The genetic code

What is the role of DNA as genetic material?

DNA carries the instructions for protein synthesis, which determines how the cell is built and how it functions.

What are proteins made of?

Proteins are polymers of amino acids, joined together in countless combinations.

How many naturally occurring amino acids are there in protein synthesis?

20.

Where does translation occur in the cell?

On the surface of ribosomes.

What part of DNA varies between nucleotides and carries the genetic code?

The bases (A, T, G, C).

Why can’t one base code for one amino acid?

There are only 4 bases, which would allow coding for only 4 amino acids (not enough for the 20 needed).

Why can’t two bases code for one amino acid?

4² = 16 combinations, still fewer than 20 amino acids.

Why is the genetic code a triplet code?

4³ = 64 possible codons, more than enough to code for 20 amino acids.

Define a gene.

A sequence of bases on a DNA molecule coding for a sequence of amino acids in a polypeptide, which affects a characteristic in the phenotype.

What is a codon?

A sequence of three bases on DNA or RNA that codes for a specific amino acid or a start/stop signal.

Why was mRNA used to work out the genetic code instead of DNA?

mRNA is smaller and easier to study; it is complementary to DNA, so the DNA sequence can be inferred.

What are the base pairing rules relevant for DNA ↔ RNA transcription?

A pairs with U (in RNA), and G pairs with C.

Why is the genetic code described as universal?

Because the same codons code for the same amino acids in almost all organisms.

How much of human DNA is non-coding?

About 98%.

What is the role of non-coding DNA?

Regulates protein-coding sequences by turning genes on or off; exact functions often still unknown.

Why is non-coding DNA thought to be important?

Similar sequences are conserved across organisms, suggesting key functions.

What percentage of human DNA codes for proteins?

About 2%.

Besides coding for amino acids, what else can codons specify?

Start signals (initiation) or stop signals (termination) for protein synthesis.

What are the three main features of the genetic code?

It is a triplet code, non-overlapping, and degenerate.

What does 'non-overlapping' mean in the context of the genetic code?

Each base is only read once in a codon, bases are not shared between codons.

What does 'degenerate' mean in the context of the genetic code?

Most amino acids are coded for by more than one codon.