David Sadava H. Craig Heller Gordon H. Orians
William K. Purves David M. Hillis
Biologia.blu
B – Le basi molecolari della vita e
dell’evoluzione
From DNA to Protein:
Genotype to Phenotype
From DNA to Protein: Genotype to Phenotype
• What is the evidence that genes code for
proteins?
• How does information flow from genes to
proteins?
• How is the information content in DNA
transcribed to produce RNA?
• How is RNA translated into proteins?
• What are mutations?
From DNA to Protein: Genotype to Phenotype - What is the evidence that genes code
for proteins?
The molecular basis of phenotypes was
known before it was known that DNA is
the genetic material.
Studies of many different organisms
showed that major phenotypic
differences were due to specific
proteins.
From DNA to Protein: Genotype to Phenotype - What is the evidence that genes code
for proteins?
The gene-enzyme relationship has been
revised to the one-gene, one-polypeptide
relationship.
Example: in hemoglobin, each polypeptide
chain is specified by a separate gene.
Other genes code for RNA that is not
translated to polypeptides; some genes are
involved in controlling other genes.
From DNA to Protein: Genotype to Phenotype - How does information flow from genes
to proteins?
Expression of a gene to form a
polypeptide:
• transcription—copies information from
gene to a sequence of RNA;
• translation—converts RNA sequence to
amino acid sequence.
From DNA to Protein: Genotype to Phenotype - How does information flow from genes
to proteins?
RNA, ribonucleic acid differs from DNA:
• it usually has one strand;
• the sugar is ribose;
• it contains uracil (U) instead of thymine (T).
From DNA to Protein: Genotype to Phenotype - How does information flow from genes
to proteins?
RNA can pair with a single strand of
DNA, except that adenine pairs with
uracil instead of thymine.
Single-strand RNA can fold into complex
shapes by internal base pairing.
From DNA to Protein: Genotype to Phenotype - How does information flow from genes
to proteins?
Messenger hypothesis—messenger
RNA (mRNA) forms as a
complementary copy of DNA and
carries information to the cytoplasm.
This process is called transcription.
From DNA to Protein: Genotype to Phenotype - How does information flow from genes
to proteins?
The central dogma of molecular biology:
information flows in one direction when genes
are expressed (by Francis Crick).
From DNA to Protein: Genotype to Phenotype - How does information flow from genes
to proteins?
The central dogma raised two questions:
• how does genetic information get from
the nucleus to the cytoplasm?
• what is the relationship between a DNA
sequence and an amino acid sequence?
From DNA to Protein: Genotype to Phenotype - How does information flow from genes
to proteins?
From gene to protein
From DNA to Protein: Genotype to Phenotype - How does information flow from genes
to proteins?
There is an exception to the central dogma.
Viruses: acellular particles that reproduce
inside cells; many have RNA instead of DNA.
From DNA to Protein: Genotype to Phenotype - How does information flow from genes
to proteins?
Transcription occurs in three phases:
1.initiation;
2.elongation;
3.termination.
From DNA to Protein: Genotype to Phenotype - How does information flow from genes
to proteins?
DNA is transcribed to form RNA (phase 1)
From DNA to Protein: Genotype to Phenotype - How does information flow from genes
to proteins?
DNA is transcribed to form RNA (phases 2 and 3)
From DNA to Protein: Genotype to Phenotype - How is the information content in DNA
transcribed to produce RNA?
The genetic code specifies which amino
acids will be used to build a protein.
Codon: a sequence of three bases. Each
codon specifies a particular amino acid.
Start codon: AUG—initiation signal for
translation.
Stop codons stops translation and
polypeptide is released.
From DNA to Protein: Genotype to Phenotype - How is the information content in DNA
transcribed to produce RNA?
The genetic code
From DNA to Protein: Genotype to Phenotype - How is the information content in DNA
transcribed to produce RNA?
The genetic code is nearly universal: the
codons that specify amino acids are the same
in all organisms. Exceptions: within
mitochondria and chloroplasts, and in one
group of protists.
This genetic code is a common language for
evolution.
The code is ancient and has remained intact
throughout evolution.
The common code also facilitates genetic
engineering.
From DNA to Protein: Genotype to Phenotype - How is RNA translated into proteins?
tRNA, the adaptor molecule: for each
amino acid, there is a specific type or
“species” of transfer RNA.
Functions of tRNA:
• carries an amino acid;
• associates with mRNA molecules;
• interacts with ribosomes.
From DNA to Protein: Genotype to Phenotype - How is RNA translated into proteins?
Transfer RNA
From DNA to Protein: Genotype to Phenotype - How is RNA translated into proteins?
Ribosomes have two subunits, large and
small.
In eukaryotes, the large subunit has three
molecules of ribosomal RNA (rRNA)
and 45 different proteins in a precise
pattern.
The small subunit has one rRNA and 33
proteins.
From DNA to Protein: Genotype to Phenotype - How is RNA translated into proteins?
Ribosome structure
•
•
•
E- site: the ribosomal site harbouring deacylated
tRNA on transit out from the ribosome
P-site: the ribosomal site most frequently occupied
by peptidyl-tRNA
A-site: the ribosomal site most frequently occupied
by aminoacyl-tRNA
From DNA to Protein: Genotype to Phenotype - How is RNA translated into proteins?
The initiation of translation (part 1)
From DNA to Protein: Genotype to Phenotype - How is RNA translated into proteins?
The initiation of translation (part 2)
From DNA to Protein: Genotype to Phenotype - How is RNA translated into proteins?
The elongation of translation (part 1)
From DNA to Protein: Genotype to Phenotype - How is RNA translated into proteins?
The elongation of translation (part 2)
From DNA to Protein: Genotype to Phenotype - How is RNA translated into proteins?
The termination of translation (part 1)
From DNA to Protein: Genotype to Phenotype - How is RNA translated into proteins?
The termination of translation (part 2)
From DNA to Protein: Genotype to Phenotype - How is RNA translated into proteins?
The termination of translation (part 3)
From DNA to Protein: Genotype to Phenotype - How is RNA translated into proteins?
Destinations for newly translated polypeptides in a eukaryotic cell
From DNA to Protein: Genotype to Phenotype – What are mutations?
From DNA to Protein: Genotype to Phenotype – What are mutations?
Missense mutations: base substitution
results in amino acid substitution.
From DNA to Protein: Genotype to Phenotype – What are mutations?
Nonsense mutations: base substitution
results in a stop codon.
From DNA to Protein: Genotype to Phenotype – What are mutations?
Frame-shift mutations: single bases
inserted or deleted—usually leads to
nonfunctional proteins.
From DNA to Protein: Genotype to Phenotype – What are mutations?
Chromosomal mutations (part 1)
From DNA to Protein: Genotype to Phenotype – What are mutations?
Chromosomal mutations (part 2)
From DNA to Protein: Genotype to Phenotype – What are mutations?
Spontaneous and induced mutations (part 1)
From DNA to Protein: Genotype to Phenotype – What are mutations?
Spontaneous and induced mutations (part 2)
Fom DNA to Protein: Genotype to Phenotype – What are mutations?
Mutation provides the raw material for
evolution in the form of genetic diversity.
Mutations can harm the organism, or be
neutral.
Occasionally, a mutation can improve an
organism’s adaptation to its
environment, or become favorable as
conditions change.