Chapter 8.1-8.3 LOs

Genetics Exam2

Complementation Test

• Complementation (mutant × mutant → wild‑type offspring)
  → Mutations are in different genes.
  → Each parent supplies the functional copy the other lacks.

• Phenotype= Wild type (normal function)

• Non‑complementation (mutant × mutant → mutant offspring)
  → Mutations are in the same gene.
  → No functional copy is present.

• → Phenotype= Mutant (defective function)

What is a complementation group? Are they in the same or different genes?

• A set of mutations that do not complement each other.

• A complementation group = one gene.

• Different groups = different genes.

How do you use a complementation table to find the number of genes and complementation groups?

• Count the number of complementation groups (clusters of mutants that fail to complement each other).

• Each group = one gene.

How do you find the groups?

• Look for mutants that all show mutant phenotype when paired → same group.

• Mutants that restore wild‑type when paired → different groups.

What did Benzer’s experiment with T4 phage show about complementation?

Complementation

• Mutations in the same gene fail to complement.

• Mutations in different genes complement.

• He split the rII region into two genes (rIIA and rIIB).

What did Benzer’s experiment with T4 phage show about mutations?

Mutations

• Mutations are points along a gene.

• Genes have internal structure, not indivisible units.

What did Benzer’s experiment with T4 phage show about Recombination?

Recombination

• Recombination can occur within a gene, not just between genes.

• He detected intragenic recombination by recovering rare wild‑type phages.

How do Mutations, recombination and complementation relate to each other

• Mutations define positions.

• Recombination can occur between those positions.

• Complementation tells you whether mutations are in the same gene.

Together, they show:

Genes are long sequences of nucleotides with internal organization.

Why did Benzer Needed a Control Group

To ensure:

• Wild‑type phages weren’t due to back‑mutation.

• Results weren’t due to second‑site suppressors.

• Recombination was truly intragenic, not contamination.

Controls validated that the experiment measured real recombination.

How can deletion mapping shows where mutations are

• A deletion removes a known chromosomal region.

• Cross a point mutant with strains carrying deletions.

• If the deletion overlaps the mutation → no complementation → mutant phenotype.

• If the deletion does NOT overlap → complementation → wild‑type phenotype.

How to solve a deletion map

1. Look at which deletions fail to complement the point mutation.

2. The mutation must lie within the overlapping region of those deletions.

3. Narrow down the smallest region where all non‑complementing deletions overlap.

(do examples)

Hotspots

• Locations in DNA where mutations occur more frequently.

• Often due to sequence features that make the site prone to errors.

Garrod’s Findings

Garrod studied metabolic diseases (e.g., alkaptonuria).

He discovered:

• Some diseases are caused by inherited defects in specific enzymes.

• This led to the idea of inborn errors of metabolism.

Beadle & Tatum’s Findings

Using Neurospora arginine auxotrophs, they found:

• Each mutant was blocked at a specific step in a biochemical pathway.

• Each step corresponded to a specific enzyme.

Conclusion:

One gene → one enzyme
(later refined to one gene → one polypeptide).

How do you use a supplementation table to find the biochemical pathway?

• Test mutants on media containing pathway intermediates.

• Growth = the intermediate bypasses the block.

• No growth = the block is after that intermediate.

What does it mean if a mutant cannot grow on something?

• The block is after that compound.

• The compound does not bypass the defect.

What does it mean if a mutant can grow on something

• The block is before that compound.

• The compound rescues the pathway.

If there is a block at any step of the pathway, how does that affect the concentration of intermediates in the pathway?

• Intermediates before the block accumulate.

• Intermediates after the block are depleted

What did Yanofsky find?

Yanofsky showed:

• The order of DNA mutations in a gene matches the order of amino acid changes in the protein.

• This demonstrated colinearity:

The gene and protein sequences correspond in linear order

What did Crick and Brenner (1955) find?

Using frameshift mutations, they found:

• +1 or –1 frameshifts → completely nonfunctional protein.

• +3 or –3 frameshifts → function largely restored.

Conclusion:

The genetic code is read in non‑overlapping triplets (codons).