Translation
• Translation occurs in the cytoplasm of cells
• Translation occurs in three stages:
• Initiation
• Elongation
• Termination
Initiation
• An initiation complex is formed
• A small ribosomal subunit binds to mRNA
• The anticodon of initiator tRNA base-pairs with the start
codon (AUG) of mRNA
• A large ribosomal subunit joins the small ribosomal
subunit
Elongation
• The ribosome assembles a polypeptide chain as it moves
along the mRNA
• Initiator tRNA carries methionine, the first amino acid of
the chain
• The ribosome joins each amino acid to the polypeptide
chain with a peptide bond
Termination
• When the ribosome encounters a stop codon, polypeptide
synthesis ends
• Release factors bind to the ribosome
• Enzymes detach the mRNA and polypeptide chain from
the ribosome
start codon
(AUG)
initiator tRNA
first amino acid
of polypeptide
1 Ribosome
subunits and an
initiator tRNA
converge on an
mRNA. A second
tRNA binds to the
second codon.
3 The first tRNA
is released and the
ribosome moves to
the next codon. A
third tRNA binds to
the third codon.
5 The second
tRNA is released
and the ribosome
moves to the next
codon. A fourth
tRNA binds the
fourth codon.
peptide bond
2 A peptide
bond forms
between the
first two amino
acids.
4 A peptide bond
forms between the
second and third
amino acids.
6 A peptide bond
forms between the
third and fourth
amino acids.
The process repeats
until the ribosome
encounters a stop
codon in the mRNA.
Stepped Art
Figure 9-11 p156
Transcription
polysomes
ribosome
subunits
tRNA
Convergence of RNAs
mRNA
Translation
polypeptide
Figure 9-12a p157
ANIMATED FIGURE: Translation
To play movie you must be in Slide Show Mode
PC Users: Please wait for content to load, then click to play
Mac Users: CLICK HERE
Polysomes
• Many ribosomes may simultaneously translate the same
mRNA, forming polysomes
mRNA
polysomes
newly forming polypeptide
Take-Home Message:
How is mRNA translated into protein?
• Translation converts protein-building information carried by
mRNA into a polypeptide
• During initiation, an mRNA, an initiator tRNA, and two
ribosome subunits join
• During elongation, amino acids are delivered to the complex
by tRNAs in the order dictated by successive mRNA codons;
the ribosome joins each to the end of the polypeptide chain
• Termination occurs when the ribosome reaches a stop codon
in the mRNA; the mRNA and the polypeptide are released,
and the ribosome disassembles
9.6 Mutated Genes
and Their Protein Products
• If the nucleotide sequence of a gene changes, it may result in
an altered gene product, with harmful effects
• Mutations
• Small-scale changes in the nucleotide sequence of a
cell’s DNA that alter the genetic code
Mutations and Proteins
• A mutation that changes a UCU codon to UCC is “silent” – it
has no effect on the gene’s product because both codons
specify the same amino acid
• Other mutations may change an amino acid in a protein, or
result in a premature stop codon that shortens it – both can
have severe consequences for the organism
Common Mutations
• Base-pair-substitution
• May result in a premature stop codon or a different amino
acid in a protein product
• Example: sickle-cell anemia
• Deletion or insertion
• Can cause the reading frame of mRNA codons to shift,
changing the genetic message
• Example: thalassemia
Hemoglobin and Anemia
• Hemoglobin is a protein that binds oxygen in the lungs and
carries it to cells throughout the body
• The hemoglobin molecule consists of four polypeptides
(globins) folded around iron-containing hemes – oxygen
molecules bind to the iron atoms
• Defects in polypeptide chains can cause anemia, in which a
person’s blood is deficient in red blood cells or in hemoglobin
Mutations in the Beta Globin Gene
Figure 9-13a p158
Figure 9-13b p158
Figure 9-13c p158
Figure 9-13d p158
Figure 9-13e p158
Sickle-Cell Anemia
• Sickle-cell anemia is caused by a base-pair substitution which
produces a beta globin molecule in which the sixth amino acid
is valine instead of glutamic acid (sickle hemoglobin, HbS)
• HbS molecules stick together and form clumps – red blood
cells become distorted into a sickle shape, and clog blood
vessels, disrupting blood circulation throughout the body
• Over time, sickling damages organs and causes death
sickled cell
glutamic acid valine
normal cell
Figure 9-14 p159
ANIMATED FIGURE: Sickle-cell anemia
To play movie you must be in Slide Show Mode
PC Users: Please wait for content to load, then click to play
Mac Users: CLICK HERE
Thalassemia and Frameshifts
• Another type of anemia, beta thalassemia, is caused by the
deletion of the twentieth base pair in the beta globin gene
• Deletions cause a frameshift, in which the reading frame of
the mRNA codons shifts
• Frameshifts garble the genetic message, just as incorrectly
grouping a series of letters garbles the meaning of a sentence
Thalassemia and Transposable Elements
• Beta thalassemia can also be caused by insertion mutations,
which also cause frameshifts
• Insertion mutations are often caused by the activity of
transposable elements, which are segments of DNA that
can insert themselves anywhere in a chromosome
Take-Home Message: What
happens
after a gene becomes mutated?
• Mutations that result in an altered protein can have drastic
consequences
• A base-pair substitution may change an amino acid in a
protein, or shorten it by introducing a premature stop codon
• Frameshifts that occur after an insertion or deletion change
an mRNA’s codon reading frame, so they garble its proteinbuilding instructions
ANIMATED FIGURE: Base-pair substitution
To play movie you must be in Slide Show Mode
PC Users: Please wait for content to load, then click to play
Mac Users: CLICK HERE
ANIMATION: Frameshift mutation
To play movie you must be in Slide Show Mode
PC Users: Please wait for content to load, then click to play
Mac Users: CLICK HERE