B4 Genetic variation Flashcards
What is a mutation?
A change in the base sequence of DNA
Explain why a mutation might lead to a change in the amino acid sequence?
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
Why might a mutation NOT lead to a change in the amino acid sequence?
Genetic code is degenerate
New triplet might code for the same amino acid
What is a substitution mutation?
Nucleotide in the DNA sequence is replaced by another.
What is a deletion mutation?
A DNA nucleotide is removed from the DNA.
Leads to a frame shift
What is a mutagenic agent?
Factors that increase the rate of mutations
Give an example of a mutagenic agent
UV light, X rays, gamma rays, certain chemicals in tobacco
What is n number of chromosomes?
The haploid number of chromosomes present in an organism eg in sperm/eggs/gametes
What is the diploid number of chromosomes?
The number of chromosomes present in a normal body cell.
What is a polyploidy chromosome mutation?
Where an individual has three or more sets of chromosomes instead of two.
What is chromosome non-disjunction?
When chromosomes fail to separate correctly in meiosis, resulting in gametes with one more or less chromosomes than normal
What is meiosis?
A type of cell division
Halves the number of chromosomes present
Usually used to make gametes
Produces 4 genetically different haploid cells
Name the cells created by meiosis
Plants - pollen and ovules (eggs)
Animals - sperm and ova
Whcih stage of meiosis is taking place in the diagram and how do you know?
Mataphase I
Homologous chromosomes in pairs
Lined up on equator of cell
Which stage of meisosi is shown in the diagram?
How do you know?
Anaphase II
Sister chromatids separated and moving to opposite poles of the cell
What happens during meiosis I
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.
What happens in meiosis II?
Sister chromatids are pulled apart at the centromere by spindle fibres.
Producing 4 haploid gametes
When does crossing over take place?
Prophase I of meiosis
Name thr process that has occured in the diagram
Crossing over
How does meiosis produce genetic variation?
Crossing over during meiosis I
Independent assortment/segregation of homologous chromosomes d sister chromatids.
Which stage of meiosis is taking place in the diagram
Anaphase I
Homologous chromosomes are being pulled to oppoite poles of the cell
What is the biological importance of reducing the chromosome number when the cell divides by meiosis?
Diploid number is restored during fertilisation
Give 5 differences between mitosis and meiosis
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
Homologous chromosomes carry the same genes but are not genetically identical. Why?
Homologous chromosomes have different alleles
Explain the role of independent segregation in meiosis
To provide genetic variation
Allows different combinations of maternal and paternal chromosomes
Produces haploid cells.
How is mitosis similar to meiosis II?
Separation of sister chromosomes
Describe how DNA nucleotide substitution can lead to a non- functional enzyme
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.
What is a chromosome mutation?
Change in the structure or number of whole chromosomes
What is the equation for calculating the possible chromosome combinations as a result of independent segregation during meiosis
No. of chromosome combinations = 2 n
(where n is the number of PAIRS of chromosomes.
How is chromosome number halved in meiosis?
Separation of homologous pairs.
One of each pair ends up in the daughter cells.
Define proteome
Full range of proteins that a cells can produce
Define genetic diversity
The number of different alleles of each genes (within a population)
What is a species?
A group of organisms with similar features that can breed together to produce fertile offspring
Describe the stages of natural selection
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
Describe how direction selections produces antibiotic resistant bacteria
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
Describe the effect of stabilising selection on human birth wieght
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
Name two types of variation and give examples
Continuous variation eg height, weight
Discontinuous variation eg blood group
What is directional selection and what does it result in?
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
What is stabilising selection and what does it result in?
Selection that favours the average characteristic
Tis preserves the characteristics of the population
Name the point on homologous chromosomes where the sister chromotids make contact and crossing over might take place
Chiasmata
