Module 4 - Genetic diversity can arise as a result of mutation or during meiosis Flashcards
(23 cards)
Define homologous pair of chromosomes
2 chromosomes that carry the same genes
6 marks
Describe what happens to chromsomes in meoisis
- Chromosomes condense
- Chromosomes associate in homologous pairs
- Crossing over
- Join to spindle fibres
- at equator of cell via centromere
- Homologous pairs move to opposite poles
- Pairs of chromatids seperated in 2nd division
4 marks
Explain crossing over
- Chromatids can become twisted around each other
- This puts tension on chromatids
- Causing pairs of the chromatid to break
- Broken parts of chromatid recombine with another chromatid
- =New combination of alleles
4 marks
Describe and explain meoisis that result in genetic variation within a species
- Crossing over between homologous pairs= variation in gametes
- Independent segration of the homologous pairs leads to variation of different alleles in the gametes
- Random fertlisation of gametes
- New combination of alleles in gametes
Explain how chromosome number is halved
- Homologous chromosomes
- One of each pair goes to to opposite poles of cell
2 marks
Explain why meosis is important in sexual reproduction, apart from producing gametes that are genetically different
- Produces haploid cells
- Restoring diploid number
- Introduces genetic variation
3 marks
Explain how crossing over can contribute to genetic variation
- Sections of chromatids exchanged
- Sections have different alleles
- New combination of alleles
5 marks
Meiosis results in genetic variation in the gametes which leads to variation in the offspring formed by sexual reproduction.
Describe how meiosis causes this variation and explain the advantage of variation to the species.
- Crossing over
- Independent segregation
- Random fertilisation
- Different adaptations
- Some survive
- To reproduce
- Pass on allele
The offspring produced from farmed trout are sterile. Suggest and explain why.
- Too many copies of chromosomes
- Homologous chromosomes do not pair
OR
Homologous chromosomes do not separate
evenly - So no meiosis
4 marks
Describe how the process of meiosis results in haploid cells.
Do not include descriptions of how genetic variation is produced in meiosis.
- DNA replication
- Two divisions
- Separation of homologous chromosomes in first division
- Separation of sister chromatids in second division
- Produces 4 haploid cells
2 marks
Explain the role of independent segregation in meiosis
- To provide genetic variation
- Allows different combinations of maternal and paternal chromosomes / alleles
Explain the importance of meiosis in the life cycles of organisms which reproduce sexually. (3)
- Meiosis halves the chromosome number from diploid to haploid
- This keeps the chromosome number constant
- Introduces genetic variation / independent assortment / crossing over;
3 Advantages of asexual reproduction
Fewer stages so quicker
Only one parent involved so can colonise new environment
Produces clone so successful geno / phenotype maintained
Advantage of sexual reproduction
Increase genetic diversity so greater chance of survival
4 marks
Healthy cells with damaged DNA produce enzyme X. This enzyme slows the cell cycle by delaying the start of DNA replication. People with the disease ataxia telangiectasia (AT) do not produce functional enzyme X. Mutations occur at a higher rate in people with AT. Suggest and explain why ‘mutations occur at a higher rate’ in people with AT
- No functional enzyme x produced
- So faster cell cycle
- Faster DNA replication
- Mutations more likely to occur
3 marks
Scientists have produced a mutated rice variety in which there is no crossing over. A population of the mutant rice variety produced by sexual reproduction shows genetic variation. Populations of non-mutant rice varieties also show genetic variation.
Suggest and explain the similarities and differences in the causes of genetic variation within these rice populations.
(Similarities)
1. Both populations have variation due to independent segregation of chromosomes
2. Both populations have variation due to random fertilisation of gametes
(Difference)
3. Crossing over causes variation in non-mutant only
2 marks
Patau syndrome is a condition caused by a mutation affecting chromosome number. All the cells of the body will have this mutation. Describe how the change in chromosome number in Patau syndrome was produced
- In meiosis
- Homologous chromosomes / sister chromatids do not separate (non disjunction)
2 marks
Patau syndrome is a condition caused by a mutation affecting chromosome number. All the cells of the body will have this mutation. Explain why all the cells of the body will have this mutation.
- Mutation / extra chromosome in gamete
- All cells derived by mitosis
OR
All cells derived from a single cell / zygote by mitosis - Mitosis produces genetically identical cells
6 marks
Meiosis results in cells that have the haploid number of chromosomes and show genetic variation. Explain how
- Homologous chromosomes pair up
- Crossing over
- Produces new combination of alleles
- Chromosomes separate
- At random (independent assortment)
- Produces varying combinations of chromosomes / genes / alleles
- Chromatids separated at meiosis II
1 mark
An old form of wheat, emmer wheat (Triticum turgidum), has a diploid chromosome number of 28 (2n = 28). A wild wheat, einkorn wheat (Triticum tauschii), has a diploid chromosome number of 14 (2n = 14). These two species occasionally crossed and produced sterile hybrid plants. Due to an error during cell division, one of these hybrid plants formed male and female gametes with 21 chromosomes. Fusion of these gametes resulted in viable offspring. These plants were a new species, Triticum aestivum (2n = 42), our modern bread wheat.
How many chromosomes would there have been in each of the cells of the hybrid plant produced by crossing Triticum turgidum with Triticum tauschii?
21
3 marks
An old form of wheat, emmer wheat (Triticum turgidum), has a diploid chromosome number of 28 (2n = 28). A wild wheat, einkorn wheat (Triticum tauschii), has a diploid chromosome number of 14 (2n = 14). These two species occasionally crossed and produced sterile hybrid plants. Due to an error during cell division, one of these hybrid plants formed male and female gametes with 21 chromosomes. Fusion of these gametes resulted in viable offspring. These plants were a new species, Triticum aestivum (2n = 42), our modern bread wheat.
Explain why Triticum aestivum is fertile while the majority of hybrid plants were not.
- T. aestivum has 2 copies of each type of chromosome / is diploid
- T. aestivum’s chromosomes can form bivalents / can assort in meiosis / can produce haploid gametes
- T. aestivum’s gametes receive a copy of every chromosome / receive all the genetic information
2 marks
Division of the nucleus by meiosis produces haploid cells from a diploid cell. Nuclei produced by mitosis have the same number of chromosomes as the parent nucleus. What is the biological importance of reducing the chromosome number when the cell divides by meiosis?
- Later fertilisation
- Restores diploid number
3 marks
Differences between mitosis and meosis
- meiosis: 2 cell divisions produced 4 daughter cells, mitosis, 1 cell division produces 2 daughter cells
- meiosis: genetically different cells, mitosis: genetically identical cells
- meiosis: crossing over, mitosis: no crossing over
- meiosis: reduces chromosome number, mitosis: maintains the same chromosome number as in the parent nucleus
- meoisis: homologous chromosomes associate in pairs, mitosis: homologues independent / do not pair
