3.4.3 Genetic Diversity - mutations and meiosis Flashcards
Give 2 possible causes of variation that result from meiosis during gamete formation
- Random segregation of chromosomes
* Crossing over and recombination of chromosomes
Explain what happens to chromosomes in meiosis
- Chromosomes condense
- Chromosomes undergo independent segregation and arrange themselves in homologous pairs
- Crossing over and recombination of sections of chromosomes (this is rare)
- Chromosomes join to spindles at the middle of the cell joined by the centromere
- Homologous chromosomes are pulled apart to opposite poles (meiosis 1)
- The pairs or chromosomes are then separated at the centromere in meiosis 2 producing 4 genetically unique gametes.
Describe how meiosis causes variation and explain the advantage of variation to the species
- Crossing over
- Independent assortment/segregation (Random assortment)
- Some members of the species will be better adapted so may survive if the environment changes
- So will be able to reproduce and pass on the advantageous alleles
What is the biological importance of reducing the chromosome number when the cell divides by meiosis?
As the diploid number is restored during fertilisation
State ways in which meiosis and mitosis are different
- Meiosis reduces the chromosome number where as mitosis maintains the chromosome number
- In meiosis chromosomes associate in pairs where as in mitosis chromosomes do not pair
- In meiosis there are 2 nuclear divisions that produce 4 gametes where as in mitosis there is 1 nuclear division producing 2 body cells
- Meiosis results in genetically unique daughter cells where as in mitosis the daughter cells are all genetically identical
Briefly go through the stages of meiosis 1
- Homologous chromosomes pair up randomly (independent segregation/assortment)
- Chromatids from each pair wrap around each other then crossing over and recombination of portions of chromatids occurs.
- One of each pair of chromosomes is randomly separated into 2 daughter cells.
Briefly go through the stages of meiosis 2
- Sister chromatids are pulled apart at the centromere by spindle fibres and separate into 4 haploid daughter cells
- Resulting gametes have varying combinations of alleles
Describe the role of the centromere in mitosis
It holds the chromatids together
It attaches the chromatids to the spindle and then splits the chromatids apart pulling them to separate poles of the cell
Homologous chromosomes carry the same genes but are not genetically identical. Why?
Homologous chromosomes have different alleles
Meiosis results in cells that have the haploid number of chromosomes and show genetic variation. How?
- Homologous chromosomes pair up
- Crossing over produces new combinations of alleles
- Chromosomes separate at random
- This produces varying combinations of chromosomes
- Chromatids then separate at meiosis 2
Explain the role of independent segregation in meiosis
- To provide genetic variation.
- Allows different combinations of maternal and paternal chromosomes.
- Produces haploid cells.
Identify the 2 ways that meiosis produces variation
crossing over
independent segregation
Name an event that occurs during division 1 of meiosis that does not occur in division 2
pairing up of homologous pairs, crossing over followed by their separation to either pole
How is mitosis similar to the second division of meiosis?
separation of sister chromosomes
Define mutation.
Any change to the quantity of bases or the base sequence of DNA in an organism.
Identify the 3 types of mutation that can occur.
- Base substitutions
- Base deletions
- Chromosome mutations
Describe how DNA nucleotide substitutions can lead to a non-functional protein.
- A DNA nucleotide of a gene is replaced by a different nucleotide with a different base.
- This change alters the DNA triplet and its resulting mRNA codon.
- The altered codon translates into a different amino acid.
- This results in a different primary structure.
- Different hydrogen, ionic, disulphide bonds will form a new tertiary structure.
- Protein with different shape results with different function/non-functional.
Describe how DNA nucleotide deletions can lead to a non-functional protein.
- A DNA nucleotide is lost from a normal DNA sequence.
- This causes a ‘frame shift’ - each triplet after the deletion is shifted to the left by one base.
- This alters the resulting mRNA codons.
- The altered codons translate into different amino acids.
- This results in a different primary structure.
- Different hydrogen, ionic, disulphide bonds will form a new tertiary structure.
- Protein with different shape results with different function/non-functional.
Describe how DNA nucleotide deletions can lead to a non-functional protein.
- A DNA nucleotide is lost from a normal DNA sequence.
- This causes a ‘frame shift’ - each triplet after the deletion is shifted to the left by one base.
- This alters the resulting mRNA codons.
- The altered codons translate into different amino acids.
- This results in a different primary structure.
- Different hydrogen, ionic, disulphide bonds will form a new tertiary structure.
- Protein with different shape results with different function/non-functional.
What is a chromosome mutation?
Changes in the structure or number of whole chromosomes.
What is ‘non-disjunction’?
- Where homologous chromosomes fail to separate during meiosis 1.
- This results in a gamete having too many or too few chromosomes e.g. Down’s Syndrome sufferers have an extra chromosome.
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 = the number of PAIRS of chromosomes
What is the equation for calculating the possible chromosome combinations as a result of independent segregation during meiosis AND random fertilisation of gametes.
No. of chromosome combinations = (2^n)^2
where n = the number of PAIRS of chromosomes
explain the difference in a cell that has under gone meiosis 1 and meiosis 2
meiosis 1 - only 2 nuclei/cells - only homologous chromosomes separated
meiosis 2 - 4 nuclei/cells - separation of sister chromosomes (haploid cells)
How is chromosome number halved in meiosis
separation of homologous pairs
one of each pair ends up in the daughter cells
In down syndrome, people have an extra chromosome (trisomy) - how does this happen?
In meiosis
homologous chromosomes/sister chormatids fail to separate
define proteome
full range of proteins that a cell can produce/that the genome can code for
