Meiosis Flashcards
What is meiosis?
A form of reduction cell division used to produce gametes for sexual reproduction whereby one diploid parent cell produces four haploid daughter cells that are genetically different to each other and the parent cell.
Why is meiosis important?
- Meiosis halves the number of chromosomes in the nucleus and produces haploid daughter cells that then act as gametes in sexual reproduction.
- Gametes need to be haploid as they fuse during fertilisation to form zygote in order to preserve original chromosome number in zygote.
What is an overview of steps in meiosis?
Meiosis I: 1. Prophase I. 2. Metaphase I. 3. Anaphase I. 4. Telophase I. Meiosis II: 5. Prophase II. 6. Metaphase II. 7. Anaphase II. 8. Telophase II.
What are the events that occur in prophase I?
- Chromatin condenses to form chromosomes that consist of 2 (identical) sister chromatids joined together at centromere. They can be seen under microscope.
- Chromosomes pair up in their homologous pairs to form bivalents, each consisting of maternal and paternal chromosomes.
- Sister chromatids on maternal chromosomes attach to points on sister chromatids of paternal chromosomes, forming points called chiasmata.
- Sections of chromatids are swapped between maternal and paternal chromosomes, about chiasmata in a process called crossing over.
- Nucleolus disappears and nuclear envelope breaks down.
- Centrioles move to opposite ends of the cell.
- Spindle fibres form.
What are the events that occur in metaphase I?
- Bivalents line up along equator in random orientations, with either the maternal or paternal chromosomes facing a certain pole (random assortment of chromosomes).
- Chromosomes in bivalents attach to spindle fibres via centromere.
What are the events that occur in anaphase I?
- Spindle fibres shorten an bivalents broken, with each chromosome in homologous pair (still consisting of 2 sister chromatids) being pulled towards opposite poles.
- Chiasmata break to leave crossed over pieces of chromosomes permanently attached to new host.
- 2 sets of haploid chromosomes are formed around each pole.
What are the events that occur in telophase I?
Animals:
- New nuclear envelopes form around each set of haploid chromosomes around each pole.
- Chromosomes uncoil.
- Cell divides into 2 daughter cells by cytokinesis.
Plants:
- Prophase II occurs straight after anaphase I.
What are the events that occur in prophase II?
- Nuclear envelopes break down and chromatin condenses in each daughter cell.
- Centrioles divide and move to opposite poles about both sets of haploid chromosomes.
- Spindle fibres form.
What are the events that occur during metaphase II?
- Chromosomes line up along equator and attach to spindle fibres via centromere.
- Daughter chromatids orient themselves randomly, with either one facing towards specific pole (random assortment of chromatids) assortment.
What are the events that occur during anaphase II?
- Centromeres divide and spindle fibres shorten.
- Daughter chromatids of each chromosome pulled towards opposite poles to form 2 sets of individual sister chromatids about each pole per daughter cell.
What are the events that occur during telophase II?
- Nuclear envelopes form around each set of daughter chromatids to form new nuclei with haploid number of chromosomes.
- Each daughter cell divides again by cytokinesis, resulting in 4 haploid daughter cells being formed.
What are the 3 mechanisms by which meiosis generates variation?
- Crossing over (during prophase I).
- Random assortment of chromosomes (during metaphase I).
- Random assortment of chromatids (during metaphase II).
How does crossing over generate variation?
- Crossing over results in different alleles being swapped between the daughter cells of maternal and paternal chromosomes in homologous pairs.
- This results in each sister chromatid in chromosomes being genetically different from each other as they contain different combinations of alleles.
How does random assortment of chromosomes (during metaphase I) generate variation?
- Random assortment of chromosomes results in random chance of maternal or paternal chromosome in bivalent facing a specific pole during metaphase I.
- This means that when bivalents are separated during anaphase I, different combination of maternal and paternal chromosomes may arise and the 2 daughter cells at the end of meiosis I will have random mixture of maternal and paternal chromosomes.
- E.g. chromosome 1 may be paternal and 2 is maternal, or 1 is maternal and 2 is paternal.
How does random assortment of chromatids (during metaphase II) generate variation?
- Random assortment of chromatids results in random chance of either daughter cell of chromosome facing specific pole during metaphase II.
- Sister chromatids are genetically different from each other due to crossing over.
- This means that when sister chromatids are separated during anaphase II, the daughter cells formed at the end will contain different combinations of sister chromatids and thus different combinations of alleles.
