UNIT 3: Molecular Genetics
Chapter 7: Genes and Proteins Synthesis
7.1: From Gene to Protein
pg. 310 pg. 312 - 318
One gene-one enzyme hypothesis - is the hypothesis, proposed by Beadle
and Tatum; that each gene is unique and codes for the synthesis of a single
enzyme.
One gene-one polypeptide hypothesis – is the hypothesis that each gene is
unique and codes for the synthesis of a single polypeptide; the restated
version of the one gene-one enzyme hypothesis.
Archibald Garrod (1896) studied Alkaptonuria, a human disease easily
detected when a patient’s urine turns black in air. Garrod determined that
alkaptonuria was an inherited. By 1908 he determined the disease was
caused by a mutation in a gene that normally codes for an enzyme to
breakdown tyrosine. If tyrosine is not broken down then alkapton
accumulates.
Beadle and Tatum (1940’s) studied bread mould, Neurospora crassa. Their
data supported the relationship between genes and enzymes. The mould was
able to grow in a minimal medium (MM). The moulds were able to synthesis
the more complex molecules for survival.
They then x-rayed spores form Neurospora crassa, and then observed them
in the minimal medium. The mould was unable to grow. They suggested that
the x-rays mutated the genes; therefore the spores could no longer produce
the complex molecules to survive. The mutated strain could resume growth
if arginine was present. The production of arginine is a multi-step process,
with multiple enzymes required, one for each step. Their conclusion stated
that one gene-one enzyme hypothesis. The hypothesis was later expanded to
one gene-one protein, because not all amino acids assembled create enzymes.
Connection between DNA, RNA, and Protein
Central dogma – is the fundamental principle of molecular genetics, which
states that genetic information flows from DNA to RNA to proteins.
Figure 2: The central Dogma of molecular genetics states that genetic information flows from DNA to
RNA to Protein.
Transcription – is the mechanism by which the information coded in nucleic
acids of DNA is copied into the nucleic acids of RNA; something rewritten
in the same language.
Translation – is the mechanism by which the information in the nucleic
acids of RNA is copied into the amino acids of proteins.
Francis Crick (1956) gave the name “central dogma” to the progression from
DNA to RNA to protein. There are two major steps to this progression;
transcription and translation.
RNA – Ribonucleic Acid
Messenger RNA (mRNA) – is the end product of the transcription of a gene;
mRNA is translated by ribosomes into protein.
Transfer RNA (tRNA) – is a carrier molecule that binds to a specific amino
acid and adds the amino acid to the growing polypeptide chain.
Ribosomal RNA (rRNA) – is an RNA molecule within the ribosome that
bonds to correct amino acid to the polypeptide chain.
Ribonucleic acid is a carrier of genetic material like DNA. RNA molecules
are different from DNA because they are single stranded, contained uracil
other then thymine, and had a ribose sugar group.
Table 1: Comparison of DNA and RNA
Deoxyribonucleic Acid
- double stranded
- adenine pairs with thymine
- guanine pairs with cytosine
- deoxyribose sugar
Ribonucleic acid
- single strand
- adenine pairs with uracil
- guanine pairs with cytosine
- ribose sugar
Figure 3: DNA and RNA are similar, with a few important differences: RNA is single stranded, not double
stranded; RNA has Uracil in place of Thymine in DNA, and RNA contains a hydroxyl group (OH) at the 2′
position of the sugar, whereas DNA has hydrogen (H) at that position (thus “deoxy” to denote the absence
of Oxygen).
There are three major types of RNA participate in protein synthesis;
messenger RNA, transfer RNA, and ribosomal RNA.
Table 2: Different Types of RNA
Type of RNA
Messenger RNA
(mRNA)
Transfer RNA
(tRNA)
Ribosomal RNA
(rRNA)
Characteristics and key functions
- varies in length, depending on the gene that has been copied
- acts as the intermediary between DNA and the Ribosome
- is translated into protein by ribosomes
- is the RNA version of the gene encoded by DNA
- Functions as the delivery system of amino acids to ribosomes
as they synthesize proteins
- is very short, only 70 to 90 base pairs long
- binds with proteins to form the ribosomes
- varies in length
Figure 4: tRNA is composed of a single strand of RNA that loops in on
Itself to form antiparallel double-stranded areas.
Transcription and Translation: An Overview
RNA polymerase – is an enzyme that reads a DNA strand and creates a
complementary strand of RNA.
Template strand – is the DNA strand that is copied into an mRNA molecule
during gene transcription.
Precursor mRNA (pre-mRNA) – is the initial RNA transcription product.
Transcription is the first step in protein synthesis. RNA polymerase is used
to synthesize a RNA molecule, using complementary base pairs to one
strand of a DNA molecule. The DNA template strand is read 3′ to 5′
direction, as the RNA strand is built in the 5′ to 3′ direction.
The DNA template strand (code) is read by RNA polymerase to transcribe a
precursor mRNA molecule. The pre-mRNA molecule is not used to produce
a protein; it must be modified before it exits the nucleus.
After the modified mRNA molecule leaves the nucleus and enters the
cytosol and binds with a ribosome, translation can begin. The ribosome
moves along the mRNA reading the codon (3 nitrogen bases). tRNA is
responsible for retrieving amino acids that are identified by the codon, which
is complementary to the anti-codon of the tRNA molecule. The tRNA
delivers the appropriate amino acid to the ribosome to be bonded to the
adjacent amino acid, forming a polypeptide chain. The number and sequence
of amino acids are determined by the gene.
Figure 5: the relationship between a gene, the codons in an mRNA, and the amino aid sequence of a
polypeptide.
The Genetic Code: Three-Letter Words with a Four –Letter
Alphabet
Genetic code – is the specific coding relationship between bases and the
amino acids they specify; genetic code can be expressed in terms of either
Codon – is a group of three base pairs that code or an individual amino acid.
Start Codon (Initiator Codon) – is the codon that signals the start of a
polypeptide chain and initiates translation.
Stop Codon – is a codon that signals the end of a polypeptide chain and
causes the ribosome to terminate translation.
The genetic code determines the amino acid number and sequence during
protein synthesis. The DNA alphabet consists of four letters combined in
groups of three letters to code for an amino acid. The four letters represent
the nitrogenous bases found in the DNA molecule, A (adenine), G (guanine),
T (thymine), and C (cytosine).
During transcription an mRNA molecule is produced based on the code for a
gene found in a strand of DNA. RNA has an alphabet made up of four letters
representing nitrogenous bases; A (adenine), G (guanine), U (uracil), and C
(cytosine). Once again only three letters are used to code for an amino acid.
There are sixty four (43) different combinations of three nitrogenous bases
that can code for 20 amino acids. Some of the amino acids have more then
one codon to represent them.
61 codons code for amino acids, while 3 codons code for a stop codon
(UAA, UAG, UGA) in the protein sequence, (a period within a
sentence).The codon (start codon) AUG codes for the first amino acid in
protein synthesis, known as Methionine.
The genetic code is universal in most organisms or viruses, which code for
the same amino acids. This indicates that the genetic code was established in
an early time in the evolution of life and remains unchanged.
Figure 7: The Genetic Code, written in the form in which the codons appear in mRNA; The AUG initiator
codon, which codes for Methionine, is shown in yellow; the three terminator codons are shown as STOP.
The triplet sequences are in the 5′ to 3′ order.