BIO 311C Chapter Notes - Chapter 17: Protein Subunit, Asparagine, Covalent Bond
The Central Dogma:
● transcription – the first step of gene expression during which a complementary mRNA
molecule is produced from the DNA template strand
● reading frame – the correct grouping of codons in mRNA that produces the intended
polypeptide sequence; determined by the start codon and can be shifted by the addition
or deletion of nucleotides in DNA
● pre-mRNA – in eukaryotic organisms, the product of transcription before any RNA
processing has occurred; also called the primary transcript
● messenger RNA (mRNA) – a type of RNA that carries genetic information from the
nucleus to the cytosol
● RNA processing – in eukaryotic organisms pre-mRNA is modified before it leaves the
nucleus: introns are removed, exons are spliced together, and a 5’ cap and poly-A-tail
are added
○ 5′ cap – a chemically-modified guanosine triphosphate that is added to the 5’ end
of pre-mRNA to facilitate binding by ribosomes and to provide protection from
enzymes that break down RNA
○ poly-A tail – a sequence of 100 to 300 adenine nucleotides added to the 3’ end of
pre-mRNA to assist in the export of mRNA from the nucleus
● RNA splicing – introns are removed from pre-mRNA and exons are joined together
during this process
○ spliceosome – a RNA-protein complex that cuts out introns and joins together
exons during pre-mRNA processing
○ small nuclear ribonucleoprotein particles (snRNP) – RNA-protein molecules that
bind to other proteins to form a spliceosome
○ exons – nucleotide sequences in eukaryotic genes that code for amino acids
○ introns – non-coding nucleotide sequences in eukaryotic genes that are removed
during RNA processing
○ alternative splicing – variation in the splicing of exons for a given mRNA that
produces two or more different polypeptide sequences from the same gene
find more resources at oneclass.com
find more resources at oneclass.com
17.1 Genes specify protein via transcription and translation
● Beadle and Tatum studies of mutant strains of Neurospora led to one gene—one
polypeptide hypothesis. During gene expression, the information encoded in genes is
used to make specific polypeptide chains (enzymes and other proteins) or RNA
molecules.
● Transcription is the synthesis of RNA complementary to a template strand of DNA.
translation is the synthesis of a polypeptide whose amino acid sequence is specified by
the nucleotide sequence in mRNA.
● Genetic information is encoded as a sequence of nonoverlapping nucleotide triplets or
codons. A codon in messenger RNA (mRNA) either is translated into an amino acid (61
of the 64 codons) or serves as a stop signal (3 codons). Codons must be read in the
correct reading frame.
Key points:
● The one gene, one enzyme hypothesis is the idea that each gene encodes a single
enzyme. Today, we know that this idea is generally (but not exactly) correct.
● Sir Archibald Garrod, a British medical doctor, was the first to suggest that genes were
connected to enzymes.
● Beadle and Tatum confirmed Garrod's hypothesis using genetic and biochemical studies
of the bread mold Neurospora.
● Beadle and Tatum identified bread mold mutants that were unable to make specific
amino acids. In each one, a mutation had "broken" an enzyme needed to build a certain
amino acid.
Garrod’s “inborn errors of metabolism”:
● Garrod worked with patients who had metabolic diseases and saw that these diseases
often ran in families. He focused on patients with alkaptonuria. This is a non-fatal
disorder where a person’s urine turns black because they cannot break down a molecule
called alkapton (which, in normal people without the disorder, gets broken down into
other, colorless molecules)
● By looking at family trees of people with the disorder, Garrod realized that alkaptonuria
followed a recessive pattern of inheritance, like some of the traits Mendel had studied in
his pea plants. Garrod came up with the idea that alkaptonuria patients might have a
metabolic defect in breaking down alkapton, and that the defect might be caused by the
recessive form of one of Mendel's hereditary factors (i.e., a recessive allele of a gene).
● Garrod referred to this as an “inborn error of metabolism,” and he found other diseases
that followed similar patterns. Although the nature of a gene was not fully understood at
the time, Garrod is now considered "the father of chemical genetics" – that is, the first to
have linked genes with the enzymes that carry out metabolic reactions.
● He hypothesized “one gene-one enzyme”, a gene dictates the production of a specific
enzyme
find more resources at oneclass.com
find more resources at oneclass.com
Beadle and Tatum: Connecting genes to enzymes:
● Beadle and Tatum worked with a simple
organism: common bread mold, or Neurospora
crassa. Using Neurospora, they were able to
show a clear connection between genes and
metabolic enzymes.
● Neurospora had a fast and convenient life cycle,
one with both haploid and diploid phases that
made it easy to do genetic experiments
● Neurospora cells could be grown in the lab on
simple medium (nutrient broth or gel) whose
chemical composition was 100% known and
controlled by the experimenter. In fact, the cells
could grow on minimal medium, a nutrient source
with just sugar, salts, and one vitamin (biotin).
● That's because Neurospora has biochemical
pathways that turn sugar, salts, and biotin into all
the other building blocks needed by cells (such as
amino acids and vitamins)
● If genes were connected to biochemical enzymes,
Beadle and Tatum reasoned that it should be
possible to induce mutations, or changes in
genes, that "broke" specific enzymes (and thus,
specific pathways) needed for growth on minimal
medium. A Neurospora line with such mutation
would grow normally on complete medium, but
would lose the ability to survive on minimal
medium.
● To look for mutants like this, Beadle and Tatum exposed Neurospora spores to radiation
(x-ray, UV, or neutron) to make new mutations. After a few genetic cleanup steps, they
took descendants of the irradiated spores and grew them individually in test tubes
containing complete medium. Once each spore had established a growing colony, a
small piece of the colony was transferred into another tube containing minimal medium.
● Most colonies grew on either complete or minimal medium. However, a few colonies
grew normally on complete medium, but couldn't grow at all on minimal medium. These
were the nutritional mutants that Beadle and Tatum had been hoping to find. On minimal
medium, each mutant would die because it could not make an particular essential
molecule out of the minimal nutrients. Complete medium would "rescue" the mutant
(allow it to live) by providing the missing molecule, along with a variety of others.
● To figure out which metabolic pathway was "broken" in each mutant, Beadle and Tatum
performed a two-step experiment.
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Transcription the first step of gene expression during which a complementary mrna molecule is produced from the dna template strand. Reading frame the correct grouping of codons in mrna that produces the intended polypeptide sequence; determined by the start codon and can be shifted by the addition or deletion of nucleotides in dna. Pre-mrna in eukaryotic organisms, the product of transcription before any rna processing has occurred; also called the primary transcript. Messenger rna (mrna) a type of rna that carries genetic information from the nucleus to the cytosol. Rna processing in eukaryotic organisms pre-mrna is modified before it leaves the nucleus: introns are removed, exons are spliced together, and a 5" cap and poly-a-tail are added. 5 cap a chemically-modified guanosine triphosphate that is added to the 5" end of pre-mrna to facilitate binding by ribosomes and to provide protection from enzymes that break down rna.
