Meiosis Flashcards
Homologous pairs of chromosomes
matching pairs of chromosomes that can possess different versions of the same genes/alleles
- One member comes from the male parent and the other from the female parent
- Maintained by the exact replication that takes place prior to each mitotic division
Prophase I
During interphase, the chromosomes replicate into chromatids held together by a centromere. Now the chromosomes condense (shorten and thicken) and become visible.
Homologous chromosomes pair up as they continue to shorten and thicken by coiling. Centrioles duplicate. 2 centrosomes start to move apart.
Breakages occur in parallel non-sister chromatids at identical points
Rejoining of non-sister chromatids forms chiasmata → new combinations of genes on the chromosomes
Homologous chromosomes repel each other. Chromosomes can now be seen to consist of chromatids. Sites where chromatids have broken and rejoined, causing crossing over, are visible as chiasmata. Nucleoli and nuclear envelope disappear.
Once pairs of homologous chromosomes have separated, the sites of crossing over are not apparent
Synapsis
pairing of homologous chromosomes
Chiasma
point of the join between different chromatids
Metaphase I
- Spindle forms
- Homologous pairs become attached to individual microtubules of the spindle by their centromeres
- Homologous pairs are arranged at the equatorial plate of the spindle framework
Anaphase I
Chromosomes of each homologous pair move to opposite poles of the spindle, but with the individual chromatids remaining attached by their centromeres (sister chromatids do NOT separate)
Telophase I
- Homologous chromosomes arrived at opposite poles = end of Meiosis I
- Chromosomes tend to uncoil and start to decondense
- Nuclear envelopes reform around both daughter nuclei
- Spindle breaks down
- These 2 cells do NOT go into interphase but continue into cytokinesis then Meiosis II
Prophase II
- Nuclear envelope breaks down again
- Chromosomes shorten and re-thicken by coiling
- Centrioles (in animals cells only) duplicate and move to opposite poles of the cell
- Spindle apparatus re-formed at right angles to original spindle
Metaphase II
- Chromosomes line up at the equator of the spindle, attached by their centromeres
Anaphase II
- Centromeres divide
- Chromatids move to opposite poles of the spindle, centromeres first
Telophase II
- Nuclear envelopes form around the four groups of chromatids so four nuclei are formed =four cells, each with half the chromosome number of the original parent cell
- Chromosomes uncoil and become dispersed as chromatin
- Nucleoli reform
- Cells then undergo cytokinesis
Haploid
an eukaryotic cell or organism containing only 1 complete set of chromosomes (only 1 of each homologous pair of chromosomes), shown as n, such as a human sperm or secondary oocyte
Diploid
an eukaryotic cell or organism containing 2 complete sets of chromosomes (2 copies of each homologous chromosome), shown as 2n, such as a human body (somatic cell)
explain the need for a reduction division process prior to fertilisation in sexual reproduction.
- Meiosis is a type of nuclear division that results in daughter nuclei each containing half the number of chromosomes of the parent cell
- In sexual reproduction, 2 haploid sex cells (gametes) fuse = fertilisation, to form a diploid zygote which will grow an develop into a new individual
- Meiosis produces haploid gametes, which keeps the chromosome number from doubling each time sexual reproduction occurs (every generation).
- When haploid gametes fuse, the resulting cell has the diploid condition again → Without reduction in chromosome number, the union of two gametes during fertilization would result in offspring with twice the normal number of chromosomes
how meiosis and fertilisation can lead to variation.
- Independent assortment of maternal and paternal homologous chromosomes
- The way the chromosomes of each homologous pair line up at the equator of the spindle in meiosis I is entirely random
- Which chromosome of a given pair goes to which pole is unaffected by (independent of) the behaviour of the chromosomes in other pairs - Crossing over of segments of individual maternal and paternal homologous chromosomes
- Results in new combinations of genes on the chromosomes of the haploid cells produced - Further genetic variation in the progeny of organisms that reproduce sexually is assured by the random fusion of male and female gametes in sexual reproduction (combination of genes from both parents)
