genes and the genetic code 8.1

What is a gene ?

A gene is a section of DNA that codes for making polypeptides and functional RNA

What is a locus?

A locus =The fixed position on a DNA molecule
occupied by a gene.

What does the gene in a base sequence of DNA code for ?

The amino acid sequence of a polypeptide
Or a functional RNA

Cracking the genetic code ?

The code for dna is a triplet code
This means that 3 bases code for one amino acid

In mRNA this triplet code is called a codem
There are 64 different codons most of the code for a

A few are nonsense codons which signal the beginning and end of a gene

Since there are only 20 aa there are more than enough codons to code for them.

What is genetic code?

The sequence of base triplets (codons) in mRNA that code for specific amino acids

What does it mean to say that the code in genetic code is ‘non-overlapping’?

And advantage ?

The triplets are read separately
Base triplets don’t share any bases.

Each codon is read as a discrete unit (only read once).

The advantage of this is if at point a mutation occurs, it will only affect one codon and therefore one amino acid

What does it mean to describe genetic code as ‘degenerate’?

There are more possible combinations of triplets than there are amino acids

There are 20 amino acids, but 64 possible triplets

This means there are different triplets that exist that code for the same amino acid

Why is degeneration an advantage if a mutation occurs?

This is an advantage as if a point a mutation occurs,

even though the triplet of bases will be different, it may still code for the same amino acid and therefore have no effect

What does it mean to say that genetic code is ‘universal'?

The same specific base triplets code for the same amino acids in all living things

this is an advantage as it has made genetic engineering is possible

Features of the genetic code ?

-Degenerate code
-non overlapping
-universal

What are introns?

Sections of DNA that don’t code for amino acids.

They are found in eukaryotic DNA but not prokaryotic DNA

What are exons?

Sections of DNA that code for amino acids

What are start and stop signals?

Triplets that are used to start and stop protein production

What is a start codon?

-3 bases at the start of a gene in DNA+mRNA.


-This codes for the amino acid methionine.

-This methionine is later removed from the protein if it not actually needed for the structure .

-Initiates translation

What is the stop codon?

-The final 3 bases on the end of DNA and mRNA.

-Do not code for an amino acid

-They mark the end of a polypeptide chain and cause ribosomes to detach and stop translation.

Why is it that 3 bases code for one amino acid?

Maths)

Equation 4 (to the power n)
n=base
4(because 4 bases ATCG)

-if one base
4 to the power 1 =4 =insufficient to code for 20 amino acids

-if 2 bases
4 to the power 2 =16 =insufficient to code for 20 amino acids.

-if 3 bases
4 to the power 3 =64 which is more than enough/

Explain how a change in one base along a DNA molecule may result in an enzyme becoming non-functional?

A different base might code for a different amino acid.

The sequence of amino acids in the polypeptide produced will be different.

This change to the primary structure of the protein might result in a different shaped tertiary structure.

The enzyme shape will be different and may not fit the substrate. The enzyme -substrate complex cannot be formed, and so the enzyme is non-functional.