B4 Genetic variation

Question 1

What is a mutation?

Answer 1

A change in the base sequence of DNA

Question 2

Explain why a mutation might lead to a change in the amino acid sequence?

Answer 2

Triplet coding for a particular amino acid on the DNA changes.
Codon coding for a particular amino acid on mRNA is changed.
Different amino acid is coded for

Question 3

Why might a mutation NOT lead to a change in the amino acid sequence?

Answer 3

Genetic code is degenerate
New triplet might code for the same amino acid

Question 4

What is a substitution mutation?

Answer 4

Nucleotide in the DNA sequence is replaced by another.

Question 5

What is a deletion mutation?

Answer 5

A DNA nucleotide is removed from the DNA.
Leads to a frame shift

Question 6

What is a mutagenic agent?

Answer 6

Factors that increase the rate of mutations

Question 7

Give an example of a mutagenic agent

Answer 7

UV light, X rays, gamma rays, certain chemicals in tobacco

Question 8

What is n number of chromosomes?

Answer 8

The haploid number of chromosomes present in an organism eg in sperm/eggs/gametes

Question 9

What is the diploid number of chromosomes?

Answer 9

The number of chromosomes present in a normal body cell.

Question 10

What is a polyploidy chromosome mutation?

Answer 10

Where an individual has three or more sets of chromosomes instead of two.

Question 11

What is chromosome non-disjunction?

Answer 11

When chromosomes fail to separate correctly in meiosis, resulting in gametes with one more or less chromosomes than normal

Question 12

What is meiosis?

Answer 12

A type of cell division
Halves the number of chromosomes present
Usually used to make gametes
Produces 4 genetically different haploid cells

Question 13

Name the cells created by meiosis

Answer 13

Plants - pollen and ovules (eggs)

Animals - sperm and ova

Question 14

Which stage of meiosis is taking place in the diagram and how do you know?

Answer 14

Metaphase I

Homologous chromosomes in pairs

Lined up on equator of cell

Question 15

Which stage of meiosis is shown in the diagram?

How do you know?

Answer 15

Anaphase II

Sister chromatids separated and moving to opposite poles of the cell

Question 16

What happens during meiosis I?

Answer 16

Homologous chromosomes pair to form bivalents
Crossing over occurs at chiasmata.
One of each pair of chromosomes is randomly separated into 2 daughter cells.
Cell divides in two.

Question 17

What happens in meiosis II?

Answer 17

Sister chromatids are pulled apart at the centromere by spindle fibres.
Producing 4 haploid gametes

Question 18

When does crossing over take place?

Answer 18

Prophase I of meiosis

Question 19

Name the process that has occurred in the diagram

Answer 19

Crossing over

Question 20

How does meiosis produce genetic variation?

Answer 20

Crossing over during meiosis I
Independent assortment/segregation of homologous chromosomes sister chromatids.

Question 21

Which stage of meiosis is taking place in the diagram?

Answer 21

Anaphase I

Homologous chromosomes are being pulled to opposite poles of the cell

Question 22

What is the biological importance of reducing the chromosome number when the cell divides by meiosis?

Answer 22

Diploid number is restored during fertilisation

Question 23

Give 5 differences between mitosis and meiosis

Answer 23

Meiosis halves the number of chromosomes number whereas mitosis maintains chromosome number
In meiosis chromosomes associate in pairs whereas in mitosis chromosomes do not pair
In meiosis there are 2 nuclear divisions whereas in mitosis there is 1 nuclear division
In meiosis 4 gametes as produces whereas in mitosis 2 body cells are made.
Meiosis results in genetically different daughter cells whereas mitosis the daughter cells are genetically identical

Question 24

Homologous chromosomes carry the same genes but are not genetically identical. Why?

Answer 24

Homologous chromosomes have different alleles

Question 25

Explain the role of independent segregation in meiosis

Answer 25

To provide genetic variation
Allows different combinations of maternal and paternal chromosomes
Produces haploid cells.

Question 26

How is mitosis similar to meiosis II?

Answer 26

Separation of sister chromosomes

Question 27

Describe how DNA nucleotide substitution can lead to a non- functional enzyme

Answer 27

DNA nucleotide of a gene is replaced by a different nucleotide with a different base.
Changes the DNA triplet and its resulting mRNA codon.
Altered codon translates into a different amino acid.
Different primary structure of protein
Different hydrogen, ionic and disulfide bonds form a new tertiary structure.
Enzyme has a different tertiary structure
Active site no longer complementary to the substrate
No enzyme substrate complexes formed.

Question 28

What is a chromosome mutation?

Answer 28

Change in the structure or number of whole chromosomes

Question 29

What is the equation for calculating the possible chromosome combinations as a result of independent segregation during meiosis

Answer 29

No. of chromosome combinations = 2 ⁿ
(where n is the number of PAIRS of chromosomes.

Question 30

How is chromosome number halved in meiosis?

Answer 30

Separation of homologous pairs.
One of each pair ends up in the daughter cells.

Question 31

Define proteome

Answer 31

Full range of proteins that a cells can produce

Question 32

Define genetic diversity

Answer 32

The number of different alleles of each genes (within a population)

Question 33

What is a species?

Answer 33

A group of organisms with similar features that can breed together to produce fertile offspring

Question 34

Describe the stages of natural selection

Answer 34

Individuals in a population show variation due to mutation
Individuals with beneficial characteristics survive and reproduce
Selection for advantageous alleles
Leads to a change in allele frequency
Occurs over a long period of time
Populations no longer able to interbreed and produce fertile offspring

Question 35

Describe how direction selections produces antibiotic resistant bacteria

Answer 35

Variation in bacteria due to mutation
Variation in bacteria due to the transfer of plasmids containing a gene for an enzyme that hydrolyses antibiotic
Antibiotics are introduced
Selection for antibiotic resistant allele
Bacteria with antibiotic resistant allele survive and increase in numbers
Bacteria without antibiotic resistant allele die
Allele frequency for antibiotic resistance increases

Question 36

Describe the effect of stabilising selection on human birth weight

Answer 36

Variation due to mutation
Individuals with beneficial characterises survive and reproduce
Large babies cannot be delivered - selection pressure Small birth weight babies fail to survive selection pressure
Selection for advantageous alleles
Leads to change in allele frequency
Birth weight remains constant

Question 37

Name two types of variation and give examples

Answer 37

Continuous variation eg height, weight

Discontinuous variation eg blood group

Question 38

What is directional selection and what does it result in?

Answer 38

Selection favours individuals that vary in one direction from the mean of the population
eg antibiotic résistance in bacteria
Causes a change in the characteristics of a population

Question 39

What is stabilising selection and what does it result in?

Answer 39

Selection that favours the average characteristic
Tis preserves the characteristics of the population

Question 40

Name the point on homologous chromosomes where the sister chromatids make contact and crossing over might take place

Answer 40

Chiasmata