Download From DNA to Proteins

KEY CONCEPT 8.5
Translation converts an mRNA message into
a polypeptide, or protein.
Genetic Code:
•mRNA is read by
tRNA 3 bases at a
time
•Each 3 base
sequence of mRNA
is called a codon
•Each codon codes
for a certain amino
acid (20 aa’s)
The genetic code matches each RNA codon with its amino acid or function.
The genetic code matches each codon to its
amino acid or function.
–three stop codons
–one start codon,
codes for
methionine
The genetic code matches each RNA codon with its amino acid or function.
• A change in the order in which codons
are read changes the resulting protein.
• Regardless of the organism, codons
code for the same amino acid.
AMINO ACIDS ARE LINKED TO BECOME A PROTEIN.
•An anticodon is a set of three nucleotides that is
complementary to an mRNA codon.
•An anticodon is carried by a tRNA.
Ribosomes consist of two subunits.
• The large subunit has three binding sites for
tRNA.
• The small subunit binds to mRNA.
AMINO ACIDS ARE CODED BY MRNA BASE SEQUENCES.
Step 2: Translation converts mRNA messages into
polypeptides.
A codon is a sequence of three nucleotides that codes
for an amino acid.
codon for
methionine (Met)
codon for
leucine (Leu)
1. For translation to begin, tRNA binds to a start
codon and signals the ribosome to assemble.
2. A complementary tRNA molecule binds to the
exposed codon, bringing its amino acid close to the
first amino acid.
3. The ribosome helps form a polypeptide bond
between the amino acids.
4. The ribosome pulls the mRNA strand the
length of one codon.
5. The now empty tRNA molecule exits the ribosome.
6. A complementary tRNA molecule binds to the next
exposed codon.
7. Once the stop codon is reached, the ribosome
releases the protein and disassembles. Protein assembly
EXAMPLE OF PROTEIN SYNTHESIS:
Strand of DNA: T A C
mRNA codon:
tRNA anticodon:
amino acids:
GCA TGG
THE STUDENT IS EXPECTED TO:
5C DESCRIBE THE ROLES OF DNA,
RIBONUCLEIC ACID (RNA) AND
ENVIRONMENTAL FACTORS IN CELL
DIFFERENTIATION;
6C EXPLAIN THE PURPOSE AND PROCESS
OF TRANSCRIPTION AND TRANSLATION
USING MODELS OF DNA AND RNA;
6D RECOGNIZE THAT GENE EXPRESSION IS
A REGULATED PROCESS;
6E IDENTIFY AND ILLUSTRATE CHANGES IN
DNA AND EVALUATE THE SIGNIFICANCE OF
THESE CHANGES
KEY CONCEPT 8.6
Gene expression is carefully regulated in
both prokaryotic and eukaryotic cells.
PROKARYOTIC CELLS TURN GENES ON AND OFF BY
CONTROLLING TRANSCRIPTION.
•A promoter is a DNA segment that allows a gene
to be transcribed.
•An operator is a part of DNA that turns a gene
“on” or “off.”
•An operon includes a promoter, an operator, and
one or more structural genes that code for all the
proteins needed to do a job.
In prokaryotes, The lac operon acts like a
switch.
• The lac operon is “off ” when lactose is not present.
• The lac operon is “on” when lactose is present.
Eukaryotic Genes
TATA box - may help to position RNA
polymerase.
Enhancer sequence – proteins bind to the
enhancer sequence and may open up
chromatin, attract RNA pol, or act as
repressor proteins
Eukaryotic genes are more complex
than prokaryotic genes – why?
EUKARYOTES REGULATE GENE EXPRESSION AT MANY POINTS.
•Different sets of genes are expressed in
different types of cells.
Transcription in eukaryotes is controlled by
regulatory DNA sequences and protein
transcription factors.
• Most eukaryotes have a TATA box promoter.
• Enhancers and silencers speed up or slow down
the rate of transcription.
• Each gene has a unique combination of
regulatory sequences.
RNA processing is also an important part of
gene regulation in eukaryotes.
• 3 major steps: Introns are removed
and exons (coding regions) are spliced
together.
• A cap is added.
• A tail is added.
ENVIRONMENTAL FACTORS INFLUENCE GENE EXPRESSION,
RESULTING IN DIFFERENT CELL TYPES.
Internal factors can affect cell
differentiation.
Uneven distribution of
proteins, mRNA, and
organelles within a cell can
affect gene expression.
Surrounding cells can influence
one another by sending and
receiving signal molecules.
Factors in an organism’s external environment can
also affect gene expression.
Too much oxygen in an organism’s environment
can prevent transcription proteins from being
made by the cells.
Temperature can influence gene expression.
Drugs or chemicals in the external environment
can influence cell differentiation.
Light can change the way that genes are
expressed.
THE STUDENT IS EXPECTED TO:
6E IDENTIFY AND ILLUSTRATE
CHANGES IN DNA AND EVALUATE
THE SIGNIFICANCE OF
THESE CHANGES
KEY CONCEPT 8.7
Mutations are changes in DNA that may
or may not affect traits.
SOME MUTATIONS AFFECT A SINGLE GENE, WHILE
OTHERS AFFECT AN ENTIRE CHROMOSOME.
•A mutation is a change in an organism’s DNA.
•Many kinds of mutations can occur, especially during
replication.
•A point mutation substitutes one nucleotide for another.
mutated
base
EXAMPLE OF A POINT MUTATION: CYSTIC FIBROSIS
Cystic fibrosis (CF) is a recessive inherited disease.
It is caused by point mutations in the CFTR gene, which codes
for a transmembrane protein that acts as an ion pump.
The CFTR gene is found on chromosome 7. It codes for 1480 amino
acids. There are over 1000 known mutations, which can affect the
function of the CFTR gene in different ways.
In around 70% of cases CF is caused by a triplet deletion, resulting
in the removal of an amino acid from the polypeptide chain
produced.
MUTATIONS
A frameshift mutation inserts or deletes a
nucleotide in the DNA sequence.
FIGURE 12–20 CHROMOSOMAL
MUTATIONS
Section 12-4
Deletion
Duplication
Inversion
Translocation
• Chromosomal mutations affect many genes.
• Chromosomal mutations may occur
during meiosis (will study this soon)
–Chromosomal mutations affect many
genes.
–Gene duplication results from unequal
crossing over.
• Translocation results from the exchange of
DNA segments between different
chromosomes.
MUTATIONS MAY OR MAY NOT AFFECT TRAITS.
Chromosomal mutations tend to have a big effect.
Some gene mutations change phenotype.
A mutation may cause a premature stop codon.
A mutation may change protein shape or the active site
A mutation may change gene regulation.
blockage
no blockage
Some gene mutations do not affect phenotype.
• A mutation may be silent.
• A mutation may occur in a noncoding
region.
• A mutation may not affect protein folding or
the active site.
Mutations in body cells do not affect offspring.
• Mutations in sex cells can be
harmful or beneficial to offspring.
• Natural selection often removes
mutant alleles from a population
when they are less adaptive.
MUTATIONS CAN BE CAUSED BY SEVERAL FACTORS.
•Replication errors can cause
mutations.
•Mutagens, such as UV ray and
chemicals, can cause mutations.
•Some cancer drugs use
mutagenic properties to kill
cancer cells.