A3: Encoding and decoding

What did Gregor Mendel work out about inheritance?

Dominant and recessive alleles, homozygous vs heterozygous states, and independent assortment unless traits are linked.

What did Garrod's work show about genes?

Genes exert their effects through enzymes.

Name the four inherited conditions Garrod studied.

Alkaptonuria, cystinuria, pentosuria, albinism.

What enzyme is missing in alkaptonuria?

Homogentisic acid oxidase.

What causes dark urine in alkaptonuria?

Accumulation of homogentisic acid.

What enzyme is missing in phenylketonuria (PKU)?

Phenylalanine hydroxylase.

What happens when phenylalanine hydroxylase is absent?

Phenylalanine accumulates in the blood and damages the nervous system.

What are inborn errors of metabolism?

Metabolic disorders caused by absence of specific enzymes.

Which organism did Beadle and Tatum use?

Neurospora crassa.

Why is Neurospora a good genetic model organism?

It produces haploid spores that are genetically identical.

What is minimal medium?

A medium providing sugar, nitrogen, mineral ions, and biotin.

Why can Neurospora grow on minimal medium?

It can synthesize all other required molecules itself.

What happens if an enzyme in a biosynthetic pathway is missing?

The amino acid must be supplied in the growth medium for cells to grow.

What mutation method did Beadle and Tatum use?

X-ray exposure.

What defines a mutant in these experiments?

A spore that cannot grow on minimal medium.

How were mutant spores characterised?

By testing growth with amino acid supplements.

What was the One Gene-One Enzyme hypothesis?

Each gene codes for one enzyme.

How has the One Gene-One Enzyme hypothesis been updated?

To One Gene-One Polypeptide.

What do most genes make?

Proteins.

What is the central dogma of molecular biology?

DNA → RNA → Protein.

Why isn't DNA directly used to make proteins?

Its information is copied into mRNA first.

What enzyme carries out transcription?

RNA polymerase.

In which direction is mRNA synthesized?

5′ to 3′.

List key differences between RNA and DNA.

RNA is single-stranded, uses ribose, contains uracil, can leave the nucleus, and is less stable.

What happens during translation?

Ribosomes decode mRNA to build a polypeptide.

How many amino acids are used in proteins?

20.

Why must the genetic code be a triplet code?

Single or double bases cannot encode 20 amino acids, but triplets can.

What is a codon?

A triplet of nucleotides that specifies one amino acid.

Is the genetic code overlapping?

No, it is non-overlapping.

What molecule translates between nucleic acids and proteins?

tRNA.

What did the poly-U experiment show?

UUU codes for phenylalanine.

Why can one mRNA encode different polypeptides?

Different reading frames can be used.

How many reading frames are possible?

Six.

What codon starts translation?

AUG (methionine).

What sequence helps define the start site?

The Kozak sequence.

What are the three stop codons?

UAA, UAG, UGA.

What does it mean that the genetic code is degenerate?

Multiple codons can code for the same amino acid.

Which amino acids have many codons?

Arginine and leucine.

Which amino acids have few codons?

Tryptophan and methionine.

How does mitochondrial genetic code differ?

It uses a different codon translation scheme.

What was demonstrated by writing messages in synthetic genomes?

DNA can store non-biological information.

What is one goal of reducing codon degeneracy in E. coli?

To make bacteria resistant to viral infection or produce novel proteins.

What is the final conclusion about the genetic code?

It is a linear, triplet code with defined start and stop points.