16. HOW GENE WORKS
What do gene do George Beadle and Edward Tatum studied genes by making them defective and observing the
effects. They found that damaged genes, called loss-of-function alleles, lead to missing traits
or functions. They used bread mold N. crassa to identify mutants unable to make certain
compounds.
Their findings led to the one-gene, one-enzyme hypothesis, which proposed that each gene
◦ one gene, one-
directs the production of an enzyme. Srb and Horowitz tested this by studying a 3-step
enzyme hypothesis
metabolic pathway to make arginine in mold. They found mutants that lacked one of the
enzymes in the pathway, confirming that each gene controls a specific enzyme. This idea
evolved into the one-gene, one-polypeptide hypothesis, meaning genes instruct the production o
proteins..
The central dogma of The Central Dogma explains how genes produce proteins. DNA stores genetic information, but it
molecular biology can't directly make proteins. Francis Crick proposed that DNA acts as a code to direct protein
production.
Jacob and Monod showed that RNA is the link between DNA in the nucleus and protein synthes
in the cytoplasm. Messenger RNA (mRNA) carries the genetic code from DNA to the site of
proteins synthesis. RNA polymerase is the enzyme that creates mRNA by copying a DNA
strand, matching complementary nucleotides.
In eukaryotic, proteins are foun inside the nucleus but proteins are manufactured in ribosome
Linking central The Central Dogma links DNA to cellular processes:
Dogma to cellular ◦ DNA replication makes copies of DNA.
process
◦ Transcription is when DNA template is used to make complementary RNA .
◦ Translation is when mRNA is used to make proteins.
Genotype is determined by the DNA sequence (bases), and phenotype (traits) depends on the
proteins produced. Different alleles of same gene have different DNA sequences, which can lead
to different proteins (differ in amino acid sequences)and traits.
Exceptions: Not all RNA codes for proteins. Some RNAs have important roles, like building
ribosomes. Gene flow is DNA → RNA.
How did the Researchers cracked the genetic code by discovering that a sequence of three nucleotides (a
researcher crack the codon) codes for a specific amino acid. Gamow predicted this triplet code, and Crick and Brenn
code confirmed it. They found that adding or removing one or two bases shifts the reading frame,
but adding or removing multiples of three doesn't.
Nirenberg and Leder identified which codons correspond to each amino acid. The code includes
start codon (AUG) and stop codons (UGA, UAA, UAG) that signal the beginning and end of
protein synthesis. There are 64 possible codons, but only 20 amino acids, so some codons are
redundant.
Key features of the genetic code:
◦ Redundant: Most amino acids have more than one codon, because of redundancy, more
than one DNA sequence could code for the same amino acide sequence.
◦ Unambiguous: Each codon codes for only one amino acid.
◦ Non-overlapping: Codons are read one at a time.
◦ Universal: Most codons are the same across organisms.
◦ Conservative: Codons for the same amino acid often have similar first two bases.
What do gene do George Beadle and Edward Tatum studied genes by making them defective and observing the
effects. They found that damaged genes, called loss-of-function alleles, lead to missing traits
or functions. They used bread mold N. crassa to identify mutants unable to make certain
compounds.
Their findings led to the one-gene, one-enzyme hypothesis, which proposed that each gene
◦ one gene, one-
directs the production of an enzyme. Srb and Horowitz tested this by studying a 3-step
enzyme hypothesis
metabolic pathway to make arginine in mold. They found mutants that lacked one of the
enzymes in the pathway, confirming that each gene controls a specific enzyme. This idea
evolved into the one-gene, one-polypeptide hypothesis, meaning genes instruct the production o
proteins..
The central dogma of The Central Dogma explains how genes produce proteins. DNA stores genetic information, but it
molecular biology can't directly make proteins. Francis Crick proposed that DNA acts as a code to direct protein
production.
Jacob and Monod showed that RNA is the link between DNA in the nucleus and protein synthes
in the cytoplasm. Messenger RNA (mRNA) carries the genetic code from DNA to the site of
proteins synthesis. RNA polymerase is the enzyme that creates mRNA by copying a DNA
strand, matching complementary nucleotides.
In eukaryotic, proteins are foun inside the nucleus but proteins are manufactured in ribosome
Linking central The Central Dogma links DNA to cellular processes:
Dogma to cellular ◦ DNA replication makes copies of DNA.
process
◦ Transcription is when DNA template is used to make complementary RNA .
◦ Translation is when mRNA is used to make proteins.
Genotype is determined by the DNA sequence (bases), and phenotype (traits) depends on the
proteins produced. Different alleles of same gene have different DNA sequences, which can lead
to different proteins (differ in amino acid sequences)and traits.
Exceptions: Not all RNA codes for proteins. Some RNAs have important roles, like building
ribosomes. Gene flow is DNA → RNA.
How did the Researchers cracked the genetic code by discovering that a sequence of three nucleotides (a
researcher crack the codon) codes for a specific amino acid. Gamow predicted this triplet code, and Crick and Brenn
code confirmed it. They found that adding or removing one or two bases shifts the reading frame,
but adding or removing multiples of three doesn't.
Nirenberg and Leder identified which codons correspond to each amino acid. The code includes
start codon (AUG) and stop codons (UGA, UAA, UAG) that signal the beginning and end of
protein synthesis. There are 64 possible codons, but only 20 amino acids, so some codons are
redundant.
Key features of the genetic code:
◦ Redundant: Most amino acids have more than one codon, because of redundancy, more
than one DNA sequence could code for the same amino acide sequence.
◦ Unambiguous: Each codon codes for only one amino acid.
◦ Non-overlapping: Codons are read one at a time.
◦ Universal: Most codons are the same across organisms.
◦ Conservative: Codons for the same amino acid often have similar first two bases.
