Genetic analysis and mutagenesis screen

Question 1

What are the restrictive and permissive conditions when looking at temperature sensitive mutants

Answer 1

permissive: mutant and wild-type phenotype are the same

restrictive: mutants arrest and phenotype seen at cell checkpoint

Question 2

How can we see mutant phenotypes

Answer 2

• temperature sensitive mutants are ‘conditional’, mutations in genes required for progression from one phase to the next cause yeast cells to arrest (stop) at the specific point in the cell cycle
• at the restrictive temperature mutations in the genes required for progression from the end of the S phase cause yeast to arrest at the end of the S phase
• each cell division cycle was identified because all cells arrested at the same point in the cell cycle
• non-cell cycle mutants can be easily distinguished as they will arrest at any point not just at checkpoints in the cell cycle

Question 3

What is a mutant

Answer 3

a mutant is something that has one gene that does not function

Question 4

How did plasmid rescue identify human cycle gene

Answer 4

stages
i. cdc2 mutants were grown at permissive temperatures (no mutant phenotype seen)
ii. mutant cells were transformed with a plasmid library of human genes
iii. shift transformed cells to the restrictive temperature

the mutant then works, as the plasmid is rescued from the human gene, this enables progression through the cell cycle

Question 5

What are weak alleles

Answer 5

• weak alleles (reduced loss of function) of cdc mutant were found, some had very low levels of the activity from the mutant gene enabling cells to very slowly progress through the cell cycle
• wee mutants were also found which made rapid progression through the cell cycle but formed a smaller cell

Question 6

What is the nomenclature of the S.pombe and what is Cdc2

Answer 6

Cdc2 is a key cell cycle regulator
- dominant and recessive mutations in the same gene have opposite phenotypes
- both phenotypes are consistent with the cdc2 gene having a key role in regulating progression through the cell cycle

cdc2+ = wild type gene
cdc2- = recessive mutation
cdc2D = dominant mutation
Cdc2 = protein

Question 7

How are cdc25 and wee1 mutants similar to cdc2 mutants

Answer 7

cdc25 and wee1 have similar phenotypes to cdc2 mutants, this indicates that cdc25 and wee1 are likely to act at the same point in the cell cycle as Cdc2
- cdc25- (recessive) and wee1- (dominant) have opposite mutant phenotypes
- wee1D and cdc25D also have opposite mutant phenotypes

this indicates that wee1+ and cdc25+ have antagonistic functions

cdc25, wee1 and cdc2 regulate entry into mitosis
- wee1 (repressing) and cdc25 (activating)
- Cdc25 and Wee1 regulate Cdc2 by a series of phosphorylation events

Question 8

What has mutagenesis done for flower regulation

Answer 8

mutagenesis identifies genes that regulate flowering. in the floricaula (flo) mutant, flowers are replaced by shoots. floral meristem identity in Antirrhinum is determined by the FLO gene

the Flo gene is expressed in young floral meristems
- DNA sequence of the FLO gene reveals that it is a transcription factor

Question 9

How do genes regulate development

Answer 9

• transcriptional regulators can be master switches for development
• the binding site can be blocked preventing the coding region from being accessed meaning the protein is then not formed

Question 10

How do botanists draw flower

Answer 10

flowers are made up of concentric rings (whorls) of four types of organs
- floral organs initiate sequentially
- se-pe-st-ca (sepals, petals, stamens and carpels)

Question 11

How are sepals, petals, stamens and carpels expressed in the ABC class model

Answer 11

there are only three major classes (three genes) of organ identify mutants: a,b,c

a - sepals
ab - petals
bc - stamen
c - carples

normal: se - pe - st- ca
class a: ca - st - st - ca
class b: se - se - ca - ca
class c: se - pe - pe - se

the activities of a and c are mutually antagonistic