2.1.6 Cell cycle - Meiosis Flashcards
What are the main features of meiosis?
The main features of meiosis are:
- The cell divides twice, forming 4 gametes
- The gametes are genetically different from each other.
Outline the first stage of meiosis
There are two main stages in Meiosis.
P1 – Chromosomes condense and homologous chromosomes pair up forming bivalents (4n)
M1 – Bivalents chromosomes line up upon the midline.
A1 – Spindle fibres contract pulling the homologous chromosomes to opposite poles crossing-over leads to recombination of chromatids.
T1 – Cell division 1, two separate cells with 46 chromosomes (2n).
Outline the second stage of meiosis
P2 – Chromosomes, of two chromatids recondense
M2 – Individual chromosomes line up upon the midline, independent assortment due to crossing – over = genetic variation.
A2 – Division of the centromeres leads to the chromatids being pulled to opposite poles.
T2 – Cell division 2, our separate cells with 23 chromosomes (n).
What happens during prophase I?
During Prophase I, the duplicated chromosomes condense and homologous chromosomes pair up to form bivalents. The bivalents are connected lengthwise by protein and crossing over can occur as chromatids entangle. A chiasma occurs at each point where a crossover has occurred. The nuclear envelope fragments as in mitosis. Spindle fibres begin to develop from each centrosome as protein microtubules extend out of them as in mitosis.
During Prophase I, the individual chromosomes of each homologous pair, come into very close contact with each other. Crossing over results in equivalent portions of the chromatid being exchanged at the chiasmata (point of breakage).
What happens during metaphase I?
Metaphase I is the same as metaphase in mitosis except that the homologous pairs line up along the metaphase plate.
During Metaphase I, homologous pairs line up with one chromosome in each pair facing each pole. Orientation of homologous pairs on the midline is random and independent of any other homologous pair. This random assortment is called independent assortment. Independent assortment can result in many different combinations of alleles facing the poles, leading to genetic variation.
What happens during anaphase I?
Anaphase I is different to anaphase of mitosis as homologous chromosomes move to the poles and chromatids stay together.
During Anaphase I, proteins holding sister chromatids together breakdown allowing homologous chromosomes to separate. The spindle guides homologous pairs towards opposite poles, centromere first. Sections of DNA on sister chromatids that was entangled, now breaks off and re-joins forming recombinant chromatids.
What happens during telophase I?
Telophase I is essentially the same as in mitosis, resulting in the nuclear membrane reforming around chromosomes at each pole.
During Telophase I, each half of the cell has a complete haploid set of duplicated chromosomes. Each chromosome is composed of two sister chromatids; one or both containing non-sister chromatid DNA. Cytokinesis usually occurs simultaneously with telophase I, to form two haploid daughter cells.
What happens during prophase II?
During Prophase II the chromosomes, which still consist of two chromatids, condense and become visible again. The nuclear envelope fragments once again, and a spindle apparatus forms and starts to move the chromatids.
What happens during metaphase II?
In Metaphase II individual chromosomes line up along the metaphase plate as in mitosis, unlike the pairs in metaphase II. The two sister chromatids of each chromosome are not identical due to crossing over in meiosis I. Independent assortment occurs again, leading to more genetic variation following metaphase II.
What happens during anaphase II?
Anaphase II, unlike anaphase I, results in the chromatids of the individual chromosomes being pulled to opposite poles. Breakdown of the proteins holding the sister chromatids together at the centromeres allows the division.
What happens during telophase II?
Telophase II is essentially the same as in mitosis, resulting in the nuclear membrane reforming around chromosomes at each pole. Cytokinesis results in the division of the cells forming four genetically different haploid daughter cells in total.
Discuss the ways in which genetic variation is produced in meiosis.
- crossing over to produce a chiasma
- independent assortment
- mutation (change in DNA sequence)
- Non-disjunction (uneven split of homologous chromosomes)
- Random fusion of gametes
What is independent assortment?
Independent assortment is the production of different combinations of alleles in daughter cells due to the random alignment of homologous pairs along the equator of the spindle during metaphase I.
