Transcript Chapter 17 From Gene to Protein

AP Biology
Ch. 17
From Gene
to Protein
How Genes Control
Metabolism




The study of metabolic defects provided
evidence that genes specify proteins.
Garrod (1909) suggested that genes
dictate phenotypes through enzymes that
catalyze reactions.
Some inherited diseases result from the
inability to produce certain enzymes.
Ex) alkaptonuria
Specific genes direct production of
specific enzymes.
One Gene-One Enzyme
Hypothesis




Beadle and Tatum studied the relationship
between genes and enzymes by studying
auxotrophs (nutritional mutants)
They determined that the mutants lacked certain
enzymes needed to produce necessary nutrients
from the food source.
One gene-one enzyme hypothesis: The
function of a gene is to dictate the production of
a specific enzyme.
This was later modified to one-gene, onepolypeptide. In most cases, a gene determines
the amino acid sequence of a polypeptide chain.

RNA- ribonucleic acid



RNA links DNA’s genetic instructions for
making proteins to the process of protein
synthesis.
It copies or transcribes the message from
DNA and then translates that message
into protein.
RNA differs from DNA in the following
ways: it has ribose instead of
deoxyribose, uracil instead of thymine,
and it is a single-stranded molecule.
Transcription and Translation
Transcription and translation are the
two main processes linking gene to
protein.
 Both nucleic acids and proteins are
informational polymers with linear
sequences of monomers—
nucleotides and amino acids,
respectively.

Transcription



Transcription is the synthesis of RNA
using a DNA template.
A gene’s unique nucleotide sequence is
transcribed from DNA to a
complementary nucleotide sequence in
mRNA.
mRNA carries this transcript to the
ribosomes for translation into protein to
take place.
Translation



Translation is the synthesis of a
polypeptide, which occurs under the
direction of mRNA.
The linear sequence of bases in mRNA is
translated into the linear sequence of
amino acids in a polypeptide.
Process occurs on ribosomes (composed
of rRNA) in the cytoplasm
The Genetic Code



The flow of information from gene to
protein is based on a triplet code.
Codons are three-nucleotide sequences
that specify which amino acids (61
codons) will be added to the growing
polypeptide.
Codons can also signal when translation
terminates (3 codons). The codon for
methionine (AUG) acts as a translational
start signal.

Genetic code is universal

Tobacco w/ firefly
genes

The genetic code must
have evolved early in
the history of life, and
is shared by bacteria
as well as complex
plants and animals.
Because diverse forms
of life share the same
genetic code, it is
possible to program
one species to
produce proteins
characteristic of
another species.
Template strand




Transcription is the DNA-directed
synthesis of RNA.
For each gene, only one of the two DNA
strands (template strand) is transcribed.
The complementary nontemplate strand
is the parent strand for making a new
template when DNA replicates.
mRNA is complementary to the DNA
template from which it is transcribed.

Template, cont.




RNA synthesis on a DNA template is
catalyzed by RNA polymerase.
Base-pairing rules are followed, except
that in RNA, uracil substitutes for
thymine.
Promoters signal the initiation of RNA
synthesis, and transcription factors help
eukaryotic RNA polymerase recognize
promoter sequences.
Transcription continues until a particular
RNA sequence signals termination.


Stages of transcription



Initiation- RNA polymerase binds to the
promoter, DNA strands unwind, enzyme
initiates RNA synthesis.
Elongation- Polymerase moves
downstream, unwinding the DNA and
elongating the RNA transcript. DNA
strands re-form a double helix.
Termination- Polymerase transcribe a
terminator sequence. RNA is released,
and polymerase detaches from the DNA.
Eukaryotes: RNA editing




Eukaryotic cells modify RNA after
transcription.
mRNA molecules are processed before
leaving the nucleus by modification of
their ends and by RNA splicing.
Most eukaryotic genes have introns
(noncoding regions) and exons (coding
regions).
In RNA splicing, introns are removed and
exons are joined.

RNA processing: addition of the 5’cap
And poly(A) tail

RNA splicing: introns are excised and exons are
Spliced together

Exons and proteins
In a number of genes, different
exons code for separate domains of
the protein product.
 Each domain, an independently
folding part of the protein, performs
a different function.
 New proteins can evolve by exon
shuffling among genes.


Protein synthesis:
Translation



After picking up specific amino acids,
tRNA molecule line up by means of their
anticodon triplets at complementary
codons on mRNA.
The attachment of amino acids to its
particular tRNA is an ATP-driven process.
Ribosomes coordinate the three stages of
translation: initiation, elongation, and
termination.

Transfer RNA (tRNA)



Molecules of tRNA are not identical.
Each type of tRNA links a particular
mRNA codon with a particular amino acid.
The tRNA bears an anticodon which base
pairs with the codon on the mRNA.
For example, if the mRNA codon is UUU
(phenylalanine), the anticodon on tRNA
would be AAA and it would carry
phenylalanine at its other end.


Ribosomes



Each ribosome is composed of two
subunits made of protein and ribosomal
RNA (rRNA).
Ribosomes have a binding site for mRNA;
P and A sites that hold tRNA as amino
acids are added to the polypeptide chain.
and an E site for release of tRNA.
Several ribosomes can work on a single
mRNA molecule at the same time,
forming a polyribosome.


Stages of Translation



Ribosomes coordinate the three stages of
translation: initiation, elongation, and
termination
Initiation: ribosomal subunit binds to a
molecule of mRNA, initiator tRNA pairs
with the start codon, AUG. This tRNA
carries the amino acid methionine.
Arrival of a large ribosomal subunit
completes the initiation complex.

Initiation of translation!
Translation: Elongation
Elongation adds amino acids to the
polypeptide chain.
 Codon recognition, peptide bond
formation, and translocation are the
steps.


Elongation:
Fill, bond, release and shift!
Translation: Termination



Elongation continues until a “stop” codon
in the mRNA is reached.
A protein called a release factor binds to
the stop codon, causing the addition of a
water molecule instead of an amino acid
to the polypeptide chain.
This frees the polypeptide from the
ribosome.

Termination of translation.
Signal peptides




Free ribosomes in the cytosol initiate the
synthesis of all proteins.
Proteins needed for membranes, or to be
exported from the cell, complete their synthesis
when the ribosomes making them attach to the
ER.
Signal-recognition particles (SRP) binds to the
leading end of the polypeptide chain, allowing
the ribosome to bind to the ER.
Other signal sequences target proteins for
chloroplasts and mitochondria.


Protein synthesis:
Prokaryotes vs. Eukaryotes



Prokaryotes lack nuclei, so DNA is not
segregated from ribosomes.
Transcription and translation occur in
rapid succession.
Eukaryotes have nuclear envelopes that
segregate transcription in the nucleus
from translation in the cytoplasm.
mRNA is modified extensively before it
moves from the nucleus to the cytoplasm
where translation occurs (RNA
processing)

Coupled transcription and translation in bacteria
Point mutations




Point mutations are changes in one base pair of
DNA.
Substitutions can cause missense (wrong
codon, wrong amino acid) or nonsense (codes
for stop signal) mutations.
Insertions or deletions can produce
frameshift mutations that disrupt the mRNA
reading frame “downstream” of the mutation.
Spontaneous mutations can occur during DNA
replication or repair, sometimes caused by
chemical and physical mutagens.

Point mutation: substitution causes sickle-cell


